| lineup |
Hello people.
I hardly ever find a chance to start a new topic in Tubes.
Tubes are nice and interesting!
This Project is about build a Pre Amplifier
to match the input of SEWA 7 Watt Class A MOSFET Power Amp.
If you post here, please RESPECT OUR TOPIC.
And find out some basic knowledge about SEWA MOSFET amp
before you make your posts and suggestions.
The SEWA thread is GIGANTIC, so try the last pages
for uptodate information about latest version of The Circuit.
Thank you.
/lineup
Reference link:
Pass Labs >SEWA - Seven Watt Amplifier
http://www.diyaudio.com/forums/show...?threadid=66822
:spin: :spin: :spin: :spin: |
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| Geek |
Great idea to start a topic here :D
I have a design in mind, but without an actual SEWA present, I can't say if it will work to perfection.
If I am correct, we need 25V P-P driving some fairly hefty Cin on the MOSFET. Driving the capacitive load isn't an issue for what I have in mind, but keeping it stable with no feedback loop.
If there is a SEWA owner within reasonable driving distance of Chilliwack, BC, I'd like to meet up with you :)
Cheers! |
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| lineup |
| quote: | Originally posted by Geek
Great idea to start a topic here :D
I have a design in mind, but without an actual SEWA present, I can't say if it will work to perfection.
If I am correct, we need 25V P-P driving some fairly hefty Cin on the MOSFET. Driving the capacitive load isn't an issue for what I have in mind, but keeping it stable with no feedback loop.
If there is a SEWA owner within reasonable driving distance of Chilliwack, BC, I'd like to meet up with you :)
Cheers! |
Thanks Geek.
My own thinking, based on what little i have seen and read about tupe preamps, is 6922 / E88CC / 6DJ8
Not a very original thought, yes,
but this these tubes has made a lot of good and many people happy.
And is easy to find and buy, to a resonable price in most countries.
-------------
This article have a very valid title:
THE SUITABILITY OF THE 6DJ8 FOR AUDIO| quote: | Russia’s Best
Today I see many premium numbers with exactly the same internals as their commercial version, sometimes with a gold "flash plate," which the marketer adds to the pins after the fact. Since the gold rubs off on the narrow line of contact with the first insertion, its value is questionable. With a superior beefy structure, the 6922s I have recently bought from Russia are the best I have ever seen from that country, and certainly deserve a premium label. |
About The Author ROGER MODJESKI
Roger Modjeski received a BS in Electrical Engineering in 1973 from the University of Virginia and was a teaching assistant while studying for his Masters at Stanford in 1975.
He has been active in electronics since the age of five watching his dad build a Heathkit Williamson.
Prior to starting Ram Tube Works and Music Reference in 1980, he opened a high-end retail store where he met and later worked for Harold Beveridge.
His other interests are music theory, piano, singing, building musical instruments, solar energy, architecture, and understanding how things work.
This article attempts to correct a few misconceptions
about some of the popular dual triodes
used in audio amplification.
With comparison of diffeent variations of 6DJ8. Original Article Link ( recommended! )
THE SUITABILITY OF THE 6DJ8 FOR AUDIO
http://store.electron-valve.com/suof6dforau.html
/lineup :cool: |
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| planet10 |
| quote: | Originally posted by Geek
If there is a SEWA owner within reasonable driving distance of Chilliwack, BC, I'd like to meet up with you :) |
Gregg... you are a SEWA owner... one of the things we didn't get aeound to talking about enuff... i have (most of) the parts for 4 monobloks (need Mosfets & thru hole passive parts) -- 2 are yours. All part of the toob-i-fication project. We did talk about using the Al tube for chassis & heatsink which was the missing ingredient...
dave |
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| johndiy |
umm lets see some schematics tube driving sewa old chaps
john |
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| Zen Mod |
both systems of 6DJ8 in parallel.CCS-ed from down (choose your poison - you can also put some nice Ni-MH in cathode ; )),screaming current, low Rp .......
it must work like this |
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| Mad_K |
| quote: | Originally posted by Geek
If I am correct, we need 25V P-P driving some fairly hefty Cin on the MOSFET. |
Cin is typically 250pF because of the common drain connection, look at the Crss curve of the mosfet for this figure (about 250pF for IRFP240 @ 15VDS) ;) |
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| Mad_K |
| Here it is. If you are planning to use the full 7W potential, I would use 30V rails and 5K6 for R1 to keep distortion down ;) |
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| Geek |
Hi Guys,
| quote: | Originally posted by planet10
Gregg... you are a SEWA owner... one of the things we didn't get aeound to talking about enuff... i have (most of) the parts for 4 monobloks (need Mosfets & thru hole passive parts) -- 2 are yours. All part of the toob-i-fication project. We did talk about using the Al tube for chassis & heatsink which was the missing ingredient...
dave |
...
Uhhh.. I think I remember what I forgot to pack it in the scramble to make the Greyhound? :zombie: :blush:
| quote: | Originally posted by Mad_K
Cin is typically 250pF ;) |
Very cool. I assume this takes into consideration Mr. Miller's contribution?
Anyway, this is what will drive it. Yes, it's an ECC88 and if you want to use a 6922/E88CC, it'll need changes.
I haven't done the gain stage yet... backed up on projects.
It uses lower voltage and higher current. I've used this as a straight buffer and is stable into a 4.7uF load, as well as complex weired inductances. It'll also drive fair good. Not as good as two trioded 6EJ7's in mu-follower, but it'll swing 25V P-P across 2K resistive+capacitive okay.
So, three tubes. One 6DJ8 for each channel and a third dual-triode for the gain stage. I'm thinking 6CG7. |
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| Mad_K |
| quote: | Originally posted by Geek
Very cool. I assume this takes into consideration Mr. Miller's contribution?
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I think we can disregard him for this Common Drain circuit ;) |
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| Geek |
| quote: | Originally posted by Mad_K
I think we can disregard him for this Common Drain circuit ;) |
*looks at schema*
D'OH! :xeye:
Time for bed. |
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| Bas Horneman |
| quote: | | This Project is about build a Pre Amplifier |
In that case why not specify the requirements first.
Ok you've given us low impedance. But what else? Should it be cheap? Or a cost no object preamp? Or somewhere in between.
Simple or complex? DHT or IDHT. Classic or modern. Kit or complete DIY?
Would you like to develop it from scratch in this thread here or would you like a proven design.? From the Sewa thread I gathered that you would want some gain. |
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| lineup |
Hello
Bas Horneman
This thread is mainly for 'The SEWA People'.
Some SEWA owners has been requesting suitable tube pre amp
for their power endstage amp: SEWA.
