Why 12 power transistors in a class AB (?)120 W (?) amplifier?

[DanielT]

Why 12 power transistors in a class AB (?)120 W (?) amplifier in .. 4-8 Ohm (?)
What is the point of 6 power transistors per channel in such an amplifier? Information and pictures below .

I would be very grateful for clarification. The stuff works, but I'm just curious.

Some facts. Sela, a Power amplifier, built sometime in the 80's. Unfortunately, there are no facts about it. It has probably mostly been used for PA. Maybe class AB, but I do not know.
Well, if I compare with a hifi intended vintage amplifier, from about the same time period, a Luxor 7082 integrated at 70 W in 4 Ohm, it has only two power transistors per channel.

Does it have to do with the fact that a power amplifier is built to withstand tough conditions and thus so many power transistors? To minimize the risk of burning them? Is so many transistors (of that type) needed in it to be able to handle what the transformer can pump out?

Will there be, in general, better hifi sound (less risk of clipping, distortion) if you use several power transistors? To distribute the effect between them, so they do not have to "work" so much? Is it not becoming more difficult to get them to interact, to integrate them into a well-functioning whole?

As you can see, fairly basic questions about construction, design regarding amplifiers. Therefore, I think this thread fits on this part of the forum.

Facts:
Power amplifier brand Sela:
Power, I guess 2x120 W (in ...? Ohm).
Transistors: 2N3773 (6 per channel)
Electrolyte capacitors in the power supply: 4700 uF, 63 V (times2, per channel, so 9400 uF, 126 V per channel),
Fuse, see picture
Class amplifier? No idea AB, maybe ..

Transistors: 2N3773:

________

Integrated amplifier Luxor 7082 A.
It was easier ..

Luxor 7082 Stereo Integrated Amplifier Manual | HiFi Engine

www.hifiengine.com

Power: 2x70 W in 4 Ohm.
Transistors: Two D718, two B688 (the green ones)
Electrolyte capacitors in the power supply: 50V, 10,000 uF

A little more pictures of them. I mostly waffle in that thread, but you see a little more pictures. I take the signal from the pre amp section of the Luxor and feed Selan with that signal.

Project sub output

An old amplifier Luxor 7082 A. I connected a high to low level converter on the speaker outputs, to be able to connect an active subwoofer. Connected in parallel, together with the cables to the speakers. The converter: My friend said: why not take the signal directly from the pre amp section...

audiosciencereview.com

Edit:
I guess in and of itself that if I took out the multimeter, measure a little plus together with Ohm's law , it is possible to get a decent idea of the effect on the power amplifier, Sela.I have not, yet, really figured out how to do it per se.

 

[tomtoo]

Wow! That first amp is a beauty of a build!!!
Unpayable!
Not compareble to uglynes of the second.

That first looks like something build for a space station. Not realy but from the quality of the build while the second one compared looks like a garage build.

 

[tomtoo]

Maybe ask @restorer-john ?

 

[tomtoo]

Just look at the quality of that trimm potis, the rectifiers, the trafo, the PCBs, All says iam a fu**ing pro.
Realy nice.
Thats industrial grade electronics. Iam amazed. Keep that treasure. And if its just for looking opened to it. It shouts quality.
Iam realy amazed.
Do you have more info about the amp?

 

[DanielT]

Thanks tomtoo for the response!

I like that power amp, Sela. I will keep it. I will do some more detective work around it.

Im thinking about eventually, possibly splitting the signal and letting one (or two, I have one but they sell them cheap now, so why one when you can have two)the t.amp E-800 takes care of my subwoofers. Up to 80-100 Hz then Selan will take care of the rest. In addition to the usual, ie miniDSP, PEQ (for subwoofers in any case)

Regarding effect, Selan. I will take it to a friend, and he will measure the effect on it. He has stuff for it.

Luxor 7082 A is nothing special. For its time a decent standard amplifier, for its effect. Developed together with Luxman. Or the Swedish company Luxor was the general agent for Luxman. It is possible that the development engineers, the designers at each company talked to each other. Exchanged ideas and thoughts. In any case, the Luxor 7082 A apparently reminds a lot of the Luxman L-30.

Luxman L-30 Solid State Integrated Amplifier Manual | HiFi Engine

www.hifiengine.com

The lid, the grid was red on the Sela. Now sprayed it with black heat-resistant spray.

 

[Vini darko]

They are probably running 4 parallel transistor sets in push pull. So three transitors handle the high side of the wave and three handle the the low side per channel.
I'd say it certainly looks like a AB design we can see two pots on the amp board for bias and dc offset i suspect.
Yes running parallel transistors can lessen the load and increase power output.

 

[tomtoo]

Could you make a picture of the back? Type shild? Connections?

Looks lile a build with thermal stability in mind. Not a thypical hifi amp.

 

[DanielT]

Do you have more info about the amp?

In a way, I have it. Bought it from a friend like last year. He got it when he worked as a PA roadie in his youth. It had been stored the amp for many years (in a warm and dry closet).

Well. That it's called Sela is because it was designed by this company:

"Sela is a company with over 60 years of experience in sound technology, stage equipment and image installations for public premises."

Sela AB - Ljudteknik - Enskede

Ljudteknik, scenutrustning och bildinstallationer är vår specialitet!

www.sela.se

I called Sela and asked them if they had schedules for it but they said that in the 80's there was a fuc*'' g quarrel between their amp designer and the owners. The designer became so angry that he resigned immediately and with that he took with him all the drawings, schedules. Since then, Sela has never manufactured any, in - house, amplifiers.

They never got hold of him after he left Sela.

