Reverb/Ringing in Leach Amp

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This is my first post and as with most first posts it has to do with a problem. I'm an engineer, and if anyone needs help with an airplane I'm the one to talk to (yes, I'm a rocket scientist), but in DIY audio I'm merely enthusistic student. I've recently finished my second Leach Amp and acoustically it has some problems. The first was built with all the spec'd parts and sounded reasonably good. So I built the second hoping to improve on the parts content and have a dirct comparison unit to evaluate mods, caps, etc.

The result is a noticable (to me) reverb for lack of a better word. (eg. if a vocalist finishes a line there is a slight sustain). Female vocals particularly sound 'airy' and as if the singer was 10 feet behind the band in a large empty room.

The only major changes were from an EI Xformer to a Plitron torroidal and from spec'd metallized poly caps to Sprauge 715P film/foil (Student's Budget mind you). Could this be it, these caps ARE cheap, but I hoped they would offer some improvement over the MFC's? Or any ideas of what else would manifest this behavior?

The amp is driven by a stock AMC-CD8B through a Pass BOZ.

Thanks you for any input and I look forward to contributing in the future.

Do you have access to an oscilloscope? I expect in your position you could find the appropriate equipment around the place. I would feed an impulse into the input and look at the output on the oscilloscope. It could be that the amp is slightly unstable. This can be caused by layout (even wire runs) in addition to component selection.


Pete Fleming
The only thing I can think of that would make an amp behave in any way like a reverb is a borderline stability problem. If you have an oscilloscope, you might try looking at, say, either impulses or square waves to see if there's any ringing.
As a related issue, you might check the feedback loop to make certain that all is well. Does the amp exhibit noticibly higher gain than the first one?


...I hit enter and see that Pete has hit upon the same suspicions that I have. There's no guarantee that I/we are correct, but it's something to check.

[Edited by GRollins on 10-29-2001 at 09:07 PM]
O'scoped, nothing obvious

I had O'scoped everything but the output stage prior to final assembly, but I just scoped the amp again as per both your suggestions, I ran sine/step/saw from 10Hz to beyond 25kHz, pulse response, and amplitude up to clipping. Everything seems ok with no ringing on the step/pulse beyond that actually generated by the function generator (infact it matched it quite well). I don't have access to any sort of distortion analyzer.

I may have overstated the problem. Its not so noticable that my significant other even notices without me pointing it out, but to me it just sounds hollow. any very annoying particullary at higher freqs.

I had to do some creative lead bending to fit the orange drops onto the board, they are not particularly well secured and the exposed leads are long in some places. My only idea so far is to revert to smaller metallized polypro caps to minimize lead length. I know this is and issue with VERY high freq. electronics, but am not sure about the audio spectrum.

Is it possible that the combination of this amp with the BOZ has a problem not present when testing the amp alone?

Thanks again for any ideas already given or to follow.
Pete, more the point, did you have your normal speakers connected when testing. Some amps will be quite stable with some speakers, yet unable to tolerate others. The fact that you mentioned high frequencies tends to further suggest stability problems. For this problem to be noticeable to the ear it will be quite obvious in testing. Ideally you would connect the entire system as you have it at home when testing.


I'll retest this weekend

Thank you to you both,
In hindsight it was a bit silly for me to test the amp with no load, but of course that is what I did. I'll go through it again when I can get my speakers and cables into the lab.

Janne, what is PSRR? and what should I be looking for on the PS side (I assumme a nice quiet, boring DC?)

Thanks again,


PSRR = Power Supply Rejection Ratio.
It is a figure that specify the circuits immunity to
ripple from the power supply.

Yes, I too assume that you will find a normal nice, quiet and boring DC, but if you see something strange under loaded signal condition something might be wrong here.
For example a damaged rectifier bridge or cap.
You should expect to find a stable jig-saw pattern with a smaller copy of the signal overlayed.

Since the Stability issue is covered i Would replace those Orange Drop caps. I do not nor have i ever liked the sound of these. While i dont think of them as Hollow sounding to me thay are Verry hard and Yes tend to recess the soundstage.These trates i noted when these caps are used as Power supply By-Passes. I dont use Coupling capacitors so i can't say what thay do if used in that posistion. My expearence with capacitors is whatever thay spund like in one Part of the Circuit is also similar to what thay do in another part Maby just to More or less of a degree.
Success with new caps

I O'scoped the system as a whole

Leach Amp
Siver Sonic T-14
PSB Century 400

and saw no obvious indications of ringing/oscillations/etc.
(I also checked on the power supply)

so I removed all of the orange drop caps and replaced with much smaller panasonic ECQP polypro and ECWF Met.Polypro.

These also are nothing fancy, but this has made a significant change for the better, female vocals are significantly more 'up front'. In general the sound is much less irritating. So I don't know if the improvement is simply due to the change in caps, or if by changing them out I eleminated the long lead-lengths and stiffened the attachment. (As I mentioned the orange-drop were too big for the board layout). It could also be a little bit of both. In any case things are that much closer to fine. Of course things will never be perfect, otherwise I'd be out of a hobby!

Is it typical for a simple swap of primarily bypass caps to have such a strong impact?

Thanks again to all who offered advice. It has been greatly appreciated and I can only hope to return the favor.

-Pete Kunz
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