Harman kardon hk 870 lacks bass punch

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wqs01

New Member
Hello,

I am new to this forum and although I have used vintage solid state equipment for years (since the time before it was considered vintage), I am no expert and I need some help.

I have a question about a problem that I have with a HK 870 amp that I picked up. I replaced an Onkyo receiver I had in my system with a HK 870 power amp and a Soundcraftsman preamp. I find that I have a lot of power with this combo. I do not have to turn the volume up much to get a good amount of volume out of the speakers. The problem is that the sound seems to really lack low end bass. The effect is similar to turning off the loudness button on my old Sony receiver. The lack of bass is the same for both vinyl and CD.

I tried the following:

1. I checked the polarity of speaker cables

2. I tried a different preamp

3. I checked the component cords and changed the cord from the preamp to the amp.

None of these things helped. Would a lack of bass be the sign of bad capacitors? The HK 870 is rated at 100 wpc and it does seem to have more power than my Onkyo receiver which is rated at 65 wpc I think, but the Onkyo has much better bass.
 
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I agree that it must be due for maintenance. All of the HK units I've heard that claim "high current design" have excellent control of the woofers.
 
Thanks for the response, Masterdraco. No, I have not checked the bias or the DC offset. I must confess that do not know how or what these terms actually mean. I have been strictly a user of vintage solid state equipment. I do not have any experience with repair or tweaking.
 
Harmon kardon hk 870 lacks bass punch

grey said:
I agree that it must be due for maintenance. All of the HK units I've heard that claim "high current design" have excellent control of the woofers.
Click to expand...

Thanks grey. I heard about good bass from the hk870. That's what got me thinking something was wrong.
 
An accurate multimeter with mini-grabber leads is standard fare for the used gear diy hobbyist. Needed to check the bias and offset.

An ESR meter is used to check for failing capacitors. Some here just replace all electrolytics at once which isn't a terrible idea.

Decent quality meters are going to run ~$100 each. Fluke low end MM and Anatec or EBV esr for examples.

These tools are great to have, but I can't rule out an impedance mismatch or exotic cabling causing problems. I've had problems from both. It reads that you've intuitively touched on this a bit already.
 
The amp has two series connected electrolytic caps on each input terminal. They are 100uF 6.3 volts and I'd replace those two in each channel first. Easy and may well be causing the problem.
 

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Maybe, just maybe, it would be good to have a follow up diagnosis before anything is done, just to show the performance deficit?

Then, at least, the change could be measured after the caps are changed, bolstering what the ears are hearing.
 
I agree. Personally, I'd simply test the amp and determine if there were any frequency response/ power bandwidth issues and hunt them down, fix and test again.

The HK870 has truckloads of bottom end and the design is excellent, so it seems odd it is getting accused as being bass light. The only caps with any bearing on the deep bass response are the input caps, and even they are rated high enough that even if they dried out and went down to 10uF each, the deep bass shouldn't really be affected. High ESR+the caps going low might cause some LF issues.
As it stands, the low freq -3db point is about 0.13Hz.
 
Steer clear of the DC bias- it has nothing to do with deep bass.

The input impedance is low, but not super low, about 22kohms or so.

Any decent preamp shouldn't find trouble with that, and generally it'd be the HF that would be affected.

I think you may be hearing a more accurate amplifier than you previously were. The onkyo may have been a fatter sounding amp stage, but less accurate. Those HKs are ruler flat in my experience.
 
Thank you all for your responses. I opened up the HK 870 just to take a peak to see if anything looked wrong. With the exception of a little dust on the top of the transformer and the caps, everything looked really clean and like new condition. I looked at the four 5 amp 125 fuses. They were not blown, but a couple looked kind of funny. One had some yellow substance visible in the glass. I replaced the fuses and I believe I got an improvement in the bass. Could this be or I am hearing things?
 
I agree with RJ, you may be hearing what a clean, flat response amp really sounds like.
You could try moving your speakers to a position that reinforces the bass to your satisfaction.
 
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