polarity flip switch "pops"?

when I flip the polarity on the pm1k channels I racked up, it pops. it is ok at low gain but is pretty loud when the gain is turned up.

I'm assuming that this is because I have used what appears to be a BBM (non shorting) dpdt toggle? the polarity flip does work fine, it just has the annoying pop. is there anything else that could cause this? any fixes besides replacing the switches?

Check your inputs for DC. Test between pin2-3 and ground. Any DC present will cause a pop when you flip.

I had that problem with a MOTU plugged into my ProVLA. MOTU had 1.82V DC on it's input (!!!) and when pushing the bypass switch caused a massive pop.

Do you have phantom on? -Is the changeover at the mic input or line or both?

Sorry for the simple question, but I'm not familiar with the PM1k.

Keith

hmm i don't remember if it was only with phantom or not and I don't have the unit here right now. perhaps you could let me know how to fix it if it's only with phantom or if it is popping with phantom on and off.

here is how I have it setup:

XLR Input > polarity flip dpdt > mic pad on a dpdt > 1:1 600ohm input transformer

I have the 48v phantom being delivered to pins 2 and 3 via 2 carefully matched 6.81k resistors.

thank you for the replies folks, much appreciated.

anybody? :green:

You'll NEVER get that to reliably flip without popping. It's in the mic line, and particularly with phantom present, it will pop LOUDLY. -It may well be dramatically quieter with phantom disengaged, but when engaged it WILL pop.

If you're thinking that you'd like a fairly 'silent' polarity switch, with the operational transparency which you find on mixing console channel strips, then this simple design is NOT the way to go about it. -It flips polarity, and it does so without introducing any active circuit complexity into the signal path... but it's not without its flaws, and this sort of pop is one.

If you want a polarity switch which will operate quietly under all conditions, you need it to happen AFTER the mic stage.

Keith

hi keith. thanks so much for the reply.

if I put the dpdt polarity switch after the output transformer, would that make it quieter? I don't need something silent, just something that isn't obscenely loud.

is it possible you have a link to some info or a schem or something that would explain how to get a quieter polarity flip like in a console?

what do you do when you are adding a polarity flip switch to a set of racked pres? I am going to rack some SSl9k's soon and any recommendations would be much appreciated. also...thanks so much for the ssl9k project!

Yep. -After the output transformer will be perfect.

Console manufacturers use a single opamp stage to invert at the unbalanced stage... just imagine an op-amp with the +input grounded, and a 10kΩ resistor between the output and the -input. Now connect the incoming signal to the -input via a second 10kΩ reistor. Arrange a single pole changeover switch to select either the signal input or op-amp output, and you've got a semi-typical console polarity flip circuit.

Keith

keith, you are the man! thank you!