You can of course join in, too, even if you have no SEWA :)
We could use all good advices from really experienced valve builders, like you.
Thanks for reading and posting. Everything is most welcome.
Many of us are just beginners in tubes.
But we can learn - like you could.
I would imagine, you were not born a valve circuit designer ??
;)
Regards
lineup |
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| Bas Horneman |
| quote: | | This thread is mainly for 'The SEWA People' |
I understand... :) But what I am curious about is what do you trashy people want? ;) Just to try anything with valves? Or the best valves have to offer?
| quote: | | We could use all good advices from really experienced valve builders, like you. |
I'm not really experienced at all...it just looks that way :) But before someone can give you good advice you have to know what you want. Hence my question...must it be cheap or expensive? Transformer coupled or cap coupled...etc., etc.
I'm experienced enough to know that it is easy to make a valve pre-amp. To make something outstanding is a whole different ballgame. |
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| Mad_K |
IMHO it should follow the lines of simplicity like SEWA and the suggested BOZ frontend. It should be in the same (or better) quality/price range as BOZ/SEWA.
It boils down to a single (/dual) tube and some resistors and caps. I presume the work/money would have to be spent on the PSU
Requirements:
+/-20V voltage swing
Able to drive 47K, 250pF
10-20X Gain
Volume control
Source selector
Low noise
Wishlist:
-Uses available parts
-Transformerless (or readily available)
-Easy to build |
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| Bas Horneman |
| quote: | | I presume the work/money would have to be spent on the PSU |
A big saving could be made if using a MR psu. (Minimal reactance psu) like one that Dave Davenport developed. After the rectifier one cap -> one CCS..voila. |
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| Bas Horneman |
My suggestion based on those requirements would be to build something like Douglas's (Sector7G' or Bandersnatch on diyAudio.com) Guinevere but with (a minimal reactance psu - not shown here yet). And instead of the relatively hard to get depletion mode mosfets use that IXYS 10m something part. Tube could be 5687 or the 2 dollar supertube 6n6p. (Or many other tubes for that matter)
One tube socket. One dual tube. 3 Single chip CCS's. (If using the IXYS jobbies and the MR psu.)
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| planet10 |
BAS,
How much current is each section drawing? (even if i wasn't half asleep i'm not yet able to look at the circuit and figure that out). The IXYS chip has a minimum of 10 mA.
dave |
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| Bas Horneman |
Around 15mA per channel.
For more info including feedback from folks who built one go to Audioroundtable.com and search for "Guinevere". |
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| Sheldon |
| quote: | Originally posted by Bas Horneman
I understand... :) But what I am curious about is what do you trashy people want? ;) Just to try anything with valves? Or the best valves have to offer?
I'm not really experienced at all...it just looks that way :) But before someone can give you good advice you have to know what you want. Hence my question...must it be cheap or expensive? Transformer coupled or cap coupled...etc., etc.
I'm experienced enough to know that it is easy to make a valve pre-amp. To make something outstanding is a whole different ballgame. |
A job for Mr. Aikido? Pretty simple, selectable gain, low output impedence, non-exotic power supply (maybe even share the supply) - 6DJ8 version? Maybe a single 6DJ8 triode for the input side and 6GM8 for the follower?
Sheldon |
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| lineup |
| quote: | Originally posted by planet10
BAS,
How much current is each section drawing? (even if i wasn't half asleep i'm not yet able to look at the circuit and figure that out). The IXYS chip has a minimum of 10 mA.
dave |
from schematic in this post
http://www.diyaudio.com/forums/show...8055#post998055
I get it the input impedance is ~50 kOhm.
The minimal output current needed we get from:
Voltage input (for maximal output) / input impedance.
The input voltage needed in relation to input impedance will give the current that must be provided.
Very correct an amplifier is always an adapter
between a source and a load.
Knowing these values, as Bas Horneman and planet10 has put it
is essential!
Read more amplifier basics:
http://www.diyaudio.com/forums/show...3062#post893062
lineup |
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| Bas Horneman |
Yes, the Aikido is also an excellent canditate...but with 4 tubes and a whole lot of resistors it is relatively difficult. But one could do a lot worse than buy John Broskie's boards and switched attenuators! http://www.tubecad.com
Even if one does not use the Aikido the switched attenuator kit over at tubecad is great to use with any tube preamp that is decided here. |
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| dsavitsk |
| quote: | Originally posted by Bas Horneman
My suggestion based on those requirements would be to build something like Douglas's (Sector7G' or Bandersnatch on diyAudio.com) Guinevere but with (a minimal reactance psu - not shown here yet). And instead of the relatively hard to get depletion mode mosfets use that IXYS 10m something part. Tube could be 5687 or the 2 dollar supertube 6n6p. |
I use something similar for a DAC gain stage after a passive I/V.
http://www.ecp.cc/5687_module.html (I might even have an extra one of these boards laying around ...)
I also have a board showing up in the next few days for a 6n6p/6n30p based one of these that uses either LED or standard resistor biasing. |
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| lineup |
| quote: | Originally posted by dsavitsk
I use something similar for a DAC gain stage after a passive I/V.
http://www.ecp.cc/5687_module.html
(I might even have an extra one of these boards laying around ...)
I also have a board showing up in the next few days for a 6n6p/6n30p based one of these that uses either LED or standard resistor biasing. |
6N30P ( 6n30pi )
is one tube i have read a lot about, and from what i have understood
it is a good preamp tube - with high current output drive capacity.
The link posted by dsavitsk shows us
this little fantastic BEAUTY
 |
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| Bas Horneman |
is a great tube. But the 6n6p is virtually the same. However it still can be bought cheap because it's fame has not spread as much. But there are many tubes one could use. With the CCS being flexible you could use any tube you fancy.
PS...dsavitsk great little pcb! That way you can change a capacitor (or psu) in a New York minute! |
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| dsavitsk |
| quote: | Originally posted by Bas Horneman
PS...dsavitsk great little pcb! That way you can change a capacitor (or psu) in a New York minute! |
Thanks. (sorry to thread ****) I have taken to doing everything modularly like this, so that project I can swap in and out one of 3 DAC boards, 5 different gain stages (several single ended, a srpp, and an opamp), a pile of power supplies, etc (not to mention swapping different coupling caps). I am also working on a headphone amp that is the same -- there are little boards that have a PCB mount socket but also a retaining ring. All the resistors/caps, etc and terminal blocks are on each board so it takes about 3 minutes to change the amp completely. I even picked up some bottlehead hole shrinkers so it can use both octal and noval tubes. |
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| lineup |
ECC86
is about as low in NOISE you can get with tubes.
Has been used in 1.000 of low noise preamps for almost as long as existed.
