Class B w/o crossover distortion (1975) - Page 2 - diyAudio
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Old 30th January 2010, 09:40 PM   #11
hahfran is offline hahfran  Germany
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Ah yes indeed that explains how no crossover distortion appears even with class B setting of the power BJTs. It appears the same design idea is realized in the old QUAD 303? But why the bootstrap? And indeed the emitter of the current source is forced to -34.4 volts same as the base. If I hadn't built in 1976 I were sure it cannot work but since I have I see it works...
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Old 30th January 2010, 09:49 PM   #12
grufti is offline grufti  United States
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There is a Dutch company in the audio business called Hawk Audio that traces its roots back to Nico Visch. They mention him in their "About" page.

Hawk Audio Design Philosophy
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Old 30th January 2010, 09:57 PM   #13
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Quote:
Originally Posted by grufti View Post
There is a Dutch company in the audio business called Hawk Audio that traces its roots back to Nico Visch. They mention him in their "About" page.

Hawk Audio Design Philosophy
Quote:
Those hairs 'fire' electrons, let's say a kind of digital '1' signals, into the brain.
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Old 30th January 2010, 10:04 PM   #14
AKSA is offline AKSA  Australia
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Yes, I'd seen that some time ago, Hawk Audio is most progressive.......

The purpose of the bootstrap around the 250uF cap is to maintain a near constant current through T1, T4 and T7. Upon the constancy of this current depends the consistent 1.3 volts drop between input and output, and hence the linearity. The bootstrap cap strongly defines the voltage at the collectors of T4 and T7, which would prevent collector of T7 going too low as it draws current from T5 on negative swings, otherwise cutting out the transistor.

The disadvantage of this circuit is the additional negative supply for the emitter of T7, which sets the quiescent. Manufacturers hate additional supplies; they really louse up the costs.

Hugh
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Old 30th January 2010, 10:24 PM   #15
Bigun is offline Bigun  Canada
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I love this circuit - it offers some intrigue that could keep me busy in Spice for weeks, if not longer. Hey, maybe I should build it, I have some of those old output devices.

I guess modern Class AB is now so darn good there's no call for this type of thing anymore. But I like the golden oldies... there really is a lot of stuff out there in the archives.
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Old 30th January 2010, 11:53 PM   #16
AKSA is offline AKSA  Australia
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Gareth,

You should acquire a copy of 'The V12 Engine', by Karl Ludwigsen. There are a LOT of golden oldies there, and they are much more interesting than the mass produced fours and V6s we see today......

Hugh
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Old 31st January 2010, 12:40 AM   #17
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I think this idea is actually very common in commercial amplifiers, I have a 1980's era amplifier that uses an amplifier chip biased into class a using a ccs as the error amplifier with very good results.
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Old 31st January 2010, 03:36 AM   #18
Bigun is offline Bigun  Canada
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Gareth,

You should acquire a copy of 'The V12 Engine', by Karl Ludwigsen. There are a LOT of golden oldies there, and they are much more interesting than the mass produced fours and V6s we see today......

Hugh
It sounds very interesting. My only experience of a V12 was riding in a friends Jaguar XJS many many years ago. What an absolutely stunning engine. It was so darn smooth I really would have believed it was something other than an internal combustion engine. And boy did it move.
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Old 31st January 2010, 05:58 AM   #19
AKSA is offline AKSA  Australia
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Gareth,

The Visch circuit can be improved with a Baxandall diode, cap and resistor all in parallel between emitter of the pnp driver and output rail; distortion is reduced from 0.0077% to about 0.005% according to the sim at 1KHz +20dBU output (14.5Vp). But you'd have to build it to find out how it sounds, of course.

The Jag V12 was a good motor, but there are better. Cooling was a major problem, particularly in Oz.

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Old 31st January 2010, 09:16 AM   #20
hahfran is offline hahfran  Germany
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Originally Posted by AKSA View Post
Yes, I'd seen that some time ago, Hawk Audio is most progressive.......

The purpose of the bootstrap around the 250uF cap is to maintain a near constant current through T1, T4 and T7. Upon the constancy of this current depends the consistent 1.3 volts drop between input and output, and hence the linearity. The bootstrap cap strongly defines the voltage at the collectors of T4 and T7, which would prevent collector of T7 going too low as it draws current from T5 on negative swings, otherwise cutting out the transistor.

The disadvantage of this circuit is the additional negative supply for the emitter of T7, which sets the quiescent. Manufacturers hate additional supplies; they really louse up the costs.

Hugh
I had solved that problem with a voltage doubling extra rectifier 2 diodes 2 caps. Voltage regulation improved the sound quality.
The amp was at first used as lead vocals amp in our high school rock band.
When I was university student the amp was always lended to someone...
after that it had driven a low frequency horn speaker...and it still works today no failure at all!
Clever rugged design indeed. The secret of its musical sound could be the spectrum of the harmonics.
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