Replacing Caps in HK775?

[mvarney]

I am new to this forum. This is my first post.

I recently acquired a couple of Harman/Kardon HK775 "Ultrawideband Monophonic DC" amplifiers. Based on what I've read here and elsewhere, these are highly regarded.

I hooked them up to the preamp output of my NAD 7240PE receiver and connected the outputs to a pair of Mirage M3si tower speakers. After listening to this setup for quite a few hours, I had several observations:

1) there is almost too much treble, and it sounds good, but the midrange and the lowest bass are lacking, compared with both the NAD's built-in power amp and a pair of Carver TFM-6CB power amps that I tested in place of the HKs. The HKs seem to have a cleaner sound overall than the Carvers, but the midrange is noticeable lacking, and the bass is somewhat lacking in definition, especially compared to the NAD. Incidentally, our old Denon PMA-750, from about the same time as the HKs (early '80s) was also lacking in midrange in this system, but it was especially lacking in bass (unlike the HKs, which do have a lot of bass; it just doesn't seem as extended as the NAD power amp).

2) both HK775s seem to sound about the same

3) they get very hot even with nothing playing. There is also a "hot component" smell (evaporating capacitor electrolyte?) that is eventually noticeable throughout the room. I realize these amps are probably supposed to get hot, but I do wonder if this is normal. The smell shouldn't be normal. I've noticed it before on other amps that didn't sound good, and assumed the caps were bad.

Before I ever turned on the amps, I assumed that I would have to replace some caps. My auditioning would seem to confirm this.

So, now my questions: 1) Am I right? Could bad capacitors cause the frequency response issues I've described?

2) If so, should I replace the main filter caps first? They are C2 and C3 (15000 uF 71 V). Incidentally, there are also four smaller 1000 uF caps (C10-C13) doing filtration (?) for a second supply, using a different transformer coil and rectifier diodes. I'm not certain what this second supply is for. Is this related to the "DC" feature of the amplifier topology? I really don't know much about amplifier design, despite having designed several simple ones for an electronics class in college.

Also, I am not certain what type of terminals are on C2 and C3. What kind of cap should I buy to replace those, or the others? I have attached a link to a picture of the capacitor terminals, as well as the service manual and a shot of the interior, partly disassembled. Any advice will be much appreciated!

Service Manual (with schematics)
Interior Shot
Capacitor base

[Netlist]

Welcome mvarney,

From your picture, the caps look fine.
Sure it’s more accurate to measure them and check for ripple.
Changing some ceramics 'might' do something.
Check the bias first to make sure things are not overheating.

/Hugo

[Leolabs]

try to resolder all the solder joints.

[mvarney]

Thank you both for your quick replies.

Leolabs says that I should resolder all the solder joints. Is this to make sure that the frequency response issues, etc., are not related to disconnected component(s)? I'm sure that's a good idea in general, but the amps sound identical to my ears, and there's nothing intermittent happening.

Hugo says,

From your picture, the caps look fine.
Sure it’s more accurate to measure them and check for ripple.
Changing some ceramics 'might' do something.
Check the bias first to make sure things are not overheating.

The big caps look perfect, as near as I can tell (it's hard to access the bottom of the PSU board because of wires soldered on; I don't want to disconnect anything until it makes sense to do something major). There is no bulging or even the usual leaking.
However, I have read and heard from several sources that most electrolytic caps go bad in about 15 to 20 years. The electrolyte evaporates or dries out, or the aluminum oxide dissolves from the plates, and things don't work right anymore. I assumed that what I was smelling (which I've smelled in other old amps) was electrolyte evaporating from the caps or from the board where the smaller filter caps have leaked. But then some people use ancient equipment and think that it's working fine, despite others' opinions that the caps should be junk by now (whether or not the equipment "appears" to work OK). What is the truth? It would be nice to have a definitive answer on this subject, instead of the wishy-washy reports often seen.

How would I test the caps for ripple? I guess I can find that somewhere else here, so don't take too much time replying to it.

What do you mean the ceramics "might" do something?

In this amp I can adjust the bias point (apparently), in order to eliminate DC offset at the output, by adjusting a variable resistor. Is this all you mean by "check the bias first"? Again, I'm acquainted with the biasing issue based on my reading and experiments in college, but I don't know what crazy varieties of biasing may be present in this amp, given its relatively unusual topology.

But could the biasing issue really have an effect on my frequency response issue?

Maybe someone who has auditioned these amps could say whether they think what I'm reporting (in the first post of this thread) is normal, taking into account what I've said about the relative characteristics of the NAD and Carver power amps.

Again, your input has been and will be much appreciated.

- Matt

[Netlist]

Electrolytes tend to deteriorate over the years so a major recap is sometimes in order to fix the amp or improve the performance - bring the circuit back to specs.
Bad power supply caps usually translate in hum at the output.
It all depends on the amount of money and time you want to spend.
In your case, and I haven’t auditioned that amplifier, I believe most caps are fine and the difference in sound with the NAD is probably inherent with the HK as both sound almost equal as you said. Ceramics are said to be bad for the sound when in the signal path so if you find them there you can replace them with good poly- caps.
Don’t expect miracles, without changing the design completely.

As for the alignments, perform the two described on page two of the manual, eventually the one on page three. The first one is for minimal DC at the output; the second sets the idle (bias) current of the output stage. The latter determines the amount of heat the amp is generating.

/Hugo

[mvarney]

Hugo: I didn't notice any hum at the output, so the power caps are probably mostly OK. I don't know how to check for ripple, though (put it on oscilloscope and view positive voltage rail at cap? unfortunately I don't have a scope).

