| dimitri |
| That pretty stupid thing is here: |
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| uniuniunium |
| quote: | Originally posted by dimitri
That pretty stupid thing is here: |
What makes it stupid? (just curious) |
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| Smiffy |
Dimitri was kind enough to send me some measurements of the unit showing it to be somewhat out with regards to its RIAA response.
Smiffy |
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| analog_sa |
| quote: | | What makes it stupid? |
An input coupling cap, an elco that seems part of a filter, a servo around the input stage - all this is a bit strange to say the least |
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| dimitri |
| R12 should be 1k5, R9 left pin should be connected to ground instead the 4580 output. And this is only for a start |
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| Zombie |
Hello!
I'm thinking about modding or improving the Phonobox.
I have some OPA604, 134 and 637 in my box, are they drop-in replacements?
There are some ceramic caps on the card, too. And elcos
:smash:
Earlier some people suggested there were some "faults" in the circuit, is there anything one can do?
What about increasing the output level a bit?
Got some experience with modding CD:s so difficult jobs don't scare me :clown: - I totally lack theoretical skills, but understand schematics, so if ou can help me - thanx!
Tom |
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| Zombie |
Thank you Mandat and Dmitri!
Lots to think about there...
:bigeyes:
Tom |
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| dioklecijan |
| My phono-box. |
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| Zombie |
Good morning Dio!
Looks cool!
Did you replace the opamps or any other capacitors? If you did the mods above (or other mods), what effect did it have on the sound?
Cheers,
Tom |
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| dioklecijan |
I replace:
R12 with 1,5kohm
R10 with 220 ohm
C1 with 1mF WIMA MKS4
cut R9 from mass
put 10microF and 100nF near OP-amps
Replace filter capacitors with 2200 microF
bypass regulators with 100nF |
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| dioklecijan |
Little mistake,
Cut R9 on left side of schematic and put it to mass. |
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| Zombie |
Hello Dio!
mass = ground? 0V?
Did you replace the opamps?
Tom |
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| Werner |
| quote: | Originally posted by Zombie
Earlier some people suggested there were some "faults" in the circuit, is there anything one can do? |
Just about everything in the Phonobox is either wrong or just stupid and wasteful engineering. Just what went on in the mind of the 'designer' is something I can not grasp, when it is perfectly possible to make with 50% of the parts and just two 5534 opamps something that outperforms this one easily.
Keeping the circuit and dropping in expensive opamps and capacitors makes things only worse, as such parts should be put to better use.
Dimitri goes some way to redressing the circuit errors present.
I would go further: throw away the innards, keep the box and the power supply, and put in any of the simple opamp-based phono circuits that circulate here on the forum.
A DC servo fed back to the cartridge input ... sjeeeez .... |
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| dioklecijan |
mass=ground
I don't replace OPamps.
It's possible to replace JRC4580 with better dual ultra-low noise OP maybe OPA2227 or something... |
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| Upupa Epops |
| This preamp comes not from fa Project, but from my friend Dr. Sýkora and I was the first, which was build it. In original was there two NE 5534 and in servo loop TL 071. As input cap was there foil polyester, any bipolar. This connection is about sixteen years old and was designed ASAP cheapest. When it was new, in many comparisions every win against all preamps in tests. Later it was published in one Czech electronic magazine and was ( and still is ) in our country very popular. Probably from this magazine it was stealed by fa Project and later reduciert to the form, which show Dimitri. This amp is very simple, but have very good sound and noise parameters thanks to " active damping " ( very low current noise ). Dr. Sýkora is not fool - to this time he develope many better preamps - in this time he's working for fa Clearaudio. |
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| Zombie |
Hello Upopa!
Very interesting information. Have you got a link to the original schematic?
Have you tried to "restore" the phonobox to the original Sykora?
Cheers,
Tom |
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| Upupa Epops |
| To Zombie : " restoration " is easy. Resolder both opamps and get there precidip sockets. If is PCB made for dual opamp, get there NE 5532 and instead 061 give 071 ( or AD 711, LF 411 etc. - any low noise JFet opamp with low offset ). Change input cap, get there PP or PES with the same value. With NE 553. family of opamps is SNR of this connection in MM mode better than 86 dB, which is 1 dB up theoretical value of noise of typical MM pickup ( 85 dB ). This preamp is noise optimalised for MM pickups, in MC mode is it " only " 68 dB ( noise floor is refered to 5 mV and 0,5 mV ). Value of others devices stay the same, but on output give serial resistor 100 R for better stability. That's all. Now exist better solutions, I will try to get it on this pages - it is not exist in electronic form. :cool: |
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| dimitri |
Dear Upupa Epops,
I never had the intention to offend Dr. Sykora, as I haven't heard anything about the man and/or his designs. I have read many times on this forum that you, Czechs, invented so many substantial things in audio in the past, but - I didn't discuss your self-doubt.
I discussed phonobox as it was manufactured. In it present state it contains obvious engineering flaws.
The owner of the Project is in Austria and the production facility is in Czech. I once found the Czech factory web site, where they wrote that they produce automotive electronics for one German company and Project turntables, phono and headphone electronics as by-product. |
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| Upupa Epops |
| To Dimitri : I take it only like explanation of all story around this preamp and I haven't anything agaist your name for it :cool: . It is only about how blatantly somebody ( and in this czech manufacture is sales manager " old bolshevic pig " ) know to stealed ideas of others. As I know, Dr. Sýkora don't get any penny. :xeye: |
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| dimitri |
>I haven't anything agaist your name for it
sorry this is not my design, I don't associate with it, I just gave an example how manufacturer rips off the customers |
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| maxg |
But if you open the phono stage you will find 3 sets of jumpers - 2 on either side at the top and one set at the bottom slightly to the left as you look at it.
The top 2 jumpers are for gain, the bottom one for selecting operation for either MC or MM.
If you are running an MM cart (or a high output MC design) you can move the top 2 jumpers and get higher gain for an MM cart.
Works very well and definitely improves the sound (even when you balance out the volume gain with the volume control on your amp).
I have tried this with a variety of cartridges (the standard K4 cart that comes from Project on the RPM4, the Blue Point Special and a Clearaudio Virtuoso 2) and all benefited.
Sharing - just in case there is anyone out there that does not know the trick. |
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| Zombie |
So...now I've done it!
Put sockets for fast opamp change. Tried both NE5554/TL071 and OPA2134/134, 134 preferred. +/- pins have 0,1uF to ground. Low noise opamps on the way.
Shorted out C7 elco and removed R9, R8 changed to 4k33.
R1 to 560 to give correct load to MC used.
