Mic Preamp impedances...For the EEs out there


Oct 17, 2012
Lancaster, PA USA
Gear owned
DM3200 UH-7000 3030R2R
Most microphone preamp gear these days has a mic input impedance of about 1.5KOhms to, say, 3KOhms. My DM3200's pres have an input impedance of 2.2KOhms. And, most of my capacitor mics have an output impedance in the 150 to 200 Ohm range. This 10 to 1 impedance ratio seems to work well as a standard across the industry. Most everything I have read says that a mic preamp's input impedance should be about 10 times more than a microphone's output impedance.
But, recently I have purchased a certain brand of mic pre (not Tascam), and it's microphone input impedance is listed at 600 Ohms max, with the option of going down to 150 Ohms. With such less resistance, will these low impedance levels possibly distort or impair my newly modded capacitor microphones?
Nothing to worry about ! It will not impair your mic, just change frequency response and signal level. I've got ADL 600 mic pre with variable impedance (150, 300, 900, 1,500Ω. Great for subtle tone shaping.

Cheers !
Thanks for the reply! I like the tone shaping options too. But how often do you use a LDC with the 150, 300 and 900 settings on your ADL? I'm going to guess that you keep it at 1500 most of the time for more than sonic reasons.
Yes, LDC on vocals 1500 or 900 sometimes. SDC on ovearheads 1500 when looking for pristine, open sound, or 300-600 to impart more character sound. On the other hand my Sebatron preamp is 600 ohm by default and it sounds great using LDC on vocals.
I agree with Jazbina - it's more about tonality and frequency response. Being able to modify impedance is one more tone shaping option in the arsenal.

Perhaps an element more likely to affect a mic-preamp's sonic potential is whether it's FET or transformer-coupled. Transformer, multi-impedance pre amps tend to work better with some lower output mics - dynamics and most ribbons. But with powered mics/condensers, perhaps it's not as much an issue.

Thanks Dan,
The pre in question is a transformer coupled pre, the Warm Audio Tone Beast. Frankly, everything sounds great to me, no distortion or issues through all of my locker. I just wanted to make sure it stays that way.

BTW, I emailed Warm Audio about my concerns. The reply from Bryce said...

"I talked to CineMag. It looks like my specs are mis-leading in regards to impedance. CineMag supplies API their input transformer for their API 512c, the input impedance of that transformer is 150 ohms, but the industry lists transformer balanced input impedances at 10 times their transformer value. So API lists their unit at 1.5k or 1500 ohms. So by industry standards the TB12 has an input impedance of 1.5k and and 6k ohms. (1500, and 6000)."

Which sounds to me like he was trying to blow off my question with a response like..."wooops, my bad, should be 1500 and 6000 because that's how the industry lists these things." Well the lower specs are in every brochure/manual/review etc. that there is. I doubt this was a mistake.

I guess I just want to make sure I can confidently hook up my condensers to this thing for years without fear. Both of your feedbacks helped. Thanks again.
Well, CineMag IS the 'go-to' high end transformer source these days. But I question that dude's claims as well.

I also question my comparatively meager understanding of resistance and how that works with - or against - microphones. It's an adventure I just undertook this past year in an attempt to get more hands on time with these variables.

I wonder if it would be good (or entertaining) to contact CineMag and 'axe' for their take on this.

A thought - or not. :)


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