Seeking a project

acceler8 · 26th January 2012, 05:15 PM

I'm looking for a new amp project that can use an existing chassis and power supply. The amp in question is a dead Adcom GFA-555 which has some nice heat sinks and about 60,000uf of power supply capacitance. The problem is that is has 81V rails (PS schematic attached), which exceeds most of the projects I've seen here. The power supply has tested well, and seems to be capable of reuse.

I've found the thread Plh Retrofit to Hafler/Adcom that specifically mentions re-using an Adcom GFA-555 but it seems to have petered out. I'd like to try an F5 amp (from the Pass Labs forum) but my high rail voltage seems inappropriate for it.

I'm hoping for a project that has a good cost/performance ratio as my SE 300B project is consuming most of my hobby funds. At the very least I want something that sounds as good pr better than a stock GFA-555, however.

Please suggest applicable projects and/or workarounds for my high rail voltage.

Thanks!

p.s. The completed project will be hooked up to a pair of stock Tri-Trix speakers (nominally 8 ohms) in a large-ish basement. Bass is handled by a dedicated sub-woofer. Fairly low volume levels are acceptable. Primary music will be rock with some techno and solo-female-vocalists.

Osvaldo de Banfield · 26th January 2012, 05:23 PM

There is an chip amplifier capable of more than 100W called "STK4044" series, but at 71V, and are easy of build. Try some more voltage IC.

Mooly · 26th January 2012, 05:23 PM

Just thinking aloud without really thinking

You're on 120V ? Amp can be used on 240V and looks to have a good selection of tappings. So wire the tranny for 220 or 240 and that will give correspondingly lower output voltage.

acceler8 · 30th January 2012, 04:31 PM

Both excellent advice!

I see that Digi-Key still has the LM4702 in stock. I'll see if I can find any listening impressions with that chip.

Mooly, will wiring for 240V input give me 1/2 output (120 to 81 vs 240 to 40.5) or is there a conversion factor?

Thanks!!!

GloBug · 30th January 2012, 04:52 PM

That is correct, 40.5v.

The ratio is 0.68V "out" for every volt "in"

Mooly · 30th January 2012, 06:01 PM

Quote:

Originally Posted by acceler8

Both excellent advice!

I see that Digi-Key still has the LM4702 in stock. I'll see if I can find any listening impressions with that chip.

Mooly, will wiring for 240V input give me 1/2 output (120 to 81 vs 240 to 40.5) or is there a conversion factor?

Thanks!!!

No conversion factor... it's as GloBug says.

es44 · 30th January 2012, 06:02 PM

I would assume at half the power, i.e. the same secondary ampere, dictated by the thicknes of the wire ?
Best regards
Ebbe

GloBug · 30th January 2012, 06:39 PM

I would assume too, but did not state it as I was not sure.

carlmart · 30th January 2012, 08:21 PM

Quote:

Originally Posted by acceler8

The amp in question is a dead Adcom GFA-555 which has some nice heat sinks and about 60,000uf of power supply capacitance. The problem is that is has 81V rails (PS schematic attached), which exceeds most of the projects I've seen here. The power supply has tested well, and seems to be capable of reuse.

Probably a stupid question but: why not repair the dead Adcom?

ingenieus · 10:59 AM

The 81V rails are good for about 400W into 8ohms. This would be quite a project.

Something like this perhaps?

Trouble is, it 'only' uses 70V rails.

26th January 2012, 05:23 PM	#2
Osvaldo de Banfield diyAudio Member Join Date: Dec 2011 Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina	There is an chip amplifier capable of more than 100W called "STK4044" series, but at 71V, and are easy of build. Try some more voltage IC. __________________ A veces pongo un circo y me crecen los enanos... "Que se llama soledad", Joaquin Sabina, el capo.

26th January 2012, 05:23 PM	#3
Mooly diyAudio Member Join Date: Sep 2007	Just thinking aloud without really thinking You're on 120V ? Amp can be used on 240V and looks to have a good selection of tappings. So wire the tranny for 220 or 240 and that will give correspondingly lower output voltage. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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30th January 2012, 04:31 PM	#4
acceler8 diyAudio Member Join Date: May 2010 Location: South East USA	Both excellent advice! I see that Digi-Key still has the LM4702 in stock. I'll see if I can find any listening impressions with that chip. Mooly, will wiring for 240V input give me 1/2 output (120 to 81 vs 240 to 40.5) or is there a conversion factor? Thanks!!!

30th January 2012, 04:52 PM	#5
GloBug Banned Join Date: Jun 2011	That is correct, 40.5v. The ratio is 0.68V "out" for every volt "in"

30th January 2012, 06:02 PM	#7
es44 diyAudio Member Join Date: Apr 2005 Location: Fredericia, DK	I would assume at half the power, i.e. the same secondary ampere, dictated by the thicknes of the wire ? Best regards Ebbe

30th January 2012, 06:39 PM	#8
GloBug Banned Join Date: Jun 2011	I would assume too, but did not state it as I was not sure.

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