I really wanted to get rid of all the ceramic caps close to the signal path, but didn't have proper sizes of values at hand.
The 4,7uF at the input gives me nightmares, but couldn't find anything better that was small enough to fit in...pointers welcome.
1000uF caps in the power supply as well as shottky diodes. Weird the the + and - sides of the psu aren't the same...
If there's any interest I'll report back what the owner thinks about the changes in terms of sound quality. |
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| dioklecijan |
| You can put 1microF Wima MKP (or MKS) instead 4.7 microF electrolytic |
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| schnullimaus |
What do you think about the NJM4580s?
Is it worth replacing them with, say NE5532 or
LM833? Are there other suggestions for
drop-in replacements?
From the specs, NJM4580 don't look too bad to me.
Thanks,
Stephan |
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| schnullimaus |
@Dimitri : when investigating my Pro-Ject Phono Box,
it looks to me like C2 and C5 is pico-Farad instead
of nano-Farad.
Also in the pictures in the PDF they look like
ceramic capacitors, beeing physically to small
to be in the nanofarad-Range.
Especially for C2 in the input section, in series
with R2 /47 kOhm this would have some impact on
the calculations of the input impedance,
as a 47kohm in series with 27 pF will not have
too much effect in the audio band...
In my Phono Box C2 is labeled 27 (underlined),
C5 is labelled 12 (underlined), if they were nano-farad,
the labelling should be n27 or 273 AFAIK.
To me it looks that the purpose of the feedback servo loop
is to create a "virtual" cartridge loading, hence the terminology
"ACTI-DAMP circuit for automatic impedance adjusting"
(badly tranclated from german product description
found on the web).
regards,
Stephan |
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| Zombie |
Hello!
If the ceramic cap has a low voltage rating, even a 0,1uF ceramic is VERY small...maybe better to measure them?
There is an improvement in sound with OPA604, but more noise, especially in MC mode. The OPA2228 (??) is better
cheers,
T |
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| schnullimaus |
Zombie, thanks for your reply. I will measure them
later because I have to unsolder them for measurement.
But I am quite sure they are pico-Farads.
OPA604 has FET inputs as opposed to NJM4580,
NE5532 etc. Many People advise against changing
BJT against FET Types. Did you try OPA604 in the Phonobox?
Also, im am very intersted in Opinions about NJM4580.
It has good-looking specs in my opinion. Some
posts above there was the advice to change them
to NE5532, to come back to the original design.
I have some thoughts that NJM4580 is somewhat
better than NE5532, from the specs.
Thanks,
Stephan |
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| Zombie |
schnulli, I made the mods for a friend. The sound in MC was more detailed etc with OPA604 but noisy. He put in the NJM and the sound was "softer" and less defined but no noise. The best result was with OPA2227 (singel ver - wrote 2228 earlier), good sound, no noise. It's even better in MM mode.
Cheers |
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| dimitri |
Stephan wrote > when investigating my Pro-Ject Phono Box,
it looks to me like C2 and C5 is pico-Farad instead of nano-Farad.
Dear Stephan,
I could not check right now, as I don't have the Phonobox at hand.
:att'n: May be other readers could check?
But R2C2 will be 47*10e3*27*0e-12=1.27us that corresponds 125kHz, so the cartridge will not be loaded at all in audio band.
Stephan wrote > To me it looks that the purpose of the feedback servo loop is to create a "virtual" cartridge loading, hence the terminology "ACTI-DAMP circuit for automatic impedance adjusting" (badly tranclated from german product description found on the web).
If you have the original design by Sykora, mentioned by Upupa Epops could you share?
With R7=330k and C4=0.33uF it will be only integrator, otherwise C6 and R5 are in the wrong place.
Stephan wrote > Also, im am very intersted in Opinions about NJM4580.
Try to add 2k2 resistors between op-amp outputs and –15V rail |
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| schnullimaus |
Dimitri,
thanks for your reply.
Unfortunately, I do not have additional Information
nor the original design.
But what could be the purpose of having the
integrator in that place. Especially, it you reconnect
R9 as suggested, there will be no DC-Path to the next stage,
so DC offset removal is probably not the intention here.
DC-Removal would only make sense if R9 would be connected
to the output of NJM4580 in my opinion
What would we loose if we remove that TL061 and
all it's associated components at all ?
2K2-Resistors force the NJM4580 into Class-A, right?
Do you think a NE5532 would sound better ?
Thanks,
Stephan |
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| dimitri |
Stephan wrote > But what could be the purpose of having the integrator in that place. Especially, it you reconnect R9 as suggested, there will be no DC-Path to the next stage,
so DC offset removal is probably not the intention here.
I start to think, that the R6C5 network should be connected in series with C4, thus TL061 stage will have -1 gain at ac, so the input resistance will be R5/(1+R4/R3)=40kOhm.
Can you check R6C5 connection in your unit? In mine I checked it triple times.
Stephan wrote > DC-Removal would only make sense if R9 would be connected to the output of NJM4580 in my opinion
Why use R9 and C7 at all if R9 would be connected to the output?
Stephan wrote > What would we loose if we remove that TL061 and all it's associated components at all?
Nothing.
Stephan wrote > 2K2-Resistors force the NJM4580 into Class-A, right?
Yes
Stephan wrote > Do you think a NE5532 would sound better?
I wouldn't like to start a usual flame here but 4580 into class-A is more then enough ;) |
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| Upupa Epops |
| Guys, who will send me email, I will send him original schematic of Actidamp Mk II, not copy by Project. Questions ( if will be ) I can explain later. |
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| bobolix |
Hi guys,
the Phonobox / Actidamp II is quite old stuff. Nevertheless, there exists a new product, the CLEARAUDIO BASIC, which is made very similar way (from the same author), and this is assumed to be a flagship of the CLEARAUDIO phono preamp program.
Concerning the opamps: The very best is the AD745 for the MM, AD797 for the MC at the front end, but they are single ones in SOIC package - there is needed an adaptor. The absolute top among duals is Harris HA5222, but it is out of production yet. By measurement, the NE5532 has lowest distortion and very good noise, the JRC/NJM4580 is similar by noise and worse by distortion, but some say it sounds better. The OPA2227 is quite slow, but very good in respect of noise. The brand new AD8672 is very good, too, but it is produced only as SOIC, so again an adaptor is needed.
Concerning the ceramic caps - they introduce a lot of distortion, but some people like the resulting sound. Originally the polypropylene caps were used, the polycarbonates are even beter, but the good polyester work well, too, although they can be found some distorting pieces - probably due the weak winding of the foil.