And many Professional equipements and microphone amps use it!
A short explanation of the schematic:
The triode is of course the actual gain device, operating at 15V 2mA.
The plate is loaded with a 2SK170-based CCS
and direct coupled to a cascoded Jfet White follower.
Your circuit setup is interesting
and really deserves some try out!
Thanks very much, Fuling.
from
lineup |
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| Bas Horneman |
| Another added advantage is the low voltage b+ one can use! |
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| Fuling |
Thanks for shown interest!
The circuit is under evaluation, but initial testings indicates that it is perfectly capable of driving an IRFP260-based follower to 50kHz without loss of gain.
My PSU is not quite up to the task, and either the cathode resistor or the 175R reistor in the CCS must be made adjustable to set the operation point for max linear output swing.
I´m definitely going to make something out of this some day when I have the time, there are plenty of more important projects in the pipeline right now. |
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| Fuling |
| quote: | | Another added advantage is the low voltage b+ one can use! |
Direct coupling to the output stage was a major design goal, if I had to use a coupling cap I´d definitely choose a more potent tube and skip the White follower. |
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| tinitus |
Excuse me fore being a bit ignorant and off topic......
But wouldnt it be posiible to simply use a low noise step-up transformer:confused:
Nah, maybe not eneough voltage ...? |
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| Zen Mod |
| quote: | Originally posted by tinitus
Excuse me fore being a bit ignorant and off topic......
But wouldnt it be posiible to simply use a low noise step-up transformer:confused:
Nah, maybe not eneough voltage ...? |
you mean-xformer instead preamp?
then you forgot impedance issues ;) |
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| MIKET |
| Bas Horneman has the right idea, a triode with a CCS on top. I have built this circuit to drive the grid of a 300B, should handle that MOSFET's without any issues. |
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| Ryssen |
| quote: | | should handle that MOSFET's without any issues. |
Even the capacistanse of the Mosfets?Some say it won´t,that´s why I decided to go for a CF..:) |
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| Fuling |
A CCS-loaded 6C45 would have about 1kohm output impedance, which might me a bit high.
The same tube with a 3:1 stepdown transformer on the output would probably be much better suited for the job! |
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| Mad_K |
What is the problem with 1K out Z?
You'd be bandwith limited to about 600KHz... |
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| Fuling |
| quote: | | You'd be bandwith limited to about 600KHz... |
So we could get to 30kHz with 20kohm output impedance? |
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| Mad_K |
| Teoretically, yes ;) |
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| Ryssen |
Wich means,for a pretty novice,no good ,or...
Trying do make up my mind wich way to go.. |
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| Mad_K |
| I would try to keep the source impedance below 5K (to keep 100KHz bandwith) ;) |
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| Fuling |
I would aim for an even lower source impedance, preferably not more than a couple of hundred ohms.
In my experience powerful driver stages always sounds better than whimpy ones, even if both technically are capable of driving the load. This goes for both tube and mosfet output stages.
Just my two cents:) |
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| Brian Beck |
My 2 cents worth also:
You need much greater bandwidth in the gate driver than you might think to avoid distortion. Here’s why:
The driving resistance, whether it’s the plate resistance of a common-cathode stage or the cathode resistance of a CF, and the gate capacitance form a roll-off together at a pole frequency of 1/(2*pi*R*C). At that pole frequency, amplitude drops by 3dB and phase is shifted by 45 degrees. At one tenth of the pole frequency (perhaps down into the “audio band”), amplitude drops by only 0.043 dB and phase shift is -5.7 degrees.
So if the pole is set to 200KHz, as a convenient example, then if we look at the top of the audio band at 20KHz, the amplitude is down by only 0.043dB and the phase is retarded (“delayed”) by 5.7 degrees. That equates to 0.8 microsecond of “delay” at 20KHz. That doesn’t sound too scary.
So what?
In the case of the IRFP240, we need to worry primarily with Crss (= Cdg) which in the follower configuration is not amplified by the Miller effect happily. But it’s bad enough. (We’ll ignore the bootstrapped Cgs for simplicity). At the operating point of around 11 volts drain-to-source (Vds), Crss is about 350pF. To achieve a 200KHz roll-off (just for sake of example) we’d need to drive it with a resistance of no higher than about 2200 ohms, which is easily achievable with a decent triode’s plate. Is all well then? No, read on...
Vds can vary by almost +/- 10 volts from this operating point as it swings with signal. The problem is that Crss varies from about 200pF at Vds = 20 volts to about 1300pF at Vds = 1. This means that the RC time “constant” that determines the pole frequency and phase shift is not constant at all; it varies dramatically with signal voltages. This variation creates a phase intermodulation mechanism that defines, I believe, the difference between the sounds of MOSFETs and tubes. I believe that some people equate phase intermodulation with detail and etching, when it fact it’s false detail and distortion.
Taking the simple math one step further for this example (bear with me): As I said, when the output signal approaches the peak positive level, Crss approaches 1300pF. Driven by our hypothetical 2200 ohm driver, the pole frequency then drops from 200KHz to about 54KHz. Looking again at the amplitude and phase shift effects at 20KHz we see an amplitude drop of 0.55dB and a phase shift of -20.3 degrees. So the gain at 20 KHz actually CHANGES as the signal swings, from -0.043 dB to -0.55dB. And phase shift grows from -5.7 degrees to -20.3 degrees at 20KHz. When several frequencies are introduced into the amp at the same time (i.e. music) these signals will intermodulate with each other in both time and amplitude.
Converting that phase shift to time at 20 KHz, the high end of the audio band is delayed by an additional 2 microseconds as the signal swings positive (and by a lesser amount in the other direction too, which we’ll ignore). That’s 2,000 nanoseconds or 2,000,000 picoseconds. You can think of this as a kind of jitter. In the digital world, we’ve learned that the ear is exquisitely sensitive to jitter. We believe that jitter down into the hundreds of picoseconds may still be audible. Granted, not all jitter mechanisms are alike. But what we have in this example is a peak “jitter” of about 2,000,000 picoseconds. Across a lesser signal swing or at lower audio frequencies we might see “only” 2,000 to 20,000 picoseconds of jitter. That range is still orders of magnitude more than we would hope for.
Design implication: The MOSFET’s gate needs to be driven from a very, very low drive resistance to force the pole to a very high frequency where lingering phase shift in the audio band is reduced. At the very least, the gate deserves a CF driver using a high mu, high gm tube, and even that may not be enough. IMO, of course.
