Search found 37 matches

by olafmatt
Mon Oct 07, 2019 2:18 pm
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

I look at it this way: From a DC perspective if the output of the mic is a few hundred millivolts out of balance and the servo forces the resistance bridge back into sub-millivolt balance how is that any worse than it being in perfect balance to begin with? It can even be a good thing! If you have ...
by olafmatt
Mon Oct 07, 2019 12:57 pm
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Olafmatt's servo that I used in the MC preamp works great at controlling the Vbe offset but I'm not sure it has enough range to also correct the larger offsets the mic presents. I would guess it would allow for large enough a correction, but it would need less attenuation at the output of the servo...
by olafmatt
Mon Oct 07, 2019 2:23 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

I think I've seen XLR chassis sockets with a switch that gets engaged once an XLR plug gets plugged in. That way a PIC (or some other logic) could detect when a mic gets connected and then run a callibration for a while and then store that value until the mic gets diconnected... but that way you wou...
by olafmatt
Sat Oct 05, 2019 5:29 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Ah, dual servo loops, now I understand. Will have to think about this, whether it makes any difference in reality. What I realized is that if you have a servo that is correcting the offset across Rg, then this correction will also show up on the transistor's bases. Not in a totally linear way (becau...
by olafmatt
Sat Oct 05, 2019 3:01 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Just checking in very infrequently, so noticed this just now... Vary the voltage on the top-side of the phantom pull-up resistors slightly to bring the mic output into balance. Duh, should've thought of that one before - it has the advantage of being able to correct large errors (tens of millivolts)...
by olafmatt
Mon Jun 11, 2018 6:32 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

To me this is not unique to an input-capacitorless topology - its a concern anyone should have using the input-correcting servos we seen thus far. Yes. I'm sorry if I didn't make it clear enough that this is a general concern and does not just apply to an input-capacitorless micpre. - An input (any...
by olafmatt
Mon Jun 11, 2018 4:57 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Yes, with a transformer input the low(-ish) DC resistance of the primary winding basically shorts the two inputs together at DC. What I'm worried about is having an active feedback system controlling the DC offset. Because with an active system you can get things like overshoot which leads to peakin...
by olafmatt
Sun Jun 10, 2018 5:58 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

What about just servo'ing the top side of the phantom resistors? That would work and be quite similar to injecting the servo correction into the bases. I think our problem is that we want zero VOLTAGE offset on the input and don't really care whether the microphone consumes unequal CURRENT. Just th...
by olafmatt
Sun Jun 10, 2018 5:12 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Just thinking aloud now: With that current imbalance spec of +/-4%, we'd need to deal with a max. of 2V DC offset of either polarity. So if we would split the gain into two stages, we could have a first gain stage (with differential output) with a fixed gain of 20dB and not need a servo. Assuming th...
by olafmatt
Sun Jun 10, 2018 4:21 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 122
Views: 120876

Re: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp

Some caps are microphonic, so even if you don't find a lot of THD on a silent lab bench, you'd probably get some if you used them in a passive speaker crossover. Probably less of a problem in a micpre, but that really depends on physical placement of the micpre. What I find amazing with those listen...