And, finally, don't fear the bipolar elcaps - at the output voltage of the phono pickup they perform quite well.
By the way, you can follow the offer of Upupa Epops - take the original Actidamp II, although they are better things now, but not as an "open source"...:smash: :smash: :smash: |
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| schnullimaus |
Dimitry,
congratulations :
"I start to think, that the R6C5 network should be connected in series with C4" turned out to be true. Here is what I found in my
Phonobox:
R7 / 330 kohm sits directly between Output 4580, Pin 1
and negative Intput TL061 / Pin 2. C5||R6 is in Series with C4:
Pin 6 TL061 --C4 --(C5||R6)--Pin 1 TL061
Also, C5 and C2 are pico-Farad, not nanofarad. I unsoldered
and measured them to be sure.
What does that tell us ??
Regards,
Stephan |
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| Upupa Epops |
| Little explanation : bobolix is Dr. Bohumil Sýkora, author of this preamp and certainly many others :cool: . |
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| zinsula |
Hey you Czech guys seem to be pretty up to the job in audio engineering! You know what you are telling. Keep up the good work.
:up:
Cheers
Tino |
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| dimitri |
Stephan wrote > What does that tell us ??
This is no dc servo, this is so called 'electronic cooling'.
From Marcel van de Gevel EW paper: "When the input stage and feedback network are properly designed, the thermal noise of the 47k resistor shunted across the input is usually the largest remaining noise contribution in the RIAA amplifier itself".
Such connection will lower noise current spectral density from 47k resistor 1+R4/R3 times, but overall s/n ratio will improved 1-3 dB |
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| schnullimaus |
Dimitri,
that's interesting. I never came across that before.
Thanks a lot for clarification. Electronic cooling ...
sounds cool ;-)
Do you think R9 should still be connected to ground
now, or does it make sense now like it is?
In the formula of calculation of time constants it does't
seem to make a difference wether R9 is connected
to gound or to the Output of NJM4580.
I'd really like to have the original Actidamp schematic
to compare it to phonobox.
regards,
Stephan |
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| Werner |
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| bobolix |
Dimitri discovered it - Marcel van de Gevel found the "Actidamp" principle about two years before me - but I can't his article (it was first published in Journal of Audio Engineering Society), so I found it independently.:clown:
The schematic shown below looks quite well - unfortunately the 2SK170 is out of production. But, it can be replaced by some switching JFET, e.g. 2N4393 - many of low level JFET switches having RDSON below 100 ohm are suitable, but they need to be selected into pair and tested for noise. |
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| Upupa Epops |
| To Werner : It is schematic of my version active damping, amp named Actidamp IV. It was published in magazine Amatccaerske Radio cca eleven years ago. There is used " separed " all pasive correction - better explanation of function give you BOBOLIX, 'cos he is author of this principle. |
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| zinsula |
| quote: | Originally posted by bobolix
unfortunately the 2SK170 is out of production. |
:confused: I checked the Toshiba Web site, and there was no information that would make me believe that the 2SK170 is out of production. It is listed in the database and the latest datasheet is from May 2003.
Cheers
Tino |
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| schnullimaus |
Hi,
in the meantime i've swapped the op-amps in my
pro-ject phonobox:
TL061 => OPA134 (this is a FET Opamp used for Actidamp in
the phonobox)
NJM4580 => OPA2228 (this is the amplification part)
Also, i adjusted a number of resistors and capacitors,
as the phonobox (in some older revisions, including mine)
is somewhat inaccurate.
Pleas refer to the schematics here (thanks to Dimitri!!)
http://www.angelfire.com/az3/dimitr...es/phonobox.pdf
DO NOT connect R9 to ground as it is suggested in the PDF,
but leave it as it is. R9|| R8 = 5003,8 Ohm, together with
C6 gives 75,06 uS time constant !!
From talks to the developer of the original design I learned
that R9, C7 are not meant as a low-pass Filter, but this
is an optimasation noise-wise! My thanks go to Pavel in
czech republic!!
What I did exactly is :
R5 : 1,32 MOhm, was 1,2 MOhm
R6, R7 : 220kOhm , was 330 kOhm
C4 : 0,47 uF , was 0,33 uF
R8 : 5.6 kOhm (selected), was 4,7 kOhm
C7 : 10uF, was 2,2uF
C6 : 15nF (selected), was 18 nF
R10 : 300Ohm, was 270 Ohm
R11 : 2,7kOhm (selected) , was 2,7kOhm
R12 : 2,2kOhm (selected), was 2,2 kOhm
R13 : 47KOhm (selected), was 39 kOhm
C9 : 68nF (selected)
Also I bypassed all electrolytics with WIMA MKS2 capacitors,
approx. 0,33 uF, and replaced the ceramic capacitors
(100pF, 27 pF, 12pF) against better ones (Glimmer type,
sorry, I don't know the english word)
Altogether if gave a real boost to the phonobox, and in
comparisation to a Lehmann Black Cube the modified
phonobox does't made a bad figure in my setup at all!
In fact the results triggerd me to replace resistors /capacitors
in the Black Cube for better RIAA accuracy. It is much better
now, though all replaced parts were in their specified tolerance.
I have a unfortunate placemant of my speakers,
I am too far away from them, so my setup is very
sensitive to deviation between channels. My original
Black-Cube was alwasy somewhat out of center, after
replacing the parts it is 100% perfectly balaced!
I am getting the impression that maximum RIAA accuracy
it most imprtant to good sound. Use selected parts, try
to get better than 1 % tolerance to the CALCULATED
values. On resistors, you could use those (somewhat expensive) 0,1 % precision resistors. At the end, it doesn't cost
so much more compared to all the other efforts in an DIY project.
I do not think that the usage of "voodoo-parts" will
greaty improve the results.
Regards,
Stephan |
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| dimitri |
>>From talks to the developer of the original design I learned
that R9, C7 are not meant as a low-pass Filter, but this
is an optimasation noise-wise!
Stephan, please, woud you like to explain this "noise-wise optimasation"?
Thank you |
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| schnullimaus |
Dimitri,
if i understood correctly, the idea is to reduce
current noise by increasing input resistance.
47 KOhm causes much less (dc) current flowing through
the opamp input compared to 5,6 kohm.
greetings,
Stephan |
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| OliverD |
As Jocko Homo told me in a different thread, some of the best Toshiba audio transistors are now being made "special order" with a minimum order quantity of 3000 pieces. This includes 2SK389 and maybe also 2SK170. They will disappear from the browsable product list but datasheets remain available.
