Heathkit AA-1800 Stereo Power Amplifier - diyAudio
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Old 15th February 2008, 10:16 PM   #1
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Default Heathkit AA-1800 Stereo Power Amplifier

Hello,

I wanted to start this thread to discuss the Heathkit AA-1800 Stereo Power Amplifier.

So, I guess my first question would be:

Does anyone know what the voltage ratings are for the power transformer? I can not find it in my manuals nor is it located on the unit itself.

I was considering perhaps making separate power supplies for each channel and locating the supplies in a separate chasis.

My Capacitors need to be replaced as they are original and I am sure are out of tolerances. So, I figured I would considerably increase the uF values ( ie, larger and/or more caps). That got me to thinking about separate supplies, basically for a project to do.

I am sure I would have to have a transformer made if I where to go with the same size. So, I was considering a new pair of toroids??? I would hate to gut a working amplifier to get a transformer from it. I own 3 now and just don't have the heart!

Recommendations/Suggestions?

Thanks!

Ed
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Old 6th March 2008, 07:44 AM   #2
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I got a phone call today from someone looking for one of the power transformers.

I remembered a fairly high DC voltage, I thought +/- 90 VDC under load, then read on audiokarma +/- 110 VDC.

Anyone know the current rating of the transformer, or the 'power rating' of the amplifier?

Thanks

Murray
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Old 15th January 2010, 05:31 PM   #3
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I may finally begin to tinker around with the above mentioned project.

Question ...

Should replacing all electrolytic caps be the only things that get changed? Or should I be looking into replacing resistors, transistors, etc.,?

The question is open to any and all even if you don't have this particular amp. It is a general question on amplifier overhaul.

Thanks!!!!

Ed
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Old 16th January 2010, 12:18 AM   #4
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That is all I would change unless I wanted to upgrade something or had a problem.

I overhauled & modded a Hafler (DH-200 or '220? I can't remember) for a friend.

It started out with a problem, thumping and muting or something (not confirmed, just took friend's description and asked Hafler support at the time). I thought it was strange, but they told me to replace the bipolar 'long-tailed pair' transistors in the input stage...that they may have become 'gain mismatched'. They were cheap enough, but I thought it was a very peculiar thing to deteriorate.

While I was in the box, I suggested some mods and he said, yeah go for it...nothing esoteric, just 'in principle' upgrades. I replaced all the carbon film resistors with 'ordinary' 1% metal film ones, replaced 10 uF electrolytic coupling caps with some 'on-hand' film caps (absurdly larger; 3 x 3.3 uF 250 VDC, no-name polyester as I had a gallon milk jug full of them from a flea market and had never used them for anything).

Then decided to take the filter caps out as they looked a little worse for the wear, and put separate rectifiers & filter cap circuit boards with the Hafler single power transformer...those came from a broken main circuit board for the Heath AA-2500/Harman-Kardon whatever model they called it.

I think all that was done without even checking to see if the original complaint was fixed and took me about 6 months as I was too busy. I was worried by that point that I'd gone overboard and would not be able to figure out what condition it was in.

But it worked! That was the main improvement. I hadn't heard it before, and I'm not sure he remembered the last time it sounded normal, so we had no choice but to call it a success. I can't remember if it was 20,000 or 40,000uF total for two channels, but it did weld the power switch contacts. I was fine with that because I left my equipment on all the time anyway. By that point he had his father's tube amp and I got to use the Hafler for a while...actually still in my basement...I should figure out a fix for the rocker switch and return it but he hasn't been asking...been 10 years at least, so don't ever let me work on anything for you :O(

Murray
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Old 16th January 2010, 02:38 AM   #5
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Thanks for the reply. I have it upstairs here and will take a look tomorrow. Gonna take her a part and apply some TLC! Already got the manual and schematics out so I can easily identify caps, etc., Since it was a kit not built by me, I will also give the boards a real good cleaning and check for "ify" solder connections.
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Old 17th January 2010, 08:55 AM   #6
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Are you building a copy of the AA-1800 or refurbishing an existing unit?

I bought one of Ebay, sans relay/protection board, as a way to build the Leach Superamp without having to scrounge up all the necessary parts and design a chassis. First I'll need to determine how to power up the amplifier and test for problems. Then start replacing the small electrolytics and resistors with better quality parts. Priority is low and more oriented towards learning about this particular version of Leach's design, rather than getting a reliable listening amplifier. I've been using Leach's regular amplifier design since the 70's with great success, maybe too much success. It's kept me from working seriously on any other large power amplifier design.

I've got a copy of the assembly manual if there are any questions.
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Old 17th January 2010, 02:37 PM   #7
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Thanks for the reply!

I am actually refurbishing one of three that I have.

"I bought one off Ebay" ... A whole non-working unit? Do you have the transformer and are willing to part with it? I would purchase it. That is, if you are going to use an new transformer for your project.

Thanks!

Ed
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Old 17th January 2010, 11:56 PM   #8
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Can anyone suggest a good brand that won't cost a bundle to replace ALL my electrolytics with? I am looking for the following:

Protection Circuit Board:

Electrolytic
(2) 2.2 F (axial)
(1) 100 F (axial)
(1) 10 F (Tantalum)
(2) .062 F (Mylar)

Output Amplifier Boards:

Electrolytic:
(4) 10 F nonpolarized (radial)
(2) 220 F nonpolarized (radial)
(4) 100 F (axial)
(4) 220 F (axial
(2) .2 F (Mylar)

Should I stay with Aluminums (OEM to this design) or has anyone had any positive and noticeable experience with films?

Thanks!
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Old 18th January 2010, 04:17 AM   #9
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I'm looking to fully restore my unit, which means eventually replacing the missing relay/protection board with a duplicate on perfboard; no way I'm parting out any of this amplifier! I plan to 'shotgun' the output boards' capacitors and resistors, but keep the power supply components unless something is defective. Maybe I will change the supply design at some point if I decide the layout/grounding is less than ideal. I haven't looked at it enough to make any decisions on that.

On parts: I use Black Gate NX or Nichicon ES for nonpolar electrolytics. If the 10uF nonpolar is an input coupling capacitor, a 4.7uF polypropylene film cap should do in its place. I wouldn't replace anything on the protection board unless it's clearly defective, but use the best quality (high temp rating) if something is bad. I've noticed over the years that many of the axial lead electrolytics that Heath used had popped their vents even though almost none of them were outright defective.

I'm having a bit of trouble identifying >good< axial electrolytics; back in the day I'd order up some computer-grade Spragues and be done with it. I guess on the output board you could use radial lead caps with the leads bent out to go into the holes, but that's a little non-neat to my eye.

Hopefully you've got some basic test equipment and access to a distortion analyzer. I can't work without an oscilloscope on the analyzer's scope output to see what the distortion/noise/hum looks like. A simple meter reading tells too little.

I'm thinking a 120 center-tapped secondary (isolation transformer, essentially) with around 750 watt power rating would do for the power transformer, but it seems like there are some other voltages required as well. Got to find the manual--I moved last year and some things still haven't reappeared.

Last edited by Damon Hill; 18th January 2010 at 04:26 AM.
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Old 18th January 2010, 05:03 PM   #10
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Thanks for the advice! I will look up Black Gate and Nichicon and see what I can locate.

Ed
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