Class B w/o crossover distortion (1975)

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
The April 1975 issue of Wireless World had a few pages on a then novel topology Class B output without crossover distortion. I had such an about 50 watts amp done it works until today ( I use it for burn-in of loudspeakers by a 24 h driving at 16 hz ) . The circuit does not require adjustment of idling current or dc offset. It has excellent stability of parameters versus operating temperature. Now I ponder to have this using contemporary transistors as a power amp for active speakers. But the copy of the Wireless World is lost.
Electronics World doesn't keep it in the archives , either. Any idea how to get an April 1975 copy of Wireless World?
 
Do a search at google.com for:

"Class B" without "crossover distortion" April 1975 Wireless World

I haven't gone through all of the search results, yet. But it looks like there are some results that might have what you want.

For example, it looks like the PDF linked below at least mentions the article. The short description includes "A novel class B output, Wireless World, April 1975", but it's still downloading, here, and I haven't looked at it yet.

http://www.hi-fi.ro/fhifi/download.php?id=10251&sid=98e9c714e51a23f25bc2138bc9aa6729

Tom
 
Last edited:
And here's a Google Books result that mentions the same article:

High perfomance audio power ... - Google Books

If that's the right article ("A Novel Class B Output"), the google books result shows the author as "Visch, Nico.M".

Maybe a better search string for google.com would be:

"Class B" "crossover distortion" April 1975 "Wireless World"

Tom
 
Last edited:
Do a search at google.com for:

"Class B" without "crossover distortion" April 1975 Wireless World

I haven't gone through all of the search results, yet. But it looks like there are some results that might have what you want.

For example, it looks like the PDF linked below at least mentions the article. The short description includes "A novel class B output, Wireless World, April 1975", but it's still downloading, here, and I haven't looked at it yet.

http://www.hi-fi.ro/fhifi/download.php?id=10251&sid=98e9c714e51a23f25bc2138bc9aa6729

Tom

Ok. That provides a hint.
Thanks.
I have recovered the schematics from the only document that made it through all those years, the PCB layout, however that has no component values except that the power BJTs are 2N3055. The circuit that prevents reverse-biasing is the current source T9 and the circuit T7,T8. Thus the NFB
hasn't to handle large voltage steps. I tested the crossover distortion with a dual channel scope in Channel I - Channel II mode ( output - inverted input, different sensitivity) which isn't accurate. However below 30 watts in 4 ohms, no crossover distortion is seen. That is not bad.
It could be interesting to combine this class B in a current dumping topology.

Here is the schematic
 

Attachments

  • Class B.jpg
    Class B.jpg
    41.3 KB · Views: 3,974
Ah..getting closer to the source. Or not...frankly I don't get it how it works, how it is class B, and what is the large 250 capacitor for? It is apparent why and how this topology provides an almost junction-temp independent setting.
This explains why it still works after more than 30 years. But not how it prevents crossover distortion...
 
Hahfran,

If you do the simulation you see that neither of the input transistors (with emitters joined) ever turns off. Thus, remaining biased on, there is a relatively simple step function (1.3V) between input and output at all points on the waveform.

Their collectors, one on the top triplet, the other on the bottom triplet, will then behave in a way which always maintains dc 1.3V between input and output, and this implies that their collector voltage will always control the triplets to ensure that the transfer function of the output triplets is neatly pasted over at the crossover point.

I'm not sure Anatoliy is correct; the 250uF is a bootstrap, not a miller cap (no gain).

Clever circuit, huh?

Hugh
 
Last edited:
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...
 
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 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.
 
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.
 
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.

Hugh
 
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.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.