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.
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.
On 15/01/2023 16:11, Tatonik wrote:
A dried out capacitor across the phantom supply smoothing is possible,
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.
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.
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
[...] 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.
The company no longer works on these, but a representative suggested
rotating the toroidal transformer to see if that would help.
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.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
....Phil
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.
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 thecover is likely directing the magnetic field instead of stopping it.
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