Heathkit Amp & Preamp


I just found this forum today. The looks like there is a wealth of knowledge among the good people posting here. Please enlighten me.

My question is about a Heathkit amp AA-1800 and preamp AP-1800 that I have laying around the house. My father built these about 20 years ago and I’ve now got them. He gave them to me several years ago and I used them for a while but the preamp started having some issues (not wanting to play both channels if you selected “stereo”). I’ve hooked up the amp in another setup and both L & R seem to be working fine. I guess my question is…are these units any good ? Is the preamp worth getting fixed? Are they quality units? Is there any market for these if I wanted to sell them? I know nothing about them so any information would be great.

Thanks for any help I can get.

Hey Murph

I'm up in Los Alamos. Would be glad to help you (gratis) with trouble-shooting the preamp. Shoot me an email if you're interested.

By coincidence, I recently came across a schematic for the amp. It's not a bad design if I remember right, but might benefit greatly from some new parts and a few other tweaks.



2001-06-01 4:53 pm
Murph, you may want to poke around or even post your questions at the Audio Asylum site (www.audioasylum.com). Although it didn't really help me getting info on my Powered Advent Loudspeakers, I noticed there was some discussion about old Heathkits.
If you don't want to go through the hassle of joining, I can ask the question on your behalf.
I like Grey's idea of fixing it, probably a great way to learn. The Heathkit manuals usually had great troubleshooting sections.
And you could always 'upgrade' it with better caps, etc.
Or you could even (sacrelige!) rip out the guts of the preamp and replace them.

Thanks for the info. I may just use the amp to power an SVS sub that I'm getting and not mess with fixing the preamp right now. If I do get a wild hair and decide to try fixing it I will give you a call Phil. Thanks for the offer to help. Have to love the way poeple on this site offer to help each other out.

BTW, djk its not that I don't believe you... but do you have a copy of that EPA warning about the AA-1800???? If the EPA says I gotta send it to you for disposal, then I guess that's what I'll do.....



2001-02-04 4:23 am
" BTW, djk its not that I don't believe you... but do you have a copy of that EPA warning about the AA-1800???? If the EPA says I gotta send it to you for disposal, then I guess that's what I'll do..... " I always wanted one.Used to own an AA1400.The AA1800 was designed by Leach and is substantially the same as the Super Leach.They bring $300+ on eBay.
Ok guys one more question. Because I don't have any of the original manuals I was wondering if anyone knew if the AA-1800 amp could handle a 4-ohm load. I want to use it to drive an SVS sub and they are rated at 4ohms.

Also, can this amp be bridged? I don't have any experience with other amps and was wondering if other amp that can be bridged have some sort of switch on them that allows you to invert the signal for the second channel? The Heathkit does not have one so I don't know how to invert the signal if I was going to try to bridge the amp. Also, if I understand correctly, bridging the amp to a 4-ohm subwoofer would mean that the amp would have to be able to handle a 2-ohm load. Could the heathkit handle it?

Would I be better off just selling the Heathkit and buying a new Sampson amp that SVS sells??? Any thoughts???

Please help a rookie...
The AA-1800 is 4 ohm stable and is rated at something like 315 watts per channel into a 4 ohm load. This info was given to me Heathkit themselves in the 1980's. About 2 years ago, I traded emails with one of the old Heathkit design engineers who worked on the Pro Series line and was part of the product testing and validation. His amp was used for the initial testing in the lab and even drove a 2 ohm load to the point of distortion.

According to him, the AA-1800 is very underrated and can nominaly drive 350 ohms watts at 8 ohms if its in good condition. Heathkit chose to under rate the product for various reasons ranging from FTC burn in requirements which they couldn't control in a kit to taking into consideration that some of their customers were novices at soldering.

Also, the AA-1800 is more correctly a Super Leach design, with a few modifications. The ones I can think of off the top of my head is the rail voltage being 100 VDC, higher than the Super Leach design and the offset and bias circuits being a bit different. Overall though, it's "the same thing" and the Motorola TO-3 transistors used in the Heathkit design can still be purchased new from Motorola and they are exactly the same TO-3 used in the Super Leach.

This amp is so transparent that it takes on the sound of the pre amp driving it. Connect a tube pre amp and you get a tube sound, but with the dynamics of solid state.

And yes, I own two of them.
I stumbled across this thread and did some research; Walt Jung reviewed the AA-1800 amplifier in the 3/82 issue of The Audio Amateur, with a full schematic posted. I looked it up and sure enough, it's a direct derivative of Leach's amp circa early 80s, with active current sources on the dual diff amps being the only major change. (there have been minor design changes to Leach's Super Amp since then).

So I emailed Leach and he said this:

"No, I didn't do any design work for Heathkit. A former student of ours went to work there in the latter '70s and copied the preamp that I had in Audio in '77. He changed all the npn transistors to pnp and vice versa, but it was essentially the same circuit. I remember a 400 W (or so) Heathkit amplifier, but I don't remember what the circuit diagram looked like."

I never knew this until now and I am kicking myself. I could have afforded the AA-1800 kit at the time. I've been using Leach's design for all these years, but while my build version is reliable, it's ugly and I've always wanted a professional-looking Leach amplifier but lack the tools and knowledge to build sexy chassis.