I have a large overstock of 2SK170-BL. Email me if you're interested. |
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| bobolix |
| quote: | Originally posted by dimitri
>>From talks to the developer of the original design I learned
that R9, C7 are not meant as a low-pass Filter, but this
is an optimasation noise-wise!
Stephan, please, woud you like to explain this "noise-wise optimasation"?
Thank you |
In fact, the lowpass of 75 us is created by complete circuit
consisting of R8, R9, C6, C7. The purpose of such complicated circuit is to achieve similar DC resistance "seen by" both non-inverting and inverting input of the following opamp and same time achieve low AC impedance of the low-pass, so that the noise introduced by the input current noise of the opamp would be reduced as well as the thermal voltage noise generated by the total resistance in the lowpass (R8 || R9). |
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| schnullimaus |
Thanks for clarification!
Stephan |
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| Upupa Epops |
| Guys, you can give to Bobolix any questions about phono preamps - in this thematic he knows absolutely ALL and I claim it quite seriously :cool: . |
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| dimitri |
>> In fact, the lowpass of 75 us is created by complete circuit consisting of R8, R9, C6, C7. The purpose of such complicated circuit is to achieve similar DC resistance "seen by" both non-inverting and inverting input of the following opamp and same time achieve low AC impedance of the low-pass, so that the noise introduced by the input current noise of the opamp would be reduced as well as the thermal voltage noise generated by the total resistance in the lowpass (R8 || R9).
Sorry Bohumil, I will interfere
1) yes, dc "seen by" both non-inverting and inverting input will be around 47k
2) > achieve low AC impedance of the low-pass - the output impedance of low-pass will be R8 4k7 with gradual decrease at hf. How R9 C7 will lower this? (R8 || R9) is only 10% reduction.
3) > the input current noise and thermal voltage noise - again, (R8 || R9) - 10% reduction is negligible.
4) dc shift of the input amp, worst case 5mV*28=140mV, will be presented at the output |
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| padamiecki |
| quote: | Originally posted by Upupa Epops
Guys, who will send me email, I will send him original schematic of Actidamp Mk II, not copy by Project. Questions ( if will be ) I can explain later. |
it is me again,
please send me a sch if possible:) |
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| Upupa Epops |
| Later, now I'm not at home, OK ? |
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| bobolix |
Hi boys,
don't forget it is a construction about 20 years old. I learned something since then and now I'd make it different - frankly said, I did it so, but it has been made by an order and therefore it is a subject of the "bussiness secret". Though, I can say, the dc-servo path needs to be modified slightly at least ... :cool: |
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| Ludwik |
| Please find attached circuit diagram from one of the Polish magazine "Radioelektronik" No 10/1991 |
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| Ludwik |
| ...and the PCB layout. |
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| bobolix |
| ... the Actidamp was made as an optimization for the MM pickup regarding noise. Recently the MCs are assumed to be the best usually and the Actidamp need not to be the optimum solution. E.g., the linear pre-preamp could be made as balanced using some of ICs determined for mic preamps - INA217 e.g. I've made even a fully symmetrical phono preamp, i.e. with completely symmetrical signal path including the equalization circs, but probably it will never be published. |
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| mzperx |
| I would share my symmetrical input version (with INA217). It was made on request of my friend. He his very happy with that... |
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| bobolix |
| quote: | Originally posted by mzperx
I would share my symmetrical input version (with INA217). It was made on request of my friend. He his very happy with that... |
I'm very curious, really, but, unfortunately I'm unable to download the attachement - I'm getting zero length file. Could you up it somewhere else - on any file hosting or imagehosting - e.g.
| code: | http://www.imagehut.eu/ |
:confused: :confused: :confused: |
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| mzperx |
| The schematic is attached again. Hopefully you can open it. |
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| bobolix |
Thanx, I did it.
Well - quite nothing to add. It seems to be intended for the MM pickup. For the MC the R6 could be lowered. The impedances around U2 could be higher (particularly R8 bigger, C7 smaller), as the OPA134 has the input noise voltage comparable with 4 kOhm resistance (1,1 uV over the audio band approx - measured by AP) and the noise voltage at the output of the INA 217 is about 20 uV in this circuit. And, in general, the INA 217 is not the optimum choice for the MM preamp due its quite high input noise current - the best solution IMHO is with a low noise FET OP, with unsymmetrical input, indeed.
Nevertheless, this circuit certainly performs very well as it is - the comments above are rather of cosmetic kind.
BTW, By some opinions I met last time, the SSM 2019 "sounds better". But, I can't judge it. |
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| Calvin |
Hi,
nice design, which reminds me of my own and which is capable of extremely good performance :-)
A few comments on parts and schematic.
The used parts are all lowcost parts. You can improve the design a bit by using similar better parts. The input receiver INA217 can be substituted by the INA163UA (SO-14 package) or the INA103 (DIP-14 or SOL-16).
The INA163 features lower distortion and lower noise than the INA217. The internal feeedback resistances are smaller -->less thermal noise- and the output stage is less noisy.
The INA103 is even better than the INA163, though only slightly (distortion figures) but at double the cost.
If You replace R6 by a series connection of a fixed and a variable resistor (e.g. discrete resistors switched by a DIP-switch or a pot) You can change the gain of the circuit such that it fits the need of Your pickup. I´ve used resistance values that allowed for an input sensitivity range from 0.1mV-5mV for nominal output voltage of 300mV and had excellent results with whatever pickup I used.
This is one great advantageof this circuit topology. It can be used with extreme flexibility. Add some R- and C- variability at the input, add some switch to change between symmetrical or asymmetrical input and You have a full fledged onefitsall solution for Your vinyl probs.
The LR-networks at the INA217´s inputs shall prevent the OP from oscillating with very low source impedances (<10Ohms) but it raises the noise figures remarkably. Most pick-ups feature a higher ohmic value anyway and their inductance will raise the impedance figure with growing frequency even more. So You can omit the input guards. Just think of using them (or test for oscillations), when You want to run an extremly low output MC with this stage. Using the INA103 for more than 12Years now in such a circuit I never had any probs driving the stage from high-output MM´s like Linn K18, to the ever popular mid-output MC Denon DL103 to the low-output enigmatic machine Koetsu Urushi. :) I just used small ferrite beads on the signal cables.
