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:: class a symetric power amplifier ::
:: class a symetric power amplifier ::
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Old 14th October 2002, 09:59 AM   #1
dhengkoel is offline dhengkoel  Indonesia
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Default :: class a symetric power amplifier ::

Dear all,

I've redesign an old schematic and I came up with this schematic ... Any comment or designs you can recommend is welcome.

Thanks:
ragil.hastomo
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Old 14th October 2002, 10:00 AM   #2
dhengkoel is offline dhengkoel  Indonesia
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Default :: the end stage ::

Here the end stage .....
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Old 14th October 2002, 11:23 AM   #3
AKSA is offline AKSA  Australia
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Hi Sdr Hastomo,

Amp anda cukup bagus - bunyinya jelas, murni......

I think it would be terrific, but can make one suggestion.

Each of the two drivers should have a 100R base stopper; and output devices should have a 10R base stopper. The 120R/100n should be connected between the two output device bases as shown, and you could delete the 270R resistors from driver base to emitter.

With these changes, it's a good design.........

Kapan anda membikinnya??

Cheers,

Hugh

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Old 14th October 2002, 11:25 AM   #4
AKSA is offline AKSA  Australia
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Oh,

I should add that R4 and R25 (both 1K5) are unnecessary. If the diff pairs use matched transistors, and if the input bias resistor matches the resistance from output to fb node, then balance in the diff pairs is automatic.

Cheers,

Hugh
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Old 14th October 2002, 12:14 PM   #5
Christer is offline Christer  Sweden
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Hugh,

I know it is common to omit these resistors in diff pairs, but is
it really true that it does not matter? From a theoretical point
of view I think it will make a difference. At the Q point, we will
have the same Icq through both transistors in the diff pair if
these are matched. However, if we omit the collecor resistor on
one of them we get different Vceq. This means the power
dissipation differs and, hence, the temperature, which will
make the transistors slightly unbalanced in practice. It will
probably not make much of a difference in this particular case,
but resistors are cheap. Probably 1k5 is not the optimal value
to achieve balance, though, since the other transistor has a
load in parallel with its collector resistor.
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Old 14th October 2002, 06:00 PM   #6
Nelson Pass is offline Nelson Pass  United States
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:: class a symetric power amplifier ::
Or you can buy an old KSA50 on ebay....



Here's a tip: put the trim pot base-emitter so that
when it gets old and dirty the bias goes to 0 instead
of infinity.

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Old 15th October 2002, 11:34 AM   #7
capslock is offline capslock  Europe
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dhengkoel,

are you sure the connections on the feedback network are drawn right? There is two lines leaving the first picture but only one is continued in the second picture. Also, I am not sure that all the connection points are correct.

What is the function of R14 by the way?

Regards,

Eric
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Old 15th October 2002, 11:14 PM   #8
Nelson Pass is offline Nelson Pass  United States
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:: class a symetric power amplifier ::
R14 is the actual feedback resistor. The other two in
series with capacitors are the compensation, one looking
at the output and the other nested to the output of the
voltage gain devices. The diagram should show R12, 14, and 18
attached together on the left, which I can't see.
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Old 15th October 2002, 11:36 PM   #9
Morello is offline Morello  Sweden
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Hello Sirs,

I suggest that VR1 i sconnected between base and emiter.

If you connect it as shown in the schematic, the bias-voltage will increase to very high levels if the whiper of VR1 fails.

Otherwise, it look like a proper symmetric design

SE-amps are best avoided, or what do you think Mr. Nelsson?

best regards
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Old 16th October 2002, 01:52 AM   #10
dhengkoel is offline dhengkoel  Indonesia
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Smile :: full schematic in one sheet ::

Quote:
Hugh: Each of the two drivers should have a 100R base stopper; and output devices should have a 10R base stopper.
Saya akan menyelesaikannya akhir bulan ini. [I will finished it at the end of this month.]
Quote:
Nelson Pass: Or you can buy an old KSA50 on ebay.... Here's a tip: put the trim pot base-emitter so that when it gets old and dirty the bias goes to 0 instead of infinity.
The diagram should show R12, 14, and 18 attached together on the left, which I can't see.
I can't buy the KSA-50, that's too much expensive for me [sorry for that].
Is this schematic too close with your design KSA-50? I don't see the KSA-50 schematic yet.
I found the PCB in an old shop. Actually it's for 100w class AB amplifier. So I redesign it and decrease the voltage rail to 25v, but increase the Iq to 1.1A .....
Is that ok if I change the Q4 to PNP? [Because the PCB is ready ...]

Anyway thanks all for the comment; Here the full schematic in one sheet ....

Regards:
ragil.hastomo
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