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Old 13th March 2006, 02:03 AM   #1
lndm is offline lndm  Australia
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Default Phono pre amp - thanks SY

SY,

I have just put together a cascode stage, valve on top, jfet underneath. It has enough gain to match two of my previous stages.

Resolution, WOW.

Allen Wright spoke of high transconductance and single electrons. I was interested in his write up but skeptical. Now, I know he knows his subject. Allen wright is in no doubt a genius, and you, SY, know a good amp when you hear one.

Dynamics, the word effortless springs to mind. I know that this is a characteristic of the cascodes vertical load line for the bottom device. I seem to have tried all the other topologies, and all the time it was a cascode I wanted.


Can anyone recommend devices to use? I am using a 12au7 and 2n5484 as I had these available. I tried a 12at7 but no thanks. I don't have a 6922 at this point. I may set up for octal if I hear it is worth it.
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Old 13th March 2006, 03:10 AM   #2
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Dynamics, the word effortless springs to mind. I know that this is a characteristic of the cascodes vertical load line for the bottom device.
Huh? Would someone please explain THAT one to me? I won't argue the subjective impression, but when someone tries to explain a desirable sonic attribute by saying that it's due to a loading condition that increases non-linearity, well... Please help me to understand why this condition would reveal dynamics more accurately than a high Z plate load. If the quiescent operating point is set where the cascode plate curves are more crowded, and you were to hit the stage with a dynamic peak, one side of the waveform would be "expanded" while the other side would be further compressed. That might sound like increased dynamics, but I think it's more like signal processing. To his credit I think, Allen tends to use his cascodes in differential configurations, and this will tend to cancel the even distortion mechanism. Is this phono amp cascode single-ended or differential?
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Old 13th March 2006, 03:19 AM   #3
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Enjoy. That's psychoacoustics. All subtle colourations and distortions (real or imaginary) are wonderful the first few times we listen to them.
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Old 13th March 2006, 04:10 AM   #4
SY is offline SY  United States
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I haven't tried Allen's particular combination of tube and FET, but with a slightly degenerated NTE458 on the bottom and an ECC88-type on top, my 25 year old old phono stage still managed to clock in at about 0.1% THD midband at 5mV in, totally dominated by second HD. The small signal swings help. I think I can do rather better now and I'd like to knock that distortion down a decimal place or two.

You might try a 6SL7 on top with an NTE458 on bottom, running at 2mA. The NTE has good transconductance at this lower current, is cheap, and easy to get.
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Old 13th March 2006, 04:27 AM   #5
lndm is offline lndm  Australia
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Originally posted by Brian Beck


Huh? Would someone please explain THAT one to me?
http://www.passlabs.com/downloads/articles/cascode.pdf

This is a good read.




Thanks SY.
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Old 13th March 2006, 11:27 AM   #6
EC8010 is offline EC8010  United Kingdom
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Whilst Nelson's article about cascodes is interesting, it is largely concerned with the problems of Class AB vs Class A transistor output stages and Early effect. Triodes are an entirely different kettle of fish and do not exhibit Early effect. As has been pointed out by Brian Beck, a triode with a vertical loadline suffers greatly increased second harmonic distortion. This distortion can be tamed either by operating at small signal levels (which is why cascodes can be useful as the first stage of an RIAA stage), or by cancelling the second harmonic in a differential pair.
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Old 13th March 2006, 12:19 PM   #7
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I know little about electrical engineering, but I do play a cascode at night: Steve Bench's 12ay7/6922 cascode input phono stage, and it's a keeper. I may try Mr. Wright's design some day, if I can find a reasonable way of purchasing matched FETs.
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Old 13th March 2006, 01:05 PM   #8
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I didn’t mean to rain on the cascode parade. If it sounds good to you, that’s great. I was just jumping on the assertion that “effortless” dynamics could be explained by the vertical load line. Cascodes have the potential for certain other advantages, such as reduced Miller capacitance (and therefore high BW) and high gain, although the latter drives one to use higher upper plate loads, which increases the lower tube’s Miller capacitance, so these advantages tend to go in opposite directions. The parallels to the pentode keep coming back, except that cascodes can be quiet due to lack of screen partition noise.

SY, as you suggest, the 0.1% THD at 5mV is not a terrible situation, especially being primarily second HD, but I would think at that exceedingly low level, a conventional grounded cathode stage with high plate load would do much better.
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Old 13th March 2006, 01:42 PM   #9
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valve on top, jfet underneath
Quote:
Can anyone recommend devices to use?
Kevin Carter uses this as well for his K&K audio phono stage and for the Art Audio phono stage (which he designed). (The first stage for a moving magnet cartridge consists of a very high transconductance JFET / triode (6N1P) cascode amplifier.)

So to answer your question on what tube. The 6n1p seem suitable. (Very little microphonics as well by the way) I never found out which JFET he uses.
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Old 13th March 2006, 03:25 PM   #10
SY is offline SY  United States
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Brian, EC, the triode is on top. FET is on the bottom. That makes things more linear and transformer-friendly.
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