mic preamp modification

"Fred Bloggs" wrote in message news: snipped-for-privacy@nospam.com...

was obviously not a consideration.

There was an error budget for channel to channel coupling, and that assembly had only a small portion of it, and easily met its obligations in that regard.

I was one of the reviewers for that design, and that was one of my concerns as well. But some simple calculations from datasheet coupling performance numbers and the circuit imedances were sufficient to alleviate those concerns. The guy had already done them and verified their applicability in the lab.

On Thu, 07 Apr 2005 22:58:13 GMT, rex put finger to keyboard and composed:

- Franc Zabkar

transistors.

Nevermind about the transistors. I opened it up again and they are not transistors. Frank's instincts were right. Two of the three device pins are connected together so they are diodes of some kind.

I'm going to see if I can learn anything about where the mic inputs go before I close it up, but right now I don't see any transistors at the input. I'm beginning to wonder if there are components on the bottom of the board.

Oh, I did get it a little further opened up and found the other information printed on the caps. They are Elna 63V 10u.

I read in sci.electronics.design that Erik Walthinsen wrote (in ) about 'mic preamp modification', on Fri, 8 Apr 2005:

Which three classes do you have in mind?

There is 'phase-control, turn on' - this is the oldest electronic technology and uses triacs of thyristors as the control elements. It can cause lamp buzz and audio interference.

There is 'phase-control, turn off', which is fairly new and uses IGBTs as the control elements. It does not cause lamp buzz but can cause audio interference.

There is 'sine-wave dimming', which is a PWM technique. It does not cause lamp buzz and is claimed not to cause audio interference, but I suspect you can't completely ignore the possibility and use the wrong sort of audio cables and poor installation techniques.

was obviously not a consideration.

Yep.

It was one digit up or down from that. If you care enough to look at a datasheet, either one should suffice to determine which is the 8:1 mux.

I used the same part for part of an RF calibration subsystem used to fine tune gains in that phase array ultrasound system. Its isolation was a significant issue in that application also.

was obviously not a consideration.

Yeah- sure- which 4052 part are you talking about?

That seems to cover it AFAICT, extrapolating from the various marketing materials I've seen. The D2/D4DMX internally have a single inductor and single TO-220 package, plus misc bits, per channel. Haven't checked what the TO-220 actually is, I'll have to do that sometime soon. It definitely causes lamp buzz however, so it's almost certainly in the first category ;-(

The trick will be to figure out if there are any current products out there with prices we can handle, that use sine-wave dimming. Or, we take some steps to isolate things properly. Problem is, the current audio runs are decent wire but are exposed runs in the attic of the building which is the same space we'd probably be putting the dimmer packs. Depending on what the mains and dimmer-channel installation is we might be able to cluster the packs inside a grounded cage or two, and with metal-clad cable as required by code, keep most of the noise contained.

Terminology keeps changing! :-( What is "phantom power"?

Ted

Hello Graham,

What I do is take a through-hole cap that's slim yet big enough to fit into my fingers. Then I leave 1/3" or so of wire on the probe side, or whatever is enough to be able to touch the SMT part pins. I use really old wire cutters for that which results in a sharp edge or I file it into a point contact.

The other thing to make sure is that the outside foil is on the ground side so the finger doesn't introduce noise. Most important is to always discharge the cap before moving on to the next pin, especially if the circuitry is somewhat unchartered. Else.....bzzzt... foomph...pop.

Regards, Joerg

It's not new terminology.

Good quality condenser mics contain amplifiers and require DC to operate. Phantom power is usually about 48 Vdc and is injected common mode down the balanced signal lines of an XLR cable. Usually limited and isolated from the signal by running it through 6.8K to each of the lines.

Hello Ted,

Audio speak for a voltage riding on the line going from amp to mike, to supply electret mikes and the like. 48V most of the time, I believe, and low current.

Regards, Joerg

transistors.

I pulled my recorder almost completely apart and got the main board out. It has lots of components on both sides. I traced the mic input circuits for one channel and have posted it as 'mic preamp' in ABSE. snipped-for-privacy@4ax.com

I think it is complete as far as I went. The first active device is one of the op amps. The hot and cold sides of the XLR input have symetrical circuitry so the one diagram shows both. There are some more comments in the ABSE message.

I'd appreciate any comments on what you think of this circuit and how I might get it working cleaner.

Hello Rex,

Maybe it's just me but I cannot see the attached file in this ABSE post.

Regards, Joerg

I suspect this design is not going to get high marks.

I was just sitting here looking at what I drew and thought, "why is that

Sorry, when you post text and graphics there it splits it into two messages with the same subject. I linked the text part, here is the image part...

Hello Rex,

Still can't see it. It could be my ISP though. With most posts I can see the attached files in a.b.s.e. but then there are a few where I can't.

If you think I might be of help just email it to jsc (the domain below in my signature line).

Regards, Joerg

Hello Erik,

That is tough. If you walk through a hardware store they have a wide spread of dimming products in terms of pricing. But I never saw any hints as to technology, except for "this one buzzes less" or so.

Did you run differentially? If not, Radio Shack has "ground loop isolators" which seem to be little audio xfmrs at around $17 for a two-channel. Not too bad. Muxlabs has high quality ones but they may also cost more.

Regards, Joerg

A lot of ISPs don't carry the binaries group referenced.

Here is a link to an ftp place I have with the same two text and gif files... ftp://ftp.sonic.net/pub/users/rexa/Preamp/

Hope that works for you and thanks for contributing.

Hello Rex,

Mine does but for some reasons a few posts come without the pictures.

Yes, that worked. The top 10uF cap seems polarity reversed. Other than that I don't see anything weird. Except that regular electrolytics in a zero bias coupling function (the lower 10uF cap) aren't a great idea. Non-polarized caps are usually better.

Regards, Joerg