1290Preamp - Dynamic mics = hum & buzz. Condenser mics = lovely & quiet. Why?

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Redtns

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I've been trying to solve this for a few weeks now, but I can't see what the issue is...

As per the title, why does my 1073/1290 style DIY mic pre work impeccably when I use phantom powered condenser microphones (with gain @ +75db it's noise floor is actually quieter than my RME Micstasy), but when I use dynamic mics it becomes a buzzy, hummy mess?

The mics I've mostly tested with are a 414 and an sm57, but I've also tried a 421 and a schoeps mk4, with no appreciable change in behaviour. The impedance of the 414 is 200Ω, the sm57 150Ω, the 421 200Ω and the Schoeps is 35Ω(!?). I thought impedance might have been something worth investigating, but they're so similar..

It's powered by a linear, 317 based supply.

I suspected power supply noise as everything is mounted in the same metal chassis, but I don't see why it would be so completely perfect with a condenser mic?? Also, when I turn off the power supply (therefore disconnecting the power toroid from the mains) the noise barely changes during the (roughly) 5 seconds or so the unit continues to be powered by the capacitors remaining charge... So it's not mains hum.

If I unplug the dynamic microphone, the noise disappears.

I have pin 1 of both the input and output XLRs going to chassis (a star-earth next to the IEC inlet), along with any cable shields, the carnhill VTB 9046M I/P transformer casing and LED return paths. The audio 0V on the preamp PCB is only used for the preamp circuit, and returns to the same star earth via the power supply.

Any clues? It's so lovely and full with the condensers.. What's the inherent difference I'm missing?


 

ruffrecords

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PIN 1 of XLRs should go to chassis right next to the connector itself - you can usually use the XLR mounting screw. Analogue 0V from the PSU should connect to the chassis by the IEC socket as does the safety earth. There is no need to connect anything else to this point. No need to connect LED returns or cable screens to a star point.

Cheers

Ian
 

Redtns

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Thanks Ian,

I just moved the pin 1 connections to chassis to next to the connectors - no noticeable change...

Just noticed something else now - I've got a 3 resistor pad circuit before the input transformer, and when I turn it on it hums and buzzes badly with both condenser and dynamic microphones... Weirdly I hadn't noticed this before - I only briefly tried it with an audio output from my RME interface and it was fine.

Any other thoughts?
 

ruffrecords

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Redtns said:
Thanks Ian,

I just moved the pin 1 connections to chassis to next to the connectors - no noticeable change...

Just noticed something else now - I've got a 3 resistor pad circuit before the input transformer, and when I turn it on it hums and buzzes badly with both condenser and dynamic microphones... Weirdly I hadn't noticed this before - I only briefly tried it with an audio output from my RME interface and it was fine.

Any other thoughts?

Try completely bypassing the pad circuit and see if it still hums.

Cheers

Ian
 

abbey road d enfer

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Redtns said:
when I use dynamic mics it becomes a buzzy, hummy mess?
It could be that the mic is picking magnetic field from a transformer? Does hum change when you move the mic?
Is phantom permanently on? Does the noise change if you cut phantom?
 

Redtns

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abbey road d enfer said:
It could be that the mic is picking magnetic field from a transformer? Does hum change when you move the mic?
Is phantom permanently on? Does the noise change if you cut phantom?

The dynamics certainly pick up horrible hum from the power toroid when they're moved near to it, but for this test I've got the mics in another room and nowhere near a power source. Phantom is switchable and was off when using the dynamic mic..
 

abbey road d enfer

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Redtns said:
The dynamics certainly pick up horrible hum from the power toroid when they're moved near to it, but for this test I've got the mics in another room and nowhere near a power source. Phantom is switchable and was off when using the dynamic mic..
Do the 57 and the 421 have the same issue?
 

JohnRoberts

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This is pure speculation but phantom powered mics with active circuitry inside will generally have lower source impedance than dynamic mics.

Is preamp quiet when you terminate the mic input with a fixed resistor, is it quieter with pins 2 and 3 shorted to each other?

Do all mics act the same?

JR 
 

Cranehazard

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Feb 3, 2016
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Yes. And phantom mics have a preamp= gain and lower impedance. Dynamic mics will reveal the shortcomings of your preamp.
 
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