Single vs Dual Supply Voltage Amplifiers

[TomCarlos]

Having owned an Acoustic Control Corp 150 amp and having worked on a few, and seeing posts here on MEF, I was curious to read up on differences between Single vs Dual Supply amps. The ACC approach in the 70's seem to go with a single +75 Vdc, while other amps used the dual +35, -35 approach. I was wondering if there are differences in Ripple, circuitry limitations, design considerations, etc.

Anyone have any links to suggested reading material?

Thanks!!

[Dave H]

One advantage of using a dual supply is that it can be directly coupled to the speaker eliminating the large electrolytic capacitor in the signal path required by a single supply. On the other hand if a direct coupled output stage fails the speaker is toast with no capacitor there to protect it from the DC offset.

[TomCarlos]

Thanks Dave...

Yes, that is one of the obvious differences. That and the huge Filter cap that a single supply amp requires. I think my ACC 150 uses a 3900uf @ 80vdc - a big cap that runs around $25 - $30 if I am not mistaken.

[nevetslab]

Thanks Dave...

Yes, that is one of the obvious differences. That and the huge Filter cap that a single supply amp requires. I think my ACC 150 uses a 3900uf @ 80vdc - a big cap that runs around $25 - $30 if I am not mistaken.

They were under $10 in production qty back when they were being built. Of course nowadays, you're absolutely right, though resorting to snap-in caps mounted in different cap clamps makes it more affordable. Expected lifetime on the computer grade caps was far longer than the snap-in style that so many mfgrs are forced to use now.

While dual supply amps eliminate the output coupling cap, you're paying for the extra power supply cap in exchange.

[pdf64]

...While dual supply amps eliminate the output coupling cap, you're paying for the extra power supply cap in exchange.

And (hopefully) some sort of protection circuit to save the speakers if something fails and the amp puts out a heap of dc

[jmaf]

As far as design considerations go: In my region NPN transistors used to be a lot more expensive than PNP and also harder to find in regular stores. PNP transistors were all over the place.

I wasn't old enough to hack electronics in the 1970's but even in the 1990's I had lots of trouble finding complementary pairs.

My guess about symmetric rails vs. single ended would be that the electronic design decision was mainly due to available components and their price at the time.

[J M Fahey]

As far as design considerations go: In my region NPN transistors used to be a lot more expensive than PNP and also harder to find in regular stores. PNP transistors were all over the place.

I wasn't old enough to hack electronics in the 1970's but even in the 1990's I had lots of trouble finding complementary pairs.

My guess about symmetric rails vs. single ended would be that the electronic design decision was mainly due to available components and their price at the time.

Maybe you mean the exact opposite?

[jmaf]

Maybe you mean the exact opposite?

Brazil is in the Southern hemisphere. Everything is the exact opposite of USA

Don't ask me why.

My former shop business partner, great tech, had drawers and drawers full of PNP transistors. I asked why. He said "Brazil in the 70's".

[J M Fahey]

I am also in the South Hemisphere, in fact sharing a long frontier with Brazil, and what you say is impossible.
In fact, the workhorse power transistor from late 60´s to the 80's, and which I used by the thousands (like *everybody* else) was 2N3055.

Its PNP mate was MJ2955 , notice that originally it was not even "2N" because it was a single manufacturer specialty, and was both expensive and very hard to find ... in Southamerica that is

Check ALL Acoustic, early Peavey, Fender, Ampeg, etc. amplifiers, ALL used NPN transistors ... because those were the cheap available ones; PNPs were as rare as hen´s teeth.

Brazil did not manufacture its own transistors and got them from the World Market.

Even IBRAPE which was the Brazilian branch of Philips, assembled theirs using European supplied dies.

[jmaf]

I am also in the South Hemisphere, in fact sharing a long frontier with Brazil, and what you say is impossible.
In fact, the workhorse power transistor from late 60´s to the 80's, and which I used by the thousands (like *everybody* else) was 2N3055.

