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tech / rec.audio.pro / Re: Mic preamp still humming along

SubjectAuthor
* Mic preamp still humming alongTatonik
+* Re: Mic preamp still humming alongJohn Williamson
|`* Re: Mic preamp still humming alongTatonik
| `* Re: Mic preamp still humming alongJohn Williamson
|  +- Re: Mic preamp still humming alongScott Dorsey
|  `* Re: Mic preamp still humming alongScott Dorsey
|   `- Re: Mic preamp still humming alongTatonik
+- Re: Mic preamp still humming alongJohn Williamson
+- Re: Mic preamp still humming alongDieter Michel
`* Re: Mic preamp still humming alongpalli...@gmail.com
 `* Re: Mic preamp still humming alongnone one
  `- Re: Mic preamp still humming alongpalli...@gmail.com

1
Mic preamp still humming along

<tq18jc$1p08$1@gioia.aioe.org>

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From: can...@cometothephonerightnow.invalid (Tatonik)
Newsgroups: rec.audio.pro
Subject: Mic preamp still humming along
Date: Sun, 15 Jan 2023 10:11:24 -0600
Organization: Aioe.org NNTP Server
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 by: Tatonik - Sun, 15 Jan 2023 16:11 UTC

I recently picked up a Benchmark MPA1 mic preamp that's about 12 years
old. I wanted one with detents on the gain for repeatable matched
levels between left and right channels for my pair of condensers, and
it's good for that. When I connected a couple of dynamic microphones,
though, I noticed a low-level hum. I changed microphones again, and the
pattern held: hum with dynamics, no hum with condensers. I disconnected
the preamp and moved it away from all the other equipment, listening on
the built-in headphone port, and it was the same.

The company no longer works on these, but a representative suggested
rotating the toroidal transformer to see if that would help. It changed
the nature of the hum slightly and perhaps reduced it somewhat, but I
wouldn't say the change was substantial. In the process, however, I
discovered that the hum was significantly reduced with the top case
removed. As soon as I put the top case back on, the hum would get
louder again.

Then, as an experiment, I swapped the original top case for one from a
DAC1 made by the same company. It is essentially the same steel top
case but is missing an extra layer of shielding that is glued to the
inside of the original (which I think is Mu-metal, based on an old
promotional video I found on YouTube). The hum level stayed about as
low as it was with the top case removed. Something about the original
case with the shielding makes the hum louder.

I'm out of my depth here, but I was wondering why the case makes a
difference, what the root cause of the hum could be, and if it's likely
something that can be fixed by a technician. The company representative
suggested it could be a dried out capacitor or something else entirely.

Re: Mic preamp still humming along

<k2io4rFflvuU1@mid.individual.net>

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From: johnwill...@btinternet.com (John Williamson)
Newsgroups: rec.audio.pro
Subject: Re: Mic preamp still humming along
Date: Sun, 15 Jan 2023 16:32:25 +0000
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 by: John Williamson - Sun, 15 Jan 2023 16:32 UTC

On 15/01/2023 16:11, Tatonik wrote:
>
> I recently picked up a Benchmark MPA1 mic preamp that's about 12 years
> old. I wanted one with detents on the gain for repeatable matched
> levels between left and right channels for my pair of condensers, and
> it's good for that. When I connected a couple of dynamic microphones,
> though, I noticed a low-level hum. I changed microphones again, and the
> pattern held: hum with dynamics, no hum with condensers. I disconnected
> the preamp and moved it away from all the other equipment, listening on
> the built-in headphone port, and it was the same.
>
> The company no longer works on these, but a representative suggested
> rotating the toroidal transformer to see if that would help. It changed
> the nature of the hum slightly and perhaps reduced it somewhat, but I
> wouldn't say the change was substantial. In the process, however, I
> discovered that the hum was significantly reduced with the top case
> removed. As soon as I put the top case back on, the hum would get
> louder again.
>
> Then, as an experiment, I swapped the original top case for one from a
> DAC1 made by the same company. It is essentially the same steel top
> case but is missing an extra layer of shielding that is glued to the
> inside of the original (which I think is Mu-metal, based on an old
> promotional video I found on YouTube). The hum level stayed about as
> low as it was with the top case removed. Something about the original
> case with the shielding makes the hum louder.
>
> I'm out of my depth here, but I was wondering why the case makes a
> difference, what the root cause of the hum could be, and if it's likely
> something that can be fixed by a technician. The company representative
> suggested it could be a dried out capacitor or something else entirely.
>
A dried out capacitor across the phantom supply smoothing is possible,
Have you switched off the phantom supply when the dynamic mics are
connected, and does it make a difference? Do you have the tools and
knowledge to temporarily disable the phantom power supply circuitry?