Edit: These numbers seem worse than even I would have predicted. I hope that someone will check my math and make sure that I didn't make a mistake. The message is still valid though, even if a decimal point is off by a space. |
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| lineup |
hi
first decision to be made
for preamp for SEWA amp
http://www.diyaudio.com/forums/atta...tamp=1157451156
Should we use the existing power supply of SEWA
or
make a separate power supply for the tube pre amp
Both ways are surely possible.
But for best results I think tube should have its own supply
a bit higher voltage and not very much current TRAFO
what you say guys? |
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| Ryssen |
| Alright then..;) I will use a separate tarnsformer for a (180v)CF and try ECC86 for voltage tube. |
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| lineup |
| quote: | Originally posted by Ryssen
Alright then..;) I will use a separate transformer for a (180v)CF and try ECC86 for voltage tube. |
no doubt this is best way for best performance of the tube
but what about this approch, used with good result
the link is http://www.customanalogue.com/diytu...clone/index.htm
as i told before in this topic |
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| Sheldon |
| quote: | Originally posted by Brian Beck
My 2 cents worth also:
You need much greater bandwidth in the gate driver than you might think to avoid distortion. Here’s why:
The driving resistance, whether it’s the plate resistance of a common-cathode stage or the cathode resistance of a CF, and the gate capacitance form a roll-off together at a pole frequency of 1/(2*pi*R*C). At that pole frequency, amplitude drops by 3dB and phase is shifted by 45 degrees. At one tenth of the pole frequency (perhaps down into the “audio band”), amplitude drops by only 0.043 dB and phase shift is -5.7 degrees.
So if the pole is set to 200KHz, as a convenient example, then if we look at the top of the audio band at 20KHz, the amplitude is down by only 0.043dB and the phase is retarded (“delayed”) by 5.7 degrees. That equates to 0.8 microsecond of “delay” at 20KHz. That doesn’t sound too scary.
So what?
In the case of the IRFP240, we need to worry primarily with Crss (= Cdg) which in the follower configuration is not amplified by the Miller effect happily. But it’s bad enough. (We’ll ignore the bootstrapped Cgs for simplicity). At the operating point of around 11 volts drain-to-source (Vds), Crss is about 350pF. To achieve a 200KHz roll-off (just for sake of example) we’d need to drive it with a resistance of no higher than about 2200 ohms, which is easily achievable with a decent triode’s plate. Is all well then? No, read on...
Vds can vary by almost +/- 10 volts from this operating point as it swings with signal. The problem is that Crss varies from about 200pF at Vds = 20 volts to about 1300pF at Vds = 1. This means that the RC time “constant” that determines the pole frequency and phase shift is not constant at all; it varies dramatically with signal voltages. This variation creates a phase intermodulation mechanism that defines, I believe, the difference between the sounds of MOSFETs and tubes. I believe that some people equate phase intermodulation with detail and etching, when it fact it’s false detail and distortion.
Taking the simple math one step further for this example (bear with me): As I said, when the output signal approaches the peak positive level, Crss approaches 1300pF. Driven by our hypothetical 2200 ohm driver, the pole frequency then drops from 200KHz to about 54KHz. Looking again at the amplitude and phase shift effects at 20KHz we see an amplitude drop of 0.55dB and a phase shift of -20.3 degrees. So the gain at 20 KHz actually CHANGES as the signal swings, from -0.043 dB to -0.55dB. And phase shift grows from -5.7 degrees to -20.3 degrees at 20KHz. When several frequencies are introduced into the amp at the same time (i.e. music) these signals will intermodulate with each other in both time and amplitude.
Converting that phase shift to time at 20 KHz, the high end of the audio band is delayed by an additional 2 microseconds as the signal swings positive (and by a lesser amount in the other direction too, which we’ll ignore). That’s 2,000 nanoseconds or 2,000,000 picoseconds. You can think of this as a kind of jitter. In the digital world, we’ve learned that the ear is exquisitely sensitive to jitter. We believe that jitter down into the hundreds of picoseconds may still be audible. Granted, not all jitter mechanisms are alike. But what we have in this example is a peak “jitter” of about 2,000,000 picoseconds. Across a lesser signal swing or at lower audio frequencies we might see “only” 2,000 to 20,000 picoseconds of jitter. That range is still orders of magnitude more than we would hope for.
Design implication: The MOSFET’s gate needs to be driven from a very, very low drive resistance to force the pole to a very high frequency where lingering phase shift in the audio band is reduced. At the very least, the gate deserves a CF driver using a high mu, high gm tube, and even that may not be enough. IMO, of course.
Edit: These numbers seem worse than even I would have predicted. I hope that someone will check my math and make sure that I didn't make a mistake. The message is still valid though, even if a decimal point is off by a space. |
Interesting analysis Brian. Your math seems correct. A couple of questions: The Crss figures are quoted at measurement conditions of f 1MHz and Vgs of 0. Do the figures change under conditions that would be typical for an audio amp? It looks like those Crss curves tend to take off around 3V or so. Couldn't one deal with the issue by considering that 3V level a kind of clipping point and design the driver to stay below it for standard input levels? This should be less of a limitation with stages driven by higher voltages. I'm looking at a different kind of mosfet output stage, so this is interesting to me.
Sheldon |
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| Mad_K |
| Very enlightening reading ;) |
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| Brian Beck |
Sheldon,
Thanks for checking my math. Yes, capacitance values would be somewhat different at a Vgs value of something other than zero, but that chart was all I had to go on. The “jitter” values were so large that a little error in calculation just won’t change matters much.
I think it was convenient to measure these capacitance values at 1 MHz, since impedances might be get too high at lower frequencies to measure accurately. I doubt the capacitance values would be much different at lower frequencies. At higher frequencies lead inductances start to factor in so they probably thought that 1MHz was a low enough frequency to allow ignoring the inductances. And a lot of test equipment measures at 1 MHz as a standard.
I don’t see anything special happening at 3 volts, since the curve smoothly rises as you move left across the whole chart. If you limited the voltage swing, it certainly would reduce the distortion, the same as with almost any other distortion mechanism, but then you have a lower power amplifier. On another thread where a MOSFET was a driver and not the final stage, it was noted that when you bias these high voltage devices to closer to their maximum spec for Vds, the Crss drops in value and becomes more constant. But here a 200 Volt supply with 2 amps flowing wouldn’t work!