Its PNP mate was MJ2955 , notice that originally it was not even "2N" because it was a single manufacturer specialty, and was both expensive and very hard to find ... in Southamerica that is

Check ALL Acoustic, early Peavey, Fender, Ampeg, etc. amplifiers, ALL used NPN transistors ... because those were the cheap available ones; PNPs were as rare as hen´s teeth.

Brazil did not manufacture its own transistors and got them from the World Market.

Even IBRAPE which was the Brazilian branch of Philips, assembled theirs using European supplied dies.

I didn't say Brazil produced these transistors. I referred to market conditions in the 1970's.

So, what I said is impossible? You sure Juan Manuel? Let's take a look.

Here's a little amplifier from early 1970's Brazilian electronics magazine, must be all NPN 2n3055 right?



Oh. Wow. Why is every single transistor in it PNP? Can't be.

What about a little metronome circuit:



Look at that, a HEP251 PNP transistor. In Brazil? Can't be. It's impossible.

A capacitance meter:



They're not 2n3055 either! They're BC214L, ACY18. This must be magic because Juan Manuel said it's impossible.

A little buffer circuit from the 1970's:



This is getting really spooky now. Every transistor in there is PNP? They're AC125, AC126 and they are PNP but that's not possible either.

A wireless microphone transmitter from the 1970's. It's gotta be all NPN transistors, right? Because Jose made this stuff up?



HEP-251 and HEP-3 both PNP.

What about an LFO from the 1970's? Gotta use 2n3055 for sure this time.



Hey, it uses OC71 a PNP. Funny but that's impossible too.

A funky beats generator. Now we'll find a 2n3055 for sure.



Whoah there. That's impossible! It uses three PNP OC35 and OC72! Not a single NPN.

But a catalog must show 2n3055 for sure since what I said is impossible. Let's see.



Wow no 2n3055 and look at all those PNP's in stock. This isn't possible, these magazines must be from a time machine or something.

A tiny little amplifier project for hobbyists. Gotta find a 2n3055 in here somewhere.



Nope. RCA SK-3004, Motorola HEP-200. All PNP.

Echo chamber? Must be 2n3055 this time.



Nope. 2n397, a PNP transistor.

This can't be! Even in the book covers they're conspiring AGAINST THE IMPOSSIBLE!!!



No wonder you left São Paulo, Juan Manuel, you really don't know the electronics history of the country you lived in!

[Enzo]

Just a thought, jmaf, a long list of sarcasm is not the way to win friends. The man disagreed with you, he didn't call you names or threaten your family.

The discussion was aimed at output transistors, so a collection of small signal circuits like metronomes, or tiny 9v battery amps is not likely to use TO3 types of either chemistry. When I was a kid learning, transistors came along and small signal types like for code practice oscillators were indeed little PNP germanium types. That was mostly in the 1950s though.

[TomCarlos]

Thank you Enzo.... and I agree.

While I appreciate the responses from everyone (and I have learned something as a result of the post), I do not see a need to take a jab at anyone. Perhaps there is a way to reply with "have you considered this???"

So please, consider offering up some knowledge, hints, learn, etc while refraining from the insults to fellow members.

Thanks, Tom

[jmaf]

I posted 9 schematics and a few silly comments. No insult at all.

[loudthud]

@jmaf, if you have a big stash of germanium transistors, they can be worth big buck$. Pack 'em up and send 'em to me.

@TomCarlos, keeping with the KISS principle (Keep It Simple Stupid, or some say Keep It Stupid Simple) I lean towards the single rail design philosophy. It's more like a tube amp and doesn't rely so much on negative feedback. Much of the bad transistor sound people complain about comes from dual rail designs.

[jmaf]

@jmaf, if you have a big stash of germanium transistors, they can be worth big buck$. Pack 'em up and send 'em to me.

I was browsing my "regional ebay" and found a mint state box of Tungsram PNP resistors for $10.

This one https://www.etsy.com/hk-en/listing/5...y-matched-pair

I bought it on sight. I see a fuzzy future.