The integrator stage around U3 could be improved with an low voltage noise, low offset OP. The OPA137 is quite high in voltage noise and offset, especially those with suffixes ´A´. Since the OP works in unity gain over most part of the freq range and low impedances/shorts are presented at it´s inputs low voltage noise figures are a plus. Low current noise is just useful below the filters (R9/C6) -3dB point. FET-types are good since they are less prone to offsets induced by largely differing impedances at their inputs which can be as high as ~1megOhm in this case! Low offset figures and offset drift figures are needed, since freqencies below 50Hz are pushed in the second gain stage with a gain of up to ~32 (30dB) and any offsets at the input of this stage are presented at the circuits output 32times higher. The typical +-2.5mV (max. 10mV) input offset of the OPA137xA could lead to an excessively high output offset at the circuits output!
OPA604, OPA134, AD711 and AD8610 are all better here, with my favourite -the AD8610- beeing ~10 to 30times better than the OPA137. Even the lowcost OPA134 is min. 5 times better!
The offset network R9/C6 has a very low crossover frequency (<1Hz). The RIAA/IEC equalization recommendation mentions a low frequency cut of 20Hz (7950µs time constant). You can use the offset circuit not only to correct for offsets but also to equalize correctly after RIAA/IEC. As a positive sideeffect You can lower the values of R9 and/or C6.
The 2120Hz network R8/C7 could be designed with slightly higher ohmic value for R8 to lower the capacitance value. It might be a bit problematic to get values of 100nF with first class capacitors like WIMA FKP2 or Mica types. Since the noise of the complete circuit is mostly dependent on the first gain stage and since the second stage is made up from a FET-type (which have lowest noise figures with impedances in the >5kOhms range) You can safely choose R8 in the 2k-5k range and reduce C7 down to 30nF-10nF without sacrificing much on noise figures.
In theory the preequalization raises the signal level till ultimo. In practise the cutterheads couldn´t withstand ever rising signal levels. So a additional time constant of 3.18µs (~50kHz) was introduced by the cutterhead manufacturer Neumann . This time constant should be corrected for within the equalization network. This can be done with a simple resistor in series with C7 (100-300Ohms). There will be a slight tonal difference between the traditional- and the Neumann-equalization. If You use a simple switch to short this resistance You can try which one suits Your taste best.
The use of an OPA134 as second gain stage OPamp is a very good choice. An AD8610 will bring just rather minor improvements.
All OPs I mentioned are available as DUALs too and will give better results than the OPA137/134 combination. My favourites here are the AD8620, followed by the OPA2134 and OPA2604. The use of DUAL-OPs eases the PCB layout and is of no bad at all in this case.
For the same reason of capacitor choice You could think of designing the equalization network R10/11/12/C8 with higher resistance- and smaller capacitance values.
The 100Ohm R7 is used to guard the output of the OP from large capacitive loads. It is useful when the circuit is connected to the next stage (preamp input or poti) by a long cable or a highly capacitive input. If the circuit is built into an preamp wit its short cable lenghts You can omit R7. Modern OP amps can safely drive such loads. ;)
jauu
Calvin |
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| mzperx |
bobolix, Calvin thanks for a valueadded comments!
This preamp was designed for high output MC.
DC resistance of the affected cartridge is 95 Ohm, so I think that it is well matched as possible to the actual input stage (INA217).
In the 1st version LR-network was fully omited, but due to oscillating I have tried to apply 330Ohm ferrite bead. It could help a lot, but it didn't kill tendency to oscillate, thereby I took the recommended stabilization network.
Availabiliy of FKP2 capacitor is really only up to 22nF from my sources, so I will recalculate resistor values.
Calvin, since your post I don't like the applied dc servo solution anymore...
Is it a good way to eliminate dc error at the circuit ouput if the feedback resistor of dc servo stage would be conncted to output of 2nd amp instead of output of 1st?
Or much more recommended to use inverting circuit in position of 2nd stage with local dc servo loop???
How would you update the circuit in order to be capable of balanced and single ended outputs (both together)?
With adding of DRV134?
thanks in advance
Zoltán |
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| Calvin |
Hi,
Yup, You´re right. This circuit is optimized for mid- to high-output MCs.
You got oscillation with an high-output MC? Well that shouldn´t happen. I used the INA103 and had never any probs. I´ll try the INA163 though with my next project probabely in January.
I use 10nF Mica 1% and parallel those (2 or 3). This reduces tolerances even further. Since the sound of a phonostage is mainly due to the precision of the equalization I usually simulate the components to an optimum value and invest some bucks on highquality low tolerance parts ;) I´m quite confident with the results (usually less than 0,1dB).
The DC-servo works very good as it is. The first stage needs some measurement against DC. Since You correct for DC in the second stage of the input device You don´t have to worry about coupling caps at the very input of the circuit (a lot of MC-headamps push the poor little signal through big nasty electrolytics). I tried to connect the DC-servo between the circuit´s output and the reference Point of the INA but this gave always stability problems. So I stuck to the original solution. Using OPA2604 for the servo and the second stage amp, I never had problems regarding DC at the output.
There are two measures to cope with the problem if DC occured
- You could add an DC decoupling cap right at the output
- You could change the DC-gain of the second stage to a value of 1 by adding a cap in series with R10
The first measure gives You relly no DC at the output and is the safest method for Your system, but it may degrade sound
With the second measure a negligible small -and safe- part of DC is still there, but You can design the values such that the 1st order equalization below the 20Hz RIAA/IEC (7950µsec) by servo No. 1 is assisted to a second order (-12dB/oct) character.
Anyway, I prefer the simple solution as it is shown in the schematic.
I had an order for a standalone vinyl solution by a former friend which offered the following features:
- symmetrical and asymmetrical Phono-inputs
- switchable input resistances and capacitances
- switchable gain to accomodate from low-output MCs to high output MMs
- one symmetrical and one asymmetrical highlevel-input.
- highest quality volume control with switched discrete resistors (64 steps)
- one symmetrical and one asymmetrical output
- IR controlled Volume, Channel and on/off/mute
- manual control by digital encoder (one knob design) and 2 digit display
- 2 SMPS, one for +-15V audio and one for the Volume controller
- casing routed from a massive aluminium block with 1/2 HiFi width (22cm)
The Phonostage and the two highlevel inputs are switched by the volume controller PCB and the signal is routed through the volume resistor cascade. Then it is buffered by two parallel working buffers, one of which giving an asymmetrical output and the second a symmetrical output. You can use the DRV134/135 or SSM 2142 for highquality, easy and space- and part saving solutions. For the asymmetrical output I use a circuit that is capable of driving large capacitive loads and uses a OP together with an discrete or integrated buffer (e.g. BUF634)
There are two PCBs, one for the volume controller and one for the complete audio circuits. The audio PCB is a doublesided layout which makes intensive use of SMD parts (I used only parts in throughhole technology where they provided for higher quality than SMD, mainly in the EQ-circuit and powersupply)
Recently I found out that Clearaudio offers a quite similar stage (named Balanced something xxx) which uses a similar topology, but with considerable less flexibility and comfort and component quality for a mere 3.000€. I found the measurements -though all in all on the good side- below whats possible with this circuit.
jauu
Calvin |
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| bobolix |
:D :D :D
The ClearAudio mentioned above is based on my older design, which I made for one czech company, which manufactures several products marked as Clearaudio (including the engraved front panel), hehe. The measurement results are not very serious, really, although the device performs quite well - but, unfortunately, although we made our own measurements, we have no influence on the marketing policy of ClearAudio.