--
Tciao for Now!

John.

Re: Mic preamp still humming along

<k2io7nFflvuU2@mid.individual.net>

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From: johnwill...@btinternet.com (John Williamson)
Newsgroups: rec.audio.pro
Subject: Re: Mic preamp still humming along
Date: Sun, 15 Jan 2023 16:33:57 +0000
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 by: John Williamson - Sun, 15 Jan 2023 16:33 UTC

On 15/01/2023 16:11, Tatonik wrote:
>
> I recently picked up a Benchmark MPA1 mic preamp that's about 12 years
> old. I wanted one with detents on the gain for repeatable matched
> levels between left and right channels for my pair of condensers, and
> it's good for that. When I connected a couple of dynamic microphones,
> though, I noticed a low-level hum. I changed microphones again, and the
> pattern held: hum with dynamics, no hum with condensers. I disconnected
> the preamp and moved it away from all the other equipment, listening on
> the built-in headphone port, and it was the same.
>
> The company no longer works on these, but a representative suggested
> rotating the toroidal transformer to see if that would help. It changed
> the nature of the hum slightly and perhaps reduced it somewhat, but I
> wouldn't say the change was substantial. In the process, however, I
> discovered that the hum was significantly reduced with the top case
> removed. As soon as I put the top case back on, the hum would get
> louder again.
>
> Then, as an experiment, I swapped the original top case for one from a
> DAC1 made by the same company. It is essentially the same steel top
> case but is missing an extra layer of shielding that is glued to the
> inside of the original (which I think is Mu-metal, based on an old
> promotional video I found on YouTube). The hum level stayed about as
> low as it was with the top case removed. Something about the original
> case with the shielding makes the hum louder.
>
> I'm out of my depth here, but I was wondering why the case makes a
> difference, what the root cause of the hum could be, and if it's likely
> something that can be fixed by a technician. The company representative
> suggested it could be a dried out capacitor or something else entirely.
>
Other things to try are moving the dynamic mics round, as they aren't
all well shielded against stray magnetic fields.

--
Tciao for Now!

John.

Re: Mic preamp still humming along

<tq1j4i$pp1$1@gioia.aioe.org>

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From: can...@cometothephonerightnow.invalid (Tatonik)
Newsgroups: rec.audio.pro
Subject: Re: Mic preamp still humming along
Date: Sun, 15 Jan 2023 13:11:14 -0600
Organization: Aioe.org NNTP Server
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 by: Tatonik - Sun, 15 Jan 2023 19:11 UTC

On 1/15/23 10:32 AM, John Williamson wrote:

> On 15/01/2023 16:11, Tatonik wrote:
>>
>> I recently picked up a Benchmark MPA1 mic preamp that's about 12 years
>> old.  I wanted one with detents on the gain for repeatable matched
>> levels between left and right channels for my pair of condensers, and
>> it's good for that.  When I connected a couple of dynamic microphones,
>> though, I noticed a low-level hum.  I changed microphones again, and the
>> pattern held: hum with dynamics, no hum with condensers.  I disconnected
>> the preamp and moved it away from all the other equipment, listening on
>> the built-in headphone port, and it was the same.
>>
>> The company no longer works on these, but a representative suggested
>> rotating the toroidal transformer to see if that would help.  It changed
>> the nature of the hum slightly and perhaps reduced it somewhat, but I
>> wouldn't say the change was substantial.  In the process, however, I
>> discovered that the hum was significantly reduced with the top case
>> removed.  As soon as I put the top case back on, the hum would get
>> louder again.
>>
>> Then, as an experiment, I swapped the original top case for one from a
>> DAC1 made by the same company.  It is essentially the same steel top
>> case but is missing an extra layer of shielding that is glued to the
>> inside of the original (which I think is Mu-metal, based on an old
>> promotional video I found on YouTube).  The hum level stayed about as
>> low as it was with the top case removed.  Something about the original
>> case with the shielding makes the hum louder.
>>
>> I'm out of my depth here, but I was wondering why the case makes a
>> difference, what the root cause of the hum could be, and if it's likely
>> something that can be fixed by a technician.  The company representative
>> suggested it could be a dried out capacitor or something else entirely.
>>
> A dried out capacitor across the phantom supply smoothing is possible,
> Have you switched off the phantom supply when the dynamic mics are
> connected, and does it make a difference? Do you have the tools and
> knowledge to temporarily disable the phantom power supply circuitry?

