Ioaudio MK47 - Background noise?

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l.e.d

Member
Joined
Apr 7, 2006
Messages
20
Location
Australia
Hi, I bought a MK47 a little while back already built and it has a white noise type hiss in the background. I've tried a couple of sets of tubes but it doesn't alter the level of the background noise.
It's not loud enough to be a problem with recording in a track full of other parts but if you were to record a solo vocal/ spoken voice you would notice the background noise. It's definitely noisier than my other mics, (neumans akg etc) but they are all solid state. A tech who does some work for me suggested it could be the carbon resistor (I think it was 100K?) but that changing it would alter the tone. Tonally it sounds great, but I've read other build posts saying theirs were quiet.
Anyone whose built one got any pointers as to what might be the cause, or comments on the self noise level of their builds? This one has a Thiersch blue M7 capsule. Is it maybe worth trying more tubes?
Thanks.
 
Hi Gus, yes I've done a search and read about gassy tubes, resistor values etc, but was wanting to see if anyone had similar experiences with this specific build in it's stock form.
I have a number of tube eq's and compressors and preamps and haven't really found noise to be such an issue before, but they are obviously
used in different ways to the mic and are different tubes. I've used original 47's a number of times and haven't found them to be as noisy either.
I could just buy a batch of 10 408a's and go through them all, but rather than drop the cash
first I wanted to get some opinions from others who've built the mic. Can 408's really be that much of a crapshoot as far as noisy tubes go?
I realize I could just have 4 tubes that are all noisy but in trying all possible combinations of the 4 the noise didn't alter.
I thought I'd ask if anyone had any other suggestions for the source of noise. I've been through the
build thread and seen comments about the resistor and using an aluminium housing one, but from memory last time I had the mic open it already has one of these.
I suppose the other possibility is some flux/schmutz in the high impedance section but doesn't look as easy to give it all a clean now that it is all built.
It's not unworkably noisy, it would just be great to drop the noise floor a little more.


Thanks for replying.
Cheers.
 
Check that the high impedance parts are very clean, use isopropyl alcohol...
check the psu too, use 10W resistors, remember that too much heat=noise
however, two tubes in parallel are noisier than one tube or a jfet
408 are also prone to microphonic noises in fixed bias...
cheers
 
Yes, tubes can be noisy.
But IMOAE these 408a's are better than many here choose to believe.
No harm in trying two others. It's a quick way to check.
But my guess is there's another problem.
Some of the possibilities have already been mentioned.



Henk
 
Simple thing to try, disconnect the capsule wires at the board not at the capsule.  Install a cap of 50pf  to 100pf (don't handle the cap(s) with your hands make sure they are very clean) for the capsule install the circuit back in the body and check for noise.  This is a way to check if the capsule is a noise source.  You could sub in a known good capsule as a test. 
It is not a bad idea to use no powder nitrile gloves

I have posted in the past that I used a method to burn in tubes for C800G like circuits.  I built 4 cathode bias circuits 100K plate, selected cathode resistor(for the plate current you want) and a grid to ground resistor.  I made the filament supply switchable between something like 6.8VDC and 5.6VDC for 6.3VDC tubes (three diodes in the ground leg of a 7805).  I would run the tubes for about 4hours at the different heater voltages sometimes for a total of a few days, then I would test the tubes in the microphone.

How many hours/days have you had the tubes powered up in the microphone?
 
Do you have the proper resistor installed in the mic for the heater voltage.  Not the value but the type.  If it's a sandbar type that is your issue. 
 
Thanks for all the replies, there's a few options there to go through. The tubes have probably run around 50 hours. I had thought about replacing the capsule with a cap to see if the circuit is quieter
without it in. The resistor for the heater voltage - that's the 1K5 one that is mounted to the mic body? If so it is a metal housed one but I'll have another look see. I hadn't thought about the resistors in the PS but will check them as well.
Thanks again for the help.
Cheers.
 
I agree about the 1k5 dropper resistor I had issues try couple different ones
 
dandeurloo said:
Do you have the proper resistor installed in the mic for the heater voltage.  Not the value but the type.  If it's a sandbar type that is your issue.

Dan are you talking about the white block ones?  Aren't those wire wound?  I think that's what's in mine...  anyone care to comment?
 
I bought a batch of 10 408s and did not regret it.

On an extremely quiet vocal with alot of compression noise might be an issue but no more than a vintage U47.
 
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