Amp/Preamp Asylum

Posted by jolin (D) on January 23, 2017 at 21:38:14

I build and measure loudspeakers at home as a hobby. But I have never had a decent mic preamp for my unmodified Panasonic ECM capsules. I use a USB sound card to accept the mic audio and analyze it with ARTA software.

After thinking about it for awhile I decided to go to the mountain and use the Linkwitz circuit for my measurement mic preamp, but with a couple changes.

The Linkwitz circuit is designed for capsules that are cut. That means the capsules are referenced from ground to V-. So I thought why not design the circuit so that it can handle uncut as well as cut capsules. My capsules are all uncut but I may try out cut ones in the future.

So, I figured a switch could be used to switch the capsule reference from "ground to V-" to "ground to V+". That change is on the schema below. Thoughts?

I believe my Panasonic capsules put out about 6 ma at 90 dB SPL which is where I do most of my testing. The Linkwitz circuit has a gain of about 10 dB. But the cut capsules are more efficient than uncut. I am uncertain what gain I need to properly feed my sound card, so I figured why not make the circuit variable gain to provide for the uncut capsules. What I figured was to use a rotary switch to change the resistance of R3 in the schema below. A four position switch with the resistances shown should be able to provide a gain variation of 10, 20, 30 and 40 dB. But at the higher gains the effective R3 starts to get pretty small. Am I in trouble there? Would it be smarter to only have two levels of gain, like 10 dB and 20 dB? How High should I go to get a proper level to my sound card? Would it be smarter to vary R4? Suggestions?

The output of the circuit will go to line-in of a sound card. Cables will be very short, a few inches. So do I even need R5 and C2?

As for the pot, it may not even be necessary given the switched gain choices in the circuit. Thoughts?

I can use any value for the Pot if it makes sense to have it at all. Suggestions?

The resistances in the circuit were chosen by Linkwitz to keep noise down. Is there any good reason to change them? Especially considering the uncut capsule interface with the circuit? I understand the uncut capsules like an input impedance of about 20k or higher. Thoughts?

The LM833N OPA ( )is a dual chip. Is there anything smart that can be done with the second amp?

And last, how long a cable can an uncut capsule push signal down before the high frequencies begin to be cut? I have seen home theatre mics with cable lengths of 20 feet or more. How far can I go with mic cable? Linkwitz suggests less than 30 ft with a cut capsule. What about a stock capsule?

Here is my schematic:

• http://www.kynix.com/uploadfiles/pdf9675/LM833N.pdf   (Open in New Window)

Edits: 01/23/17