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Old 6th February 2008, 05:37 PM   #1
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Default Larger transformer for Adcom GFA555ii

I would like to use this amp to drive a new horn loaded bass bin.

Although the bass bin will be quite efficient, it will have two 5-ohm drivers wired in parallel and the Z can drop to about 3 Ohms. I am quite impressed with my Adcom 555 for delivering bass (thanks to DJKs suggesting of bypass caps on the PS caps).

However, I am wondering if it could use a bigger transformer for this load (it is currently rated at about 800 & believe it is 52 volts - I need to check that). The stock transformer is in an enclosed case.

I am told that a transformer from a Hafler DH500 is a straight replacement and will fit.

What would be a good Transformer?

Thanks,
-Tom
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Old 8th February 2008, 10:48 AM   #2
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"I am told that a transformer from a Hafler DH500 is a straight replacement and will fit."

It's a perfect mechanical fit, but way too much voltage. This was done with the Ryan regulator (Audio Amateur project) and dropped the raw voltage (95V) to a regulated 67V.
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Old 8th February 2008, 06:21 PM   #3
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Thanks DJK, I wil cross that one off the list.

Any suggestions for a bigger transformer? I assume others have done this, since the transformer seems a bit smallish for this amp.

Thanks,
-Tom
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Old 9th February 2008, 03:30 AM   #4
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Hey Tom
I have been playing Mr I think I know a little about audio, and just received a transfo from a GFA-555. I checked the voltage on the rails, and I get 70vac per secondary, and 65vdc per secondary. Please let me know what kinda voltage you are getting.
Thanks
Ben
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Old 9th February 2008, 01:40 PM   #5
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The stock transformer is 52-0-52 at 700VA with about 5% regulation.

The no-load DC voltage will be about 75V with 120V input.

This is adequate for the 325W/4R x2 the amp is rated at for a 30% duty cycle (rock music).

If you want to push it for just bass you will want about 1.5KVA total at the same voltage. That will usually be a stacked pair of 700VA~800VA toroid. Check and see if they will fit, not all brands will.
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Old 12th February 2008, 07:22 PM   #6
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While I never question DJK's postings...

I have routinely measured 555ii ' s Raw DC rails to be anywhere form 82 to 84vdc and the AC to be about 59-0-59. AC mains I have is 122vac.

I have used many Avel 60-0-60 1kva parts as replacement in the Adcom 555ii and since the 60-0-60vac by Avel, is rated at 117vac, I get unloaded rails of 87-0-87dc with my 122mains... I have not blown an Adcom with this Mod however I never use it below 4 ohms nominal.

I get over 370 watts at clipping at 8 ohms and over 500 watts into 4 ohms..., with mantaining my mains at 120vac minimum, I can the 8 ohm clip at about 420 watts RMS! With the 800va part it stays below 400 watts even if I regulate the mains. This is 1 channel running not both...

Please ensure the sinks stay below 50C while you rig the amp this way.

Let it be known the amp is not designed to be run at 87vdc... so extra precautions or controlled usage is strongly recommended.

Avel's are available directly or via Partsexpress to name 2 sources.
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Old 12th February 2008, 10:29 PM   #7
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60-0-60 will blow a GFA 555 to smithereens, the driver transistors will NOT take it.

Just because you measure 59-0-59 does not mean that is what the transformer is rated at, that's no load.

Typical line voltage in the USA is 123V (light load), and the utiliy company does not consider it high until it hits 127V.

With Avel using 115V primaries and applying 123V, the correction factor would be 1.07 (rounded). Divide by 0.96 for regulation, times 1.414, and subtract .7V for rectification.

52V*1.07=55.62V 55.62V/.96=57.93V 57.93V*1.414=81.92V 81.92V-.7V=81.22VDC (no load).
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Old 12th February 2008, 11:23 PM   #8
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True...

I usually do change the drivers to something in the 1302/3281 region, but even with the 1047/817's I have not blown a pair even at 87 vdc rails....

the measurements I did were as follows:

DC measured on the caps 87vdc.

AC measured at the Trafo's secs, 62-0-62vac

Mains 122vac... Used the Y23 series.
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Old 13th February 2008, 02:46 AM   #9
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The original GFA 555 used 2SA968/2SC2238 (160V, 1.5A, 25W). They blew up whenever you drove it hard into low impedances.

They tried 2SA1011/2SC2344 (180V, 1.5A, 25W), same problem.

Next they went to the 2SB633/2SD613 (100V, 5A, 40W) that had to be selected for voltage. These worked (mostly).

I always replaced mine with MJ15030/31, only 150V, but they worked. Today I would use MJ15032/33.

Later, the GFA 555II went to a different design with TO-3P drivers.

The difference in maximum output of 55-0-55 and 50-0-50 is less than 1dB, I am not going to recommend that to anyone.

If you want to go to MJ21193/94 outputs, MJE15032/33 drivers and fans you could run a 60-0-60, that would be 2.5dB more than running the stock 52-0-52.
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Old 13th February 2008, 03:35 PM   #10
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Thats typically what I do, Mj21195/96's in the outputs if 4 ohm loads are used otherwise the original OP's can do 90vdc rails (Heck I have tested them at 100vdc rails without blowing and some guys have done 125vdc rails with the Toshiba 424/554's... but NOT recommended in normal use.)

I would not raise the rails on the original 555's as I would easily with the 555ii
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