Adcom 5500 class A (more not less) - diyAudio
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Old 11th June 2010, 05:00 AM   #1
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Location: Portland, OR USA
Default Adcom 5500 class A (more not less)

So I have been browsing the passdiy site and I have been thinking class A for awhile. But, I like the 5500 but the damn thing is too powerful for my Heybrook HB4's.

The volume pot on the Adcom (I like Adcom) 345 is barely off 0 and the common wall neighbors start running up and down the stairs (pretty stupid).

Then I get into Mr. Pass' designs and I'm thinking jack up the bias, rip out a couple of follower pairs and settle for a decent 20W of class A that lets the 345 preamp get into a better space without having to double the cubes of the stock 5500 heatsinks (and really ******* off the guys next door).

I take it I may have reduce the gain of the 5500? Anyone tried this?

I have professional electronics experience but as a mere pleb rather than designer (and in audio).

Please don't tell me to get a smaller amp; I like the physical presence of the 5500 and it gives the cat somewhere to lay (but not for long ).


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Old 11th June 2010, 07:12 PM   #2
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Join Date: Jun 2005
Location: Sacramento, CA
I do have a thought - it may be insane, but I'll let you judge.

1) Reset the power transformer's taps as if it's running off 220VAC, yet run it on 110VAC. This will reduce your rail voltage by 1/2 decreasing output to 1/4 original. Also reduce F3 (primary fuse) from 12A to 6A to keep the current protection accurate.

2) To reduce the gain, simply use a voltage divider on the input to reduce the signal seen by the amp. That, or add a 49.9K resistor in parallel to R12 (49.9K Rfeedback) to reduce the amp's gain by 1/2.

3) Play with P2 (5K pot) to slide the bias up towards class-A. You could go as far as altering R23 (2.74K) and P2 to further change the bias.

All of these mods will effect the amp's performance, DC offset, rails, efficiency, stability, etc. But if everything is done right, you just may reach your goals and end up with a big, hot, stable class-A amp.

Any comments from others on this forum?

If you need the 5500's schematics, email me and I will send them to you.

"Believers cling to the myth despite the evidence, reinterpret the myth to suit the evidence, or lie about the evidence to support the myth." "To err is human; to blame errors on external factors is even more so."
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Old 11th June 2010, 10:24 PM   #3
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Location: Florida
I have a even better idea I'll send you my Adcom GFA-555 with new caps and you send me you GFA-5500. On the other hand a smaller Adcom,Rotel or Parasound can be a lot simpler than making a mod on such a fine amp.
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Old 11th June 2010, 11:51 PM   #4
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Thanks Paul, I like those ideas. However, just read the service manual and the fixed power lead model is 120 only.

I got the schematics after I shorted the damn power supply with a 'scope probe a few years back.

This could be an interesting project but I may have to get another amp 'cause I see smoke in my future.

Another way to reduce voltage would be too disconnect the neg rail and connect the drains to ground? This would leave the front end voltage intact.

I don't want to increase the amount of feedback to limit gain. I will play with the input voltage divider.

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Old 11th June 2010, 11:55 PM   #5
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Location: Portland, OR USA
Gotcha embrown. I really like the amp I would just like it to work a little harder.

I may just try building a couple of class A mono-blocks.....
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