 

[DanielT]

Could you make a picture of the back? Type shild? Connections?

Looks lile a build with thermal stability in mind. Not a thypical hifi amp.

I can unscrew it later this week.

I took some pictures. Note that my friend changed the inputs and outputs, hence the stickers (he knows electronics so it's a good solder job).

Those heat sinks, sharp as hell. No ridiculous feathers (is that what they are called in English, those paper-thin heat sinks you can see on non-lavish amplifiers) heat sinks, if I say so..He he.

 

[tomtoo]

In a way, I have it. Bought it from a friend like last year. He got it when he worked as a PA roadie in his youth. It had been stored the amp for many years (in a warm and dry closet).

Well. That it's called Sela is because it was designed by this company:

"Sela is a company with over 60 years of experience in sound technology, stage equipment and image installations for public premises."

Sela AB - Ljudteknik - Enskede

Ljudteknik, scenutrustning och bildinstallationer är vår specialitet!

www.sela.se

I called Sela and asked them if they had schedules for it but they said that in the 80's there was a fuc*'' g quarrel between their amp designer and the owners. The designer became so angry that he resigned immediately and with that he took with him all the drawings, schedules. Since then, Sela has never manufactured any, in - house, amplifiers.

They never got hold of him after he left Sela.

Interesting story. If the engineering behind is the same good as the build quality they made a big mistake.

 

[solderdude]

What is the point of 6 power transistors per channel in such an amplifier? I

Each transistor has its own emitter resistor.
This is to prevent thermal runway and wattage can be lower as it is divided over several resistors.
1 resistor per emitter ensures each transistor gets to handle 1/3 of the current/power.

Its the most common and best way to do this.
Schematic from a different amp but also has multiple output devices and only shows an output stage, not the obligatory gain stage.

The amp below has even more OP devices, MOSFETs in this case, the principle and reasons are the same.

The bottom one also has one resistor per emitter.

 

[tomtoo]

Hand number punching for the serial number. I told space station.

 

[DanielT]

Each transistor has its own emitter resistor.
This is to prevent thermal runway and wattage can be lower as it is divided over several resistors.
1 resistor per emitter ensures each transistor gets to handle 1/3 of the current/power.

Its the most common and best way to do this.
Schematic from a different amp but also has multiple output devices and only shows an output stage, not the obligatory gain stage.

The amp below has even more OP devices, MOSFETs in this case, the principle and reasons are the same.

The bottom one also has one resistor per emitter.

Aha, thanks for the information Solderdude.That was interesting

Is it a common way to design amplifiers? Or was it common in the past? In PA and / or Hifi for home use?

Edit:
See that you wrote:
"Its the most common and best way to do this."

.. so that was the answer. He He.

 

[tomtoo]

Ah thats the company that build that soundboks. Interesting.

 

[tomtoo]

No pesty fans. You need more "iron" to hold it thermal stabel. Nice build.

 

[DanielT]

Ah thats the company that build that soundboks. Interesting.

It's no bullshit sensible PA Sela deals with and not fuzzy esoteric "high end" Hifi so... He he.

 

[DanielT]

You need more "iron" to hold it thermal stabel. Nice build.

I will fix that. Thanks for the tip.

 

[sq225917]

It's built like that so it can run 24/7 simply cruising along miles away from its thermal limit. Paralleling them Increases its load driving ability, for lower ohm speakers.

It's a class a/b quasi complimentary output stage, taken from the rca/Hitachi application note design for those transistors and shares a lot with naim offerings of similar vintage.

When it was made there were no decent matching pnp devices of suitable ruggedness to build a complimentary output stage.

A nice little bit of history.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ampslab.com/RECOMMMENDED%2520READ/QUASI%2520COMPLIMENTARY%2520OUTPUT/QUASI%2520OUTPUT%2520STAGE.pdf&ved=2ahUKEwj1srvumOPzAhWPxYUKHTq6DEMQFnoECAUQAQ&usg=AOvVaw23QxI7Qb46h2lkzfPS5gAt

 

[Hiten]

Why 12 power transistors in a class AB (?)120 W (?) amplifier in .. 4-8 Ohm (?)

Newbie answer.
Probably to share load and run long hours./?

 

[DanielT]

It's built like that so it can run 24/7 simply cruising along miles away from its thermal limit. Paralleling them Increases its load driving ability, for lower ohm speakers.

It's a class a/b quasi complimentary output stage, taken from the rca/Hitachi application note design for those transistors and shares a lot with naim offerings of similar vintage.

When it was made there were no decent matching pnp devices of suitable ruggedness to build a complimentary output stage.

A nice little bit of history.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ampslab.com/RECOMMMENDED%2520READ/QUASI%2520COMPLIMENTARY%2520OUTPUT/QUASI%2520OUTPUT%2520STAGE.pdf&ved=2ahUKEwj1srvumOPzAhWPxYUKHTq6DEMQFnoECAUQAQ&usg=AOvVaw23QxI7Qb46h2lkzfPS5gAt

I seems sensible considering what its purpose was / is.

Thanks for the link sq225917. I think the radio and hifi history is very interesting. I tell myself that I should get to my main modern hifi system, but I still always ends fiddling with my secondary, more, vintage like system.

I really like modern sensible class d amplifiers..Good clean pure power, but to be honest not that hm.. exciting.
I think, anyway. I say that now when I'm not standing and swearing at something old that has broken. You can probably have a little both. Vintage and new.

Edit:
Speaking of Hifi history:

14 of the longest-running hi-fi brands still going strong today

Legendary hi-fi veterans that have stood the test of time

www.whathifi.com