Thanks for the suggestion. I just tried switching the phantom power on
and off with a dynamic mic connected, and it didn't seem to make a
difference. I don't have the tools or know-how to mess around with any
circuitry, but I did find a jumper on the board that's supposed to
prevent phantom power from being sent to the mic even if the exterior
switch is engaged. I don't know if it actually disables the supply
circuitry. Anyway, removing that jumper didn't alter the hum.

I took a photo of the circuit board and posted it to imgur. The phantom
power jumper is in the lower left hand corner:

https://imgur.com/f5Pao2J

One thing that does appear to take care of the hum is inserting a
Cloudlifter between the preamp and the mic. The weird thing is, though,
if I turn off phantom power (which means the Cloudlifter has no power
and the mic is effectively disabled) then I hear the hum again.

Re: Mic preamp still humming along

<k2j3viFhe5fU1@mid.individual.net>

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From: johnwill...@btinternet.com (John Williamson)
Newsgroups: rec.audio.pro
Subject: Re: Mic preamp still humming along
Date: Sun, 15 Jan 2023 19:54:25 +0000
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 by: John Williamson - Sun, 15 Jan 2023 19:54 UTC

On 15/01/2023 19:11, Tatonik wrote:

> Thanks for the suggestion. I just tried switching the phantom power on
> and off with a dynamic mic connected, and it didn't seem to make a
> difference. I don't have the tools or know-how to mess around with any
> circuitry, but I did find a jumper on the board that's supposed to
> prevent phantom power from being sent to the mic even if the exterior
> switch is engaged. I don't know if it actually disables the supply
> circuitry. Anyway, removing that jumper didn't alter the hum.
>
> I took a photo of the circuit board and posted it to imgur. The phantom
> power jumper is in the lower left hand corner:
>
> https://imgur.com/f5Pao2J
>
> One thing that does appear to take care of the hum is inserting a
> Cloudlifter between the preamp and the mic. The weird thing is, though,
> if I turn off phantom power (which means the Cloudlifter has no power
> and the mic is effectively disabled) then I hear the hum again.
>
>
All I can think of then, is an earth loop, defective cables or a
microphone that is not wired correctly. I have seen (Admittedly cheap
and nasty) microphones that use an XLR connection, but which are not
balanced, using one of the signal pins as the signal earth, ignoring the
normal earth pin altogether, or, even worse, to power the preamp in the
condenser section.

--
Tciao for Now!

John.

Re: Mic preamp still humming along

<tq1vpd$1lh$1@panix2.panix.com>

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From: klu...@panix.com (Scott Dorsey)
Newsgroups: rec.audio.pro
Subject: Re: Mic preamp still humming along
Date: 15 Jan 2023 22:47:09 -0000
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 by: Scott Dorsey - Sun, 15 Jan 2023 22:47 UTC

What condenser mikes are you trying? Maybe Neumann mikes with pin 1
problems?

If your mike has a pin 1 issue, it'll pick up noise from stray magnetic
fields... and those fields will change a lot when you pull the cover off
the preamp and expose the transformer.

Also of course it would not hurt to make sure all ground connections inside
the preamp are clean and tight.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Re: Mic preamp still humming along

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Newsgroups: rec.audio.pro
Subject: Re: Mic preamp still humming along
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 by: Scott Dorsey - Sun, 15 Jan 2023 23:35 UTC

Wait, wait... Hum with dynamics, not with condensers?

Does the hum change when you move the dynamic mikes around? Because that
sure sounds like typical magnetic pickup.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Re: Mic preamp still humming along

<tq6kuj$1p9o$1@gioia.aioe.org>

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From: can...@cometothephonerightnow.invalid (Tatonik)
Newsgroups: rec.audio.pro
Subject: Re: Mic preamp still humming along
Date: Tue, 17 Jan 2023 11:12:51 -0600
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 by: Tatonik - Tue, 17 Jan 2023 17:12 UTC

On 1/15/23 5:35 PM, Scott Dorsey wrote:

> Wait, wait... Hum with dynamics, not with condensers?
>
> Does the hum change when you move the dynamic mikes around? Because that
> sure sounds like typical magnetic pickup.
> --scott

Yes, the pattern has held with what I've tried so far. No hum with a
pair of Oktava MK-012 and an AT2035, hum with an E/V RE20 and a
Sennheiser 441. Moving the dynamic microphones around doesn't seem to
change it, although I'll try that again to be sure. I've done it in a
couple of different rooms, with some different cables, and with and
without a plug-in RFI/EMI power filter. So far the only thing I've
found that substantially changes the nature of the hum is removing or
changing the top case of the preamp.

