Axiom Home Page Forums General Discussion Technical Questions Feedback?

Looking for some advice. Something curious has crept up in my system. When one of my kids was loudly making an "Ooooohh," sound my left front speaker would mimic it in a sort of feedback response. If I hit pieces of metal together the speaker responds as well. Has anyone experienced this and if so what is the culprit?

I am using a Marantz SR 5005 as a pre/pro that was modded by removing the amp pathway by Outlaw Audio. I then have the L/R off the Marantz going to HT inputs to a Modwright SWL 9.0 being used as a stereo preamp and bypass for home theater. Oppo -83 HDMI to Marantz for home theater, coax to Eastern Electric MiniMax DAC for stereo on to Modwright via RCA. M22's, VP100, and QS 8's.

Thanks, hope I provided enough info.

Sips

Hi power window,

What source are you using when you hear the feedback? I'll guess that somewhere in your system you may have vacuum tubes, which can be quite microphonic with higher sound pressure levels. To check for a microphonic tube, just remove the chassis and tap on the tubes in the amp (or preamp). You'll hear a "ping" sound if the tube is microphonic.

If you were playing vinyl on a turntable, the disc itself can become microphonic, vibrate, and pass on loud acoustical sounds to the phono cartridge and to your system. I have a collection of various weights and clamps for vinyl on a turntable (back when I played vinyl) to prevent feedback.

Regards,
Alan Lofft

You are right about tubes. My stereo preamp with bypass (which is used with all sources) has power and pre tubes. If I perform your diagnostic, does that mean the tube is bad and needs replacing as I do not recall this occurring when this piece of gear entered my system? Thanks for the quick reply and nice to hear from you Alan.

Alan, side note. I haven't done your diagnostic yet, but the fact that the feedback is only present in the left speaker leads me to believe the left channel tube in my preamp is the culprit. The only question then, is it a bad tube or will a tube damper solve it?
Thanks,
Sips

or get a solid state amp.

New tube.

Sips, yes, you've provided enough info and Alan's suggestion about a tube is likely the problem. Even in a pre-amp, using obsolescent tube audio technology can lead to unnecessary problems.

If you haven't yet checked all the tubes, you might first connect the 5005 directly to the amplifier to verify that the problem you describe doesn't then occur.

Hi sips,

I think that's a reasonable conclusion about a tube in the left-channel part of the preamp, since that's the channel you're hearing the microphonic feedback.

I forgot to say that you should replace the microphonic tube. I never tried tube dampers back in the tube era, but perhaps one would be effective.

Alan

Thanks all for the reply/feedback :), as well as the jabs on tube technology. Old technology is not necessarily worse, and conversely new is not better. Variety is what makes this hobby so interesting. Now I'm off to see if someone can wipe the dust off one of those old tube boxes and see if it's the right one.

Cheers

My tube amp brings me great joy.