Anyway, I tried the alignment procedures. I should have let the amps warm up longer before initially testing them, however, one of them was showing 46 mV across the bias current test points (instead of 36 mV), and the other one had more than twice the recommended voltage. I easily fixed this. The DC offset appeared to initially be over 100 mV on one and over 400 mV (!!) on the other. Although this was before they had settled completely, I have little doubt that the high offset was negatively affecting performance.

Unfortunately, stabilizing the DC offset has proved to be nearly impossible. I had it fairly satisfactory last night. This morning, when they were cold, they started around 50 mV and dropped to some decent values, about +/- 5 mV on one and +/- 8 mV on another. But the DC offset on these things varies constantly, marching up and down. And this morning I found that the bias current on both was a bit off, so I adjusted that, and then the DC offset was way off (down to -30 mV on one and some other bad value on the other). I know that none of these values are terrible, but it's supposed to be possible to align this amp to +/- 10 mV DC offset, and I do want to hear the high quality sound that these things are supposed to be capable of.

I am convinced that the offset adjustment pots are junk. If it was easy to replace them, I would. Unfortunately there is a frame/heat sink directly behind the board, which is attached to two transistors on the board, and the whole thing has about seven connections to the main board below. I'd rather not mess with it if I don't have to.

Right now one of the amps is running about +/- 10 mV but it occasionally goes up to 20 mV. What could this be caused by? Line voltage fluctuation? Bad power transformer? Bad caps? or simply the bad potentiometer?

The other amp continues to be pretty flaky (the DC offset pot is more touchy in that one). It appeared to be operating in a decent range but then went beyond + 50mV. I readjusted it and it's a bit better now, in the -20s (at least briefly).

Thanks for all your input. I'm going to audition the amps now and see what effect my adjustments have had.

BTW, I tested the DC offsets in the NAD 7240PE in the main system and found 2.4 mV in the left channel and 4.5 mV in the right. Outstanding! Now I'm going to have to check that on all the other amps around here...

[anatech]

Hi mvarney,
Hugo is right, but I'll add that ceramic caps impart an evil sound to an amplifier. Replace with mica or some plastic film type.

HK's always run hot.

From looking at the schematic, I'm not surprised that the offset drifts. Ignore it, new trimmers won't help. You can use contact cleaner on the rear in the middle to improve things. You are adjusting the tail current in one of the diff pairs (dual differential).

This amp does not have the usual cap to ground in the feedback network. This means two things. It has full gain at DC and therefore no reason for a loss in bass. The DC offset will tend to change a lot as it heats up. It will also tend to drift around.

-Chris

[lndm]

Originally posted by Netlist
Electrolytes tend to deteriorate over the years so a major recap is sometimes in order to fix the amp or improve the performance - bring the circuit back to specs.

Furthermore, if a cap deteriorates and its capacity goes down, and its ESR goes up, it could also result in an apparent lack of bass and a muddiness that might be caused by a reuction in damping factor.

Originally posted by mvarney
the bass is somewhat lacking in definition

Originally posted by Netlist
Ceramics are said to be bad for the sound when in the signal path so if you find them there you can replace them with good poly- caps.

If you find ceramics in other parts of the amp, it's often best to keep them there as ceramics have other properties that make them useful.

[mvarney]

anatech:

From looking at the schematic, I'm not surprised that the offset drifts. Ignore it, new trimmers won't help. You can use contact cleaner on the rear in the middle to improve things.

I did use contact cleaner on VR401 (the one that adjusts DC offset) but it had no effect. If you barely bump it, the offset goes way off somewhere, and sometimes not in the direction expected. Maybe these were damaged by the heat in that area; most of the heat in this amp (when idling) seems to come from that pre-driver board, where VR401 is. What do you mean by "the rear in the middle"?

OK, so I should "ignore" the drift. But certainly some average value is better than another, e.g. one of these amps was "drifting" near 400 mV before I adjusted it. Should I let it warm up to a typical temperature and then set the offset near zero? And doesn't a large offset negatively impact the sound? Or is this amp relatively immune to that?

I auditioned them for a while yesterday, and notice little change except that the bass might have been better. I was more pleased with it at the time, anyway, but that might be only psychological (as, unfortunately, many impressions of this kind of stuff are). I can't A/B it. It seems that there should have been some significant distortion with the DC offsets where they were, but I didn't notice it if there was.

These amps sound good to me, especially the treble, but I don't think there is enough midrange. But maybe that's how they are supposed to sound. I know that the room sounds too bright anyway; hard, smooth floor, masonite walls, and a smooth particle board tile ceiling: the designer must not have been thinking about acoustics. It's not really my listening room, anyway, so I haven't been too concerned about its sound. I am primarily interested in knowing whether my equipment is working right. If I know that the biasing is right, and it sounds good, then I am fairly satisfied.

netlist says:

Ceramics are said to be bad for the sound when in the signal path so if you find them there you can replace them with good poly- caps.
Don’t expect miracles, without changing the design completely.

There are very few ceramics in this amp. Most of the caps appear to be either polyester film or electrolytic, but I'm not familiar enough to say for certain what they all are.

[Netlist]

One can appreciate the knowledge of the guys who really know what they’re doing.

As Chris said, the DC will keep drifting; don’t worry about it. Anything between +50mV and –50mV is quite acceptable and won’t change the sound.
Bias is another thing.
In Pass class-A amplifiers, the sound tends to improve with increased bias.
Of course these are mosfet designs, this one is BJT.

Here is a good page on different types of capacitors:
http://www.interq.or.jp/japan/se-inoue/e_capa.htm

/Hugo