questions about this amplifier

[demons_wing]

I just have a couple of questions about this amplifier...its a pretty neat design and from what ive read, its quite good to listen to...

http://www.astro.uu.se/~marcus/private/schm250.jpg

1)
i was wondering about the input stage LTP current...its labelled 1.8ma, now if im not mistaken (and i probably am!!!)
if the voltage drop of a green LED is 3.5V, then the current would have to be (3.5-0.7/715) = 4mA...therefore it would be 2mA per leg

2)
is the VAS current (3.5-0.7/200) = 14mA...how do you work out the required current for the VAS??..

3)
shouldnt R2 and R14 be equal?..and how do you calculate C4?

any suggestions would help heaps

[janneman]

The LED only runs 5.5 mA, so it's voltage may well be less than 3.5V. Anyway, it's only 10% difference, that's larger than the production spread probably.

Yes, R14 should be equal to R2. I recommend increasing R14 and R15 by factor 10, decrease C4 by factor of 10 (but see below). Makes it even cheaper, like having your cake and eat it too.

C4 determines lf roll off with R15. The time constant of R15 and C4 should correspont to the lowest freq you like to play: R15 = 1/ 2.pi.f.C, so C should be 1/2.pi.f.R15. For 10Hz, R15 being 750 Ohms (see above), f = 10 Hz you get someting like 10uF.

Jan Didden

[demons_wing]

thank you very much for the reply...i really appreciate it considering that i am very new to audio..

how do you ascertain the required current for the VAS circuit??...does it have to do with the charge current required for the Miller capacitor C5?..or does it have to do with the output stage?...

the power output is 250W @ 55V rails

[Nelson Pass]

It's a classic circuit, but I would consider putting some resistance,
say 100 ohms or more, in series with the base of T4 to clean up
the clip recovery on the positive wave. When T4 saturates, it
tends to dump the current fed to the reference diode which is
also feeding the diff pair.

[demons_wing]

thank you nelson

i was wondering if you could answer this question for me..
is the VAS current (3.5-0.7/200) = 14mA...how do you work out the required current for the VAS??..does it have to do with the charge current required for the Miller capacitor C5?..or does it have to do with the output stage?...

the power output is 250W @ 55V rails

[boholm]

Originally posted by demons_wing
is the VAS current (3.5-0.7/200) = 14mA...how do you work out the required current for the VAS??..does it have to do with the charge current required for the Miller capacitor C5?..or does it have to do with the output stage?...

Trying to answer instead:

You most surely have to make certain that there will be enough current for the output stage. Otherwise the output stage will take all the current from either T4 or T5 depending on the signal, if you do not take that into account. How that would sound I don't know but it won't be pretty.

The other thing - I would say - would be the operating point of T5. For that you need to look at the data sheets to see if there is a specific "place" - quiecent current - that will give the best performance regarding linearity/distortion and maybe noise.

As for Miller effect AFAICS (S=see): Nothing to do with that whatsoever.

[marcus66]

Hello Demons_Wing,

The voltage drop across the LED is less than 3.5V (As far as I remember, abt 1.9V). The value of C4 is also related to stability when clipping. The changes proposed by Janneman are probably OK.
I'm working abroad for a year, and the amp is put into storage along with everything else, so I cannot make tests or changes at the moment.
My idea was to make an amp with output power above 100W, with a simple
schematic and low component count. If someone comes up with tweaks or changes that improves the performance I'm very interested in hearing
about it. Increasing VAS current is probably a good idea, especially
for low speaker impedances. For 2Ohm speakers the driver stage
and overcurrent protection must be re-designed.


Marcus