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Thread: Single vs Dual Supply Voltage Amplifiers

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    Supporting Member TomCarlos's Avatar
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    Single vs Dual Supply Voltage Amplifiers

    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!!

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    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.

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    Supporting Member TomCarlos's Avatar
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    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.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by TomCarlos View Post
    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.

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    Quote Originally Posted by nevetslab View Post
    ...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

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    Last edited by pdf64; 03-16-2019 at 01:41 PM.

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    Valvulados.com jmaf's Avatar
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    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.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by jmaf View Post
    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?

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    Juan Manuel Fahey

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    Valvulados.com jmaf's Avatar
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    Quote Originally Posted by J M Fahey View Post
    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".

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    Old Timer J M Fahey's Avatar
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    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 60īs 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 henīs 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.

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    Juan Manuel Fahey

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    Valvulados.com jmaf's Avatar
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    Quote Originally Posted by J M Fahey View Post
    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 60īs 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 henīs 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?

    Click image for larger version. 

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

    What about a little metronome circuit:

    Click image for larger version. 

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

    A capacitance meter:

    Click image for larger version. 

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    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:

    Click image for larger version. 

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    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?

    Click image for larger version. 

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    HEP-251 and HEP-3 both PNP.

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

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

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

    Click image for larger version. 

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    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.

    Click image for larger version. 

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    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.

    Click image for larger version. 

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

    Echo chamber? Must be 2n3055 this time.

    Click image for larger version. 

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    Nope. 2n397, a PNP transistor.

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

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

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    Lifetime Member Enzo's Avatar
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    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.

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    Supporting Member TomCarlos's Avatar
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    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

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    Last edited by TomCarlos; Yesterday at 12:08 AM.

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    Valvulados.com jmaf's Avatar
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    I posted 9 schematics and a few silly comments. No insult at all.

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    @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.

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    Valvulados.com jmaf's Avatar
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    Quote Originally Posted by loudthud View Post
    @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.

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    Quote Originally Posted by jmaf View Post
    mint state box of Tungsram PNP resistors
    those might be worth big buck$

    Click image for larger version. 

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    Last edited by shortcircuit; Yesterday at 03:10 PM.

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    Quote Originally Posted by loudthud View Post
    @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.
    So if single supply design is more like a tube amp and more simple, did it eventually just get ditched in an attempt for more power? say using single supply 75V vs +/-75 for twice as much signal swing and more power ?

    If that's the case, and maybe the transistors of the 60s/70s were limiitng how much voltage you could use for a single supply. Aren't there output transistors that can handle 200V or so now? Or maybe more?

    If this is possible, it makes me think it could be fun to try to make a very loud bass amp with modern output transistors and a 200V (or whatever ) single supply design.

    If Sunn concert series is 35V for the output section and it does 120-130W clean into 4 ohms it seems like it would not be that hard to make a 300W-ish bass amp with 100-200V supply ?

    you got my mind going

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    Member mhuss's Avatar
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    I worked on a number of power amps in the 80s that were (1) split supply and (2) had all (identical) NPN outputs. See Peavey CS400, CS800, etc. These were DC-coupled outputs, but had protection circuits that would blow fuses when outputs failed -- and it was always at least two, I used to bring outputs to gigs like spare fuses . Even though they blew up once-twice a year, we never had a damaged speaker (though to be fair, they were old JBL D140Fs!) I suspect the repeated failure was due to the design depending on closely matched outputs and we never could afford to replace all 10 at a time...

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by TomCarlos View Post
    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!!
    I think you may be looking at things the wrong way. It's the amplifier topology that matters.

    For example you can have the common push-pull with direct coupled output, or single supply with a capacitive coupling. These can use complementary or quasi complementary designs. But you can also have dual rail designs with flying rails , single supply with transformer coupling, single supply DC coupled bridged and many others too, I'm sure.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by J M Fahey View Post
    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 60īs 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 henīs 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.
    Not to take sides at all, but I do recall my first experiences with red spot /white spot transistors in glass envelopes. These were all PNP. So, I'm not entirely surprised to see PNP designs in an old book. IIRC it was only when silicon came along that it switched to NPN predominance.

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    Senior Member nevetslab's Avatar
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    Back in the mid-70's and for the next 10 years or so, you tended to find mostly quasi-comp output stages, all using NPN xstrs. The higher power amps tended toward 2N3773's, even 2N6259's, and the lower powered ones using 2N3055's or their higher-voltage screened parts that we'd get from RCA or Motorola with in-house numbers. The PNP's just didn't have the same Safe Operating Area characteristics, so you tended to not build the output stages with PNP's.

    In the early 80's, BGW had made a purchase for around 10 million pairs NEC complimentary parts....4-leaded 2SB706 PNP's and 2SD746 NPN's. We had taken a few hundred samples to Motorola to have them tested, and being impressed enough, went ahead with the deal. For the next many years, we made a lot of product using the NPN parts in quasi-comp designs, and a number of products using complimentary parts (BGW 750D/E). As time moved on, and the pile of 2SD746's was rapidly dwindling, we had to make a change. Turned the schematic upside down and began using the PNP's, until we finally switched over to the MJL21193/MJL21194s and other faster parts (Sanken 2SC3858 NPN, 2SA1494 PNP) in full complimentary circuits.

    Sure, there were a lot of products out there fully complimentary, but....the earlier PNP's just weren't as strong, and they'd fail when pushed hard.

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