The amp is a direct couple design with no impedance matching between stages. I can tell you with absolute, perfect accuracy that an overload on the input will cause a cascade failure of every power transister in the channel plus a few more for good measure.
never, never, never make or break any connection to this amp when it's turned on.
Experience has spoken.
Thankfully, Heathkit still existed when I learned this lesson.
Reading the above note, this amp does indeed have a DC offset. Offset is set to zero VDC with bias set to 50mv, once offset is set, adjust bias to 35mv. Both are set after the amp has been on for no less than 20 minutes. My expereince has been an hour or more is best.
never, never, never make or break any connection to this amp when it's turned on.
Experience has spoken.
Thankfully, Heathkit still existed when I learned this lesson.
Reading the above note, this amp does indeed have a DC offset. Offset is set to zero VDC with bias set to 50mv, once offset is set, adjust bias to 35mv. Both are set after the amp has been on for no less than 20 minutes. My expereince has been an hour or more is best.
I actually have Three (3) of the AA-1800's. There is a total of Five (5) in the family! We love them!
You would have to spend a serious amount of money to get an amplifier built on this platform, that is a non switching power supply type design.
Heathkit's power ratings where very conservative to say the least. There was an old Stero Review article on this amplifier in the early 80's by Julian Hersch who bench tested it. He got over 500 Watts per channel in 8 ohms just prior to clipping. The amplifier continued to output over 800 watts per channel in 4 ohms.
I am using one for a pair of Bohlender Graebner RD 75 Ribbons and another for a pair of Speakerlab 30's as subwoofers (since they have the Nesterovic woofer system, the bass is AMAZING!) My third one is idle at the momet since we are in an apartment and I could not take my Speakerlab DAS 8's for the rear channel.
I would like to get 2 more once we are in a place where I can support a theter set up. Running 7.1 surround with over 4,000 watts should be intersting.
True, the components are older but with some cleanup and reconditioning, I think they still rival all the computer controlled nonesense out there, unless of course you are looking at McIntosh or Krell!
If you have one of these amps, they are keepers for sure.
Enjoy!!!
You would have to spend a serious amount of money to get an amplifier built on this platform, that is a non switching power supply type design.
Heathkit's power ratings where very conservative to say the least. There was an old Stero Review article on this amplifier in the early 80's by Julian Hersch who bench tested it. He got over 500 Watts per channel in 8 ohms just prior to clipping. The amplifier continued to output over 800 watts per channel in 4 ohms.
I am using one for a pair of Bohlender Graebner RD 75 Ribbons and another for a pair of Speakerlab 30's as subwoofers (since they have the Nesterovic woofer system, the bass is AMAZING!) My third one is idle at the momet since we are in an apartment and I could not take my Speakerlab DAS 8's for the rear channel.
I would like to get 2 more once we are in a place where I can support a theter set up. Running 7.1 surround with over 4,000 watts should be intersting.
True, the components are older but with some cleanup and reconditioning, I think they still rival all the computer controlled nonesense out there, unless of course you are looking at McIntosh or Krell!
If you have one of these amps, they are keepers for sure.
Enjoy!!!
Finally nabbed one of these amps on eBay; it's missing the top cover and the relay/control board in addition to the usual scratches and dings. But I expected a fixer-upper and will get to it in a couple of months when there's time for this new project. Don't know if I will attempt to build a new board or just jumper out as needed.
I plan to replace all the carbon film resistors with high quality metal films and similar other quality parts, so long as they don't break the budget. Also soft recovery rectifiers and a snubber. Any modifications to the circuit design will have to wait until I've thoroughly tested it. I'm very curious to compare the sound to my 'regular' Leach amplifier.
Yeah, you couldn't buy a new amplifier of this power and quality these days for less than $600 - $1000.
Who makes high quality axial lead electrolytic capacitors these days?
I plan to replace all the carbon film resistors with high quality metal films and similar other quality parts, so long as they don't break the budget. Also soft recovery rectifiers and a snubber. Any modifications to the circuit design will have to wait until I've thoroughly tested it. I'm very curious to compare the sound to my 'regular' Leach amplifier.
Yeah, you couldn't buy a new amplifier of this power and quality these days for less than $600 - $1000.
Who makes high quality axial lead electrolytic capacitors these days?
