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Harman Kardon HK770 Thread

Connor G

Connor G

Ear conditioned
Subscriber
Ever since restoring and listening to the Harman Kardon hk770, I have been an extremely avid proponent of them. I can't recommend them enough to people looking for a relatively cheap power amplifier that can drive efficient tower speakers, bookshelf speakers, and even headphones with ease.

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The HK770 was one of Harman Kardon's highest end consumer models in the early to mid 80's. Being designed by Matti Otala, it's design was based around his findings on T.I.M. and I.I.M. distortion.

More info can be found here:
http://www.thevintageknob.org/harman_kardon-hk770.html

I have restored a few of these amplifiers so far, and have somehow managed to convince enough friends to buy them that I currently have 3 more on my workbench for a full restoration.

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These amplifiers are DC coupled, ultra high bandwidth, use minimal NFB, are fully discrete from end to end, dual mono, have high slew rate (150v/μs!), minimal THD/TIM/IIM, protection circuit out of the signal path... Not to mention a breeze to work on, and they fly very low under the radar, often selling for less than $200!

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Here is a 100hz square wave with the amplifier loaded down into 8 ohms:

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Pretty impressive if you ask me.

If you are looking for a bill of materials for restoration, I have one here which includes new output relays, electrolytic caps, as well as the film caps for the input coupling modification:

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One of the main issues with the HK770 (as well as most other amplifiers from Harman Kardon of this era) is bad soldering. This is one of many broken solder joints on the (very hot) driver boards of the amplifiers:

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This is easily remedied by reflowing all of the joints on the driver boards.
Here is a before and after view of the reflow:

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As for the sound, the HK770's are extremely dynamic (even given their conservative 60WPC power rating)
They lay out a very precise and believable stage with great imaging, crystal clear and non-fatiguing highs, liquidy midrange and very fast and authoritative bass. They aren't the last word in absolute resolution, but they are much more than enough for most people looking for a sub $500 amplifier all-in.

I have also personally used HK770's to power high end headphones very successfully, as the noise floor is almost non-existent. They especially work well with power hungry planar headphones. Even electrostatic headphones with a transformer based energizer sound exceptional on the HK770.


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The hk770 is an extremely versatile amplifier. It has no issues running down into 4 ohms, and have had multiple speakers which dip closer to 2 ohms that run perfectly as well. The hk770 is a class AB amplifier which has an idle current of around 24mV. This means that the amplifier while being able to run in class A for the first few watts, actually runs cool until pushed. I’ve noticed the hk770 is very revealing of the signal chain and is very dependent on which preamplifier you have hooked to it. If you’re using the power amplifier with a passive preamp or plugged directly into your source (DAC, CD player, etc…), you will not get as good of a performance as you will with a preamplifier that has an active buffer and gain stage.

Here is a quick demo of a 700 series set I restored recently:


Alright, rant over. Hopefully I have convinced at least one person to give an HK770 a shot if they're in the market for a very cheap and very competent "High end" amplifier :)

Anyone else have experience with the HK770's (or 775's)?

I have since started offering my restoration services for the hk770 (and any other amplifier for that matter) for people looking to have theirs restored the right way. Shoot me a PM if you’d like me to take a look at yours.
 
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There's an as acquired hk770 here that was way, way less than 200 smackers (albeit quite a few years ago). EDIT: Just to be clear, it sounds fine to me, but it's utterly untouched so it's certainly not up to snuff.

Thanks for the BOM; maybe I'll tackle a rehab of it one of these days.

DSC_9173 by Mark Hardy, on Flickr
 
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Wow, nice looking gear.
I had an HK receiver on my office system that was nice - but intermittent.
Probably just needed a good deoxit cleansing to make it right, but now its on a shelf downstairs until I find time,
while an Advent Model 300 Receiver takes its place feeding my computer monitors (JBL L20Ts).
 
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Certainly true of the slightly later hk670i receiver (in terms of troublesome switches).

Yeah, like most of the soiled state stuff here -- it's not like I actually use it... but there's still some of it I like. I've had a fondness for hk stuff for as long as Yamaha & Polk Audio: college days.

wouldn't mind having a Citation Two, but at today's prices, that's not likely to happen.
 
I have a 770 and two 775s. The latter are keepers, if I find any more I'm keeping them! Sadly I have no place to use them at the moment.
 
I had the whole stack of 700 series components back in the day. Bought it all new with the exception of the 775 mono amplifiers, and they were a demo. I had the 705 cassette deck, 710 tuner, 725 preamp, 775 mono amps and flirted a bit with the equalizer from same series-cannot recall its number. I also had a 720 turntable with Dynavector 10X4 as well.

The mono amps were very nice, was driving a pair of Magnepan MGI at the time. Also, the 705 cassette deck made some very good tapes too, was a nice piece with the unusual loading drawer. All in all, those pieces were very nice, sounded great, and made a very nice visual stack of components. I miss them and wish I had kept them!

About the only issue I had with them were the little green bulbs in the switches. They blew with regularity and were a pain to replace. At one point I went through all of them and mounted green LEDs. Problem solved, but it wasn't a fun project.
 
