| dorkus |
the WM8816... i know some don't like it because it requires an opamp, but was wondering how it sounds in combination with a "good" IC, e.g. AD825 or what not... if the chip is capable i may try it with a discrete opamp design.
thanks,
marc |
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| sonnya |
I guess i have to etch some more boards next week!? Actually i have five pieces laying around. They are stereo devices so i could mail two of them too you!? Should be enough.
Sonny |
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| pmkap |
the WM8816... i know some don't like it because it requires an opamp, but was wondering how it sounds in combination with a "good" IC, e.g. AD825 or what not... if the chip is capable i may try it with a discrete opamp design.
Absoluted no need for an op amp whatsoever, discrete or otherwise. All that is needed is a inverting amp, you could do it with a plate output triode if you so desired, a single transistor...
Sonny,
Have you tried the ad8610 with the wolfson yet? |
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| sonnya |
No Paul ... i have not been able to find time for this right now... But in this summer i will have 4 weeks vacation so there should be plenty of time...
For the inverting and no feedback...
The performance of the WM8816 is based on a feedback design... But if the output of the WM8816 is held onto virtuel ground then this should be possible!?
Nice idear Paul ... Worth a try! ... With the right circuit the output will also be in phase with the input.
Drawback ... The resistor network inside the WM8816 will change a lot from unit to unit .. ~30% So you have to compensate for this gain variation
Sonny |
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| dorkus |
i'm running out of attenuator ideas...
other than the wolfson, i have maybe one other scheme. we could use a dual-stage circuit, as i saw posted on some german website... there would be a 10-position attenuator at the input of the preamp (0dB to -9dB in 1dB), followed by the gain stage, then followed by a 8-position attenuator (0db to -70dB in 10dB steps), then followed by a buffer or another gain stage. this would effectively give us 0dB to -79dB attenuation in 1dB steps. suggested gain stages are Borbely balanced SE JFET, Borberly "super buffer", maybe even a simple single-ended FET buffer for the 2nd gain stage. the actual attenuators would be fixed-series, variable shunt using solid state switches to select the shunt resistor (e.g. AD SSM2404). since one end of the switch will be connected to ground, an FET switch should be fairly linear here, with low on-resistance.
thoughts? |
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| mlloyd1 |
dorkus:
sounds good.
now get busy, I want to see some results!:D
mlloyd1 |
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| AuroraB |
Anyone got some real down to earth thoughts on the placement of a single attenuator...input or output..???
Seems like putting it in the output leg makes you more dependent on a higher load impedance..??
Any other pros and cons....?? |
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| mlloyd1 |
The Dallas/Maxim DS1808 attenuator is also an "op amp less" part. It might be worth investigating. I haven't been successful yet at getting any samples though. So, I wonder if it's a real part or vaporware.:confused:
mlloyd1
| quote: | Originally posted by dorkus
the WM8816... i know some don't like it because it requires an opamp, .... |
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| tvi |
There are Japanese manufacturers of Volume Control ICs, alot include bass, treble and loudness functions, the following don't:
<a href="http://www.semicon.toshiba.co.jp/eng/prd/mpsig/ft_mpsig.html">Tohiba Electronic Volume Control ICs</a>
<a href="http://www.semicon.toshiba.co.jp/en/bucat_2/bucat_3/bucat_14/td_7/TD.pdf">TC4959</a> is 91 step 0 to -89dB
<a href="http://www.semicon.toshiba.co.jp/en/bucat_2/bucat_3/bucat_14/td_8/TD.pdf">TC9482</a> is 97 Steps 0 to -95dB, 6 channels in one chip.
<a href="http://service.semic.sanyo.co.jp/semi/ds_pdf_e/LC7536M.pdf">Sanyo LC7536 </a></body> is 81 step 0 to -79dB
As to sound ????
James |
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| tvi |
<center><a href="http://164.195.100.11/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1='3983421'.WKU.&OS=PN/3983421&RS=PN/3983421">US Patent 3,983,421
Yogore September 28, 1976
Remote audio attenuator</a></center>
Abstract
A remote audio attenuator in which a volume control variable resistor of the attenuator regulates a volume control variable resistor of a distant audio system with complete isolation between the circuit of the two volume control resistors, and with rapid response between the two control units. The attenuator employs a lamp controlled by a differential operational amplifier, with the lamp set to equally illuminate slave photoresistors, one or more of which serve as the volume control units of the distant audio system, and one of which is connected to a resistance bridge to provide feedback to the differential operational amplifier, the output of which is also controlled by the remote control variable resistor which is also connected to the resistance bridge.
