JFET input, MOSFET VAS, LATERAL output = Perfect!! - Page 235 - diyAudio
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Old 9th August 2014, 12:57 PM   #2341
danspy is offline danspy  Thailand
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Hi guys,

Click the image to open in full size.

I was away for a while.
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Old 17th November 2014, 03:23 AM   #2342
AKSA is offline AKSA  Australia
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And where was that, Danspy?

Hugh
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Old 17th November 2014, 10:32 AM   #2343
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by AKSA View Post
And where was that, Danspy?
He's just mis-posted. The real question is, where's Line Up?
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Old 17th November 2014, 11:51 AM   #2344
mikelm is offline mikelm  England
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Wow this thread just woke up again - long time, no see.

I had been thinking of posting an update here as some people have been asking how the DC feedback & servo, choke regulation version of this amp has progressed.

Many years ago I read these web pages from Peufeu

Memory Distortion Philosophies

Thought it was very interesting but thought there was probably a lot more going on besides thermal distortion and because at the time I was labouring under ( what I now regard as ) a misconception that simplest is best, I never got around to trying any of these mods.

However, since then a couple of people here have commented on how much a strategically placed compound feedback pair or a cascoded CFP can transform the sound an amp. Most recently this advice came from Sonnya who posted about a year ago with regard to his nice TSSA design.

So I decided to try a CFP cascoded with a Jfet as the i/p transistor of my DC linked fetzilla. This means I can no longer call it a fetzilla ! and technically it no longer falls within the remit of this thread. Never-the-less I though I would post an update here as it may be of interest to all the fetzilla enthusiasts here.

There is one technical problem with adding this cascoded CFP - it introduces a 0.63V offset compared with the Jfet and this needs to be addressed. However my amps already had a Jfet follower driver stage so I changed to a higher IDSS Jfet here to give me a 0.63V offset feeding into the new compound i/p device - the servo took care of any minor anomalies.

I don't want to appear too over enthusiastic but in the context of this design, this is a very significant modification. Here are the main subjective changes:

The amps now sounds like it has more effortless power particularly in the bass which now sounds much deeper & fuller. The effect of this when listening to music is that instruments like saxophones sound fatter and more rounded - more like real instruments.

The other main area of improvement is low level detail which improved dramatically - and this is the good kind of unexagerated detail that just makes everything sound natural & realistic.

Sound staging is also improved.

From my limited theoretical understanding I think this cascoded compound FB pair comes close to being an ideal transistor and I am begining to think that the reason many transistor amps with global FB do not sound as natural as amps without, may be because the devices used to mastermind the operation of the feedback most often are not ideal enough.

So all in all I think this is a very significant mod.

If anyone would like to pursue this we can decide whether to start a new thread.

Last edited by mikelm; 17th November 2014 at 11:54 AM.
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Old 17th November 2014, 12:28 PM   #2345
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I see two major distortion mechanisms that would be eliminated by this.

A: The rectified load signals from the rails are no longer leaking into the output through Early effect on the input transistor.
B: A 2nd harmonic caused by Miller compensation currents which rises with frequency will be eliminated.
C: Both of these can be worsened by the high source impedance from a volume pot. If this is the case, it would be interesting to see if the sound improves if a buffer is used after the pot.

I don't know which of these effects is stronger (they are certainly not equivalent sonically), and I don't know where they stand in relation to distortions produced by the rest of the circuit.
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Old 17th November 2014, 01:04 PM   #2346
mikelm is offline mikelm  England
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Hi KT,

Adding a buffer to give low impedance drive to the input is a definite plus for several reasons including stability. I had the idea that the drive impedance should be significantly lower than the gain resistor in the feedback path.

Also adding a buffer enables the use an input HF filter with lowering input drive impedance.

Subjectively all these factors amounted to a definite improvement when I added the buffer.

Last edited by mikelm; 17th November 2014 at 01:10 PM.
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Old 17th November 2014, 02:22 PM   #2347
kees52 is offline kees52  Netherlands
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I am happy that people see the worthy of cascodes/ cascades etc, because of the uge headroom and little memory effect, however stability has to be care for, use of big emittor resistors wil help, the sound of a casode is always very nice, I use them in the hybrid.

regards

kees

Last edited by kees52; 17th November 2014 at 02:32 PM.
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Old 17th November 2014, 02:51 PM   #2348
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Amplifiers with degenerated BJT input stages (and the Fetzilla counts) will be less sensitive to source impedance than amps without, such as a plain LTP input stage. That is to say, the source impedance necessary to double distortion at the output will be larger for amps with degenerated BJT input stages (and much larger for FET input stages).

Often I find myself wondering why most BJT amps don't already have a buffer built into the input, if it's that much of an improvement. A single-transistor buffer can be sufficient and have pretty much no distortion to speak of.
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Old 17th November 2014, 03:08 PM   #2349
mikelm is offline mikelm  England
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Quote:
Originally Posted by keantoken View Post
Amplifiers with degenerated BJT input stages (and the Fetzilla counts) will be less sensitive to source impedance than amps without, such as a plain LTP input stage. That is to say, the source impedance necessary to double distortion at the output will be larger for amps with degenerated BJT input stages (and much larger for FET input stages).

Often I find myself wondering why most BJT amps don't already have a buffer built into the input, if it's that much of an improvement. A single-transistor buffer can be sufficient and have pretty much no distortion to speak of.
The buffers brought benefits even with a Jfet i/p

I don't see a down side with i/p buffers on any amp.
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Old 18th November 2014, 01:53 AM   #2350
AKSA is offline AKSA  Australia
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Mike,

Quote:
decided to try a CFP cascoded with a Jfet as the i/p transistor of my DC linked fetzilla
This is very interesting. Could you please give me a sliver of the input stage in schemat so I can be absolutely sure what you are meaning?
I take your points very seriously.......

Thank you for your posts, Kean too, much appreciated!

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