Despite good intentions about including tubes, if one were insistent on using MOSFETs as in this amp, this is one case where I drive them with a low impedance solid-state source. Nelson Pass seems to have taken this road. You will notice that almost every node in his circuits is at a very low impedance. Even so, you will see rising distortion at higher frequencies in his test data, indicating that capacitance is starting to “weigh down” the amp stages. |
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| MIKET |
Design Goals:
1. Gain 20-40, this will alow a cd player to drive the MOSFET to full output.
2. Low output impedance. Wide Band performance.
3. Low parts count. We are using this to drive SEWA, otherwise would be wrong from a purist standpoint.
4. Output Swing 20V p-p minimum to assure we can get all 7 watts out of the MOSFET.
5. SE design no NFB.............thats my goal, but don't think it's unreasonable.
Those are my goals.......your thoughts |
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| Tyimo |
Hi Dsavidsk!
| quote: | | I use something similar for a DAC gain stage after a passive I/V. |
Very nice preamp! Could you tell us the specs?
Gain, output voltage, etc?
Tyimo |
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| Ryssen |
| If I use a E81L as a CF how do I know what the R out will be?:rolleyes: |
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| Geek |
| quote: | Originally posted by MIKET
Design Goals:
1. Gain 20-40, this will alow a cd player to drive the MOSFET to full output.
2. Low output impedance. Wide Band performance.
3. Low parts count. We are using this to drive SEWA, otherwise would be wrong from a purist standpoint.
4. Output Swing 20V p-p minimum to assure we can get all 7 watts out of the MOSFET.
5. SE design no NFB.............thats my goal, but don't think it's unreasonable.
Those are my goals.......your thoughts |
Reasonable. I second ;) |
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| Sheldon |
| quote: | Originally posted by Brian Beck
I don’t see anything special happening at 3 volts, since the curve smoothly rises as you move left across the whole chart. If you limited the voltage swing, it certainly would reduce the distortion, the same as with almost any other distortion mechanism, but then you have a lower power amplifier. On another thread where a MOSFET was a driver and not the final stage, it was noted that when you bias these high voltage devices to closer to their maximum spec for Vds, the Crss drops in value and becomes more constant. But here a 200 Volt supply with 2 amps flowing wouldn’t work! |
Oops, my mistake. Misplaced a zero. The steep uptick takes place around 30 volts for this FET. I have been looking at STB24F10 and STP20N20, where the Crss is pretty low and begins a steep rise arounng 3-5Volts. Sorry for the confusion. So for the Ir transistor, the input limiting strategy for a low power amp would be completely useless. For my application, I'm looking at a reflected impedence of the tube stage to be about 2K and the voltage at the FET's to be about 45V. So the Crss would be below 50 or so, until Vds dropped below about 3-4V, for a corner frequency of about 160K.
Sorry for the slightly off topic excursion.
Sheldon |
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| DougL |
| quote: | | 1. Gain 20-40, this will alow a cd player to drive the MOSFET to full output. |
My understanding of CD output levels are about 2 V RMS.
In my design notes, I assume a nominal 1 V RMS input, or about 3 V pp.
Given that, a minimum gain of 7 would give the 20V pp that is required.
I was doing a paper design inspired by your requirements:
CSS loaded ECC99 with an EDCOR 10k:600 interstage transformer parafead between Cathode and Plate (Anode).
Current tentatively set to about 10 Ma.
with a B+ of 240V, Plate Signal swing should be about 132 Vpp (6 Vpp input * mu 22), while output signal swing should be 1/4 of that, or about 33 Vpp. Output Z should be Rp/ 16, or 125 ohms.
Compromises:
Gain is a bit light for 1 Vrms, which "only" giving 16 Vpp.
The $12 EDCOR transformer is only good to 25 Vrms @ 30hz, which maxes out @ about 18 Vpp @ 30Hz
The 240V PS may be a culture shock for the intended audience.
I'll draw a schematic later today.
Any feedback welcome.
Doug |
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| Zen Mod |
| quote: | Originally posted by DougL
My understanding of CD output levels are about 2 V RMS.
In my design notes, I assume a nominal 1 V RMS input, or about 3 V pp.
Given that, a minimum gain of 7 would give the 20V pp that is required.
I was doing a paper design inspired by your requirements:
CSS loaded ECC99 with an EDCOR 10k:600 interstage transformer parafead between Cathode and Plate (Anode).
Current tentatively set to about 10 Ma.
with a B+ of 240V, Plate Signal swing should be about 132 Vpp (6 Vpp input * mu 22), while output signal swing should be 1/4 of that, or about 33 Vpp. Output Z should be Rp/ 16, or 125 ohms.
Compromises:
Gain is a bit light for 1 Vrms, which "only" giving 16 Vpp.
The $12 EDCOR transformer is only good to 25 Vrms @ 30hz, which maxes out @ about 18 Vpp @ 30Hz
The 240V PS may be a culture shock for the intended audience.
I'll draw a schematic later today.
Any feedback welcome.
Doug |
hehe
my kind of preamp |
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| MIKET |
Doug L said "I was doing a paper design inspired by your requirements:
CSS loaded ECC99 with an EDCOR 10k:600 interstage transformer parafead between Cathode and Plate (Anode).
Current tentatively set to about 10 Ma"
I am interested in how this turns out.
I will pursue the 6C45Pi for now since I have a few in my tube stock.
Another possiblity might be a 12B4, it has a gain of about 7 but could improve with a transformer in front of it. I was thinking possibly something like this Jensen 1:1.4 transformer:
http://www.jensen-transformers.com/datashts/11p41.pdf
Its a bit pricey but very nice specs. |
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| Shoog |
Forget about the tube buffered LM3875 approach. Its not the buffer he uses in his commercial project, and its strickly limited in performance.
A CCS loaded 12B4 with a step up xformer infront sounds very promising.
Shoog |
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| Tyimo |
Hi!
Check this out!
This monster pream could drive everything......!:D :D
Greets:
Tyimo |
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| MIKET |
| quote: | Originally posted by Tyimo
Hi!
Check this out!
This monster pream could drive everything......!:D :D
Greets:
Tyimo |
It may drive everything but with a typical cd player it won't drive SEWA to 7 watts. The gain of this tube is about 2.4 per section, see data sheet http://www.jogis-roehrenbude.de/Rus...%20datasheet%22 |
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| Tyimo |
| quote: | | The gain of this tube is about 2.4 per section |
Thanks MIKET! It was only a nice dream.......
| quote: | | SRPPed 6H30 Line Preamplifier |
It is nice!! I like it! Thanks! :)
Tyimo |
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| lineup |
:cool:
What you tube people and SEWA owners think of this RJM circuit?
| quote: | Parallel Triode Line Stage.
Primitive ART and a nod of appreciation to Conrad Johnson.
If I needed a line stage, this is the circuitry I'd use.
The parallel tubes lower the noise and output impedance.
With four 6DJ8 triode sections per channel, the output impedance is 850 ohms.
Gain is about 26 dB. |
The gain is too much for a line preamp,
but what about the idea, of parallelling fairly cheep triode tubes
to get better drive in the output?
