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AN AS-BUILT SCHEMATIC A BIT DIFFERENT FROM THIS ONE CAN BE FOUND AT POST #12 BELOW. SUGGESTIONS ARE STILL WELCOME.
This is a major edit of the original post. I will bump it below to hopefully get it going.
After thinking about it I decided to go to the mountain and use the Linkwitz circuit for my measurement mic preamp, but with a couple changes. Will you all please check me out?
The Linkwitz circuit is designed for capsules that are cut for greater headroom. The capsules are then referenced from ground to V-.
Trouble is that the WM61A capsule is obsolete and I understand that the ones available for purchase are often not genuine. But there are WM6X series capsules that cannot be modified and they are available and cheap. 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.
By putting all the front end stuff in an XLR connector, a simple switch of mics on the end of the cable and a flip of the preamp switch can accommodate a variety of capsules and modes of operation.
The Linkwitz circuit has a gain of about 10 dB. The circuit I show is a clone of the Linkwitz version. But R3 could be replaced with switched resistors and provide a gain variation of 10, 20, 30 and 40 dB. At 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? Or is a fixed 10 dB just fine?
I also decided to throw in a "floating capsule" front end too. The idea comes from the Naint Studio site (look under "Schematics" for "basic" and under "The 'Linkwitz' mod") here: http://naiant.com/about/microphonedesign.html
The output of the preamp will go to line-in of a sound card (M-Audio Transit in my case).
The resistances in the circuit were chosen by Linkwitz to keep noise down. Is there any good reason to change them? I understand uncut capsules like an input impedance of about 20k or higher.
And last, how long a cable can can these front ends push signal down to the preamp 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 or a floater? I could figure it out if I knew the output impedance of the capsules in each mode of operation. But I don't.
What have I missed?
Of course a simple one BJT or one FET buffer for each of the front ends would allow longer cables. But I do not know how to design them. So any ideas are welcome. Probably BJT versions are better cause no fiddling is necessary to make them work (and I have 2N5210/2N5087s on hand). Ideas?
BTW, I call this the Heresy Project because it is unbalanced from alpha to omega and it incorporates a floating capsule as one of the front ends. Both of these options are considered heresy and an abomination by certain folks at certain forums that cater to the Pro Audio crowd.
Thank you all for whatever help you are able to give.
Here is the circuit:
This is a major edit of the original post. I will bump it below to hopefully get it going.
After thinking about it I decided to go to the mountain and use the Linkwitz circuit for my measurement mic preamp, but with a couple changes. Will you all please check me out?
The Linkwitz circuit is designed for capsules that are cut for greater headroom. The capsules are then referenced from ground to V-.
Trouble is that the WM61A capsule is obsolete and I understand that the ones available for purchase are often not genuine. But there are WM6X series capsules that cannot be modified and they are available and cheap. 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.
By putting all the front end stuff in an XLR connector, a simple switch of mics on the end of the cable and a flip of the preamp switch can accommodate a variety of capsules and modes of operation.
The Linkwitz circuit has a gain of about 10 dB. The circuit I show is a clone of the Linkwitz version. But R3 could be replaced with switched resistors and provide a gain variation of 10, 20, 30 and 40 dB. At 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? Or is a fixed 10 dB just fine?
I also decided to throw in a "floating capsule" front end too. The idea comes from the Naint Studio site (look under "Schematics" for "basic" and under "The 'Linkwitz' mod") here: http://naiant.com/about/microphonedesign.html
The output of the preamp will go to line-in of a sound card (M-Audio Transit in my case).
The resistances in the circuit were chosen by Linkwitz to keep noise down. Is there any good reason to change them? I understand uncut capsules like an input impedance of about 20k or higher.
And last, how long a cable can can these front ends push signal down to the preamp 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 or a floater? I could figure it out if I knew the output impedance of the capsules in each mode of operation. But I don't.
What have I missed?
Of course a simple one BJT or one FET buffer for each of the front ends would allow longer cables. But I do not know how to design them. So any ideas are welcome. Probably BJT versions are better cause no fiddling is necessary to make them work (and I have 2N5210/2N5087s on hand). Ideas?
BTW, I call this the Heresy Project because it is unbalanced from alpha to omega and it incorporates a floating capsule as one of the front ends. Both of these options are considered heresy and an abomination by certain folks at certain forums that cater to the Pro Audio crowd.
Thank you all for whatever help you are able to give.
Here is the circuit:
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