Search found 67 matches

by ilya
Sun Feb 11, 2018 5:48 am
Forum: Pro Audio Design
Topic: Strange simulation results
Replies: 9
Views: 7860

Re: Strange simulation results

Wouldn't you want to put it in front of the transformer (or along side it) in order to preserve the high impedance input? Otherwise the transformer is going to load the instrument you're trying to DI. In "after the transformer " case it will be either transformer or DI. DI is a physically different...
by ilya
Sun Feb 11, 2018 4:39 am
Forum: Pro Audio Design
Topic: Strange simulation results
Replies: 9
Views: 7860

Re: Strange simulation results

I'm planning to use this DI circuit in the first post in the transformer input class-a mic preamp. Where is it better to put this circuit - before the transformer or after? Aside from color and "free" gain before the transformer is there any other pros/cons? Pardon me for resurrecting this topic wit...
by ilya
Sat Jan 20, 2018 3:16 pm
Forum: Pro Audio Design
Topic: Low noise grounding tips in mic preamp
Replies: 11
Views: 11398

Re: Low noise grounding tips in mic preamp

Well, gents. I was wrong. The problem was not the reference gnd. What eliminated the hum was a temporary wire that went from the op-amp gnd pin to transformer gnd (op-amp "-" pin doesn't have direct connection to GND, instead it goes through the RC). When looking carefully I noticed that gnd pin on ...
by ilya
Sat Jan 20, 2018 12:28 pm
Forum: Pro Audio Design
Topic: Low noise grounding tips in mic preamp
Replies: 11
Views: 11398

Re: Low noise grounding tips in mic preamp

The output stage gets connected to the PSU output ground point which is the main filtering cap right after the diode bridge. This statement raises a lot of questions. What do you mean by the output stage gets connected to the PSU output ground? Where does the preamp's common mode rejection stage re...
by ilya
Sat Jan 20, 2018 11:24 am
Forum: Pro Audio Design
Topic: Low noise grounding tips in mic preamp
Replies: 11
Views: 11398

Re: Low noise grounding tips in mic preamp

While still guessing about the gain stage, a typical non-inverting op amp or even discrete gain stage will have an effective - input in series with the gain leg ground... so that gain leg ground needs to be the same ground as the input transformer secondary ground or 0V. Thanks for that info! This ...
by ilya
Fri Jan 19, 2018 2:23 pm
Forum: Pro Audio Design
Topic: Low noise grounding tips in mic preamp
Replies: 11
Views: 11398

Re: Low noise grounding tips in mic preamp

One obvious question, does the hum increase with preamp gain? If yes the hum has corrupted the input signal. If no it is corrupted in the differential to single ended and output driver circuitry. This particular preamp I'm working on is class-A, transformer in and transformer out. 3 stages. When I ...
by ilya
Fri Jan 19, 2018 5:53 am
Forum: The Peabody Lounge
Topic: Links in topic reply notifications
Replies: 1
Views: 2007

Links in topic reply notifications

The links in the e-mail that I get when the forum notifies me about a reply in the topic is not HTML formatted. I can't click on it. Only copy/paste works. This worked some time ago though - I saw the active link in the e-mail that led me to the topic page.
by ilya
Fri Jan 19, 2018 5:51 am
Forum: Pro Audio Design
Topic: Low noise grounding tips in mic preamp
Replies: 11
Views: 11398

Re: Low noise grounding tips in mic preamp

I agree with JR: You need to find the vector first. Electrostatic, magnetic or conducted? I think that I've found the vector, which is "conducted". It's not magnetic because the source of magnetic field is removed (external PSU, no power coils in the circuit). It's not electrostatic - it's hum "foo...
by ilya
Thu Jan 18, 2018 8:36 pm
Forum: Pro Audio Design
Topic: Low noise grounding tips in mic preamp
Replies: 11
Views: 11398

Low noise grounding tips in mic preamp

I've been building quite a bit of different mic preamps. They are reasonably quiet, but most of them have a very small amount of hum to them. The kind of hum that I hear if I really crank the volume of my headphones AND apply a gain of around 20-30 dB. I'm talking about the filter cap ripple as this...
by ilya
Tue Dec 12, 2017 1:33 pm
Forum: Pro Audio Design
Topic: Incorporating LPF in the FET input stage
Replies: 9
Views: 6945

Re: Incorporating LPF in the FET input stage

Tried several things and cap on the output worked the best. No traces of high frequency garbage that was otherwise getting inside the circuit (even with the cap on the input). Thanks gents! I'll insert the 1K protecting resistor at the input as well.