Here is the circuit:
And here is the link to RJM project page:
Parallel Triode Line Stage
lineup |
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| planet10 |
Hi halo,
CJ does seen to get kudos for their paralleled line stage. Benefit -- a 1/4 the output impedance and 4 x the current drive.
The downside is that you get the average of the transfer curves of 4 tubes (just like paralleling any device) which tends to average out the lowest level of information.
So far i for single stage (what i'd like to see) are the totem pole designs or the idea of a 12b4... the last migt not have low enuff output impedance... this leads to thinking 2 stages, gain stage followed by a follower, but that doesn't have the elegance...
dave |
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| lineup |
| quote: | Originally posted by planet10
Hi halo,
....
The downside is that you get the average of the transfer curves of 4 tubes (just like paralleling any device) which tends to average out the lowest level of information.
So far i for single stage (what i'd like to see) are the totem pole designs or the idea of a 12b4... the last migt not have low enuff output impedance... this leads to thinking 2 stages, gain stage followed by a follower, but that doesn't have the elegance...
dave |
thanks planet10
And when we parallel 4 transistors, Output Stage MOSFETs for example,
many are happy about output feature / figures
and
they do not understand, that C-iss, input capacitance will be 4 times higher
and of course will require more of a previous stage transistor
For the price of 4 x 6DJ8 / E88CC / 6922
I will get 2 other more expensive high class Triodes
that may have higher current drive ( lower output impedance )
and also better audio quality.
thanks again
lineup :cool:
i walk on - a little bit more wise, with tubes
and with a little bit more heavy weight of my knowledge
... this my burdon to I have to carry with me
----------
For in much wisdom is much grief;
and he who increases knowledge increases sorrow.
[ Ecclesiastes 1:18; Author unknown ~250 years before Jesus Christ ]
:cool: |
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| Ryssen |
| Why not a Cathode follower?:rolleyes: |
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| planet10 |
Cathode follower has no gain, so it would have to be used with a preceeding voltage gain stage. Ideally the cathode of the follower would be a CCS or a significant nagnitude B-.
You then have basically a "ForePlay" style preamp in front. Certainly an option.
dave |
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| lineup |
post #53
http://www.diyaudio.com/forums/show...9317#post999317| quote: | Design Goals:
1. Gain 20-40, this will alow a cd player to drive the MOSFET to full output.
2. Low output impedance. Wide Band performance.
3. Low parts count. We are using this to drive SEWA, otherwise would be wrong from a purist standpoint.
4. Output Swing 20V p-p minimum to assure we can get all 7 watts out of the MOSFET.
5. SE design no NFB.............thats my goal, but don't think it's unreasonable. |
I think even Mad_K, the master of puristic simplicity, could live with a 2 tubes pre amp.
This is his comment on a Kaneda legendary 5 transistor schematic. ( Kaneda Schematic here )| quote: | Originally posted by Mad_K
Looks nice,
but I think it has about 5 times too many transistors in the signal path ;) :smash: ;) |
yes
looks as a good alternative, planet10
and, by the way, this is how I build my transistor pre amps
always a follower output buffer
to reduce load onto and make it easy for voltage amplifier to operate as good as possible.
I suggest we keep away all thoughts on global negative feedback.
Personally I wouldnt dream of using anything like this, in a tube amp.
If I wanted to reduce some harmonic distortion using a feedback loop,
I wouldnt go for Them Nice Tubes in the first place.
What I want by sometimes using tubes is a character of sound
that only tubes can fully give.
Those who wants the '100% distortion-free sound', whatever this is
have plenty of other good alternatives for THE SEWA.
lineup :) leader of The Mad_K official fanclub |
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| johndiy |
linu up keep up the good work;)
john |
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| lineup |
| quote: | Originally posted by johndiy
linu up keep up the good work;)
john |
Thanks john - I work hard to become a master - but long way to go, still .....
I like to read your posts, johndiy
they have more of a positive funny twist and spirit, and you show you have good audio understanding,
what we can not say about everything posted at this board.
I mean the positive and friendly approach in respect of others.
See my opinion here:
I managed to set a nice end commentary to discussion in
Class B - high feedback
http://www.diyaudio.com/forums/show...773#post1006773
lineup - far away from you |
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| Ryssen |
| Sitting here listening to my newly made Sewa amp,I am driving it with my Mu folower with 6C45 and 6888,I have in a box as a preamp.I think it´s working fine can´t see what the problem should be of driving the mosfet.If it shouldn´t be so fine should the frekvensy response be bad or what,maybee I could test it with RMAA and see. |
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| MIKET |
Ryssen, care to share the schematic with us.
I think this should be a great choice, I like the 6C45 a lot, and have used a Solid State CCS, but a pentode on top is a very good choice indeed. |
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| Geek |
Hi guys,
Did some benchwork tonight on I design I've been meaning to get to.
So far, it seems to fit the bill, is simple, provides great performance, ONE TUBE per channel and can be built with commonly available parts. Schematic attached with the values I used in the test.
Transformer can be had from Radio Shack, their 12VCT, 2A seconday piece. 12V back-to-back. One ECC88 filament on one 6.3V leg, the other on the other 6.3V leg, CT grounded.
Since the SEWA has a Zin of 100K || 250p, I did the tests with a 2:1 margin, or 47K || 470p. Here's some pics of the results.
5KHz Squarewave
15KHz Squarewave closeup
Full tilt - that's 160V P-P at 1KHz |
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| lineup |
| quote: | Originally posted by Geek
Since the SEWA has a Zin of 100K || 250p, I did the tests with a 2:1 margin, or 47K || 470p. Here's some pics of the results. |
The input impedance of SEWA Rev_d ( see attached schematic )
which i think is latest published version,
is the sum of 3 impdances:
- R2 100 k , this is needed to bias SEWA to 1/2 supply voltage
- input impedance of MOSFET Q1, this is probably so high we can put almost infite value = not needed to count this
- R9 100 k, this is not really necessary, but is good for charging input cap
as well as terminate signal wire closer to input jack
100k in parallel to 100 k, 100k//100k = 50 kOhm
Rev D as seen in picture has got an input impedance a bit lessthan 50 k
say 47 kOhm
Now R9, if we do not want to exclude it ( I dont )
can be made having higher value,
say 470 kOhm .... ( or even maybe 1 MOhm )
will give
Zin is 100//470 ... <= 82.4 kOhm
and this Zin, input impedance, will better fit your circuit, Geek |
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| Brian Beck |
| quote: | Originally posted by planet10
...The downside is that you get the average of the transfer curves of 4 tubes (just like paralleling any device) which tends to average out the lowest level of information... |
(Late response) I was reading back through this thread and saw this. Dave can you explain this phenomenon? |
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| Tyimo |
Hi Geek!
| quote: | | So far, it seems to fit the bill, is simple, provides great performance, ONE TUBE per channel and can be built with commonly available parts. Schematic attached with the values I used in the test. |
Nice circuit!!!!!
