Search found 32 matches

by olafmatt
Mon Jun 11, 2018 6:32 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 106
Views: 111343

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: 106
Views: 111343

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: 106
Views: 111343

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: 106
Views: 111343

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: 106
Views: 111343

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...
by olafmatt
Sat Jun 09, 2018 2:44 pm
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 106
Views: 111343

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

Wouldn't "conventional" servo's at the bases also have an issue about who was in control even with AC-coupled inputs? There are two servos operating against (or with) each other on opposite sides of the input coupling cap. That's a good question. - And it is not just about "conventional" servos... ...
by olafmatt
Sat Jun 09, 2018 12:35 pm
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 106
Views: 111343

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

What do your instincts tell you about which topology would be affected the most by moving DC conditions? What would be the audible effects? With microphones with caps on the output you can get interaction between the servo and those caps. You can look around and find the mic with the worst-case cap...
by olafmatt
Sat Jun 09, 2018 2:49 am
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 106
Views: 111343

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

From experience, what value do you see in the technique? Lower noise, increased transparency? What are the audible benefits realized in practice? This is tricky to answer because I never compared the exactly same preamp with flying frontend and no caps to how it sounds with flying disabled and caps...
by olafmatt
Fri Jun 08, 2018 5:58 pm
Forum: Pro Audio Design
Topic: New: A Direct-Coupled Input-Capacitorless Active Mic Preamp
Replies: 106
Views: 111343

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

One of the major considerations is servo pulling imbalanced phantom currents. A quick experiment shows that Olaf's servo used in the MC preamp will have enough range to correct mic imbalance. Yes, that servo can probably be made to have enough correction range... I'm just not sure whether the pream...
by olafmatt
Tue May 15, 2018 1:24 am
Forum: Pro Audio Design
Topic: Mic Preamp DC-Servo: The "backwards" way
Replies: 48
Views: 86108

Re: Mic Preamp DC-Servo: The "backwards" way

I like this circuit. Glad it works not just for me. In the thread about the MC preamp you mentioned the reduced power supply rejection caused by the fact that the two non-inverting inputs of the opamps are not connected together. This is basically something I was already mentioning as a drawback of...