Harman Kardon HK-870 power amp: What you say?

[dlucy]

When I got the 870, the bias and idle current where way high, and the top plate was cooking, enough to burn your hand after a hour or so. The original owner had never had it serviced.
The PA2400 is a real sleeper.
Both the PA2400, and 870 (Cit 22 and 24 as well) bare minimum should have all their solder connections reflowed, outputs re-heat sinked, and all the push on connections cleaned and checked, as they run higher current then most amp, and the extra heat generated takes it's toll after awhile.
They run a wide open, direct coupled design, so it's something to think about.

I'm glad this thread popped back up as I've got a PA2400 running my Kappa 8's and a hk870 sitting on the to-do stack. I need to add the above advice to my list. Thanks again @automojo

 

[automojo]

Cool!
The 2400 it's a much more cleaner and dynamic amp in my opinion.
Nit harsh by any means but less colored then the 870.
But to be fair most 870's are fairly old and being they run even hotter a full recap and redo would have to be done for a fair comparison.
I use a thermostatically controlled fan on top of my 2400.
Either amplifier could benefit from this.
I love Harman Kardon amps... for the money you can't beat them and you would have to spend thousands more to get this type of performance and circuit design.
But both of these amplifiers could've benefited from larger heat sinks.
I'm thinking that had more to do with the bean counters then the designers.
H/K seemed to always have a battle with this. Good news is they rarely compromised on the sound. The other stuff you can easily work around.

 

[dlucy]

I'm keeping the PA2400 (based on your opinion here and my testing said opinion against my ears) for the Kappa 8's (they sound great together) and one day I'll buy a second PA2400 to do biamp or similar craziness.

I got a hk870 in a great deal and I just need to recap and beef it up.

A pm665 also fell into my lap (back when I had a regular paycheck) and I'm keen it get it recapped and repaired and compare them.

Keep the experiences with these coming in. They're great!

 

[Randy Houk]

Hi, folks. New guy here to the group, but very long time interested in audio. I'm also a musician, clarinet in a local community band.

Back in 1990 I ordered from Heath an AA-2500 kit, but received one that was fully assembled. It's always worked well, though I haven't used it continuously these 28 years. A couple of years ago I found out that it's really an HK 870, so I downloaded the service manual. I'm an electronics engineer, so I cannot get enough of looking at amplifier schematics. First thing I did was adjust the offset and bias current. That brought the temperature down quite a bit, and it has stayed that way since. It sounds really fine with my Magnapan 1.7 speakers.

I'm now reading the Douglas Self amplifier book, so of course I've been analyzing the 870 circuit against the Self design preferences.

Anyone know of a Harmon Kardon document describing the design details of the 870, or the PA2400 which also appears to be a favorite?

 

[automojo]

Hmmm, hard to say, maybe something stabilized. The 870's run hot. I would check the bias, and idle current. It's probably drifted.

 

[automojo]

Yes, 4 pairs: 2SD845/2SB755

These are no longer available.



From my experience - replacing main caps has no impact on the sound, provided that new caps are the same capacity and good quality. Changing capacity may somehow affect the sound, depending on the design - some amps could have a little too low capacity there so increasing their value might help, but there are units where increasing power supply filter caps gives nothing.

Regarding quality, basically any new, modern capacitor should be at least as good as the best capacitors available 20 or 30 years ago. I usually buy ordinary but branded caps from trusted source to avoid Chinese fakes.

I have heard the same....the bigger caps help prop up the supply rails and prevent sags, but aren't going to really make a huge impact on bass, if any.
Some amps seems to be able to provide more current/power, or basically drive speakers better-some of this has to do with ESR, in both the amp and speaker. So some amps drive your speaker better on the lower end, usually the more difficult region.

 

[dlucy]

Anyone know of a Harmon Kardon document describing the design details of the 870, or the PA2400 which also appears to be a favorite?

Welcome to AK, Randy!

I have collected and restored a good number of Heathkit test gear and now an audio gear piece. One really nice thing about the kit form Heathkit gear is they usually have a "theory of operation" section in the assembly manual. Really good reading if you're like me and can solder and run a DMM and know how to lookup modern replacement parts from the original devices' datasheets. Thank you, Heathkit!

If there _was_ a kit form Heathkit hk870, it may have a theory of operation section. Maybe try finding an assembly manual on eBay or the web.

I have torn down and started the restore process on a pair of hk870s. They are not complicated, so replacing the electrolytic caps is fairly easy. Pulling and testing all the semiconductors is a relatively easy second step. Whatever you find in there will have a modern equivalent. AK in general can help with that and I'll be happy to share if needed.

Both of my hk870s had poorly operating power switches. THAT was actually a little hard to discover and once I'd opened the first one up, the scorching on the inside made the problem clear: high current arching inside the switch at every power on. Cleaning the switch up, reassembling it and installing the "triac power switch mod" is one very good way to push a repeat of this problem decades and decades into the future.

There are a few 870 threads on AK. You may want to search deeply, perhaps use Google with "site:audiokarma 870", and ask anyone who has posted good info in the past.

One thing you'll want to do right away is to make your own thread in the Solid State forum. That will let your questions and AK answers get concentrated in one spot. Ive had the best success when Ive done that.

Keep on asking questions! An excellent way to go!

Doug

 

[Randy Houk]

Welcome to AK, Randy!

Thanks for the welcome, Doug!

The 870 (aka Heath AA-2500) and my Adcom GFA-535II have been reliable performers here for many years.
Haven't yet had any trouble with the 870 power switch, but I power it on via the Adcom preamp. I did recently
have a power switch issue with a much older Akai AA-1010 receiver. I just soldered across that power
switch, and control via power strip.

I've recently rekindled my interest in valve audio with the acquisition and restoration of a lovely Dynaco ST-70.
There are certainly lots of ways to improve that old room heater! Surprisingly, it drives the Maggie 1.7s to a
reasonable SPL.

I also repaired a Quicksilver Audio preamp. That's built like a tank!

Randy

 

[Western ape]

Rwcently I purchased an amp that had problems. I am new to everything and am not really capable of repairing items as yet, amd at the price I paid it is fair to expect a fully functiong piece. So...he has a coupke other pieces available. A combo deal of a Harman/Kardon 870 amplifier and Harmon Kardon 725 preamplifier, for basically twice what I spent. Or a Marantz 1060 by itself for three times what I already spent? Any idea?