Greets:
Tyimo |
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| planet10 |
| quote: | Originally posted by Brian Beck
(Late response) I was reading back through this thread and saw this. Dave can you explain this phenomenon? |
4 tubes with (at least) slightly different transfer curves, each producing the smallest levels slightly differently, then add them all together... you get a blurring of the small detail.
dave |
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| Geek |
| quote: | Originally posted by planet10
4 tubes with (at least) slightly different transfer curves, each producing the smallest levels slightly differently, then add them all together... you get a blurring of the small detail.
|
The sound is similar to a homemade DAC using an R, 2R ladder with the values slightly out of alignment. Best I can describe it....
| quote: | Originally posted by Tyimo
Nice circuit!!!!! |
Thanks! :D |
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| Ryssen |
This is the one I´m using,6C45 as the lower tube the Led is an IR 1,2v.
Can also use a 6AG7 as pentode B+ is about 300v.Doe´s also drive my 300 ohm HD650..:) |
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| Brian Beck |
| quote: | Originally posted by planet10
4 tubes with (at least) slightly different transfer curves, each producing the smallest levels slightly differently, then add them all together... you get a blurring of the small detail. |
I've heard this notion before, but I'm afraid I don't get it. Maybe if you could define "blurring" in math terms it would help. I'm not saying there is no such phenomenon, just that I haven't observed it, heard it, nor can I imagine a mechanism. If there is one, let us know.
There was a fine article published in either Glass Audio or AudioXpress several years back by a group of authors about paralleling tubes. They showed, using both math and some tests (if memory serves) that the combined transfer functions of paralleled triodes have no peculiar traits. If I recall it wrong, I hope someone will correct me. I've got my back issues stored in boxes. Maybe I can find the article. |
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| maxw |
| quote: | Originally posted by Geek
Hi guys,
Did some benchwork tonight on I design I've been meaning to get to.
So far, it seems to fit the bill, is simple, provides great performance, ONE TUBE per channel and can be built with commonly available parts. Schematic attached with the values I used in the test.
Transformer can be had from Radio Shack, their 12VCT, 2A seconday piece. 12V back-to-back. One ECC88 filament on one 6.3V leg, the other on the other 6.3V leg, CT grounded.
Since the SEWA has a Zin of 100K || 250p, I did the tests with a 2:1 margin, or 47K || 470p. Here's some pics of the results.
5KHz Squarewave
15KHz Squarewave closeup
Full tilt - that's 160V P-P at 1KHz |
In your schematic it says "Voltage Gain, 27-28", isn't that far too high for a preamp?
I like your design because those parts are easily available.
Would it be a suitable preamp to any power amp? or just the SEWA? |
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| Geek |
Hi Max,
| quote: | Originally posted by maxw
In your schematic it says "Voltage Gain, 27-28", isn't that far too high for a preamp? |
The SEWA needs around 20. Gives some headroom for quiet sources.
| quote: | Originally posted by maxw
Would it be a suitable preamp to any power amp? or just the SEWA? |
You can use it with anything. Just reduce the gain a wee. This can be done by either a -6 to -10dB pad or by replacing components.
By replacing the LED's and 470 ohm resistor with 680 ohms and making the 10K resistor 1K. Adjust power supply resistor accordingly. Disadvantage of the component change is is higher output Z and increased noise. Personally I'd use the atten. pad. |
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| planet10 |
| quote: | Originally posted by maxw
Would it be a suitable preamp to any power amp? or just the SEWA? |
A very vesatile stage... the prototype is sitting in my tuner as an output stage. I'm going to add a set of inputs and sacrifice the AM to have a CD input, for a very simple pre-amp/tuner.
dave |
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| planet10 |
| quote: | Originally posted by Brian Beck
Maybe if you could define "blurring" in math terms |
if you have 4 tubes, you have 4 similar but different transfer curves. draw them. then compute the average. Any detail small enuff to fit inside the envelope defined by the 4 curves will be lost and signals near will be blurred by the averaging process.
dave |
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| Zen Mod |
| quote: | Originally posted by planet10
if you have 4 tubes, you have 4 similar but different transfer curves. draw them. then compute the average. Any detail small enuff to fit inside the envelope defined by the 4 curves will be lost and signals near will be blurred by the averaging process.
dave |
I agree with P10 here
I tried once iteration of ART -lot's of muscles ,but without finest sparkles of good musak ;it doesn't matter I drove each system with 3-5-10-whatever mA
for my taste,no matter which tube is in question ,paralleling two systems is max |
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| Brian Beck |
| quote: | | Any detail small enuff to fit inside the envelope defined by the 4 curves will be lost and signals near will be blurred by the averaging process. |
I'm not arguing with anyone’s subjective impressions, but I don't think we've got the technical reason in what Dave has said. Transfer functions usually relate Vout to Vin (or sometimes Iout to Vin). The slope on any point of the transfer function is the gain (dVout/dVin). Curve, bends and ripples in the transfer function create distortion. But the only way that low-level information can be completely lost is if there is a flat spot (zero slope = zero gain), such as with cross-over distortion or hard clipping. Below hard clipping, triodes typically exhibit smooth and monotonic transfer curves with a gentle curve. The curve gives rise to the distortion character dominated by second and third harmonic that is easy on the ears, with only a bit of higher harmonic content. If you add together four similar but slightly triode different transfer curves, the result is a new transfer function, also fairly smooth and monotonic, which will have a subtly new distortion character. This is what is likely to be audible, since the harmonic series may have shifted. But I don’t see how there could now be a flat spot, which is required for the output to disappear altogether for low level signals. Dave, I think that maybe your concept of an envelop which is somehow defined within the spaces between the 4 drawn curves is throwing you off. When the four curves are combined into one new transfer function, there is no envelop. I’m really not trying to be relentlessly argumentative, but either I want to be corrected and learn something new, or dispel what I suspect might be one of those “urban myths”. |
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| Zen Mod |
| quote: | Originally posted by Brian Beck
I'm not arguing with anyone’s subjective impressions, but I don't think we've got the technical reason in what Dave has said. Transfer functions usually relate Vout to Vin (or sometimes Iout to Vin). The slope on any point of the transfer function is the gain (dVout/dVin). Curve, bends and ripples in the transfer function create distortion. But the only way that low-level information can be completely lost is if there is a flat spot (zero slope = zero gain), such as with cross-over distortion or hard clipping. Below hard clipping, triodes typically exhibit smooth and monotonic transfer curves with a gentle curve. The curve gives rise to the distortion character dominated by second and third harmonic that is easy on the ears, with only a bit of higher harmonic content. If you add together four similar but slightly triode different transfer curves, the result is a new transfer function, also fairly smooth and monotonic, which will have a subtly new distortion character. This is what is likely to be audible, since the harmonic series may have shifted. But I don’t see how there could now be a flat spot, which is required for the output to disappear altogether for low level signals. Dave, I think that maybe your concept of an envelop which is somehow defined within the spaces between the 4 drawn curves is throwing you off. When the four curves are combined into one new transfer function, there is no envelop. I’m really not trying to be relentlessly argumentative, but either I want to be corrected and learn something new, or dispel what I suspect might be one of those “urban myths”. |
try it ,and let your ears-brain combo decide - what's better for you.
