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hahfran 21st March 2011 09:54 PM

Old and surprisingly good amp
 
1 Attachment(s)
This 1975 design of a quasi complimentary power amp suffered somewhat
from the poor properties of the 2N3055: very poor ib/ic characteristics,
low ft ( 15 kHz) , high leakage currents.
A big surprise: replacing the 2N3055 with 2SC2922 makes an amp featuring
second harmonic at -60 dB and higher harmonics not measurable at 80% max power ( here 25 Vp in 5 ohms) . Fantastic triode- like sound. Very easy to built.
Temperature bias compensation is dynamic Q6 needs not ( and should not!) be in thermal contact with heatsink.

Elvee 22nd March 2011 08:16 AM

Hi

Why do you call the compensation dynamic? To me, it looks like a pretty standard compensation, albeit with a slope correction.
BTW, R4 seems misplaced.

AndrewT 22nd March 2011 09:36 AM

What's wrong about R4's location?

I don't like the relative sizes of C6 & C4.
I would experiment with C4 moving towards 2200uF and reducing C6 towards 470nF.

I would also look at modifying the quasi so that the drivers each see the same loadings. Baxandall diode & JLH capacitor.

None of these mods may give any improvement, the designer may well have got the sound just right with the choices he made.

tiefbassuebertr 22nd March 2011 10:03 AM

Quote:

Originally Posted by Elvee (Post 2512277)
Hi

Why do you call the compensation dynamic? To me, it looks like a pretty standard compensation, albeit with a slope correction.
BTW, R4 seems misplaced.

R13 seems to be a special compensation function - perhaps therefore the actually not right estimate for a dynamic compensation ?
I am looking for a basic article (Vbe multiplier) arround this actually not necessary R13 - who know that paper?

in this case also this threads are of interest:
http://www.diyaudio.com/forums/solid...-1970-may.html
http://www.diyaudio.com/forums/solid...e-cascade.html
http://www.diyaudio.com/forums/solid...ans-amp-6.html (post #41 and #52)
http://www.diyaudio.com/forums/solid...est-class.html

AndrewT 22nd March 2011 10:22 AM

R13 is particularly useful with the bootstrap VAS current source.
It is the D.Self resistor used by a few.
Except here we see it pre-dating Self by a couple of decades.

Elvee 22nd March 2011 10:24 AM

Quote:

Originally Posted by AndrewT (Post 2512321)
What's wrong about R4's location?

It should be in the emitter of Q3. In the collector, it is useless, it just wastes a bit of headroom.

Quote:

R13 seems to be a special compensation function
R13 compensates for the residual I/V slope of the Vbe multiplier. A pretty standard trick.

AndrewT 22nd March 2011 10:30 AM

there is R3 on the emitter matching R8 in the upper half.

The arrangement of R6 & R7 look odd. But that may be deliberate to account for the unequal loading that the quasi imposes on the drivers.

Elvee 22nd March 2011 12:23 PM

Quote:

Originally Posted by AndrewT (Post 2512356)
there is R3 on the emitter matching R8 in the upper half.

When I say in the emitter, I mean in series with it.

AndrewT 22nd March 2011 12:47 PM

You are not wrong.
R3 is in series with the emitter, exactly like R8.

R4 does a different job. I don't see it as being wrong located. There is no missing emitter resistor for it to be moved to.
The voltage drop that you mentioned across R4 is only 80mV to 90mV. That just reduces Vce of the lower driver by 80mV to 90mV. I don't see that as an issue.

A bigger issue may be the completely different loads seen by the emitters of both drivers. That's why Shaw, Baxandall & JLH changed the load seen by the lower driver's emitter.

hahfran 22nd March 2011 01:06 PM

The design IS Quad303 triple cascade. The dynamic compensation of junction temperature of the power BJTs is via current feedback with R5, R10.
I have simulated that with the temp statement of LTSPice and it works well
up to 100 junction temperature of Q1 Q2 Q3 Q8. The qc of Q1 Q2 goes from 50 mA at 30 to 90 mA at 100 that is excellent. There is no time delay as it is in the case of the Vbe multiplier thermally coupled with heatsink .
The performance of this very simple design is amazing when Q1 Q2 are (as tested) 2N2922 which feature good ib/ic linearity up to ic 6 amps and ft 40 mHz.
The compensation C3 can be reduced to 15 pF and C2 to 10pF thus that distortion at 20 kHz is only second harmonic.
Btw making the triple cascade complimentary does not improve the performance, h2 becomes smaller but odd harmonics appear.


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