James |
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| dorkus |
| aurora, you probably always want the attenuator followed by a buffer stage of some sort to keep output impedance low. so yeah, it should be at the input. however, some people even prefer to put the attenuator between stages, so it is completely isolated, as this give you better impedance characteristics and thus lower noise. i think putting a fine attenuator (0db to -9dB) at the input and the coarse attenuator inter-stage is a good compromise. |
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| dorkus |
tvi, my denon surround preamp uses a japanese volume control IC, i forget who makes it... it's not toshiba or sanyo but another one of the big semi companies there... maybe NPC or something? anyway it is surrounded by tons of opamps (BB OPA2604 i believe) but to be honest, considering how "dirty" the signal path it, this thing sounds pretty good. it can't compare to my passive preamp though. all those ICs seem to be buffered internally, probably with an opamp of some sort, so many think that will be a limiting factor.
the Maxim part looks ok, but it does not have 1dB resolution throughout its range, which is a requirement for me. |
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| Petter |
How about this topology? Related post on active circuitry is here
http://www.diyaudio.com/forums/show...15&pagenumber=5
So what is good about this?
1. It is balanced
2. It uses minimal number of components
3. As shown it is a shunt
Any switching method can be used to set the volume -- from semiconductor switches to relays. My personal favourite would be a set of mercury wetted relays, perhaps 12 relays in sets of say 8, 2, 2. With a delay on the switching + some form of Gray code (one changing position at a time), one could get very long life, minimal "clapping" and serious range.
Of course, a regular pot is simpler to implement and cheaper.
JFET switches are also cheap and probably good enough, especially since this is fundamentally shunt operation with minimal output loading + it is acceptable to set them up in pure binary mode since there are no moving parts. 10 such sets yield 1024 levels.
What do you think?
Petter |
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| Helix |
I would like to go with who ever suggested this idea, just have about 8,9 or even 10 FETS and have the signal biased at about 30V (ideal to dangle on the end of my single ended pre-amp and then put the output cap after it).
would it work? one would have to build 2 for each channnel if you want balanced. |
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| dorkus |
| it's a neat idea, another great way to use the dc offset of a SE design to your advantage. my only issue with this is that it would have to use a buffer after it to keep the output impedance constant and the attenuation consistent. so, if i need a 2-stage attenuator, i would need 3 active stages total... 2 SE stages with this attenuator, followed by another buffer. it would be nice if i could get rid of one attenuator stage but i don't know of a simple way to get the range and resolution of attenuation i need w/a single stage. i was toying with a design that could switch arbitrary combinations of shunt resistors in parallel but it will take a lot of calculation to figure out the required values and combinations need... i should probalby just write a program to calculate it for me. =p |
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| AudioFreak |
| I start holidays in 2 weeks, I'm going to put in some serious time on Wayne's secret attenuator which would be perfect for this and would fit between the 2 stages. |
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| Helix |
| thats the idea, you have to control the FET's from a micro or whatever you choose. Then you can arange the R values so that you have a MSB (most attenuation) and a LSB (least attenuation) you can get the values of R's from the Pass preamp i think. that way with 8 FETs you have 255 diffrent level settings, and just drive the FETs with bineary. One has to be careful that ALWAYS at least one FET on! or otherwise no attenuation and the full preamp gain might be a bit high for the amp. |
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| Petter |
So what is needed is pull-up resistors to ensure that FET's are on unless specifically switched off.
I believe BJT's can be just as good as FET's -- and JFET's are possibly better still. Perhaps choosing a device which shuts itself off more or less completely is as good idea as one that has low on-resistance/noise/forward drop
One thing I don't like is that unless you attenuate the signal against a significantly lower voltage, you will not have symmetrical load for positive and negative swing. However if you do load down against a significantly lower voltage you stand the risk of messing with gain stages be they previous or next.
One way to avoid the asymmetric loading would be to pull up as well as down, but I still favour direct attenuation across phases instead of single attenuation to ground (or significant negative voltage).
It would probably also be useful to use capacitors to slow down the device switching somewhat.
