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-   -   Reverb/Ringing in Leach Amp (http://www.diyaudio.com/forums/solid-state/1015-reverb-ringing-leach-amp.html)

pjkunz 30th October 2001 12:42 AM

Hello,
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.

Pete Fleming 30th October 2001 02:03 AM

Pete,

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.

Cheers,

Pete Fleming

GRollins 30th October 2001 02:04 AM

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?

Grey

...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]

pjkunz 30th October 2001 03:43 AM

O'scoped, nothing obvious
 
Hi,
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 Fleming 30th October 2001 11:33 AM

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.

Cheers,

Pete

Hedlund 30th October 2001 12:11 PM

Leach amp have very high PSRR so a problem in the power supply might not be visible at the output, so you should measure there too.
And as Pete says, you better make all measurments with your loudspeaker connected.

/Janne

pjkunz 1st November 2001 09:34 PM

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,

Pete

jam 1st November 2001 09:52 PM

Pete,

Did you check Cdom? The compensation capacitor.

Jam

Hedlund 1st November 2001 10:41 PM

Pete,

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.

/Janne

ppl 3rd November 2001 12:21 PM

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.


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