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High Power Regulate Power Supply for Audio Amplifier

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MRacerxdl

Sat Dec 20 2008, 10:43PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Hey all, I am making my definitive Power Supply for my 800W Audio amp.
What do I have:
40+40V 16A Transformer with 20% higher current capacity (it can do 21A RMS without any problems, that what I asked when I ask to made it at factory)
2 10.000uF 63V Epcos Caps.
35A 1000V Bridge retifier.

Ok, My amp needs Symetrical 55V to do 400W per channel @ 4 Ohms, with 40V RMS I get a 56V peak at the ouput. After the bridge I get 55V.
Ok no problem until that. But when I get near the 400W output, the voltage at capacitors drop to about 44V, so I never will get that 400W output.
I can buy a 55+55 V Transformer, but I will get a 70VDC at ouput, that will blow a few things on my audio amp.

So, what is the best way to I regulate the output to symetrical 55V? The current needs to be arround 16A, and I dont want to use much 2n3055 with big heatsinks (the heatsinks of my Amps are small, so I dont wanna anything bigger than that)

Pics from my amp:
The old setup, the actual transformer but another cap bank and another bridgE:

One of the two boards:

Only the PCB:

Thanks!

El_Roberto

Sun Dec 21 2008, 12:49AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

I cant help you sorry, but Im having a similar problem with my powersupply Im trying to build (using a 24v 42A transformer) when drawing a 32v 14amp load the voltage drops to about 28v, The only thing I can think of is to get the 55v + 55v transformer and use a regulator to get the voltage you want (But as you said this will give you a whole lot of big heatsinks with the transistors you would need)

Hazmatt_(The Underdog)

Sun Dec 21 2008, 08:26AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Audio designs for high power aren't going to have regulated supplies for the problems you mentioned already.

I suggest you get a heavy duty toroid transformer that will either give you the voltage you need from the get-go, or get a 500-600 VA unit that you can double-up on secondary windings so you can source enough current without the sag you're experiencing.

Steve Conner

Sun Dec 21 2008, 10:06AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Regulation is a waste of money, since any half-decent audio amp rejects ripple as well as any regulator would.

It's simpler just to fit higher voltage transistors so you don't "blow a few things". The MJ15023 and MJ15024 are the classic transistors for high-power audio amps, and MPSA42, MPSA92, MJE340, MJE350 etc. for the driver circuit. These are all rated 250V or more, so you can use them for designs up to 1kW @ 4 ohms, like Rod Elliot's Project 117

Or settle for less than 400W. You won't hear the difference between 400 and 300.

Dr. Dark Current

Sun Dec 21 2008, 11:26AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Btw. what do you need such MONSTROUS power for? Do you organise large concerts or what?

P.S. is your amp class-D with those miniature heatsinks for such power?

MRacerxdl

Sun Dec 21 2008, 04:02PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Hmm, the things that blow up on my amp isnt the power transistors (the power transistors are TIP142 and TIP147, that can handle 100V) :P its a few resistors, I can adapt but, the gain of the amplifier drops much when the voltage drops from 55 to 44v. My amp rejects the ripple too, and its not a problem :D
Toroidal transformer have a less voltage drop than a normal steel-silicon ones? I was thinking about increasing the cap bank, I am using only 10.000uF for each symetrical voltage, some commercial amplifiers that I see uses 40.000uF and up.

Dr. Kilovolt: That amplifier is for testing big sound boxes, like that have 18'' speakers for 200W :D
And its not Class-D, its a Class AB, the heatsinks are too small indeed, but two 8x8 coolers get the heat away easily, but I plan to buy a big heatsink to get away that coolers.

The amp is 400W per channel @ 4 ohms. I can run it a 2 Ohms too, and I get around 600W without distortion, but with such small heatsinks, I get a problem in a few minutes running.

Dr. Slack

Sun Dec 21 2008, 10:33PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Regulation is a waste of money, since any half-decent audio amp rejects ripple as well as any regulator would

and not only that, it erodes "music power". This type of specification got a very bad press when puny amplifiers capable of 100mW continuous got advertised as 50watts (peak music power). But, the important thing is that music *is* impulsive, not continuous, and the ability to let the rail build during the rest of the bar and sag during the bass kick means that you can deliver double the peak output for any given heatsink size.

Steve Conner

Mon Dec 22 2008, 11:44AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The TIP142/147 are only rated 100V. If you go above +/-50V rails they are almost certain to explode.

To paraphrase that Dilbert cartoon: "Here's a nickel kid, get yourself some real transistors." :D

Dr. Slack

Mon Dec 22 2008, 12:28PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I've just re-read your Sunday 21st reply, do you really mean ...

the gain of the amplifier drops much when the voltage drops from 55 to 44v.

... or do you mean the maximum power output? If you mean the gain, then that's not a nice design.

MRacerxdl

Mon Dec 22 2008, 09:56PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Steve: I run it at 55V (that is the caps charge), I think the transistors only see the supply that is attached, because its +55 for the TIP142 that goes through it and goes to the speaker, and -55 goes through TIP147 and to the speaker. The speakers is at Ground (0V), so I *think* that is not the problem, because I was run for a few minutes the amplifier at 70V too (the first time for test, I used a Variac as a center tap, to make the 0V, so it gets two 55V edges that get almost 70VDC symetrical).

Dr Slack: Yeah, sorry its not the gain, its the output power for a same load. Actually, its doesnt change too much, only make distortion worse, as the onboard "pre-amplifier" (my amp oscilates 300W with only 3Vpp input) when the voltage drops, I get a little distortion (because the calculation of the gain, as the voltage gets lower, all currents get lower in the circuit, so I get a few problems)

Here is the original schematic of the amp:

That was for a 250W amplifier, so I made a few changes:
Instead TIP140/145 I used TIP142/147 (Higher voltage, 100V instead 60V) and put it 4 of it, not 2 as the original. And increase the voltage from 45V to 55V (not much, but it gaves me a 100W more)

Its working very good now, it stays at +-52V in most of time (I dont run things more than 100W per channel here :P), I didnt have any problems until now.

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