Max voltage across microphone signal transformer.

[50AE]

Hello,

I have some NOS mic preamp microphones with a 15k:600R impedance ratio. I was thinking of using them in a parafeed configuration as an anode load for a tube preamp. The problem I thought of is when B+ is applied, the parafeed capacitor conducts until charged and a big voltage impulse swings across the primary. For now, I have three "audiophile" ways to resolve this problem.

1. Rectifier tube heater (auto) soft start
2. Smart soft start - using somewhat larger PSU capacitors, so the B+ ramps up slowly enough to not let an excess swing on the parafeed capacitor
3. Shunt resistor across the primary with an automatic "mute" NC relay.

And lastly, do you think these microphone transformers can withstand voltages such as 100-200V, or I shouldn't risk?

Regards,
Alexander.

 

[JMFahey]

I'd worry more about DC passing through that winding and saturating the core.

It's absolutely NOT rated or designed for that.

 

[rayma]

I'd worry more about DC passing through that winding and saturating the core. It's absolutely NOT rated or designed for that.

I think he means that the transformer is after the coupling capacitor.

 

[rayma]

the parafeed capacitor conducts until charged and a big voltage impulse swings across the primary.
do you think these microphone transformers can withstand voltages such as 100-200V

A true slow warm-up rectifier may be ok, but check the turn-off transient as well.
A muting relay across the primary, enabled after warm-up, and muting upon switch-off, would be safest.
I have a 555 timer circuit that is well tested, if you'd like to do that.
Don't assume anything about the voltage rating, as it could be as low as 50V to 100V.

 

[abraxalito]

Microphone transformers are designed to handle mV signals, not tens or hundreds of volts.

 

[DF96]

I would be very surprised if a microphone transformer works OK and survives as a parafeed interstage. The most likely outcome is a magnetised core and significant second order distortion.

 

[Merlinb]

And lastly, do you think these microphone transformers can withstand voltages such as 100-200V, or I shouldn't risk?

Nope. A typical mic transformer can't cope with low frequency signals larger than a few volts (RMS) before saturating.

 

[50AE]

The transformer will see no more than 6V RMS of swing (17V p-p) across its primary. On the secondary this will appear as 1,2V RMS. I will measure it if it saturates. But I doubt the insulation will break on even 20V p-p.

The only thing that worries me are the transient voltages when the capacitor charges. A mute resistor and relay is pretty easy to do.

 

[Merlinb]

The only thing that worries me are the transient voltages when the capacitor charges. A mute resistor and relay is pretty easy to do.

Or just a couple of Zener diodes across the primary...

 

[Tom Bavis]

Altec 15335 (a 15K:600 bridging transformer) is rated for 6V RMS (+18 dBm, 600 Ohms) at 20 Hz and .03% distortion. That would be 30V RMS on the 5K winding. If you're using something similar, no problem with the signal levels you're using.