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Old 23rd March 2009, 10:05 PM   #1
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Default Adcom transformer for Leach Amp?

I have a dead Adcom GFA 545-ii amplifier. The transformer seems fine and I am wondering if this could be used for a Leach amp (ver4.5 board)

The DC immediately after the rectifier reads +/- 56.5 volt and the the AC at the output of the transformer reads 41.6 volts.

In the Leach article, the recommendation is for a 80 VCT at 4 or 5 amps.

My question: Is this transformer worth using? I don't know what its VAC is? I thought the Adcom was at 400 VAC or so.

Thanks
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Old 24th March 2009, 10:14 AM   #2
jaycee is offline jaycee  United Kingdom
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80VCT means 40-0-40V, so yes your transformer will be suitable.

Transformers put out AC voltage, hence VAC. VDC would be the DC voltage after rectification and smoothing.
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Old 24th March 2009, 12:05 PM   #3
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Jaycee, Thanks for responding.

Yes, I understand that 80 VCT is 40-0-40 (80=2*40). I also understand that 40 *1.1414 = 56, for the rail voltage (approximately). I measured the values and reported them since a search of the threads indicated some folks had provided incorrect numbers. Anyhow ...

So let me re-phrase my question: Does anyone know the VA rating for the transformer on an Adcom 545ii. I am shooting for at least 400 or so for my Leach project (ver4.5 boards).

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Old 24th March 2009, 12:19 PM   #4
jaycee is offline jaycee  United Kingdom
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A good indicator is it's size. What are it's dimensions?

Is it a toroid or an EI type?
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Old 24th March 2009, 12:36 PM   #5
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Quote:
Originally posted by jaycee
A good indicator is it's size. What are it's dimensions?

Is it a toroid or an EI type?
Hmmmm, that is a good question.

The beast is potted. The size of the case is 3.5 inch tall and 4.2 inch diam. Since it is potted, I assume it is not EI and they had to deal with noise (just my guess).

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Old 24th March 2009, 01:28 PM   #6
jaycee is offline jaycee  United Kingdom
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Yep, toroidal as you said "diameter"

If I had to guess I'd say it's around the 400-500VA mark. Certainly suitable for two 100W leaches.

You could probably build the whole lot right into the Adcom case, reusing many of the parts as well.
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Old 24th March 2009, 01:39 PM   #7
AndrewT is offline AndrewT  Scotland
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40Vac gives between 150W and 171W into 8r0.
Two channels need between 300VA and 680VA.
If the Adcom is in that range then you'll be just fine.
At the higher VA range use +-20mF/channel.
At the lower end of VA increase smoothing by upto 50%.
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Old 25th March 2009, 02:41 PM   #8
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Well, I am one step closer to figuring out the VA on the Adcom 545 transformer, I think.

According to specs, the Adcom 545 is 100 watt into 8 ohm and 150 watt into 4 ohm. So it does NOT double.

Does this usually indicate the transformer does not have a "generous" VA? Or is it more complicated than that?

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Old 25th March 2009, 06:51 PM   #9
infinia is offline infinia  United States
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Quote:
Originally posted by WithTarragon
Well, I am one step closer to figuring out the VA on the Adcom 545 transformer, I think.

According to specs, the Adcom 545 is 100 watt into 8 ohm and 150 watt into 4 ohm. So it does NOT double.

Does this usually indicate the transformer does not have a "generous" VA? Or is it more complicated than that?

Thanks


Here's another data point - 545 back panel says do not exceed 600W.
My guess would be closer to calling it 500VA with derating. Seems it should work perfectly for the leach amp if it has similar class AB efficiency to the Adcoms. I wouldn't get too hung up on VA numbers unless you plan on driving really low impedance speakers (or bridging outputs) hard for long periods (ie pro use).


edit> the ouput power rating is not yet at clipping so it's not too easy to use those number for VA calcs. but it gives good info on the XFRMR regulation!
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