you don't really need technical explanation and argumentation.
listen stage with one toob in setup ( with x Ua,xx Ia and gate stopper) ,then make 4 or 6 paralleled stages ,with exactly same x,xx ,and gate stopper per system.
you can try mutual and separate biasing schemes (common cathode resistor,separate cathode resistor) to deepen test a little....
been there ,done that
in case where one system struggled with load , ART (say it like that) approach is clearly better sounding;
in case where one system already have enough muscle for load,one system is clearly better.
from analytical point, or even speculative or even philosophic point-there are numerous ways and analogies which we can use for
either "side" ,pro at contra .
but my engrish really sucks for this task
EDIT:
hehe....not "gate stopper".......grid ......naturally |
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| planet10 |
| quote: | Originally posted by Brian Beck
I'm not arguing with anyone’s subjective impressions, but I don't think we've got the technical reason in what Dave has said. Transfer functions usually relate Vout to Vin (or sometimes Iout to Vin). The slope on any point of the transfer function is the gain (dVout/dVin). Curve, bends and ripples in the transfer function create distortion. .... |
A transfer function also has a frequency axis, so you don't really have a curve you have a 2D manifold in 3-space.
When you average 4 TF together you get a single transfer curve but it now has a downward dynamic range limited to those signals larger than the difference between the outer edges of the envelope defined by the original TFs. Each tube still has its own unique TF and outputs according to that... then it passes to the summer which blenderizies everything below a threshold defined by that TF envelope. The closer you can match the tubes over all the points in that 2D sheet, the better your DDR... but then come back in a month and that matching no longer holds because the tubes have aged at different rates.
On a gross scale as usually extracted by measuring kit, you won't detect it by the usual means... but it is still there... you need to imagine (or measure) what is happening 30 or 40 or 50 dB down from the reference level.
A grosser analogy that might make it clearer... in big SS amps the outputs have to be matched to even run... |
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| Brian Beck |
Zen Mod,
As I said, I am NOT debating the subjective impressions. I am interested in the technical explanation that Dave offered for them however.
I have tried multiple parallelled tubes in MC front ends. I liked up to four in parallel that we tried. Also, fairly recently, I tried first two pairs of 7119s in a headphone amp output stage and then compared that sound to just one pair. I thought the two pair sounded a bit punchier, but not by a lot. I ended up with just one pair for simplicity. There are a lot of other variables floating around that confound making absolute conclusions about this. I also know that the ART is well received by many others. |
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| Ryssen |
| Been thinking..;) If the Mu stage with 6C45 and pentode can drive the Sewa,then maybee a 6c45 and a SS CCS would work? |
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| planet10 |
| quote: | Originally posted by Ryssen
Been thinking..;) If the Mu stage with 6C45 and pentode can drive the Sewa,then maybee a 6c45 and a SS CCS would work? |
I don't see why not.
dave |
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| Geek |
| quote: | Originally posted by Ryssen
Been thinking..;) If the Mu stage with 6C45 and pentode can drive the Sewa,then maybee a 6c45 and a SS CCS would work? |
Should work fine. Just use enough current through the tube to keep good linearity across the range into the SEWA :) |
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| Brian Beck |
| quote: | Originally posted by planet10
A transfer function also has a frequency axis, so you don't really have a curve you have a 2D manifold in 3-space. When you average 4 TF together you get a single transfer curve but it now has a downward dynamic range limited to those signals larger than the difference between the outer edges of the envelope defined by the original TFs. Each tube still has its own unique TF and outputs according to that... then it passes to the summer which blenderizies everything below a threshold defined by that TF envelope. |
I hope we can be excused for hijacking this thread for a bit. I figure after 10 pages the original thread has had its run anyway :) ...
I found the reference I was looking for. In Glass Audio 2000/5, authors Kamna, McDonald and Boehlke methodically explore the claim that paralleling tubes does not give good sonic results. They used a three pronged approach: test data, listening tests and mathematical analysis. While not exhaustive, their work was pretty complete and well thought through. Their conclusion was that paralleling tubes caused no harm – just increased the power. No “envelops” or “blenderizing” were noted. They also note (significantly) that any one tube can be considered as a parallel combination of smaller sections of tubes. Imagine the long 6SN7 plate structure covering its underlying grid and cathode. Imagine slicing it into 4 smaller sections through planes perpendicular to the longitudinal axis of the plate. Each section is a smaller triode, with nominally the same mu, but with 1/4 the gm and 4 times the rp. Each section would be slightly different, however, since mechanical tolerances are not perfect across the length of the structure. But since they are butted together (in parallel) we get one tube that combines these slight differences into one working 6SN7. Nothing untoward is observed in the combined result. If paralleling separate tubes is inherently bad, then we’d have to explain how one tube can work.
I think that if there are any parallel tube effects, these would be at the large signal extremes, near cut-off or at the onset of grid current, where the “worst” tube of the group stubs its toe before the others. However, for low-level signals, you can zoom in to look at any smooth curve (a curve that can be defined with a finite set of polynomial coefficients) very closely, to see what a low level signal sees, and you’d see only an increasingly linear tiny piece of a bigger curve.
Dave, you certainly could develop transfer functions at all frequencies and make a 3D shape out of them. Then you’d have surfaces for each tube that can still be combined by summing/averaging. And the result is just another surface, with no “envelops” or “blenders” that I can imagine. Perhaps if you could sketch out what you’re thinking and label the graph axes, or develop some more rigorous mathematical explanation, it would be easier to understand what you mean, especially with regard to frequency effects and “blurring’.
Again, I won’t fuss with those rushing in to claim that they tried paralleled tubes and didn’t like the sound. There are a lot of variables in those kinds of trials besides transfer functions. But I am interested when a new technical explanation is offered which deserves further exploration. |
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