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UA LA2a Re-issue HUM Dynamic Microphones
Old 27th March 2017
  #1
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JayTee4303's Avatar
UA LA2a Re-issue HUM

Purchased new, just over a year ago. -48 dB hum on my meters, from day one. With or without input cable attached. Spec sheet says S/N greater than 70dB. Furmann conditioner, on an exclusive studio AC drop, installed for the purpose, everything else here is dead silent, including the UA 1176 right beside it in the rack. Any circuit in the place, no difference.

Shipped it back to UA, they claimed it was "in spec", claimed they didn't touch it, but it came back quieter and about 15% hotter for a given gain setting.

Now it's noisy as hell again. Main hum component at 120 hz, lesser components at 60, 240, 480 etc.

Izotope De-Hum effectively vanishes it, but I want it in the monitor chain, sans buzz.


This morning, I noticed a very slight hum reduction flipping the meter select switch. Further investigation uncovered significant microphonic effect from light tapping, especially in upper left area of front faceplate. along with a very significant hum reduction...for now.

Bad tube? Worth swapping? Which one(s)? Will it need re-biased?

Help me out, please. I love what this does, but the harsh buzz between takes is driving me nuts, and there's NO WAY I'll subject clients to noise like this from supposed high end equipment.
Old 27th March 2017
  #2
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Oldone's Avatar
You need a tech.
Old 27th March 2017
  #3
Sounds like a tube. But I'd say you need a tech as well.

I love LA2A's. I swapped my out for expensive EV tubes (because I thought it would give me more love) but was no better. I didn't re bias. I suppose I should have.
LA2A's are simply awesome. I want more and more of them. They do make a slight noise though. Thats part of the mojo and each one is a bit different.
I feel for you. Hopefully its something as simple as a bad pin connection on a tube.
Old 27th March 2017
  #4
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hasbeen's Avatar
Quote:
Originally Posted by JayTee4303 View Post
Bad tube? Worth swapping? Which one(s)? Will it need re-biased?
I agree with audiokid.

Try the 12AX7 for starts. My LA2A is so sensitive to 12AX7 that I use it to sort out quiet ones from noisy ones.

Mine is dead quiet. However I have switched out the tubes and the opto-attenuator for a Kenetek.

Thats weird about the meter thing though.

Old 27th March 2017
  #5
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JayTee4303's Avatar
I appreciate the replies so far. I will select a tech, several options available, but I want to poll long time local engineers, and talk to one in particular, an exclusive tube amp business, guitar and audiophile, with a pretty wide following.

I also want to fine tune something from the original post. By accident, I discovered that tapping on the faceplate of the comp, quieted it down considerably, in addition to producing an audible..."microphonic echo"...kind of a sound. Kind of like dropping a pebble down into the water at the bottom of a well. (Very much like inducing a transient into the mouth of a 3 foot diameter corrugated culvert, if you've even had that dubious pleasure.)

I discovered this because the detent clicks on the meter select knob had a similar, but much lesser effect, it's still a tap, but not as much amplitude as a fingernail flick. Since that tapping, say, 6 hours ago...the hum dropped and remains about 12 dB below where it was... it isn't as quiet as I'd prefer it (that would be minus infinity, natch), but it's SO much quieter than where it was, I am tempted to call it "fixed for now."

However... the overall behavior reminds me of a light bulb that has gone out, or become intermittent, when you go to unscrew it, and the slight vibration buys you a couple extra days or weeks out of a loose filament. Filament.... tube problem guess... that's the connection.

