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Old 21st March 2012, 03:58 PM   #1
Stratt is offline Stratt  United Kingdom
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Default amplifier rebuild - power supply single or dual

Hi all

I'm about to rebuild an old stereo amplifier I've had for years. It's based on two Maplin Mosfet modules (Dave Goodman - 1982 - the 1997 spec version with Exicon Mosfets).

I've got all the parts to build a 500va toroidal based power supply with 4x10,000u/80v Caps, but I don't know if it would be better to do two split supplies each with their own bridge rectifiers and 2x10,000u caps, one for each amp, or a shared supply with two br's and 4x10,000u caps. What's best practice?

Thanks

Stratt
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Old 21st March 2012, 06:02 PM   #2
jaycee is offline jaycee  United Kingdom
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That's a can of worms for just 100W amps with that kind of capacitance, i wouldn't bother. You might want to use seperate bridge rectifiers for the + and - supplies if your transformer has two seperate secondaries though.
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Old 21st March 2012, 07:21 PM   #3
AndrewT is offline AndrewT  Scotland
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How is the existing amplifier set up?
How quiet is the hum and/or buzz at the speaker?
Have you measured the hum and noise at the speaker terminals?
Have you measured the output offset at the (empty) speaker terminals?
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Old 21st March 2012, 07:31 PM   #4
GoranB is offline GoranB  Poland
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Its always better to have separated PS for each chanel
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Old 21st March 2012, 10:20 PM   #5
Stratt is offline Stratt  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
How is the existing amplifier set up?
How quiet is the hum and/or buzz at the speaker?
Have you measured the hum and noise at the speaker terminals?
Have you measured the output offset at the (empty) speaker terminals?
When I built it all those years ago I just used what I had to hand; a 32-0-32 2A transformer and a couple of 4700u/80v caps with a 25A br. It's very quiet with little or no offset at the speaker terminals. Just lacks oomph.

Now I have the wherewithal to finish the job properly. I've always been curious to see what difference it will make to have a power supply with enough grunt, but I wasn't sure whether to isolate the dc parts of the supply for each amp.

Quote:
Originally Posted by jaycee View Post
You might want to use seperate bridge rectifiers for the + and - supplies if your transformer has two seperate secondaries though.
That's what I would have done anyway if I didn't do the separate supply stuff. I'm leaning toward this method.

Thanks

Stratt
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Old 21st March 2012, 11:16 PM   #6
jaycee is offline jaycee  United Kingdom
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I think that should be adequate. There should be no hum with the right grounding scheme.
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Old 22nd March 2012, 01:15 AM   #7
Stratt is offline Stratt  United Kingdom
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Quote:
Originally Posted by jaycee View Post
I think that should be adequate. There should be no hum with the right grounding scheme.
I'm using a 3U rack type case and I'll be using a typical star grounding scheme with Rod Elliot's safety loop breaker circuit using a 35A bridge rectifier in parallel with a 10R resistor and 100nf capacitor. I'll also keep the power supply over to one side so I can route mains power away from audio circuitry.

Cheers

Stratt
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Old 22nd March 2012, 05:50 PM   #8
AndrewT is offline AndrewT  Scotland
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You have confirmed that the existing layout of cabling works.

Don't change that layout.

You can swap to different bits of each component, but keep that working layout.
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Old 8th April 2012, 04:56 PM   #9
Stratt is offline Stratt  United Kingdom
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Default Finished!

So I finished my amp rebuild. It's got two old Maplin mosfet modules that I put together back in 1997. I have no idea where the heatsinks came from.

I've replaced the old 2A 32-0-32 laminated core transformer with a brand new 500va 35 + 35 toroidal. It had just two 4700u 80v power supply capacitors that I've replaced with six 10,000u 80v types, three per side. It has a Velleman loudspeaker protection module (wait and error leds on the front) and a cheapo chinese inrush limit module with a massive ceramic resistor. Switching on is absolutely silent, apart from the satisfying relay clicks. Check out the literal star earth!

I've also put in a chinese 23 step switched attenuator. Note the replacement resistor because of a faulty resistor value.

When I turned it on with speakers plugged in for the first time I thought it was broken. Turned out it was working perfectly; it's just absolutely silent with no signal. Then when I cranked it up I hadn't realised these amplifiers could sound so good with so much grunt!

It's not particularly pretty (probably prettier on the inside!) but it does a good job.

Cheers

Stratt
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Old 8th April 2012, 07:08 PM   #10
jaycee is offline jaycee  United Kingdom
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Looks like it'd kick some serious sh*t

One thing I would do though - improve the heatsinking. I'd get a 200w, 40d, 100h black aluminum extrusion and bolt the L-angles of the modules to it.
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