How good is harman/kardon hk 725 preamp (when it works)?

So here's the thing:

I got myself this INKEL set:

http://www.audiokarma.org/forums/showthread.php?t=581328

Having fixed some issues it had, I listened to it a bit and decided I liked the power amp, pre - not so much. Firstly, compared to other preamps I have it sounds a bit veiled, with mids recessed. Secondly, I hate the push button volume control, it's a real pain in the butt to set low volume just right on this thing.

So I went on a hunt for a nice ~1980 silver pre with a headphone socket (Inkel power amp doesn't have one and that was actually one thing I did like about the Inkel pre). I got a hk 725 that has pretty much everything I wanted, plus nice specs and some cool features like resistor-based tone controls.



Some more info here: http://www.thevintageknob.org/harman_kardon-hk725.html

It was sold as 'works fine, front panel needs some more cleaning'. Yeah, right. Only the second bit was true.

So, one channel was silent to begin with, but cleaning the output relay took care of that. The issues that remain:

- R channel has significantly (some 30%) less gain than L channel. I have to turn balance knob two bars to the right to get equal levels.
- Tone controls work fine on the quieter R channel. They have much less effect (but not zero effect, each button works and changes sound a bit) on the louder L channel.
- With tone control off, the louder L channel has a different tonal balance than the quieter R channel: it is brighter, with less bass. Going by ear, I would say that the quieter R channel is the one that sounds like it should.

It seems that the quieter R channel is working correctly and the problem is in the L channel. Neither channel is distorted.

I checked loudness and hi/low filters, they affect each channel, so apparently not the case of filter switch with bad contacts. I cleaned volume and balance pots just in case, no change. Obviously I flipped all cables between channels, etc. The problem is in the preamp. It's not the relay that was dirty, the issue is also present on headphones, which don't go through the relay. Basically it looks like there's a bug down the signal path that will not be remedied by simple cleaning. For me this probably means a trip to a tech, my soldering skills are not good enough to extensively work on this thing.

I am curious though, what could cause this symptoms (apart from things I've already checked)? On the off chance I could fix it myself

The thing is, I was willing to pay what I did for this pre, but with the issues it has, I'm not sure if I will like the sound, and I can't really A/B it with the pre it was supposed to replace. I need to decide whether it's worth the bother of getting it repaired or not.

So, do you guys know this pre? Is it as good as Harman advertised? Get it repaired or try to return it?

I owned this pre along with the 775 mono block amps, 705 tape deck, 710 tuner for a number of years and thought they were very good pieces. No idea on the issues you have, but if you didn't pay too much and can afford to get it fixed, I would say well worth a bit of money. It is a good preamp. Sure, there are always better out there, but I always liked this pre. Wish I still had that stack of HK! Tim

Junk especially when dropped.

You're chasing a sound dream.. get a decent pre and you'll be pretty happy.

I have one and like it a lot, although I think the output relay is just a problem waiting to (continue to) happen. Next time mine gets crusty I'm going to bypass it.

My advice is to use this as a reason to improve your soldering (and desoldering) skills, and recap it. It's a fairly friendly amp to work on, and there is a free service manual available. Plenty of users on here have done this and can help you.

I upgraded the power supply caps and changed out everything in the signal path. There are a few bipolar caps, so it requires close attention to the schematic. AK user named Toroechado helped me a lot. The tone defeat lamps were a little tricky to redo, and the push buttons get stuck too easily.

Let us know if you decide to work on it.

RWood

Thanks for your opinion, guys. I think I will get it fixed, but let my tech give it a shot. I normally pay less than $20 per repaired unit, plus parts if he needs to buy any (and he harvests a lot from the huge pile of PCBs he has, so that's not often), so I have a pretty sweet repairs deal. And there's a lot of cheaper and more common gear I can practice soldering on and possibly kill in the process.

I will definitely get that idiotic relay bypassed. What's the point of having it there in the first place?

Binkman - I already have a fairly decent pre (more than one, actually) and I'm quite happy with my two permanent rigs. My sound dream got chased and caught But I like trying out gear I don't know, play with it for a while in 'experiment' system.

RWood, you still have your 725, so maybe you could help me confirm that on mine R channel is fine and L channel has problems (this is what I suspect). Apart from the behavior I described in my first post, (L channel louder, tone '+' and '-' buttons have less effect on this channel), I noticed few more things.

1. Bass 'Defeat' button has no effect on L channel, meaning if one of '+' or '-' buttons is pressed, its effect is not cancelled by the 'Defeat' button. Works fine on R channel, it cancels cancels the effect of '+' and '-' buttons, and with 'Defeat' pressed the sound is pretty much the same as with none of '+' or '-' buttons pressed and 'Defeat' out.

2. Treble 'Defeat' button has an effect on both channels, but it's different for L and R.
On R, it works like I think it should, so it cancels the effect of '+' and '-' buttons, and with 'Defeat' pressed the sound is pretty much the same as with none of '+' or '-' buttons pressed and 'Defeat' out.
On L, it does cancel the effect of '+' and '-' buttons, but when all '+' and '-' buttons are out, pressing 'Defeat' changes sound (it gets a bit louder and with a bit more treble and high mids).