The original design was with SSM2017 and after discontinuing production it has been redesigned for the INA163. Theoretically, it exists true replacement for SSM2017 made by THAT, but it is very hard to obtain. Now I'm finishing a fully symmetrical design using INA 103.
Some notes more: The HF instability at open input is caused by capacitive coupling between input and output usually. The remedy is rather capacitive shunting between inputs than using of LR decoupling.
And why fear the DC servos ? The OPA177 (and similars) is a very good choice for this purposes. The output noise of the DC servo presents no problem and it can be easily filtered out.
The second stage need not to be FET, if some DC servo is used with sensing of the end output. The AD5671 may be the optimum compromise. |
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| arnoldc |
hi bobolix! Did you design the Clearaudio Basic Symmetry as well? This is their MC-only phonostage with RCA input and very small 5-pin connector with cartridge auto loading.
I was wondering if I can rip it up and make improvements? Although my warranty says I still have to wait 4 more years before it expires :D |
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| bobolix |
| quote: | Originally posted by arnoldc
hi bobolix! Did you design the Clearaudio Basic Symmetry as well? This is their MC-only phonostage with RCA input and very small 5-pin connector with cartridge auto loading.
I was wondering if I can rip it up and make improvements? Although my warranty says I still have to wait 4 more years before it expires :D |
Yep, it's my child, except the input RCA - this is a nonsense degrading the symmetry, indeed. There should be the 5pin only, but it would be incompatible with usual connections, so this ... hmmm ... emergency solution. You can make any modification in order to get the input symmetry back, but, about warranty, it is at your risk. Concerning other improvements, you can consult them with me (recommended). |
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| arnoldc |
| quote: | Originally posted by bobolix
Yep, it's my child, except the input RCA - this is a nonsense degrading the symmetry, indeed. There should be the 5pin only, but it would be incompatible with usual connections, so this ... hmmm ... emergency solution. You can make any modification in order to get the input symmetry back, but, about warranty, it is at your risk. Concerning other improvements, you can consult them with me (recommended). |
I'd take you up on that offer! Should I send you and email and we'll take this offline? |
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| mzperx |
Hi bobolix and Calvin,
could you please look at the attached version and give any further commnets?
I have changed the input termination circuit according to Federico Paoletti's schematic. I feel that it's matched better to RCA inputs.
Impadances are also changed in equalization circuits.
In order to keep dc error at the output low as possible, gain of 2nd stage was decreased. As per my calculation it can be max 40mV in worst case.
And balanced output is introduced too.
regards
Zoltán |
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| Calvin |
Hi,
so far the circuit does what it is expected to do in a simulation. The lower mids are slightly elevated (+0,5dB@200Hz) and -1dB-freqs at 36Hz and 22kHz, resp. 75kHz (switch at 50kHz position). Gain @1kHz is ~40dB, giving 775mV Output voltage for 5mV of Input voltage.
Reducing R2 to 10Ohms will give a gain of 60dB and 775mV Out for 0.5mV In. The second stage doesn´t support the gain above 1kHz so the gain of the first stage (@1kHz) is basically the gain of the whole stage. While the INAs of BB work very good in a gain window of 50-500 (best with a gain of 100) and this is what You need for MM-use, I prefer to give the second stage some gain too (around 10dB) to supply for enough gain even for low output MCs without stressing the INAs with too high gains.
I find the passive EQ-circuit quite clever. While the classical RC-circuit (with additional switchable resistor connected to ground) gives slight deviations (0.1dB) between Standard- and Neumann-EQ (50kHz), the design in the above PDF gives identical frequency responses independent of the switch´s position.
What do You expect from R3/C7 at the Input? It earthes the RCA-ground but leaves only asymmetrical signal handling left. In this case a symmetrical input device like the INA is of no better use than a classical OP. Indeed are the noise figures higher than with an OP which states the same noise figures in the datasheet(!) since there are two equal inputs connected in series noisewise. With uncorrellated noise signals the actual noise figures of an instrumentation amp has to be multiplied with SQR2 to give the correct numbers. The use of an INA makes really sense when the signal is fed to the circuit in a symmetrical fashion! If You just want to use MMs -which mostly are no symmetrical generators because of internal grounding of the coils- You´d be better of with devices like LT1028 or AD797!
jauu
Calvin |
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| mzperx |
Hi (Calvin),
I intended to ensure lower impedance for shielding of RCA cable by adding C7/R3. I knew that this kind of solution will degrade capability of INA, but I thought that it's a best what I can do in case of RCA inputs.
Of course the optimal solution would be to apply XLR connectors for phone cartridge inputs where the shielding is conected independently from signal wires.
In practice it can be achieved with a simple connector change on the end of phono cable so I have decided that I will populate only XLR type input connectors on PCB. Also the RCA asym. output connector is omitted.
At version 1.3 the frequency response deviation was due to my faulty calculations at 2nd stage. I applied new EQ circiut where componenet values can be easier to determine and and 3x gain was added to this stage also.
I have rechecked/simulated the new response with TINA-TI (and with help of inverse RIAA network from Lipshitz), and it seems that the overall accuracy is in +/- 0,1dB tolerance now. (Unfortunately this version allows to simulate only 2 OPAs at a time. So I could simulate the fully preamp only partially or in more steps.)
There are few new features introduced as selectabe input impedance and gain seetings.
Please find the SCH of modified preamp and please don't hasitate to reflect on design!
Happy New Year! And lot of succesful projects!
regards
Zoltán |
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| Calvin |
Hi,
now that looks very fine to me ;) Add a really good PCB layout and You´ll have a smashing good phonostage, that will give a lot of commercial phono-pres a hard run for their money :D
jauu
Calvin |
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| mzperx |
Hi Calvin,
I would like to say thank you for your helps.