Well, I scored a modular instrument chassis for $2 at Boeing Surplus that could be the basis for an amplifier project. That at least looks like serious kit.

There >were< several commercial versions of Leach's amplifier; I briefly worked for Electronics One which built a few dozen in two models before abruptly going out of business. Leach himself felt very burnt by the experiences and is leery of ever trying it again. He's not keen on MOSFET designs either, but perhaps I'll nudge him a bit on the subject and see what happens.

If your still pinng for one of the Heathkit amps, they occasionally become available on Ebay and their price range is generally between $250 and $500. The price seems to depend more on the time of year than the condition of the amp.

Thankfully, Heathkit for the most part used generic parts which were still available the last time I checked. Out of necessity, I located an excellent repair facility that specializes in repairing Heathkit equipment, which has greatly extended the life of my amps.

The ONLY issue I've had strictly with the amp has been with the internal audio wire. For whatever reason, the copper wire severely oxidized in one amp but not the other.

The local power company was responsible for blowing out my amps at least twice before I invested in some very expensive power filtering and surge protection devices. About 6 months before we moved, the power company replaced the main lines, pole mounted surge suppressors and caps, and all insulators coming into the subdivision.
Taking the thread back to it's origination point, I still have my AP-1800, as well as the manual and schematic.

There were at least two versions of the AP-1800, mine being the second (I had a friend who had the entire Pro Series rack). The difference between the two was extremely small - something like a DC blocking capacitor (10 uF, NP electrolytic) added between two stages.

The front panel switches (especially the mode switch IIRC) need to be used fairly often to clean off oxidation.

Audio Amateur never reviewed the AP-1800; there was a letter in issue 5/82 titled "Unsung Preamp" that addressed that issue. The letter was written by my wife.
Pro Series Rack

I have the whole series pro rack AJ 1600 , AP 1800 , AA 1800 and the AD 1701.
The most trouble I have had is with channels dropping out intermittently. After a lot of hair pulling and cuss'in and lots of other stuff , I found the in and out RCA jacks are ABSOLUTE CRAP.
The solution is to take the unit apart so that you can get at the jacks with a soldering iron. You will notice that the jacks are two piece , the solder tab is crimped to the shell . I take a soldering iron and flow a little solder into the joint. To do this heat up the jack so the solder flows when you touch the jack with the solder. A little goes a long way , don't short the jack , ask me how I know , just enough to make a good connection. This cured the problem.
On the pre there is not enough room to change jacks but on the tuner and the amps I will replace the jacks with something better.
Hope this helps
There was an AA-1800 on Ebay this last week, but I had to drop out of the bidding as I didn't want to go over $300; it finally went for about $437, I think. Oh well...more will surely turn up in time and maybe I'll get lucky.

Not too surprised by corroding wiring, although that really shouldn't be happening. Had I won the thing, I was prepared to do a major cleanup and rewiring as necessary, plus replacing bypass and coupling caps, and doing a few tweaks like fast recovery rectifiers. I would leave the basic amplifier circuit alone, other than thoroughly testing it. Though it would be tempting to replacing the existing output transistors with MJ21193/94s to see if there was an audible difference.

I ended up blowing all my money (and then some) on this:


The IMD capability was what I needed, and the AA-1800 would have been an ideal test subject. However, I'll just put my existing Leach amplifer, and Brian Bell's PCB implementation of Leach's design through the wringer instead.

I've found a few schematics on several other of Heath's 'high-end' amplifiers, but the AA-1800 seems to me to be the best design of the lot.
For whatever it's worth, the AA-1800 has a total of 1 rectifier and no voltage regulators. Voltage stability on the rails is very dependent on AC feeding the unit. For the most part, this isn't uncommon for high power amplifiers and doesn't have a major impact on sound reproduction.

The bias and offset adjustments have little effect on sound quality, but do have a major effect on how warm (hot) the unit runs. These settings are very temperature sensitive and from experience, don't even think of setting them until the unit has been allowed to idle for no less than an hour.

As for bypasing caps. You may gain something, but there isn't a single cap in the audio path except for the 10 uf cap connected to the input terminal in each channel. Even at 26 years of age, my two amps are absolutely dead silent at idle.

Sound wise, they favorably compare to the high end paladium amp.
>>Hey rrcrain:
>>Where is the Heathkit Forum? Its like it went awol.
>>Have you done any re-capping on your AP 1800? I am still in >>the process of deciding waht to use and what to replace. Prob. >>just the 100 mf 16V caps to something better. Everything else >>sounds ok. What do you think?

The Heathkit forum has been extremely inactive ever since EZ-board crashed a few years ago and lost a wealth of knowledge. For all intents and purposes, it no longer exists.

I've considered recapping my AP-1800 but haven't done any more than that. Given it's age, I have absolute faith that some, if not most of the electrolytics have drifted out of tollerance and given the space constraints, a recapping would be limited to replacement with high grade electrolytics only.

I recently replaced the 10uf electrolytic coupling caps in my two AA-1800 amps with 10uf poly. I became aware that certain quencies/instruments were no longer present and this restored the missing frequencies.