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Old 18th November 2013, 07:53 AM   #111
juma is offline juma  Germany
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Quote:
Originally Posted by audiok View Post
... 20db gain on BJT version...
Two ways to do it:
- raise the value of R17 (it sounds better but raises the Zout too)
- lower the value of R12 (makes the pre sound murky if you go too low with it)

You can experiment with these two values to find what you like. I would make R17=2k and add a buffer at the output.
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Old 18th November 2013, 08:16 AM   #112
audiok is offline audiok  Hong Kong
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Thanks Juma.
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Old 18th November 2013, 11:50 PM   #113
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Juma,
On the schematic from post#66, how do you figure the gain of the folded cascode? Am I correct that the FE gain is about 14 of the total 19dB?
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Old 19th November 2013, 12:02 PM   #114
juma is offline juma  Germany
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Originally Posted by buzzforb View Post
... schematic from post#66, how do you figure the gain of the folded cascode? ...
Gain of this preamp is determined as a ratio of output I/V resistor (R9=2k2) and degenerative resistance of the input stage: R6 (220R) plus parallel Source resistance of Q1 and Q2 plus series portion of P1.

Source resistance of JFET is reciprocal to its transconductance (Yfs, gm). So if 2sk170V and 2sj74V have gm of 40mS, they will have source resistance of 25R each. And if the P1 (20R) is set to its middle position we add another 10R to it and the result is that we have two times 35R in parallel connection which makes 17.5R in series with R6 which gives about 240R and your gain will be a bit less than 2k2/240R (because I neglected the emitter resistance of BJTs, which is very small anyway...)
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Old 19th November 2013, 12:57 PM   #115
juma is offline juma  Germany
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Buzz,
reading your question again, I realized that you assume that this is a two stage design (JFETS having gain separate from BJTS) - it isn't .
This is a complementary single stage push-pull design where cascodes are just that, not the separate gain stage. You have to look at is as a whole.
True, the current through cascodes is separately setable and that's what makes the topology of this transconductance (pre)amp specific, but it's still one-stage design.
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Old 19th November 2013, 02:18 PM   #116
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Thank Juma. As always, both post were very helpful in understanding the stage.
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Old 19th November 2013, 02:19 PM   #117
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Quote:
Originally Posted by buzzforb View Post
Juma,
On the schematic from post#66, how do you figure the gain of the folded cascode? Am I correct that the FE gain is about 14 of the total 19dB?
Measure with AC meter [say at 100Hz] the voltage at all nodes. Calculate Vg and dB normally, and then connect with Juma's theoretical analyses he showed above.
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Old 20th November 2013, 11:11 PM   #118
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Today I tested Indra1's proposal (post #76) and finally there's an amp without feedback loop that satisfies me completely (no objections to sound at all). It drives the 4 Ohm, 3-way sealed boxes magnificently.
I set the bias (R21, R22) at about 1.2A (LatFET's Vgs of about 2V).
Of course, the input stage PSU voltage should be higher, to use the full power potential of output FETs, but this was enough for the scope of the test. I used small LM317L as CCS (they do a surprisingly good job at 19mA - most thermally stable one active component CCS that I know of). DC offset at speaker is OK (+/-2.5mV). Zout is acceptable - measured at 100Hz and 10kHz it's about 0.25R

After all the tests I've done with it, I think I'm in love with this gain stage - a pity that I can't use it with VFETs due to their small gm which results in high Zout without feedback loop.

Once again, thank you Mr. Pass for bringing this great topology back to life.
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Old 21st November 2013, 12:10 PM   #119
Skorpio is offline Skorpio  Denmark
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Default Questions to circuit in post #66 (Juma)

I need no more than approx. 6dB gain and my current pre has 500R output resistance and this will also be ok.

So my idea is to change R9 from 2k2 til 500R, giving lower Rout and gain. Will this be ok? Is the a price for lowering R9, and what would be the limit?

An idear would be to use 3 1k resistors for R9, one direct in circuit and the two selectable via jumpers. This would enable:

R9=1k => 12dB gain/Ro=1k
R9=1k//1k => 6dB gain/Ro=500R
R9=1k//1k//1k => 3dB gain/Ro=333R

Another issue is supply voltage. Would e.g. +/-12V ruin performance?
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Old 21st November 2013, 12:35 PM   #120
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Quote:
Originally Posted by juma View Post
Today I tested Indra1's proposal (post #76) and finally there's an amp without feedback loop that satisfies me completely (no objections to sound at all). It drives the 4 Ohm, 3-way sealed boxes magnificently.
I set the bias (R21, R22) at about 1.2A (LatFET's Vgs of about 2V).
Of course, the input stage PSU voltage should be higher, to use the full power potential of output FETs, but this was enough for the scope of the test. I used small LM317L as CCS (they do a surprisingly good job at 19mA - most thermally stable one active component CCS that I know of). DC offset at speaker is OK (+/-2.5mV). Zout is acceptable - measured at 100Hz and 10kHz it's about 0.25R

After all the tests I've done with it, I think I'm in love with this gain stage - a pity that I can't use it with VFETs due to their small gm which results in high Zout without feedback loop.

Once again, thank you Mr. Pass for bringing this great topology back to life.
What sort of output power can we have with this circuit?
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