The phono stage is fully completed and it sounds really impressive.
greetings
Zoltan |
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| Calvin |
Hi,
it was a pleasure to help ;-)
Its probabely hard to pull Yourself away off Your gear for some minutes, but if You could, I´d like to hear more about Your impression of the sonic character of the preamp. ;-)
jauu
Calvin |
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| sek |
Hi Zoltán,
a very clean board, congratulations. :cool:
Just like Calvin, I'd also like to know how it sounds and how it compares.
And I've got another couple of questions, if you don't mind... ;)
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I can see you used Wima FKP2 capacitors in the front end.
In that range and size you can buy MKT, FKP and FKC types (despite mica and ceramics).
Any particular reason you chose the FKP dielectricum (despite the availability in better selection grades)?
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I can see you attach power to the board by another XLR plug, right? Funny. :)
Why do you go for no local power supply decoupling near the ICs at all?
I assume this detail is just a difference between the posted schematic and the board you made, as the additional caps are easy to spot near the INA, OPA and DRV...
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What are the ferrite beads doing, and why?
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Are you aware of the 'pin one problem'? How is noise immunity with your board and your connections, now that you seem to mix signal and power supply ground in the high-gain stage?
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Thank you for your effort and sorry for all the bothering questions. :D
Cheers,
Sebastian. |
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| mzperx |
Hallo Calvin und Sebastian,
First of all I must say that this phono stage isn’t working in my system (I prefer only digital sources), but one of my friend’s quit expensive chain. He has ordered it, because his new 2nd hand ML38 preamp doesn’t contain phono input. In order to satisfy your interest, I have already requested a short description/comparison concerning sound quality from my friend. As soon as I have it I will share with you.
To return to the questions:
The choice of FKP2 was based on Cavin’s suggestion, and in the other hand the easy availability of this WIMA series at local suppliers.
Between the outputs the 5pin XLR male connector can receive the external PSU, because I didn’t want to put the transformator in same case together with the preamp. (And optional the user can use batteries as well…) Currently one modified Jung regulators is used as power source in same size ALUBOS housing then the preamp.
Electrical point of view the ferrite beads don’t play to much role in this low frequency application (increasing impedance of bypass loops), but their usages and lengths allow to increase the surface of ground plane around ICs.
The ‘pin 1 problem’ was absolutely new for me. But I’m sure that it isn’t issue in case of this design because I/we didn’t find any hum & buzz.
And finally please find a picture about a topview… And don't hasitate to tell me your critisms!
Tschau
Zoltán |
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| bobolix |
| I spent some time at designing completely new circuitry - in cooperation with my business partner, so I can't publish it, unfortunately. Note it is a MM-MC switchable fully-active-equalized phono preamp with tolerance ±0.2 dB inside the audio band. They were made extensive tests around distortion using Audio Precision System 2712 and it was found, that certain dual opamps exhibit strange behavior concerning odd harmonics. Evidently there originates certain error voltage at internal IC traces of one subsystem and this error is transferred into signal path of the other subsystem of the dual opamp. Particularly the new AD 8599, although being very good concerning noise, is quite susceptible to this kind of distortion (frankly said, they are some hundredths or thousandth percent THD in the game). In contrary, the NE 5532 seems to be very good from this point of view. But, the work is not finished until now... |
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| JesseG |
Hi all
Since this thread started on the Pro-Ject Phono Box, if we could come back to it for a moment...
Does anyone have the schematic for the Pro-Ject Phono Box MkII?
This is a whole different animal from the original - MM/MC switchable and TL 071 MC input gainstage.
The only thing it dosen't offer is variable loading, so I would like to make suitable changes for my DL103. This gizmo uses a small, double-sided PCB, so the component placement is not all that obvious.
Any help would be greatly appreciated :D
Jess |
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| bobolix |
| Oh, God, why TL 071 ??? This IC is absolutely unsuitable for this purpose. Recently the AD 8671 is definitely the best choice for the phono preamps either MM or MC. |
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| mzperx |
Hi Bobolix,
technical parameter point of view the AD8671 is very similar to OPA227. Did you perform any (listening) test between these competitors?
In the meantime OPA2134 was upgraded to OPA2227. This change definitely improved the resolution in high range.
Zoltán |
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| JesseG |
| quote: | | Oh, God, why TL 071 ??? This IC is absolutely unsuitable for this purpose. |
I certainly don't know :xeye:
There are 2 8-pin chips per channel - one is the TL 071 and the other is a JRC 2068D.
Maybe we could ask someone from Pro-Ject?
Jess |
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| bobolix |
I afraid the OPA 2227 is not the optimum choice, because it has fairly low slew rate. For the purposes, where the final gain is higher or equal to 5 the OPA 2228 is definitely better - in fact this is an undercompensated - and so far not unity gain stable - version of the 2227. The AD 8671 (in fact AD 8672 as dual) lies somewhere between - it has similar (or lower) noise, the SR is lower than 2228 but higher than 2227 and it is unity gain stable, so it is definitely better than OPA 2227 for the low gain applications.
Even substantially better should be the OPA 2827, but this is only planned for now. |
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| PMA |
| It is just high time to design Mk III, or SE Mk II ... ;) |
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| bobolix |
| quote: | Originally posted by PMA
It is just high time to design Mk III, or SE Mk II ... ;) |
Maybe, but recently I'm oriented on the design of the phono pramps for the MC pickups, which is slightly different kind of problems ... |
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| PMA |
| I am afraid that you did not get it ;) |
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| bobolix |
| Are you sure you did ? |
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| PMA |
| Nechame si to do auta. |
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| raffells |
Hi from a new member.
I too would also like to get a gopy of the circuit for the SE and possibly the SE"2".
Having recently aquired the SE version of the Pro-ject and noticed it has 3 NE5532s and 1 JRC opamp inside.I pressume it has been repaired sometime..
I was initially a bit unhappy at the performance until I opened up the box ,I am considering certain improvements , some of which have been discussed here .
The first thing I have started is replacement of the resistors with Vishays. Already 8 random replacements due to instant availabilty from my workshop have resulted in a better sonic.2 Schottky diodes helped.
Firstly the Wallwart power supply gets fairly ward and I suspect the quality of sound drops. Has anyone tried a better torroid power supply with improved results.
I intend to try the following as I need a bit more out of this unit.
1)Snubber caps accros the schottky diodes
2)Larger value smoothing caps and better quality.I dont realy want to use Blackgates if possible as I find their sonic signature un musical.I have used thm in many mods for nearly 15 years.
Im looking t 1000 for 470mfs and 100 for 47mfs.Im assuming I can use 25v type as this is only single rail.I prefer Slmics.
3) Bypass caps on Electrolytics.Possibly under the board.
4)Possibly increase gain by a smidgeon.
