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Low frequency oscillation in Eiclone monoblock

PostPosted: Fri Mar 07, 2014 10:50 am
by Greg Smith
Hi,
One of my cathode-biased Eiclone monoblocks has taken to going into very low frequency oscillation lately. The oscillation usually starts about a minute after the amp is turned on. I noticed the woofer on that channel moving in and out, probably around 5hz. It isn't a huge cone excursion, but enough to be clearly noticeable - probably a few volts P-P. The amp still sounds normal, but clearly something is not right.

I built these amps point-to-point, not PCB. I need to crack them open and compare wiring layouts for obvious descrepancies. Otherwise I'm looking for likely causes of LF oscillation in an amp. Anyone?

The amps both worked normally for a few years. This is a recent development. I recently replaced the output tubes, but the oscillation occurs with both old and new tubes.

Cheers,
Greg

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Fri Mar 07, 2014 2:53 pm
by Geek
Hi,

Given that it's just started, I'd say you have a bum power supply or decoupling cap.

Unless you have an ESR meter, change the lot of them with good quality caps from a reputable supplier.

Cheers!

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Mon Mar 10, 2014 3:18 pm
by Shannon Parks
Is the first stage DC coupled, or AC coupled (ie using a LM334Z in the LTP)?

Shannon

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Mon Mar 17, 2014 1:45 pm
by Greg Smith
I'd be surprised if the coupling caps were leaking already, since I used brand new Orange Drops. The power supply filter caps were salvaged from a -48 volt SMPS unit for telecom switching equipment. They were high quality (Nichicon) units, but I suppose they could be faulty. Is it possible that the cathode bypass cap on the output tubes could also be the culprit?

I had been planning to convert these amps from cathode bias to fixed for some time. I'm going to go ahead with the conversion and examine the amps for bad caps at the same time.

Greg

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Mon Mar 17, 2014 1:47 pm
by Greg Smith
Shannon, to answer your question, the first stage is DC coupled. Has that been identified as a problem in other builds?
Greg

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Tue Mar 18, 2014 12:54 pm
by Shannon Parks
Greg Smith wrote:Shannon, to answer your question, the first stage is DC coupled. Has that been identified as a problem in other builds?
Greg


No, DC coupling improves low frequency stability so this is fine (conversely multiple AC-coupled stages with lots of feedback needs careful consideration).

Are you using the LM334Z or LM234Z? If so, what is the bias voltage across them?

Shannon

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Tue Mar 18, 2014 12:59 pm
by Greg Smith
I am not using the LM334Z on the LTP - just a cathode resistor.
Greg

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Tue Mar 18, 2014 1:01 pm
by Shannon Parks
Could you post some pix, Greg?

Shannon

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Fri Dec 12, 2014 9:56 am
by Greg Smith
Nine months later, prompted by urgings from my wife (yes, you read that correctly), I've finally made time to investigate this problem.

I opened up the two monoblocks and compared them side-by-side to see if there was a discrepancy in the wiring/layout that would cause LF oscillation in one of them. Sure enough, the one that was oscillating is lacking a connection from the first filter capacitor ground (PT centre tap) to the chassis earth ground. Oddly enough, the amp functioned this way (with the amp's PSU/signal ground floating - the only earth ground would have been through the input patch cord shield to the preamp earth) for over two years before I noticed one of my woofer cones moving in and out, at about 5 Hz. I have not categorically determined that this is the fix but it seems like a likely cause. Any opinions?

In the meantime, I am in the process of converting both amps from cathode bias to fixed. (Why didn't I do this to begin with?) Shannon, I noticed that the Eiclone docs no longer include the cathode bias option schematics. Are you backing away from supporting this option? There is no real benefit to building with cathode bias - negligible difference in component cost, the cathode resistors generate a lot of heat, and you're wasting headroom).

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Sun Dec 14, 2014 7:35 am
by Shannon Parks
Greg Smith wrote:In the meantime, I am in the process of converting both amps from cathode bias to fixed. (Why didn't I do this to begin with?) Shannon, I noticed that the Eiclone docs no longer include the cathode bias option schematics. Are you backing away from supporting this option? There is no real benefit to building with cathode bias - negligible difference in component cost, the cathode resistors generate a lot of heat, and you're wasting headroom).


When I updated the Eiclone manual last year I tried to prune it down and make it less messy (and also to not have to rev/update two BOMs and schematics), so I ditched the cathode bias part. I can email anyone the old manual if requested.

FWIW, the original pair of Eiclones are in my Uncle's workshop and they are indeed cathode biased. Maybe ten years old with daily operation during that time and over 10,000 operating hours.

Shannon

Re: Low frequency oscillation in Eiclone monoblock

PostPosted: Tue Dec 30, 2014 12:32 pm
by Greg Smith
So I completed converting the amps to fixed bias, and fixed the grounding problem in one amp. No further problems with LF oscillation. I'm guessing the grounding problem was the cause. Set the bias to 38 mA per tube. Back in business!