severe microphony and 50hz hum...

tubo · 2021

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Offline tubo

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on: October 09, 2014, 01:14:53 PM
Hello all,

I just finished assembling my smash and it is amazing... for the first hour there was no microphony and it sounded really gdI then as the tube started heating up, there is now a 50hz hum on both channels and severe microphony... it is like feedback sound that increases on volume then goes away and comes back. It is very terrible and annoying.all voltages and resistance are gd.

The smash is isolated well from vibations and is still very microphonic. My system consist of a 3 stage pp 6l6 amp with enough gain for a preamp not to be use but i still have a lot of usable range with the volume pot. My speakers are kef ls50 with an 85db sensitivity.

I have the same problem like the one of our fellow member and have read the thread... i really dnt want to put an inline attenuator  as i think its a bandaid for the problem...

Pls help..... I dnt know what to do anymore.... I really like the preamp, but I won't be able use it if the problem persist... its my first kit from bottleahead. I wished bottlehead did more to.prevent stuff like this from happening.

Pls help as the wifey will be upset if I dnt.fix the problem... as she is excited to paint the wood chassis.....

Thank you



Offline Doc B.

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Reply #1 on: October 09, 2014, 01:44:07 PM
The solution for using an amp that has too much gain for a preamp and has no attenuator built in is indeed to use an inline attenuator at the input of the amp. You may find that the microphony is being set off because of the location of the preamp. Moving it might get it out of a room mode that is setting off the resonance.


Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline tubo

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Reply #2 on: October 09, 2014, 07:04:05 PM
Hello Doc,

Thank you very much responding.  I will do what you recommend. My amp is a three stage pp amp with 12au7 as the input 6cg7 as the splitter with 6l6 as the finals. What value of inline attenutor will be adequate for my application?

Thank you



Offline Doc B.

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Reply #3 on: October 10, 2014, 05:15:36 AM
It's a little difficult to judge without seeing and hearing your setup. I'm guessing that you are not turning up the volume knob very high due to the sensitive amp. If that is the case I would guess that at least -12dB would be useful, and maybe more attenuation might work even better.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline Stedes

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Reply #4 on: October 17, 2014, 12:27:56 PM
I had excellent results using 10db attenuation between the Smash and my (quite high gain) power amp.