Petter |
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| dorkus |
| you can get 255 combinations with 8 switches in a ladder config, but i'm not sure if it will be possible to optimize the values to get the desired consistent 1dB or .5dB steps, at least in a variable-shunt-only config. |
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| Petter |
A word of warning. MOSFET's have built in body diodes ...
I am seriously considering using emitter source to source n-channel JFET's for the balanced deal since JFET's are pretty good in both directions.
As far as using BJT's or MOSFET's I just don't know. Time to read some more in "The Art of Electronics".
BTW: the way to get a log response is to use more bits and a lookup table.
Petter |
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| dorkus |
| so, i was at a stereo shop w/some friends the other day, and i got to hear a Rowland Synergy preamp. i'm not sure if it was the latest version using the PGA2310 or the older CS3310 but i have to admit, the system sounded pretty decent, even though i am not a huge fan of B&W 802 speakers. the amps were Rowland as well. i did feel the system did not capture the subtle textures and shadings to the music that simpler systems do but i'm not sure how much of that is the electronics (e.g. preamp) and how much is the rest of the system. in any case i think the TI/Crystal volume chips are pretty darn good all things considered, i did not hear too much of the dynamic compression or glazing-over that i often hear from opamp ICs etc. better than i expected, but still not sure if it's good enough for Son of Dork. i'm sure Harry, HPotter, Jocko, and Jam would probably concur. :p |
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| Petter |
but only if used as shunting element, not pass element.
However, bearing in mind the lengts to which we are going to achieve great sound quality, I would suggest that electronic volume controls of the integrated ilk are not good enoug for Dork period.
My $.02
Petter |
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| Apogee |
I know that the Coda 04R preamp uses the CS volume control chip and it sounds very, very good. A buddy of mine owns one and together we've put it up against some of the best on the market (not a pass unit yet....) and it usually comes out on top...
The upshot is, if it is designed well that chip really sounds good. I know the latest versions also only allow volume changes at the null point so they have eliminated the occasional clicking that was heard in the earlier versions...
This has been very interesting to follow. You guys are doing a great job!!!
Just a couple of thoughts, |
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| dorkus |
petter, i would have to agree with you, though w/o actually trying the circuits i really can not say for sure. but the PGA2310 does use a bipolar opamp as the output buffer, which many of us consider less than ideal. i do not think it can be used as a shunt either - the Maxim and Wolfson parts are passive and could be used that way though.
i'm glad we're setting the goals for Son of Dork very high. :) a lot of people would kill for a preamp as good as a Coda or Rowland, and we are saying it is not good enough! i was never a fan of big high-end names though - audio is one thing, music is another. a lot of audiophile products get the former but miss the latter.
steve, i agree the Crystal/BB ICs can still sound very good when properly implemented. i am actually going to build a mini Son of Dork for my friend, a simple stereo preamp, that will probably use the PGA2310. for the full-fledged Son of Dork though i will probably go with a custom scheme using discrete components.
btw, the stereo version for my friend is called Bastard of Dork. catchy, eh? |
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| Petter |
Look inside a "high-end" unit costing lots of money and love and behold what do you find?
**** component quality.
As a Son of Dork participant, I cannot live with that.
BTW: If you must go with surface mount resistors, consider the new Vishay Bulk Metal foil units :)
Petter |
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| dorkus |
yeah, i've seen the innards of a lot of "high-end" gear, and it is pretty appalling at times. i was at the shop listening to the Rowland w/my friend and i whispered in his ear "i can build something better than that for $500."
actually, i was sort of interested in these SMT resistors:
http://www.venkel.com/pdf/MELF_TFCR.pdf
i hear Vishay bulk foil can be quite bright-sounding. haven't heard tantalum before but they seem to be well-regarded. |
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| Ian Macmillan |
Some while ago now Audiofreak wrote:| quote: | | I start holidays in 2 weeks, I'm going to put in some serious time on Wayne's secret attenuator which would be perfect for this and would fit between the 2 stages. |
What happened to this? I, along with many others I'm sure, would love to know more about Wayne's secret attenuator. I've been racking my brain for some while trying to guess at the topology but with little success. Any ideas would be greatly appreciated.
Ian. |
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| AudioFreak |
Ian,
Other things came up and I've put this on the back burner for the time being. |
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| Ian Macmillan |
Please, how can I persuade you or indeed others to make a start on this? Given that Nelson has already thrown down the gauntlet, I'm a little suprised that nobdy has yet risen to the challenge.
