Cool simple "clean" CFA

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#103
vzaichenko said:
Hi LC, R26 voltage drop goes like this:

10s - 0.79V
30s - 0.80V
1m - 0.82V
2m - 0.84V
3m - 0.85V
4m - 0.86V
6m - 0.87V
9m - 0.88V and then it stays at this level

I simulated higher VAS current (14mA with 3.3K input buffer CCS emitter resistors) - it doesn't show any advantages... almost 9mA seems to be enough for driving this OPS smoothly. I will probably try it live a bit later, just to make sure...

Attached is the schematic "as built" for now.
Updating the PCB 😉

Cheers,
Valery
Click to expand...
Thanks Valery

I see many caps have left the building, also some are higher in value, as predicted.😉

C13 should be 10 uF since it influence SQ quite a bit. :yes:

VAS gain also plays important role in shaping SQ. 😎

Trimming some resistors would probably eliminate DC servo too. 🙂

All in all nice CFA amp.

L.C. :wave:
 
#104
jwilhelm said:
Is it still lively at low volume levels? I find most amps go flat.
Click to expand...

Are you sure it's not subjective perception, caused by the fact that human's ear hears highs and lows not so good at low volumes? Also, some amplifiers, especially if it's a slightly under-biased class-AB ones, have got noticeable THD increase at low power, so that at 10W or higher it performs better than at 1W or lower. Just wondering 🙂

This one has got THD decrease at low volumes, and THD is almost constant throughout the audio bandwidth (although the profile contains more visible components than most of my other designs)...
 
#105
Lazy Cat said:
Thanks Valery

I see many caps have left the building, also some are higher in value, as predicted.😉

C13 should be 10 uF since it influence SQ quite a bit. :yes:

VAS gain also plays important role in shaping SQ. 😎

Trimming some resistors would probably eliminate DC servo too. 🙂

All in all nice CFA amp.

L.C. :wave:
Click to expand...

Thanks LC, I will look closer at C13 and will test some 14mA VAS on the prototype (simulation gives very rough estimation with regards to SQ)...
And yes - compensation became simpler but harder to keep the creature under control 🙂 I like higher C35, C36 as well, as you suggested :cheers:
 
#106
Updated PCB

Just updated the PCB according to prototype adjustments.
Silk screen is also significantly improved.

Gerbers are available on request as usual 😉

Cheers,
Valery
 

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#107
vzaichenko said:
Are you sure it's not subjective perception, caused by the fact that human's ear hears highs and lows not so good at low volumes? Also, some amplifiers, especially if it's a slightly under-biased class-AB ones, have got noticeable THD increase at low power, so that at 10W or higher it performs better than at 1W or lower. Just wondering 🙂

This one has got THD decrease at low volumes, and THD is almost constant throughout the audio bandwidth (although the profile contains more visible components than most of my other designs)...
Click to expand...

I've noticed a definite difference in higher frequency reproduction at lower volume levels in many amps. I have 105db efficient speakers so I usually play at a lower volume level than most. My hearing is damaged too so it may be more pronounced for me. I guess I need to try it out and see for myself.
 
#110
Increased VAS gain

Tried decreasing R26, R29 to 22R, leaving all the rest as is.
VAS current comes close to 40mA - cascodes are running pretty hot.
With R26, R29 = 47R, VAS current is 18.5mA. Optimal quiescent current for the output pairs is 35mA. Re-measured THD / IMD.
No noticeable change in listening tests.
Sounds equally good 🙂
 

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#112
vzaichenko said:
Benefits of this kind of amp are especially noticeable at "difficult" material, like Gordon Goodwin's Big Phat Band. Lots of instruments - trumpets, saxophones, drums, cimbals in combination with well-distinguished piano and tight base...
Click to expand...

That's the benefit of high slew rate amplifiers, either CFA, VFA or opamps. Very audible but many say that more than "theoretically adequate" SR is not necessary 🙄
 
#114
vzaichenko said:
Tried decreasing R26, R29 to 22R, leaving all the rest as is.
VAS current comes close to 40mA - cascodes are running pretty hot.
With R26, R29 = 47R, VAS current is 18.5mA. Optimal quiescent current for the output pairs is 35mA. Re-measured THD / IMD.
No noticeable change in listening tests.
Sounds equally good 🙂
Click to expand...

To preserve VAS bias, R10, R13 have to be decreased at the same time as R26, R29.

Are R14, R20 neccessary for proper compensation, without them amp is unstable? R3, R4 are somehow strange too.
 
#115
Yes, I know. However simulations don't show any significant improvement when I simultaneously decrease R10, R13 and R26, R29.

Those base stoppers are there on "just in case" basis 🙂 In fact, jumpering R3, R4 and decreasing R14, R20 - worth trying...
 
#116
Jay said:
One of a few things that interest me from this thread is the CFP output. I have no success using (bipolar) CFP in a CFA. Some people said: using slow output (topology) in a fast amp is not logical (defies the purpose)...
Click to expand...

Well, CFP is a "dangerous creature", especially in CFA design, but this MosFET arrangement runs smoothly and sounds great. I'm especially happy with this "dual-CFP" arrangement. Auditioning it for a few days already 😛
 
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