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Old 16th September 2014, 12:31 PM   #1
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Default Need Schematic for PA Stereo amplifier

Hello there, i have a stereo professional amplifier, old school 80's with about 150wrms @ 8ohm per channel. The amplifier was fail last year and the only good parts is output devices.
It have a 800watt power transformer with 40-0-40 secondary and aux coil 10vAC, approx. +/-56vDC with 10.000uF per rail. and +13.8vDC for preamps.
My opinion is to disable the aux coil and fit a second transformer to get +/-15vDC for preamps and protection circuit.

The output devices are 3xMJ15024 per rail, total 6xMJ15024 per channel. The original schematic is nowere, i cant find it, it's diy job.

Maybe suitable sch. is quasi nbip300 amplifier.....?
I've changed the power supply caps with new and i make a dc protect, thermal shutdown, and preamp circuit on one board from zero.
I dont know what i do with output. Witch schematic can i choose?
Chassis is 2U standard rack and i have only the output transistors and heatsinks.
almost forgot, i've change the fans from 230vAC to Corsair 12vDC with controller board (thermistors on heatsinks).
With 150w RMS @ 8ohms and 200wRMS @ 4 ohm i will be happy!
That amp i want it to drive woofers at 4ohms (2way system with electronic crossover) and i have another amplifier to drive tweeters (2x LM3886)

Thanks for the help!
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Old 16th September 2014, 01:24 PM   #2
JMFahey is offline JMFahey  Argentina
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Check the Apex Audio amplifiers in this same Forum.

He generously published many, well tried and tested, with lots of backup information, PCBs, etc.

Last edited by JMFahey; 16th September 2014 at 01:52 PM.
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Old 16th September 2014, 02:18 PM   #3
JMFahey is offline JMFahey  Argentina
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I had earlier suggested an all NPN one, but rechecked it and didn't find it very good.

This one is full complementary but much better.

500W PA amplifier with Limiter

You'd have to buy 6 x PNP complementary to what you already have, no big deal compared to what the full project is
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Old 17th September 2014, 06:37 AM   #4
djk is offline djk
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"My opinion is to disable the aux coil and fit a second transformer "

So just use two half-wave rectifiers and you can regulate the single AC winding to ±12V for opamps.
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Old 17th September 2014, 06:45 AM   #5
djk is offline djk
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Click the image to open in full size.

A very good sounding quasi-comp professional amplifier, Altec 9440 output stage. It has four pair of outputs on ±80V, three pair should be fine for your ±57V. It has three pair of 35W drivers, one pair of 100W~150W devices should be fine (A1302/C3281 or similar)
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Last edited by djk; 17th September 2014 at 06:49 AM.
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Old 17th September 2014, 12:53 PM   #6
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hello there,
JMFahey the idea to use npn/pnp is better i know but i want to keep only npn for each channel or to use 3x pnp and 3x pnp for each channel. for the apex b500 what is your opinion? do you find errors about stability etc?

djk with zeners / resistors at main rails right? Thats good and better solution i think.
did you have the full schematic of Altec 9440?

this is the nbip300 I said at the beginning of post.
https://sites.google.com/site/quasis...series/nbip300
so i'm searching something about this topology to keep the original heatsinks (3 and 3 output devices) and make only new pcb for outputs.
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Old 17th September 2014, 02:33 PM   #7
wg_ski is offline wg_ski  United States
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Another good staring point would be the original (1979) CS800. On +-56V the 3 outputs are enough (5 used on +-80). All NPN with 15024's and it works no muss no fuss. Use MJE340/350 for the Unobtainium MPSU series predrivers/VAS. It's conceptually similar to the PL400/700 Series 2 but the front end is a bit more refined for reliability.

As far as stability with the quasi vs. full comp - for supply voltages under +/-60V there really isn't much difference. Above that the quasi can start to give you trouble. The only insurmountable stability problem I've ever run into was trying to do class H with a quasi triple. I ended up giving up. On just a single set of rails it ran perfectly, but switched it would only work full comp. Just a regular class B (AB) on +/-56V should be no issue at all.
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Old 17th September 2014, 10:36 PM   #8
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did you mean that amp right?
Click the image to open in full size.

i found service manual for newer versions of amp. with npn-pnp output devices....
I found your idea very good if we found the schematic to start the construction!
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Old 18th September 2014, 04:13 AM   #9
wg_ski is offline wg_ski  United States
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Yes that amp. It was a pretty foolproof QC circut. Take the new one's front end and graft on the PL400's output stage (with MJ15024's) - and you have the old one.. more or less.
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Old 18th September 2014, 06:58 AM   #10
JMFahey is offline JMFahey  Argentina
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Different from other amps posted in DIY Audio the Apex has been actually been built, many of them are working for years, and you can read the threads for comments, ideas, even actual pictures.
And they are modular, you can build the basic power amp PCB, add speaker protection, etc.

I had originally suggested the Apex "simple amp" which is all NPN, but it has one mistake (it works anyway, but because of that I deleted it) which is that on the bottom (negative rail) transistors he puts ballast resistors in the collectors while they must be used on the emitters.
Very easy to correct but you should redraw the PCB, don't know how comfortable do you feel about that.
If you do it, make the thin tracks thicker, there's a lot of space on the PCB anyway.

As of the amount of transistors, you use as many as you want, even a single pair, it's explained in the thread.
Those amps can be scaled down as needed.

Quote:
or to use 3x pnp and 3x pnp for each channel.
I guess you mean 3 NPN and 3 PNP per channel.
That's what I said: buy 6 PNP for the whole amp, 3 per channel
And mount 3 NPN + 3 PNP on each channel, using original heatsinks.

Personally I used all NPN for ages to build my 300W Bass amplifiers, but because of the terrible counterfeiting problem and lack of reliable supplier choices, I had to convert to all NCh IRFP250 (or 240) MosFets.

EDIT: just checked the NBip300 .
Looks good and you already have the PCB .
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