Re: Mic preamp still humming along

<k2skcqFp9pU1@mid.individual.net>

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From: dmic...@prosound.de (Dieter Michel)
Newsgroups: rec.audio.pro
Subject: Re: Mic preamp still humming along
Date: Thu, 19 Jan 2023 11:29:43 +0100
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 by: Dieter Michel - Thu, 19 Jan 2023 10:29 UTC

Hi,

> [...] When I connected a couple of dynamic microphones,
> though, I noticed a low-level hum. I changed microphones
> again, and the pattern held: hum with dynamics, no hum
> [...] listening on the built-in headphone port, and
> it was the same.

do you compare condensers and dynamic mic at the same
gain settings or at gain settings appropriate for the
respective mics? If the latter, the hum might be present
also with the condensers, but too low in level to be
audible. Maybe some issue in the power supply then?

What does happen when you replace the dynamic mic with
just an XLR plug with a 200 Ohm metal film resistor
soldered from Pin 2 to Pin 3 - still humming?

Best,

Dieter

Re: Mic preamp still humming along

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Date: Thu, 19 Jan 2023 14:11:54 -0800 (PST)
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Subject: Re: Mic preamp still humming along
From: palliso...@gmail.com (palli...@gmail.com)
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 by: palli...@gmail.com - Thu, 19 Jan 2023 22:11 UTC

Tatonik wrote:
>
>
> The company no longer works on these, but a representative suggested
> rotating the toroidal transformer to see if that would help.

** Very suspicious that a rep would suggest that idea straight off.

Try unbolting the toroidal and moving it up and away as far as possible.
Its location next to all the XLRs is *rediculous*.
IMO the background hum is always there but the high output of condenser mics hides it.

BTW have a Chandler " Germanium pre-amp" on the bench now- what an odd ball.
Doesn't hum but.

.....Phil

Re: Mic preamp still humming along

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Subject: Re: Mic preamp still humming along
From: makol...@yahoo.com (none one)
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 by: none one - Tue, 24 Jan 2023 13:07 UTC

On Thursday, January 19, 2023 at 5:11:56 PM UTC-5, palli...@gmail.com wrote:
> Tatonik wrote:
> >
> >
> > The company no longer works on these, but a representative suggested
> > rotating the toroidal transformer to see if that would help.
> ** Very suspicious that a rep would suggest that idea straight off.
>
> Try unbolting the toroidal and moving it up and away as far as possible.
> Its location next to all the XLRs is *rediculous*.
> IMO the background hum is always there but the high output of condenser mics hides it.
>
>
> BTW have a Chandler " Germanium pre-amp" on the bench now- what an odd ball.
> Doesn't hum but.
>
> ....Phil
Its been many years since I posted here. To the OP, yes you have described exactly a case of magnetically induced hum. The source is the power transformer. Since it appears there is no audio input transformer, it is not clear where the susceptibility is. But Phil is correct, loosen the bolt on the torroidal power transformer, try rotating it, try moving the wires that are not twisted. Torroidal transformer are supposed to minimize the magnetic leakage but they are not perfect. The Mu metal shield on the cover is likely directing the magnetic field instead of stopping it. If you can break a piece of it off and shape it so that you can put it BETWEEN the torroidal transformer and the input circuits, you might find a spot that helps. This will definitely be a case of experimental poke and hope. Good luck Mark

Re: Mic preamp still humming along

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Subject: Re: Mic preamp still humming along
From: palliso...@gmail.com (palli...@gmail.com)
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 by: palli...@gmail.com - Wed, 25 Jan 2023 22:43 UTC

none one wrote:
palli...@gmail.com wrote:
> > Tatonik wrote:
> > >
> > >
> > > The company no longer works on these, but a representative suggested
> > > rotating the toroidal transformer to see if that would help.
>
> > ** Very suspicious that a rep would suggest that idea straight off.
> >
> > Try unbolting the toroidal and moving it up and away as far as possible..
> > Its location next to all the XLRs is *rediculous*.
> > IMO the background hum is always there but the high output of condenser mics hides it.
> >

> Its been many years since I posted here. To the OP, yes you have described exactly a case of magnetically induced hum.
> The source is the power transformer. Since it appears there is no audio input transformer, it is not clear where the susceptibility is.

**Any loop area in the wires connecting the mic to the pre amp can have a small voltage induced into it by the power transformer WHEN is that damn close. Even a fraction of a microvolt can be audible.

>But Phil is correct, loosen the bolt on the torroidal power transformer, try rotating it, try moving the wires that are not twisted. Torroidal transformer are supposed to minimize the magnetic leakage but they are not perfect. The Mu metal shield on the cover is likely directing the magnetic field instead of stopping it.

** Not likely Mu-Metal but just a strip of toroidal lamination material. Better than nothing I guess.

....... Phil


tech / rec.audio.pro / Re: Mic preamp still humming along

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