How does 'Defeat' work on yours? When you have all '+' and '-' buttons out and you press 'Defeat', does it change the sound?

One more issue I spotted is that pressing the subsonic filter button results in some crosstalk between channels, but that affects both channels and doesn't make other problems go away or worsen, so I won't worry about that now.

And it's fixed!

It turned out that left channel tracks on the main PCB were broken in 3 places, in the area right below the tone amp boards. Looks like someone accidentally pressed this area of the board while trying to clean switches or something. After closing the gaps, tone control and defeat buttons work fine, channels have even gain, no crosstalk anymore.

I listened a bit tonight and liked so far. Harman did a nice job designing this preamp. Seems like it will be a good match for the Inkel power amp. I will still have to A/B it with the Inkel pre, but my first impression is good (and it wasn't with the Inkel).

RWood, how much would you say recap improved yours? It does look recap-friendly, there are only few electrolytic caps and it might indeed be easy enough for me to try it.

Glad it is fixed. Recap on mine really wasn't necessary - the problem I was chasing turned out to be the relay - so recapping didn't improve the sound noticeably but probably extended the life of the amp.

OK, thanks. In that case I'll wait with recap till it becomes necessary. The preamp sounds good as it is to me, no sound deterioration, no audible hum or weakness in any frequency.

Just got a 725 and 770 amp. My right channel is also softer with no bass really. I hope I can find these gaps as easily as you were.

Is this the relay that is so problematic? Is it there to not allow a signal to pass through until the rest of the circuit is powered up? Some type of safety for other sensitive equipment down the line?



Also, on the high cut PCB... Is this brown crap some type of old glue or is this a blown cap?


I don't have much for electronic skills, only that its best to discharge capacitors using a large resistor. This is really my first project since high school.

Am I reading things correctly? These two muddy things are 0.1mF 50V Mylar caps?

There is a lot of this brown goo places... funny enough around other caps. But Also around plastic to plastic retaining clips, and power binding posts. It made me wonder if its a type of cement or hot glue used to hold pieces.

brown goo

It probably is hot glue. The idea was to use a pretty quick setting hot glue on parts that might vibrate while going thru solder baths -usually larger/taller inserted parts.. This would cause cold solder joints -so just lock 'em down with glue -one problem gone.

It is glue. Yes, looks like the caps in your pic are C709, 710.

The relay (yes, that's the one that likes to lose contact) is there to prevent startup noises from going to your power amp. It's not a bad idea, power switches alone can make hellish noise, in extreme cases enough to damage tweeters - but the relay should be much more sturdy. The one in 725 is too small and very delicate. I had to clean and adjust contact points twice already.

If I remember correctly, the gaps in my case were on the main board, not on the tone control board. They looked like they resulted from pressure on some component on the main PCB, could be the large caps. So don't limit your search to the tone control board.

My 725 also had bad solder joints on the inputs/outputs board (ground of one of the output jacks was especially problematic, causing intermittent hum), so best lift that board and reflow the joints.

One more thing I've noticed, this preamp does not like to be placed directly on top of or below power amps, it is not well shielded and picks up interference from large power transformers and hums a bit when it's too close. Best to put another unit (like tuner) or distances (high feet) between power amp and this pre. 10 cm distance was enough in my case.

Thanks guys! I'll try bypassing the relay - The 770 has those speaker system buttons, I'll just turn them on last.

I'm wondering if bad continuity (hopefully in the relay) would affect the minimal bass signal in the right channel too. I guess I'll find out.

That's unlikely. But bypassing the relay should be quite easy, so no harm checking.

A. O. K.

Welp,

It was the relay. AND the inter-connects between the pre & power amp. I soldered in some jumpers on the under side of the board, leaving the relay in place.

Things improved, but as I listened more, it still seemed the right channel was weak in the woofer. The headphone jack sounded balanced, but through the speakers was still problematic.

I should have tried this earlier but I swapped in a different pair of rca cords, for the nicer, yet older set that came with the units. The new cords made the rest of the difference I was hearing.

I'm new to HI-fi, and I've just always thought a functioning set of wires was fine. This really proves it for me. The bad cord was operating but at a different level than its left channel counterpart.

Both channels are equal now thanks to the relay bypass and the new cables. Any differences I may hear now are close enough to perhaps be illusions in my brain.

Thanks guys! I got my set-up working. Now for a little A/B listening with the old unit, and some more room voicing.

Great news!

Actually cables between pre and power amp are probably the most important cables in the system, because they carry a very low level signal. you probably wouldn't hear the same difference if the same cables were between the source and pre amp.

I own the 700 series still in excellent condition -- I am interested in selling these items.

Let's put it this way. I have the 725 and 770. Before those I had the 5600 integrated playing for 6 months. I don't think the 725/770 does a much better job than the 6500. Slighty better, but not much. Not Hi-End....
The Citation 11 is a far better pre amp.

The 725 is/was - considering the price- a very nice pre-amp. Of course there a lots of pre's that are much better, but in this price range it is a very nice sounding amp. The phono stage is the real winner in this amp. Silent, good soundstage, fluid sound. Some people liked the sound of the 725 pre more than that of the 770, so it was often paired with a different power amp. Enjoy listening to it!