The relay control board is missing? That's not exactly good news given what it does. This is/was a patented design that performs several functions to protect the amp as well as the speakers. It's a time delay circuit that connects the output after 8-10 seconds to prevent thumps. It monitoes output and disconnects the output in the event of a massive transient or extreme low bass approaching DC (4 HZ I think). Without this protection circuit, the amp is very capable of cooking your voice coils. My opinion is to somehow replace it.
Over the last 5 years, I discovered the coax wire was corroded and the input coupling caps finally needed replacing. The power supply caps will probably be next on my list.
There's no voltage regulation on the rails so changes in line voltage will affect rail voltage.
Over the last 5 years, I discovered the coax wire was corroded and the input coupling caps finally needed replacing. The power supply caps will probably be next on my list.
There's no voltage regulation on the rails so changes in line voltage will affect rail voltage.
For initial testing, I won't need the relay board but for general use I will have to rebuild one on perf board. I have a good copy of the manual with the layout/component photo, so I think I can duplicate that. Something will have to be done since the chassis relays are powered from that board.
The unregulated supply isn't much of a problem since the design should have very good power supply decoupling and rejection, and a totally regulated supply would be impractical. I've thought about a regulated supply for Leach's circuit designs up to and possibly including the driver stage, but the additional complexity hasn't motivated me to do anything about it.
I've never worked with a solid-state amplifier with 100 volt(!) rails before; that's almost scary. Conveniently, the individual channels unmount easily as a unit, so it should not be difficult to service and modify them. Some adapter plugs and cables will have to be built up, assuming I can find the mating connectors.
The unregulated supply isn't much of a problem since the design should have very good power supply decoupling and rejection, and a totally regulated supply would be impractical. I've thought about a regulated supply for Leach's circuit designs up to and possibly including the driver stage, but the additional complexity hasn't motivated me to do anything about it.
I've never worked with a solid-state amplifier with 100 volt(!) rails before; that's almost scary. Conveniently, the individual channels unmount easily as a unit, so it should not be difficult to service and modify them. Some adapter plugs and cables will have to be built up, assuming I can find the mating connectors.
The mating connectors are still current, or were 4 years ago. I rewired my two amps and installed new connectors while I was at it. Check Radio Shack and they should have what your looking for. Most of the old copper in one amp was turning green so I redid both of them. Best guess is salt air is to blame on the one amp. I assembled it while in the Navy onboard ship while at sea.
If you have the manual, there should be instructions on building test cables to extend each channel comfortably from the chasis. Also, the amp should be initially tested with a light bulb in series with the main. That's also in the manual.
The rails will drift if your house voltage drifts. Some places (like where we live), this is an issue. We are distinctly rural and have invested in a Monster Power voltage stabalizer and line noise filter. Can't say that the stereo sounds better but the TV absolutely looks better.
My two amps are absolutely dead quiet, no hum or hiss and take on the sound of whatever preamp drives them. I just purchased a Rogue 99 Magnum and it's quite the improvement over my AP-1800 which I'll need to list on Ebay at some point.
Good luck with your project, and if all else fails, I may know where you can have the amp professionally repaired.
If you have the manual, there should be instructions on building test cables to extend each channel comfortably from the chasis. Also, the amp should be initially tested with a light bulb in series with the main. That's also in the manual.
The rails will drift if your house voltage drifts. Some places (like where we live), this is an issue. We are distinctly rural and have invested in a Monster Power voltage stabalizer and line noise filter. Can't say that the stereo sounds better but the TV absolutely looks better.
My two amps are absolutely dead quiet, no hum or hiss and take on the sound of whatever preamp drives them. I just purchased a Rogue 99 Magnum and it's quite the improvement over my AP-1800 which I'll need to list on Ebay at some point.
Good luck with your project, and if all else fails, I may know where you can have the amp professionally repaired.
Thanks for the info, guys. I sent Spectrum an email but got no response. Oh, well. DJK ... could you give me the contact information for who you used in Boston? MIght I ask what you had him do to your amps? What did he end up charging? I think putting a few hundred into each of my amps would be well worth it. THANKS!
Hi Sithlord2007,
I've rebuilt a couple, and I can say a couple things about them.
The performance is very good.
A proper overhaul is extremely time consuming. I can service one quickly, but that will not peel back the years as far as performance goes. If you go the extra step, the original design can sound better than new. I have very, very pleased customers.
Understand I'm not trying to get your business. I hate cross border stuff for one, and I'm really busy. The matching AP-1800 is undergoing a mild redesign in a couple places and has the promise of being a really great preamplifier. To be honest, I wasn't interested in these until I got one in for overhaul + improvement. There are 5 in the wings now. I'm looking forward to this project's end.