Ian. |
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| AudioFreak |
| I've got to build some equipment for university 1st. I'm also trying to organise a mass MOSFET purchase which has gone on for months now with one problem after another but things are starting to look up on that front and I should see the FET's within 2 months (anyone out there that has expressed interest... it is going to happen soon ... sorry it's taken so long). Next chance I'll probably get to fiddle with Wayne's secret attenuator will be summer vaction which starts late November and runs for 3 months. |
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| Ian Macmillan |
Oh dear, that seems a long way off. Unfortunately I can understand the problem with lack of time - seems to aflict most of us.
Come on guys (and gals) - there must be others with both time and the wherewithall to have a crack at this! I have a little time but not the wherewithall without some advice or hint as to how to begin.
Ian. |
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| dorkus |
| why don't you do it then Ian :p |
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| Peter Daniel |
| quote: | Originally posted by dorkus
why don't you do it then Ian :p |
Maybe it's time for another prize?;) ;) |
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| Ian Macmillan |
I thought I already said: I am prepared to make the time and do any donkey work but I really don't know to start on this one. Believe me, I have been trying (and will continue to do so).
Prize? Do you mean I should offer one? I didn't think it took the lure of a prize to get folks going on this forum.
Ian. |
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| dorkus |
i wrote that msg. before your other one was on the thread.
peter was referring to Nelson Pass's last prize to someone for figuring out some element of his super-symmetric circuit, i believe. |
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| Peter Daniel |
| Yes, I was sugesting that somebody might try to win another prize from Nelson (if such would be offered of course). And I'm not saying it couldn't be Ian again.;) |
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| dorkus |
brian's got a nice prototype there, looks very cool. i especially like the VFD. unfortunately the switched relay scheme is not very practical for SoD (for 1dB steps over the required range i'd need dozens of them) but it is neat nevertheless.
i am still considering using CMOS switches as shunt elements. also looking into the Wolfson chip, but it doesn't seem anyone has tried it yet. |
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| BrianGT |
| quote: | Originally posted by dorkus
brian's got a nice prototype there, looks very cool. i especially like the VFD. unfortunately the switched relay scheme is not very practical for SoD (for 1dB steps over the required range i'd need dozens of them) but it is neat nevertheless.
i am still considering using CMOS switches as shunt elements. also looking into the Wolfson chip, but it doesn't seem anyone has tried it yet. |
There is a whole lot of range with the volume control that I am working on. 256 steps of volume control to be exact. This should easily give the percision that you desire. The number of steps is 2^n where n is the number of relays that you have. Adding 2 more relays (10 total per channel) gives 1024 steps total. Here is the schematic:
http://www.diyaudio.com/forums/attachment.php?s=&postid=93632
As for chips, I tried the PGA2310 chip, and was not satisified with it at all.
--
Brian |
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| dorkus |
yup, you can get plenty of attenuation permuations, but the problem is getting the precise 1dB steps i require. i have yet to derive an algorithm to calculate the minimum # of resistors for the 70dB attenuation range i want.
i agree the PGA2310 is not ideal, though there are some decent-sounding preamps that use it. i heard it's pretty musical for what it is. i will have to get my hands on some AD CMOS switches and try using that for shunting, instead of relays... could sound pretty good. |
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| BrianGT |
| quote: | Originally posted by dorkus
yup, you can get plenty of attenuation permuations, but the problem is getting the precise 1dB steps i require. i have yet to derive an algorithm to calculate the minimum # of resistors for the 70dB attenuation range i want.
i agree the PGA2310 is not ideal, though there are some decent-sounding preamps that use it. i heard it's pretty musical for what it is. i will have to get my hands on some AD CMOS switches and try using that for shunting, instead of relays... could sound pretty good. |
I picked up some of those switches. They switch better then relays, but I see no difference when they are not switching. They also cost more. You can get 2 free samples from analog.com
I see no reason to compromise on a volume path, The pga2310 isn't simple by any means, and isn't very clean. You can try all you want by adding filters and cleaning it up, but nothing will beat the simplicity and preserve the signal path as good as resistors and a relays, except for maybe that patent-pending passlabs volume control by Wayne.
