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1970s Design Indulgence

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2019 at 8:53am
I once had a Ford Cortina. It was one of the best driving cars I ever owned.

Years later I bought another Ford Cortina and at 60 MPH the steering wheel and whole vehicle vibrated so much it was unbearable.

I replaced just about everything on it including engine/gearbox mounts, prop-shaft, wheel bearings, ball joints, void bushes, springs, shock absorbers, etc. I had the body checked for twisting and had it professionally tracked three times. I even made templates to align caster and camber. Over the period of 1 year it cost me a fortune. I'd have been better getting shut and buying a different car.

No matter what I did, it made no difference.

There's an analogy here somewhere...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2019 at 11:01am
So today I'm trying to find out if the bias spreader is the culprit, and I'm doing that by replacing it with 3 series wired signal diodes in series with a 100 ohm trimmer between driver bases, and a 47uF bypass capacitor.

Two of the diodes receive thermal feedback from the output transistors to prevent thermal runaway - they're simply stuck on them using epoxy two pack adhesive - so as the base emitter junction of each transistor conducts harder and their temperature increases, the attached diode gets hotter and conducts harder, reducing base emitter voltage.

Unfortunately there will be thermal lag because of the plastic transistor case. The thermal track version of the MJL21193/4 transistors, prefixed NJL, has a diode inside the package for precise thermal tracking...

the data sheet exclaims

"Superior Sound Quality Through Improved Dynamic Temperature Response
Significantly Improved Bias Stability
Simplified Assembly
Reduced Labor Costs
Reduced Component Count
High Reliability"

...except you can't get them because they're not made anymore, because the amplifier industry didn't want...

Superior Sound Quality Through Improved Dynamic Temperature Response
Significantly Improved Bias Stability
Simplified Assembly
Reduced Labor Costs
Reduced Component Count
High Reliability

Wink
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Aug 2019 at 6:51pm
Circuit update 16/08/2019

Audio Power Amplifier 160819

(click view image to be able to read circuit clearly)

R5: the new collector load for T1 for it to assume 16V such that the transistor collector-emitter voltage is increased to 13.1V from 3.5V.

C5 and R14 form the level translator.

C1 increases to 10uF to preserve LF phase.

The bootstrap: R11, 12 C8, returns with R12 3k instead of 3k9. It wasn't guilty of distortion as thought.

D5 to D7 plus trimmer R13 replace the transistor bias spreader as it was thought, being an amplifier in its own right, could have been unstable.

D5 and D6 are thermally coupled (stuck) to T5 and T6 to provide thermal tracking.

The diode bias spreader impedance is shunted from mid-bass frequencies by C9.

R19 is increased from 180 ohms to 360 ohms to lower the DC load on the VAS.

R19 is shunted by C11 (1uF) to sweep charge from the power transistors at high frequencies (from about 16kHz when output is 1W). This value might be increased but ought not to have a longer time constant to that of C9 to avoid switch-on stress to T3 and T4.

So far the "icy-S/squeaky treble" problem has not manifested itself... (yet)


Edited by Graham Slee - 16 Aug 2019 at 6:53pm
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