I have the HK770 amp and it's a fully functional, full meal deal. The HK 725 pre is in a coma, unfortunately. The amp is the heaviest electronic contraption per cubic inch that I've ever picked up,
I love the pragmatic layout of it. It ony got relegated to the backup mode because a Nikko Alpha III mosfet amp became, well, the alpha unit. Otherwise it'd be the top dog in the pack. It's not the most powerful unit but it is highly capable of presenting clear and virtually distortion free, top to bottom spectral music. I cannot imagine selling mine.
My first stereo purchase was an HK330i from the same era and I loved it until somebody burgled it. (also took my turntable with Springsteen's Born In the USA on the platter. I still have the sleeve - just cuz.) Great little receiver for apartment life with Minimus 7 speakers.
 
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Shhh... don't talk about it ..

Harman is one of the few high quality brands that continues to fly under the nostalgia tax radar ..

I've got 3 sitting in the basement .. they produced some nice sounding stuff, and the Otala amps are a prime sample ..
As I recall they were the first mass marketer to include high current power supplies in their products also ..
 
I really like the HK770’s and 775’s. I have chased them for a few years and always came up short. I own a few pieces of 80’s H/K gear and they have been reliable so far as well as my Citation 12 (overdue for an recapping). Always on the prowl for a clean 770, 775’s and an 870. Good stuff with gobs of real power for their ratings.
 
blademan13 said:
I really like the HK770’s and 775’s. I have chased them for a few years and always came up short. I own a few pieces of 80’s H/K gear and they have been reliable so far as well as my Citation 12 (overdue for an recapping). Always on the prowl for a clean 770, 775’s and an 870. Good stuff with gobs of real power for their ratings.
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Clean 770's regularly come up on eBay, but they usually sell quite fast. Good luck!
 
Matter of fact, I'm pickup up an HK725 pre and HK770 amp this weekend from the original owner. Back in the day, he ran a pair of AR-3A's after he sold his AR2ax's to a friend (still has them) and bought the 3A's. Also included is an HK tuner, which not of the same vintage but similar look, although I don't listen to the radio at home. Would be nice to have the matching tuner but it would never get used...just decoration.

@Connor G Was the HK870 a generation after? Looks like it's rated at 100wpc but drops the meters. Is it of similar design like the 700 series? Also, do you update these if a person was to send one to you? Just curious. ;)
 
avguytx said:
Matter of fact, I'm pickup up an HK725 pre and HK770 amp this weekend from the original owner. Back in the day, he ran a pair of AR-3A's after he sold his AR2ax's to a friend (still has them) and bought the 3A's. Also included is an HK tuner, which not of the same vintage but similar look, although I don't listen to the radio at home. Would be nice to have the matching tuner but it would never get used...just decoration.

@Connor G Was the HK870 a generation after? Looks like it's rated at 100wpc but drops the meters. Is it of similar design like the 700 series? Also, do you update these if a person was to send one to you? Just curious. ;)
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Hi Kip, sorry for not getting back to your PM sooner. I read it the other day and was too busy to respond.

The HK line of gear from the 70’s was mostly pretty good, and the 700 line was some of the best of it’s pack. The 770 is no slouch, and would benefit from a restoration. Common issues include the typical bad caps, dry and cracked solder joints, and conductive glue. I do offer restoration services for these amplifiers, as well as the preamplifier. I am also able to custom retrofit 5 way binding posts and high quality RCA inputs onto the back of the 770 as well. If you’d like, shoot me a PM and we can discuss what you would like to do. I have a video on my YouTube channel outlining the work done to a couple of 770’s I have worked on. It can be found here:
https://youtu.be/UwIiv_JkXXo

I have restored roughly 12 of these amplifiers in the past year, so I’m very familiar with them. When restored, they will last another 5 decades with no issues.


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As for the hk870, it was the direct successor of the HK770, sharing much of the same design. Unfortunately, it was rather sloppily laid out, more complex, and suffered from many more issues than the HK770 did.
 
Does anyone have any hints on the 725 preamp? Mine has a scratchy volume control but it appears to be one of those Alps sealed units. Also, the cover is missing from the relay and I had to kind of abusively clean it to get it functional. I actually am not a huge fan of that unit as it is very limited on inputs, but it would be cool to get the matching stack working again.
 
N8Nagel said:
Does anyone have any hints on the 725 preamp? Mine has a scratchy volume control but it appears to be one of those Alps sealed units. Also, the cover is missing from the relay and I had to kind of abusively clean it to get it functional. I actually am not a huge fan of that unit as it is very limited on inputs, but it would be cool to get the matching stack working again.
Click to expand...

I saw a YouTube video where someone had gone into that potentiometer and did some serious cleaning on it and got it back in order, it seems. I believe he mentioned that there are other pots that might work but might not be as good as the original one was. When I rebuilt my Carver C-1, I had another gentleman take it and add wireless remote volume and mute to it which has been awesome to have on an almost 40 year old preamp. Wonder if it could be done with these...

Here's the video. The volume control part starts around 28 minutes and 30 seconds.

 
Here was some of the work I did on the Carver C-1 preamp with all new back panel RCA's and all the caps, resistors, and IC chips replaced. A lot of parts...

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