5)Quieter nicer op amps.Ive tried many in varying mods in the last 40 years .Some nice suggestions already picked up in this thread.
6)Continue upgrade to Vishay resistors.
7) Upgrade some ofthe smaller caps.I do have a range of esoterics from previos designs spare.
8)Anything anyone else suggests..
Ive even considered dual/seerate power supplys as a possibilty.
Thanking all those who have already posted I look forward to any helpfull comments and advice. |
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| richie00boy |
Schottkys don't need snubbers, that's the whole point of using Schottkys...
I think using a toroid instead of the wall wart will hurt sonics as they will let much more noise and spikes through. Just because the wall wart gets a bit warm does not mean it's bad in any way. It's mostly due to the magnetising current. |
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| cddumat |
| quote: | Originally posted by Calvin (long ago, i know :) )
If You just want to use MMs -which mostly are no symmetrical generators because of internal grounding of the coils- You´d be better of with devices like LT1028 or AD797! |
That to me is not clear: I always thought that a mm pickup is as symmetrical as a mc one, at least in principle.
Where is the grounding done? Inside the pickup or the turntable? Always or only with some/many MM pickups? And why? I ask because i can not detect any grounding in a Shure V15 III i have here in a Dual tt. At least to me it appears perfectly symmetrical.
Maybe someone can shed some light for me on this matter, or point me to the right direction, because i found nothing (yet) about this on the net.
Thanks, Clemens |
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| Calvin |
Hi,
You´re right..MMs are symmetrical by nature too. But a lot of MMs are desymmetrized by the manufacturers by simply connecting one pin to ground. You can often see this connection from the outside as a small band of metal at one of the pins going to the pickups casing. You could try and break this node to have a symmetrical pickup again, or use a MM that is not connected to gnd.
jauu
Calvin |
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| cddumat |
Hi Calvin,
thanks, that clears up some things but brings up more questions :)
Do you know how important the grounding of the casing is? I guess it's grounded because of shielding issues. But why are the cases of MC pickups not grounded then (making them symmetrical)? They should be equally susceptible to em fields.
And what would or could happen if i just connect an mm pickup symmetrically to a symmetric preamplifier, without breaking the connection between the cold line and the casing? The casing at least with my pickup is completely isolated from everything else, with the exception of that node directly at the cold pin (i think it's rg). Could i pickup interference because of that? I guess that leads back to my first question.
Does that mean that without modifying most of the mm pickups it's a bad idea to connect them to a symmteric preamplifier?
As i see it now there are some options to connect a magnetic pickup to a preamplifier:
1. Just the standard, asymmetrical way to an asymmetric preamplifier.
2. Do just some recabling and connect it symmetrical to an appropriate preamplifier without modifying the cartridge. That will most often lead to a pickup case that is connected to one cold signal line.
3. Similar as 2 but break the connection of the case and the cold. The case now is not connected to anything, so maybe shielding issues could arise.
4. Similar as 3, but now connect the case to ground. Now everything should be fine, the sky will open up, the sun will shine and everyone will live happily ever after.
I definitely can see an advantage in going symmetric, that being common mode signal rejection, less capacitance of the cabling, maybe other things as well. But how far down the line would one have to go? Point 4 will be a bit awkward because it will make the changing of pickups much harder. Point 3 will damage (change) the pickup permanently. Maybe 2 would be enough?
Thanks, Clemens |
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| raffells |
| quote: | Originally posted by Zombie
So...now I've done it!
Put sockets for fast opamp change. Tried both NE5554/TL071 and OPA2134/134, 134 preferred. +/- pins have 0,1uF to ground. Low noise opamps on the way.
The 4,7uF at the input gives me nightmares, but couldn't find anything better that was small enough to fit in...pointers welcome.
1000uF caps in the power supply as well as shottky diodes. Weird the the + and - sides of the psu aren't the same...
If there's any interest I'll report back what the owner thinks about the changes in terms of sound quality. |
Hi I have just started my upgrades and noticed that Vishay have released a 2.2mf and 4.7mf Polyprop cap 160V which will go into the space,even on the smaller SE version.Available from Farnell in the UK. |
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| Pasul |
Hi bobolix,
pleas can you tell me something about clearaudio balanced reference?Or this is not your design?It´s look realy good.Two separate channel in two box and external power supply.Inside is some special topology?I still looking some phonopreamp.
Or what about Borbely MM/MC phono preamp? |
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| Upupa Epops |
| Two separate box, crosstalk between channels 100 dB or more... it all for pickups with crosstalk around 25 dB... somebody is mad... Mr.Suchy ? :D |
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| Pasul |
| Ya I think this only little help for sell this product.And looks good in promo brochure.Who is Mr.Suchy? |
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| Upupa Epops |
| Owner of Clearaudio... :cool: |
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| tubenut |
I have just messed with a Phonobox 1 today. I think there are definately more then one version of even box 1. The firts I had in my shop a few years ago had a painted box, the one I have now looks more like it is powdercoated. Also, the first unit had cheap panel mount RCAs linked to the PCB with "wires"/leads, the newer one has PCB mounted RCA sockets. Board layout seems differnt too with all metal film resistors, the legends do not seem to correspond with data previously posted.
I have upped the PSU caps from 470 mfd to 1000 mfd and replaced diodes with BYT53. Also swapped out op amps TL01>OPA134 and 4580 > LM4562. I had these around...
What is the difference in operation when these devices are set to MC vs MM? Does an additional opamp section get used for MC or just the gain adjusted? I can figure out that the one set of links on the PCB brings in a 100 ohm resistor for MC loading. (I fitted cut up IC tulip socket here, so the link can be made with wire or a resistor that will be added to the 100 ohm allowing a bit more variance.)
Does the TL071 "cooling circuit" remain active for MC? What would be the best way to adjust load resistance for my Sumiko high output MC 2.5 mV? In previous set ups I got best sound when playing in to MM gain stages but set to a load of about 22K. Would I need to swap out the 47K resistor for a 22K one or can I add the MC load resistor + additional one as described above?
One thing nobody seems to have discussed is the silly PSU arrangement. I mean half wafe rectification for a device requiring clean smooth power. This imo seems to have been forced by wanting to use the std 16V project wall wart but still allow +/- 15V rails.
I suspect a serious upgrade for this unit would be an external DC supply with full wave rectifiers and say at least 4700 mfd smoothing per rail feeding the built in regulators.
I will probably also change the little 78L05 79L05 regulators for T0220 packages. In previous projects I have found these little regulators to restrict subjective dynamics. I may even run "zener set" 317/337 from a little extra vero board with short fly leads. |
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