--
Brian |
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| dorkus |
| the primary advantage of the CMOS switches is you can pack a lot of them into a small area, much more practical if you need a lot of switches and minimizes the signal path length as well. an SMT switch combined with SMT tantalum resistors could make a very sweet attenuator circuit using no more space than a few relays alone. they're also easier to control w/digital logic, don't need any current amps as you might with most relays. a mechanical relay has better performance than a CMOS switch, but when used for shunting CMOS is quite good (Nelson Pass has said so himself). some people would argue that the magnetic field from a relay also adversely affects the sound, but i have yet to confirm it and it's still a minor factor compared to a CMOS. |
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| AuroraB |
What about on-resiStance when using CMOS switches..
The SSM2402 has appx 60ohms, but is reputed to have very stable and signal independent on resitanse, which is not the case with many other CMOS switches.....
Here in Scandinavia the SSM2402 retails for appx 10USD, maybe less if you have good connections.....but this is also the price of any decent quality relay....???? |
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| SDF |
It'may be possible to use a simple digital potentiometer in input. By example the DS1802 is a real digital potentiometer used by Audio Research last preamplifier, IMHO, it's not a bad argument...
The principal weakness of this componet is it maximum signal input, +-5V max rating. |
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| dorkus |
the on-resistance of a CMOS switch is normally high but when used in a grounding or virtual ground configuration, where there is zero potential at the leg of it, it is actually very low - on the order of tenths of an ohm i believe. this is why they work well as shunt elements or into the inverting input of an opamp. it is not a cheap chip, but there are 4 switches to a chip which makes it comparable to or cheaper than relays, and the space savings are considerable.
the DS1802 is a nice chip, better than most in that it's passive, but it does not have the attenuation steps i desire, and i would rather not have an IC series element. using switchable shunt elements with fixed series resistor is the route i prefer. |
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| SDF |
OK! I think it's a good way...
I think too, relays (good parts, of course) will be very suitable in this case; with 7 relays by channel we have 128 steps (I know you know :) ) and the serial resistor is very low, one more time , there is a weakness: the relay noise... |
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| dorkus |
| again, the problem with doing relay combinations is calculating the optimum resistor values for precise 1dB steps. not all possible permuations may provide useful attenuation. however, with some thought, it may be possible to come up with values such that at least some combinations are usable. i insist on precision however, as the individual channels will be trimmed in 1dB increments and this trimming must remain consistent at all volume levels. assumming we want 60dB of control range and +/-6dB of trimming, that means we need 1dB steps from 0dB to -72dB. |
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| SDF |
| I read , in this thread, the PGA 2310 was not suitable, why? |
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| dorkus |
| PGA2310 sounds ok but it is not great because it has internal opamps which limit its sound quality. it's actually an ok sounding chip, the editor of the magazine i write for reviewed the McCormack preamp which i believe uses it and said it sounded nice, but not true high-end transparent. our ideal attenuator should be entirely passive, leaving the active circuitry at our discretion. the Wolfson attenuator is passive, but it still has a lot of circuitry in series with the signal. PGA2310 is good if you want something flexible and don't need the very highest level lof transparency - its sound is equivalent to a decent-quality IC opamp. |
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| SDF |
OK, thank for your clear answer, I mean want you want ...
In reality, I never hear PGA and I never compare it with alps potentiometer . I am in late concerning Wolfson attenuator, is it an patent concerning attenutor, doesn't it ?
:confused: |
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| dorkus |
a good potentiometer like Alps Black Beauty will always sound more transparent than an IC like the PGA2310, particularly if you use the Alps as a shunt element only. passive is always cleaner than active, at least when the active is as complex as the PGA. however, it is not a truly valid comparison as the PGA has an output buffer, so the IC by itself constitutes a complete preamp... the Alps is passive so you'd need some sort of active stage with at least 12dB gain to be comparable. i believe Alps followed by a good IC like the AD8610 would be better than the PGA; if followed by a good discrete stage, even better. however, this would not give you remote control, unless you got a motorized pot, which does not have the precision of the PGA.
i heard the latest Rowland preamp, which uses the PGA, and it sounded fairly good. i think if you want something quick and dirty it's a very nice chip - it has decent output drive and some voltage gain, so it's essentially a preamp on a chip. keep in mind you'll need a microcontroller to implement it properly, but it's definitely a convenient way to get a decent-sounding digitally controlled preamp. |
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| SDF |
I know what you mean and I really like it, :) it have been mine too.