The message is that these are well worth some extra effort, as long as the technician is good at what they do - and clean.
-Chris
I've rebuilt a couple, and I can say a couple things about them.
The performance is very good.
A proper overhaul is extremely time consuming. I can service one quickly, but that will not peel back the years as far as performance goes. If you go the extra step, the original design can sound better than new. I have very, very pleased customers.
Understand I'm not trying to get your business. I hate cross border stuff for one, and I'm really busy. The matching AP-1800 is undergoing a mild redesign in a couple places and has the promise of being a really great preamplifier. To be honest, I wasn't interested in these until I got one in for overhaul + improvement. There are 5 in the wings now. I'm looking forward to this project's end.
The message is that these are well worth some extra effort, as long as the technician is good at what they do - and clean.
-Chris
Thanks Chris. What would you recommend in terms of the best bang for the buck? Obviously cap replacement on the boards and cap replacement on the PS? The amps are 30 years old. Any transistor and resistor upgrades worth it? I do have two bad output transistors in the left channel and will just replace all 16. Other than that, not sure what else I can do besides clean up poor solder connections and replace some wiring here and there. The amp, as you know, does not have many components to it. So, again, in terms of that, where have you gone with yours?
Thanks,
Ed
Thanks,
Ed
Hi Damon,
The fT on those is lower than the MJ15024 types. The SOA on those you mentioned are better. You only need to do one channel, the other is fine.
Hi Ed,
Before you start, you must have new thermal compound and new mica insulators for the outputs or any other power devices you test and remount. Go ahead and replace the electrolytic capacitors. Make sure the replacements fit on the PCB!!! Replace axial capacitors with like, same with the radial capacitors.
-Chris
The fT on those is lower than the MJ15024 types. The SOA on those you mentioned are better. You only need to do one channel, the other is fine.
Hi Ed,
Before you start, you must have new thermal compound and new mica insulators for the outputs or any other power devices you test and remount. Go ahead and replace the electrolytic capacitors. Make sure the replacements fit on the PCB!!! Replace axial capacitors with like, same with the radial capacitors.
-Chris
Murph,
I owned a very similar Heathkit AA-1640 stereo amplifier many years back and it was excellent at powering up my AR9 speakers, which had 4-12 in woofers in two large vertical columns. The combination of the amp/speaker put out unbelievable base that would really put a 'thump' on your chest every time - mostly due to the many parallel output transistors this amp had on each channel...therefore it can swing large amperage (good for moving big woofers), but as stated it doesn't do low impedance very well. The AA-1640 was with a silver face and had beautiful 'internally lit' meters that the AA-1800 lacks, but other than that the circuits were nearly identical, I believe.
I also found that by bypassing the small 'pre-driver' transistors on the 'pre-driver-circuit-board' in the very front of this amp, it produced a slightly sweeter and cleaner sound. These amps don't command BIG prices, but they would, if working properly, be a formidable sub-woofer amp on any modern surround sound system.
You should make sure there is no substantial DC offset present on the outputs before hooking it up. A DC offset at the outputs of + or - 40 mv or less is fine but large DC offset can damage (blow out) speakers. Amps that are not working properly or have not been serviced for many years can exhibit damaging DC offset.
I owned a very similar Heathkit AA-1640 stereo amplifier many years back and it was excellent at powering up my AR9 speakers, which had 4-12 in woofers in two large vertical columns. The combination of the amp/speaker put out unbelievable base that would really put a 'thump' on your chest every time - mostly due to the many parallel output transistors this amp had on each channel...therefore it can swing large amperage (good for moving big woofers), but as stated it doesn't do low impedance very well. The AA-1640 was with a silver face and had beautiful 'internally lit' meters that the AA-1800 lacks, but other than that the circuits were nearly identical, I believe.
I also found that by bypassing the small 'pre-driver' transistors on the 'pre-driver-circuit-board' in the very front of this amp, it produced a slightly sweeter and cleaner sound. These amps don't command BIG prices, but they would, if working properly, be a formidable sub-woofer amp on any modern surround sound system.
You should make sure there is no substantial DC offset present on the outputs before hooking it up. A DC offset at the outputs of + or - 40 mv or less is fine but large DC offset can damage (blow out) speakers. Amps that are not working properly or have not been serviced for many years can exhibit damaging DC offset.