Will this work as audio amplifier?

[MikeB]

I would say pnp+cap, to avoid the biasing of the input.

One suggestion would be to use a CFP in the input ?

It seems that you use heavy currents through all parts (~40ma).
This excludes the use of small signaltransistors, if i'm right.
Why not reduce currents below 2ma and use a predriver ? (Tripledarlington output)
This enables the use of signaltransistors, which have much better
linearity, speed and gain.
Which transistortypes are you using ?

Mike

[lumanauw]

Hi, Mike,
The transistor I use for front end is D669 and B649 attached in small heatsink. The final uses C5200.
I use manual adjusting, using 100K VR attached to +/-15V, because my servo type didn't work as expected.

About triple darlington, it still confuses me. Some designer said to advoid triple bipolar darlington, and use double bipolar darlington, or use double darlington with mosfet as driver. Triple darlington gives too much shift, and prone to oscilation, they say (I dont know in SIMS)

CFP in input? Wouldn't that be the original design at post #1 ? (I put 25K as CFP resistor)?

[MikeB]

Hmm, the leachamp uses tripledarlingtonoutput...
In my sims the tripledarlington helped increasing bandwidth
and reduce oscillation. Only with tripledarlington i was able to
use 10pF-cdoms. And from the audiophile suspect, i think you can hear
that the amp has "more" power. It drives my 4ohm speakers with ease,
like it is "bored" with that high impedance.

Your ckt in the 1st post is quite similar to a cfp, but different.
You should use a 2k at the collector of the input, connect emitter/base
of the complementary to this 2k, and connect collector of the CFP-bjt
to the emitter of the input-bjt.
You should look to a CFP as a single transistor.

Mike

[lumanauw]

Hi, Mike,
What happens if we put double darlington, but use mosfet (like IRF540-9540) as the driver for the final transistor (bipolar?)
Mr.John Curl uses this configuration, he said this configuration can "block" what ever happens in the final stage (like heavy/strange speaker load) from entering the front stages of amplifier.
Triple bipolar transistor can make VAS need only low current, but still can be affected by the strange speaker load.

Yes, you are right. At first I design it like Graham Maynards design, the upper 25K is attached to the emitor of second transistor.

But then I see the design of Cyrus amp. It has the 25k attached to rail instead to the emitor of the second transistor.

[MikeB]

Hi !

Hmm, i can do some simus with this config. But you will loose
some power, due to the 3.6v Vgs-treshold.
I do not fully agree that a mosfet blocks what ever happens in the
final stage. This is true for low frequencies, but with high frequencies
mosfet's no longer block. With higher frequencies the inputimpedance
of mosfet can get very low.
I need to check, because in the driver stage you don't have large
currents. But with these low currents the mosfets operate in a very
unlinear range, but on the other hand, if driverstage operates ClassA
with high enough currents, this looks different.
I need to simulate...

In theory, the predriver stage reduces loads from output by ~300.
(300 = hfe of bc546/556)

For CFP, you might have a look at "my" design. It's easy to implement
and gives very good results. (You can simply replace the jFet by bjt)

I was able to increase bandwidth AND ol-gain in my ckt, this reduces
thd by factor 5 to 2 (1khz-20khz), and imd by factor 3. I will apply
these changes the next days for listeningtest. 3rd harmonics are
completely gone for lower freqs. This means that i was somehow
completely wrong about currentmirror... (damn it !)
THD at 1khz nearly full power into 4ohm is now 0.00012%.
(2nd harmonic only) Whatever this means...

Mike

[thanh]

Oh! lumanuaw! Does your amp work? I'm surprise at it

[lumanauw]

Thanh!,
Yes, it works with good sound

Mike,
What happens if you put all CFP transistor in your design (diff already CFP, then put CFP for VAS and output stage, instead of darlingtons)? Will the harmonics (especially the 3rd cancelation) be better

[MikeB]

Hi lumanauw !

I tried CFP in the VAS, but had no big change. (In simus)
I played with CFP-outputstage, but it seems to increase 3rd harmonic,
and also seems to be bad choice for ClassAB, as turnon/off times
increase and heavy crossoverdistortions occured. (In simus)

I wanted to try a ClassA with CFP-output, CFP-VAS needs listeningtest.

Mike

[lumanauw]

I tried CFP in the VAS, but had no big change

OK, so there is no need for CFP in VAS

I played with CFP-outputstage, but it seems to increase 3rd harmonic,

Really? So CFP works in opposite ways if put in final stage than in differential?

and also seems to be bad choice for ClassAB, as turnon/off times increase and heavy crossoverdistortions occured. (In simus)

Yes, I read this in DougSelf book. He didnt mentioned about the fit for class A, class B, class AB. But is it really bad for Class B or AB? Randy Slone usually using CFP output stage for class AB amp. He's using K1058 as the final pairs.

[thanh]

lumanauw! Have you ever built a symmetry amp?