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Author Topic: U87 Ai - Noisy with some preamps  (Read 1947 times)

Moisie

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U87 Ai - Noisy with some preamps
« on: January 25, 2022, 09:13:30 AM »

Hello:

I've got a situation which has really stumped me: I've got a U87 Ai (Circuit Diagram 04) which gives me a lot of noise on its output compared with another U87 Ai (Circuit Diagram 06), when used with some preamps.

For example, here are the two mics feeding a Focusrite Red 1 (same preamp channel, one recording made after the other with the mic in the same place in the room):



Yet feeding a UA M610, the response is much more even:



I've swapped the capsule from the 'noisy' U87 onto the body of the 'clean' one - and it remains clean.

I've measured the polarising voltage at the top of C6 and C9, and it looks ok (accounting for the loading of the multimeter):

  • Noisy U87:            ±54.6V
  • Working U87:         ±55.1V

Am I chasing a ghost here, or does this seem like genuinely unexpected behaviour?...

Thanks!
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RuudNL

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Re: U87 Ai - Noisy with some preamps
« Reply #1 on: January 25, 2022, 10:30:17 AM »

If you measure at C15 / C32 you will get a more reliable value, because you don't have the voltage drop over the 1 M.ohm resistors.
(There is no current through these resistors in the U87ai circuit.)
But I don't expect a problem here.
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Jim Williams

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Re: U87 Ai - Noisy with some preamps
« Reply #2 on: January 25, 2022, 11:23:47 AM »

Some mic preamps use a series resistor in front of the 6.81k distribution resistors. If that resistor is higher than about 500 ohms there will be a voltage drop before the 6.81k resistors. I've seen those resistors in some preamps as high as 6.81k, some are 2200 ohms.
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RuudNL

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Re: U87 Ai - Noisy with some preamps
« Reply #3 on: January 25, 2022, 04:38:03 PM »

Specified current consuption of the U87ai is 0,8 mA.
An extra series resistor with a value of 6800 Ohm would cause a voltage drop of 5,44 V.
This would bring the supply voltage for the microphone under the lower acceptable voltage of 44 V. (Phantom power=48 V, + or - 4 V.)

But would this introduce extra noise?
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Moisie

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Re: U87 Ai - Noisy with some preamps
« Reply #4 on: January 26, 2022, 04:10:41 AM »

Hello all:

Many thanks for your input - very much appreciated.

I'm not in a position to make more measurements with the U87 in question connected to the Red 1 right now, but a couple more bits of information that I already have:

  • The Red 1 schematic shows the 48V resistors as being 3k6 - not 6k8 - and with no series resistor from the regulator output;
  • I measured ~45.6V at the junction of R115/R116 (the 2k2 phantom-extraction resistors) in the U87 - so I don't think it's a question of the 48V supply from the preamp sagging under load.

...so I'm a bit lost as to what else from the preamp might be causing the radically different behaviour!
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Moisie

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Re: U87 Ai - Noisy with some preamps
« Reply #5 on: January 28, 2022, 08:08:17 AM »

[Reposting from memory]

Quote
I suggest, you take your two U87Ai and plug them into at least two additional, different brands of mic pres.

Aside from the Focusrite Red 1 and the UA M610, I also tested with a RME Fireface UFX - which showed much the same response as the M610. (I left this out in the name of brevity - apologies if I was being excessively brief!)

Quote
If any noise floor difference is observed during these additional tests, have someone knowledgeable look at the noisy U87Ai.

Yeah - I think it's heading down that path. A bit unfortunate as logistics are a bit difficult (shipping from the UK); I sent it to Sennheiser a while ago for service - whilst they cleaned the capsule fine, they didn't attend to this noise - and were extremely difficult to liaise with, so I don't hold out much hope they'll be useful option again...


RuudNL had also commented on this:

Quote
Yes. You drop a 60VDC polarization voltage to 40-some volts, and you are in serious noise land.

But I'm seeing ~55VDC (slugged down by the multimeter), and only 0.5V difference between the noisy and working mics - so I'd be surprised if the problem was along this line.

