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Old 21st March 2011, 10:54 PM   #1
hahfran is offline hahfran  Germany
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Default Old and surprisingly good amp

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.
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Old 22nd March 2011, 09:16 AM   #2
Elvee is offline Elvee  Belgium
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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.
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Old 22nd March 2011, 10:36 AM   #3
AndrewT is offline AndrewT  Scotland
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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.
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Old 22nd March 2011, 11:03 AM   #4
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Quote:
Originally Posted by Elvee View Post
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:
Edwin 20 watt (Elektor 1970 - May)
Quad 303 triple cascade
That curious extra knob, or: "Trans-Amp" (post #41 and #52)
Collection of Class B topologies <100mA Idle and Sound closest by Class A
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Old 22nd March 2011, 11:22 AM   #5
AndrewT is offline AndrewT  Scotland
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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.
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Old 22nd March 2011, 11:24 AM   #6
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by AndrewT View Post
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.
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Old 22nd March 2011, 11:30 AM   #7
AndrewT is offline AndrewT  Scotland
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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.
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Old 22nd March 2011, 01:23 PM   #8
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by AndrewT View Post
there is R3 on the emitter matching R8 in the upper half.
When I say in the emitter, I mean in series with it.
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Old 22nd March 2011, 01:47 PM   #9
AndrewT is offline AndrewT  Scotland
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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.
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Old 22nd March 2011, 02:06 PM   #10
hahfran is offline hahfran  Germany
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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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