how will smash up affect microphonics? more gain?

R

rogerfederer

New member
my build has been largely successful.  however, it has been noisy and microphonic at times.  i have tried attenuators at the amp inputs (helped a lot) and blue-tac between the top plate and the wood frame (prob helped as well).  i tried new 4 P1L and OD3 tubes (those 4P1Ls sure are a mixed bag; 1 new set no good; other new set not really an improvement).  the latest trick i have used is to turn it on and let it warm up before switching on the rest of the system.  this has made the most difference.  still, every once and a while (seemingly out of the blue) the tubes get all noisy and i have to jiggle them -- and they then quiet down.

will the smash up address this issue in any way?  how much will the gain be increased?  for my system it already has way too much gain...
 
The gain increase is not significant, as the gain of the Smash preamp is quite low to start with.  The major improvement is in distortion, noise, and drive capability (voltage, not impedance).

One very helpful thing to do for microphonics is to add mass.  It may also be helpful to consider a wall shelf vs. floor rack if you are on a wood floor built on joists.  (IE - treat the preamp like a turntable)

-PB
 
I am not having noise issues.  The microphonics are a challenge.  I  have temporarily added mass on the top plate between the input and attenuator, as well as behind the tubes.  ( I took the balance pot out of the pre.)  I used four 1" D X 2" H stainless steel cylinders sitting on cork & rubber isolation pads, (2"x2"x 7/8").  This has pretty much eliminated acoustically related microphonics.  My stand is well damped and the shelves sit on small spikes. I don't really have room on my shelf to put much on top of the tranny.  I replaced the output caps with two Russian 2uf KBG caps.  They are much larger and heavier.  They are mounted to a small acrylic plate elevated on standoffs on the tranny screws, so that added mass below the PT. 

I will admit that adding the stepped attenuator and Grayhill switch made matters worse when they are switched.  They require more
torque to turn, are just "more mechanical" and cause more 4P1L "singing", but it quickly subsides with the damping.

I do think any method that damps the plate well to the base will be helpful.  I will likely not do much more until the Smash-up kit arrives. (Unless the microphics take a turn for the worse.)  Then I will try to isolate the tube sockets with rubber washers and silicon rings. I will also look to do some constrained layer damping on the underside of the plate.  Until then....

I am really enjoying the Smash!

Cheers,
Geary

 
Yeah, when we do the attenuator upgrade for the Smash, I plan to just add instructions for making the switching force very light. 

In the past, I've also put a piece of thick marble tile under sensitive components.
 
We've done a lot of playing around with various means to reduce microphony. I think it's a three part program.

First, keep the energy from getting into the gear. Keep gear away from speakers and out of room modes and put it on a stable shelf if possible. Use some kind of squishy stuff as feet under the gear. Sorbothane works, and Herbie's Tenderfeet are very nice. I've used folded bubble wrap in a pinch.

Second, try to keep energy from moving from the chassis into the tube. A silicone o-ring under the tube can help as long as the tube pins still make good contact with the socket. Damping the chassis panel itself can be helpful.

Third, and maybe the most important, damp the tube envelope itself. IME mass is your friend. A lot of you have seen the vinyl dipped lead rings I use on my BeePre. Something similar should work well on the 4pills. Maybe some lead tape or long thin fishing weights wrapped in Teflon tape and bent into a circle fit on the tube like a crown? Stay with temperature resistant materials. One user here tried the vinyl covered rings on a Paramount, which runs the tubes at much higher temps than BeePre, and got a nasty surprise when the vinyl started to cook. I have found that Herbie's dampers work pretty well at higher frequencies, and that a really massive weight seems to be most effective for lower stuff. Filament/heater length will probaby have some bearing on where the resonances occur and thus what kind of treatment will work best. Small tube, higher frequency resonance. Big tube, lower.
 
i'm not finding those vinyl covered lead rings in the right size for the 4P1Ls. 

however this item in the cabelas catalog might work:  IK-319935

 
Other than a slight ringing when cd draw opens and closes no issues at all.
Very Pleased with the Smash.
Steve.
 
going back to my original question, i'm guessing the answer is NO, the smash upgrade will not affect the microphonics issue one way or the other.
 
rogerfederer said:
going back to my original question, i'm guessing the answer is NO, the smash upgrade will not affect the microphonics issue one way or the other.
Click to expand...

Good guess!  If I understand the upgrade, (not my first BH preamp upgrade, so better than a guess?), it adds C4S, (constant current source), and dual triode B+ regulation.  Neither of those circuit upgrades specifically address the microphonics which is really  a mechanical issue with DHT tubes.

I will use the effort in installing the upgrade to address the mechanical/mass remedies for the base, plate, tubes and sockets.  Probably even go all loctal with a 7N7 for the B+ regulation. PB and Doc are rigt on the basic solutions.  Although "starving the filaments" has been discussed for other designs, (primarily Bartola linestage/DIY audio), the Smash is really a different circuit...no OPT's.

Sounds as if you have been blessed with less than stellar performing 4P1L's.  I plan to get a couple of spare pairs.  Did you buy yours from eBay?  Suggested sellers? None of my usual Russian/Ukrainian vendors are offering any.

Cheers,
Geary

 
i bought from seller marekmagda1

the 1st set were not great and he quickly sent me a new set for free. 

i'm trying a 3 lb weight from my home gym on top of the transformer.  will see how that performs.  if it helps then i'll look for one of those lead weights divers put on their belts. 
 
well, the 3 lb weight has made a huge difference :D
NO microphonics at all since adding the weight.  so i will now look for a better weight than the temporary one i have in use right now.
thanks for the great suggestions!
 
For me the best thing, although folks at Bottlehead do not recommend, was to lower the heater voltage. I do not remember well, there is thread on diyaudio, but I think I lowered the by like a half a volt. This stopped ringing without any music or movements I had. In my case on some tubes it was like a high frequency bell every 2sec for like a second. Very strange and very funny. I added some silicon rings as well this improved further but a small effect. What stayed is on my stepped shunt pot when I turn I have a short and very quiet zink....

This is a config I have now since summer no problem with tubes...

Pred
 
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