Concerning the remote control, I let a note in the appropriate thread but I think I can, may be, help you concerning the µC prog. |
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| Variac |
| I really like relay noise. I t sounds really high tech to me.Those little clicks only happen when you adjust the thing, and the sound isn't coming from the speakers. For me the noise is a plus. (I think Nelson agrees with me so there!!......) At the least I doubt most people will be bothered by it. The Pass gear has this "feature" |
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| BrianGT |
| quote: | Originally posted by Variac
I really like relay noise. I t sounds really high tech to me.Those little clicks only happen when you adjust the thing, and the sound isn't coming from the speakers. For me the noise is a plus. (I think Nelson agrees with me so there!!......) At the least I doubt most people will be bothered by it. The Pass gear has this "feature" |
Agreed, I love it when you are sitting across the room, and change the volume with the remote, and you hear a faint ticking from the relays. I like the feedback. If it really bothered you, you could dampen the inside of the chassis that they are situated in, and I am sure that they wouldn't be too loud.
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Brian |
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| dorkus |
FYI i just got in for review an interesting multichannel preamp from MSB, their GOLD Multiple Volume Control (MVC). basically an 8 channel volume control/gain stage using a "DAC-based" attenuator. not sure exactly how it's implemented but i'll take a look inside later. yeah, it's ful of IC opamps and other nasties, but it actually sounds really, really, really good... extremely transparent in fact, with great speed and articulation. i'll compare it to my borbely JFET buffer and passive preamp soon but all things considered this is an impressive-sounding piece, and it has the attenuation flexibility i've been looking for. more info here:
http://www.msbtech.com/ |
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| MJ |
I'm deciding on whether to use a top notch motorised pot or a relay based attenuator. No silicon for me. The balance has to be between cost and performance but my budget is quite healthy.
Another compromise is between topology and size, the layout will affect perfomance, therefore all relays should be of the SIL variety (I don't know how reeds compare to regular relays - having never used them). This is to keep the signal and ground paths as linear and short as possible.
So, why not just use a ladder attenuator, and get the benefit of bulk buying for both relays and resistors. It may not be as perfect as a proper a fully switched resistor pairs attenuator but is easier to implement control etc..
I am torn between 1dB steps or 3dB, maily in an attempt to keep the signal paths short. What if there was a primary ladder stage with 8dB steps (say upto -64dB), followed by a buffer, then another ladder to implement 1dB steps (upto -7db) giving a total range from 0 to -71dB. Now my question is would the extra buffer do more harm than the extra long chain to give 1dB steps (cost of course will be better). Of course I intend the buffer to be a very high quality discrete design.
Unfortunately I cannot make my mind up and the proof through prototyping can only be done using the final PCBs so the cost hit will be full.
Any guidance or guesses as to the correct path? |
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| BrianGT |
| quote: | Originally posted by MJ
I'm deciding on whether to use a top notch motorised pot or a relay based attenuator. No silicon for me. The balance has to be between cost and performance but my budget is quite healthy.
|
I am also looking into building a great volume control with relays . I have the control worked out for it (microcontroller), but I haven't decided on the best way to use the relays for attenuation. My roommate and I are working together on this, and our first prototype is based on the Pass Labs Aleph 1.7 relay circuit:
http://www.passlabs.com/pdf/aleph/apserv17.pdf
It seems like it doesn't have enough attenuation, and the steps are on a linear scale, as to say that it takes 3-4 db steps at -48db and when it gets around 0, it changes in .1 dB steps.
I have resources to get pcbs done here for myself at college for prototyping for no cost, so I am planning on trying out a few different designs, then if I find one that works great, I can get real pcbs made.
As for the 1db steps, how many relays were you looking to use. Any input into this is appreciated.
--
Brian |
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| MJ |
I looked at the aleph circuit, but was not too impressed, I wanted a log circuit so as to ease the control side. I could use a PIC or other Micro-Controlled but it is just a big head ache, as to get true 1dB steps down to -72dB the attenuator stage actually needs to be at least 12bits not 8bits as in the aleph and that does not help.
With the twin ladders there would only be 16 relays and 32 resistors per channel so cost is reduced. A pure ladder would require 72 relays and 144 resisitors per cannel. The control logic is simple up/down counting with no need for linear to log lookup. Zipper noise is virtually eliminated by making the relays arrangement make before break - this can be done using a single RC chain parallel with each relay.
The extra cost is in the buffer stage, use something like the Borberly buffer or a simple emitter follower buffer with very high quality components (or for experimentation a simple opamp buffer using an OPA627 would be superb).