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Moisie

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Re: U87 Ai - Noisy with some preamps
« Reply #6 on: February 07, 2022, 05:19:16 AM »

Hello:

Sorry for the delay in following-up on this - it took a while to arrange some more tests...

So - to accompany the initial charts showing the response of the U87s via a Focusrite Red 1 and UA M610, I can now offer up tests via a Focusrite Red 8 and RME Fireface UFX:





Obviously the Red 8 is going to be the same design as the Red 1 - but it's useful to know that it's not a unit-specific fault with the original Red 8.

I've also run a couple more tests in far-from-ideal circumstances: the U87 in question compared with a 'vintage' U87 (Circuit Diagram '930- 20') that's in need of servicing in a not-very-quiet office - but it gave me chance to compare the response via a RME Fireface UFX and a UA Apollo x4. I'm not sure these are going to be very useful, but here they are:






I also took some more voltage readings - which showed the noisy U87 as performing the same as the Circuit Diagram 06 model:

- Junction of R115/R116:
    - Noisy U87 on Red 8: 45.7v
    - Clean U87 on Red 8: 45.7v
    - Noisy U87 on UFX: 44.2v
    - Clean U87 on UFX: 44.3v
- C32 / C15:
    - Noisy U87 on Red 8: 60.1v / -60.2v
    - Clean U87 on Red 8: 60.2v / -60.2v
    - Noisy U87 on UFX: 60.2v / -60.3v
    - Clean U87 on UFX: 60.2v / -60.3v


Any further thoughts? Would it be 'expected' behaviour that a Circuit Diagram 04 model would produce excessive noise with some preamps?
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David Satz

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Re: U87 Ai - Noisy with some preamps
« Reply #7 on: February 07, 2022, 01:01:44 PM »

I have no good guess about what's going on here and am definitely following this with interest. We might all learn something from whatever you eventually find out. Good luck!
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RadarDoug2

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Re: U87 Ai - Noisy with some preamps
« Reply #8 on: February 07, 2022, 03:10:02 PM »

As usual when people paste screen shots of audio apps, these are pretty much unreadable.
What would be more informative would be broadband and A weighted signal to noise figures.
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klaus

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Re: U87 Ai - Noisy with some preamps
« Reply #9 on: February 07, 2022, 04:49:46 PM »

As I finally read your posts once more, this time thoroughly, I pondered the variables you introduced, then I deleted my previous, rather sloppy responses.

To say it right up front: I hate the U87Ai version with the daughterboard. It never sounds good, even under the best of circumstances; besides, it does not allow for custom-biasing of the FET for lowest noise, and makes parts substitution impossible in case of a component failure.

In my mind, this is a case of the daughterboard mic (04) having inherent midband noise/hiss, likely generated by a component on that dreaded board. I don't even care whether some pres work better with this mic than others - as you have demonstrated, all pres you used were capable of feeding enough current to both mics, i.e. there is no excessive current draw of the bad mic's processor; and polarization voltage was not an issue either.

I see two solutions:
I can send you a couple of daughterboards (I throw them out as soon as I lay eyes on them), and you can install one or the other.

Or, the better suggestion in my opinion, convert the 04 circuit to 06. It's easy, all parts are readily available, and you can enjoy a less crunchy, more refined, more musical U87 experience, with an opportunity to custom-bias each FET to lowest noise floor.

If I can be of further help, call me.


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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Moisie

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Re: U87 Ai - Noisy with some preamps
« Reply #10 on: February 08, 2022, 08:03:56 AM »

I have no good guess about what's going on here and am definitely following this with interest. We might all learn something from whatever you eventually find out. Good luck!

Thanks David! I'm baffled by it too - I can't understand what the mechanism is that would be causing such a different response...
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Moisie

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Re: U87 Ai - Noisy with some preamps
« Reply #11 on: February 08, 2022, 08:06:36 AM »



Or, the better suggestion in my opinion, convert the 04 circuit to 06. It's easy, all parts are readily available, and you can enjoy a less crunchy, more refined, more musical U87 experience, with an opportunity to custom-bias each FET to lowest noise floor.


Thanks Klaus - I really appreciate your input here, and I'm reassured that I'm not overlooking something obvious.

I'm going to investigate the FET upgrade you describe - I'll report back here with the results!
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