The one reason I hesitate to start swapping tubes, especially the 12ax7... that's a preamp tube, yes? Well, the hum, though quieter now, is constant, at any and all gain settings, and whether or not the input jack is even populated. Seems to my un-educated brain, therefore, that none of the audio tubes has bearing on this. Fair assumption or no?
Old 27th March 2017
  #6
Its called microphonic. Its the sound of a bad tube or a problem.
https://www.google.ca/search?q=micro...A-TU0gKpuYLQCA.
Old 28th March 2017
  #7
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JayTee4303's Avatar
Quote:
Originally Posted by audiokid View Post
Its called microphonic. Its the sound of a bad tube or a problem.
https://www.google.ca/search?q=micro...A-TU0gKpuYLQCA.
Think it's time to take it out of service. Beats blowing something up. Remember something in the manual or online about tubes and bias on these, but I'm on pretty shaky ground with all that. I'll look into service...locally...tomorrow.
Old 28th March 2017
  #8
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just find someone with a tube tester and check them...
Old 28th March 2017
  #9
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LA2A biasing as described on the UA site is not really the same thing as biasing for example a guitar amp which has a dedicated bias control. In the case of the LA2A it's a procedure to calibrate the unit after a tube swap. Using R37 you can calibrate the units high frequency response setting to get flat (or if you want to experiment some other audio response) settings.
Old 29th March 2017
  #10
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JayTee4303's Avatar
Quote:
Originally Posted by Oldone View Post
LA2A biasing as described on the UA site is not really the same thing as biasing for example a guitar amp which has a dedicated bias control. In the case of the LA2A it's a procedure to calibrate the unit after a tube swap. Using R37 you can calibrate the units high frequency response setting to get flat (or if you want to experiment some other audio response) settings.
Thinkin smiley face EQ ana Beats sticker for the faceplate...

Sorry man, it just got away from me... :-)

Microphonics... and explosions. How does a bonehead know that a tube swap corrected a bad tube, and not a death dealing problem that led to a bad tube?
Old 29th March 2017
  #11
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Salty James's Avatar
I have 2 La2a's reissues. Both slightly different levels of ambient unit hum.
My hum does not get into the recorded signal though.
Really sounds like you got bad tubes.
Anyway, welcome to the world of hunting down great sounding QUIET!! tubes. It's a depressing miasma of unfulfilled $25-75 purchases.

When my units were "new" (actually recent model used purchases) both of them had Teletronix tubes in them. And were basically silent. Quieter than my power supplies. Pretty sure UA spec'd each unit/tube combo for quality. This is part of the service as selecting tubes is a mess.

Like someone else said. You can use the unit to test tube quality. It's very sensitive to tube quality.

Tracking with them is an awesome experience and good tubes last 5+ years easy. But finding the right tubes is tough.

I went NOS and broke down and called Bowie. He's deals high quality tested NOS tubes. Not cheap but worth it!!

La2a's do need actual tube calibration/biasing. If you replace the 6AQ5 (or 6005W) tube.

La2a's rock!
Two knobs.. (Actually, I have 4 )
Go!

Last edited by Salty James; 30th March 2017 at 12:53 AM..
Old 7th April 2017
  #12
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JayTee4303's Avatar
Quote:
Originally Posted by Salty James View Post
I have 2 La2a's reissues. Both slightly different levels of ambient unit hum.
My hum does not get into the recorded signal though.
Really sounds like you got bad tubes.
Anyway, welcome to the world of hunting down great sounding QUIET!! tubes. It's a depressing miasma of unfulfilled $25-75 purchases.

When my units were "new" (actually recent model used purchases) both of them had Teletronix tubes in them. And were basically silent. Quieter than my power supplies. Pretty sure UA spec'd each unit/tube combo for quality. This is part of the service as selecting tubes is a mess.

Like someone else said. You can use the unit to test tube quality. It's very sensitive to tube quality.

Tracking with them is an awesome experience and good tubes last 5+ years easy. But finding the right tubes is tough.

I went NOS and broke down and called Bowie. He's deals high quality tested NOS tubes. Not cheap but worth it!!

La2a's do need actual tube calibration/biasing. If you replace the 6AQ5 (or 6005W) tube.

La2a's rock!
Two knobs.. (Actually, I have 4 )
Go!
Greedy 4 knob pig...

:-)

Swapped the AX7s for new Mesa Boogies I had on hand. The markings on the ones I removed from this new LA2a exactly match some JJ Electronics AX7s.

She is now 4.3 dB quieter. I gain matched two vox tracks, before and after tube swap. I pushed both pretty hard, and after 25 years on construction sites I can distort an SM-58 when need be.

The tracks top out at -12.2dB, with the hum at -59.4 before the swap, -63.7 after.

Manual specs say "Noise: 75 dB below +10dBm output level", whatever that means. I read my noise at 51.5dB below peak signal, but, I have noted, the hum is constant regardless of LA2a gain setting, or input level.

Perhaps I have to drive it harder to reach the specified -75 figure?

In any event, next up for swap is the 12BH7. My local tube amp gurus didn't return my calls regarding biasing the 6AQ5, so that's on hold atm.

Probably grab some higher end AX7s with the12BH7, spares can't hurt, right?
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