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Old 7th June 2004, 09:00 PM   #1
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Default Amplifier Worth Repairing?

I got a broken amplifier here, that looks like it is probably very easy to repair. The output devices are MJ2955 and 2N3055. There are two devices in parallel per channel.

My end goal is to have a 200-300W (8 Ohm) amplifier, are these devices suitible if the rails are +/- 52V

The caps look small, but that is easy to change.

-Paul Hilgeman
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Old 7th June 2004, 09:36 PM   #2
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You have a lot to learn before jumping in to design an amp. But to answer your query about the 2N3055, this has a breakdown voltage of 60v, and should not be used with supplies above about +-25v. It's the wrong part for the higher powered amp you want to make. This question gets asked about once a week, so you have good company.

To select an amp design, you might want to search on "first amp" or some such topic, to see what others here have already recommended. God luck
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Old 7th June 2004, 09:56 PM   #3
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Thanks for the reply.

Well, this was a working amplifier at one time.

All of the fuses were blown, so I replaced them and check the output. It goes straight to -48V and the 2N3055 Transistors get quite hot quickly.

I did a basic check of everything in the amplifier but couldn't find anything wrong.

Is there any hope, or should I scrap the guts and use the chassis.

Thanks!
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Old 7th June 2004, 10:11 PM   #4
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I have no first hand experience with your amp, but I'd like to suggest taking a look at a chip amp.

Anyhow, I recently got some free samples of the LM4780 from National, and now I'm just waiting to stumble across a suitable power supply. The supply range is +/- 20v to +/- 84v, so your rails seem to be up to it. The chip is bridgeable, too.

I haven't built any amps yet (just did a very basic repair on a hafler dh-500)- but in my eyes the chip amps seem like an easy project. Anyway, if you can get the free sample it's a cheap experiment.

I'm kind of new here, so I don't know if that's a helpful suggestion- I see seperate forums for solid state and chip amps. I assume that chip amps are a sub-category of solid state.
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Old 7th June 2004, 10:13 PM   #5
jaycee is offline jaycee  United Kingdom
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Assuming youve checked those output transistors for shorts, check the driver transistors. It sounds like someone has repaired the amp and used incorrect parts if there is a -48V on the output and those transistors are only rated for use with +-25V.

Might be worth trying MJ15003/04 instead of the 2955/3055. They're a bit slower ft but have a lot higher Vceo and can handle more power.

If you're designing an amp from scratch and don't want to spend too much, again MJ15003/04 are popular, along with OnSemi's MJL4281/4302 pair. You may want to look at the AmpsLab c200 schematic for an idea.
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Old 8th June 2004, 06:40 AM   #6
djk is offline djk
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"Anyhow, I recently got some free samples of the LM4780 from National, and now I'm just waiting to stumble across a suitable power supply. The supply range is +/- 20v to +/- 84v, so your rails seem to be up to it. The chip is bridgeable, too."

The maximum voltage is 84V from + to -, not 84V.

Lots of typo's on data sheets.

"My end goal is to have a 200-300W (8 Ohm) amplifier, are these devices suitible if the rails are +/- 52V"

First off, a 200W/8R amplifier with a moderately stiff supply runs off 75V (Adcom GFA555 with 60,000F of filter caps)

The Adcom GFA535 runs off 52V, its rated at 60W/8R

The Adcom GFA545 runs off 55V, its rated at 100W/8R (with a much beefier supply).
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Old 8th June 2004, 07:07 AM   #7
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Quote:
Originally posted by joe carrow
I The supply range is +/- 20v to +/- 84v, so your rails seem to be up to it. The chip is bridgeable, too.
yeah, like whats his name just said, its +-10 to +-42 volts...... but +-35 is really about all I'd be happy running them at... and if you were intending on making the amp happily 4 ohm stable, then +-30 would be better..
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