For control I intend to use a simple optical encoder. The control can be setup with simple discrete logic, same as for the source switching or use a uP. Remote control can then be simply applied as well. |
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| BrianGT |
| quote: | Originally posted by MJ
I looked at the aleph circuit, but was not too impressed, I wanted a log circuit so as to ease the control side. I could use a PIC or other Micro-Controlled but it is just a big head ache, as to get true 1dB steps down to -72dB the attenuator stage actually needs to be at least 12bits not 8bits as in the aleph and that does not help.
With the twin ladders there would only be 16 relays and 32 resistors per channel so cost is reduced. A pure ladder would require 72 relays and 144 resisitors per cannel. The control logic is simple up/down counting with no need for linear to log lookup. Zipper noise is virtually eliminated by making the relays arrangement make before break - this can be done using a single RC chain parallel with each relay.
The extra cost is in the buffer stage, use something like the Borberly buffer or a simple emitter follower buffer with very high quality components (or for experimentation a simple opamp buffer using an OPA627 would be superb).
For control I intend to use a simple optical encoder. The control can be setup with simple discrete logic, same as for the source switching or use a uP. Remote control can then be simply applied as well. |
Is this the dual ladder that you are speaking of?
http://www.diyaudio.com/forums/show...19490#post19490
ftorres made his with 16 relays per channel.
It might be interesting to make it possible to control with just an optical decoder. I will look into this.
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Brian |
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| MJ |
Actually I think you're correct, and as an added elegance, because the second stage is a ladder the impedance is constant and so can be factored in, negating the requirement for the buffer stage. Only one thing to add to the given design the OdB setting on the -1.25dB stage has a different impedance, I would add a resistor to make sure the impedace is constant what ever the setting of the stages.
Thanks and well spotted. Very usefull. and I think I might be able to take the priciple even further!?! eventually the whole deign may be reduced to a direct binary count and be able rid the design of the 74HC238s and reduce the relay and component count down even further. Just thinking aloud - will get back to you on that one. |
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| Kongen |
| Hey, you should check LcAudios sidewinder preamp and the http://www.lcaudio.dk/sidewind.pdf on page 20 or so there are schematics for a relay based attenuator with precise db steps... You need to recalculate the resistor values because as i remember they use steps of 1,5dbs... |
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| BrianGT |
| quote: | Originally posted by Kongen
Hey, you should check LcAudios sidewinder preamp and the http://www.lcaudio.dk/sidewind.pdf on page 20 or so there are schematics for a relay based attenuator with precise db steps... You need to recalculate the resistor values because as i remember they use steps of 1,5dbs... |
You need to capitalize PDF. Here is a working link:
http://www.lcaudio.dk/sidewind.PDF
Also, it is kind of hard to read (page 21 on pdf), is there a better version of this schematic available?
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Brian |
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| Kongen |
| Sorry about the non-capital pdf - should have picked up on that... On page 10 of the pdf there is a schematic of the amplifier priciples or maybe try to contact lcaudio... |
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| dorkus |
i've had the MSB MVC Gold preamp ($1699) in my system for a while now and i can't say enough good things about it. this is a really great piece - transparent, musical, with lots of slam. despite being a very IC-heavy design it doesn't suffer from many of the ills of IC preamps i've heard in the past - the glazing, grain, hard upper midrange, congestion, etc. makes me wonder if ruling out ICs entirely is pig-headed on my part... i'm starting to feel more and more that it matters more how you do it than what you use. of course you can't make a silk purse out of a sow's ear to repeat that cliche, but i don't think we should assume a preamp is doomed because it uses ICs, even when we are talking the highest levels of transparency here. compared to even a passive preamp the MSB aquits itself very well in the areas of reproduction that most active designs fall short - and terms of dynamics and slam it'll trounce most passives. i'll eventually get around to opening up the unit to see what's inside... right now i'm enjoying some Ella & Louis so the last thing i want to do is unplug it from my system!
cheers,
dorkus |
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| Petter |
I would like to draw your attention to the Apox work going on here http://www.diyaudio.com/forums/show...3331#post163331
You could end up with a 6 channel relay based attenuator unit with front panel display and remote (without attenuation resistors) for about $350. The input switching scheme is quite costly if you are going to go full monty with all channels - I don't need input switching given that I am using digital source switching and digital crossover.
Petter |
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