Here is a simple Measurement MIC system I want to put together. It will feed line in of an M-Audio Transit then to a computer running ARTA. I might try REW later.
There lots of WM 6x chips available right now that cannot be cut a la Linkwitz. I bought some off eBay. Ten of 'em for four bucks postage paid from Florida. Got them in three days. They are spec'd to be very flat and have specs similar to the old WM60A capsules. Here is a typical datasheet http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000CE11.pdf
So I drew up the following for them. Will you please check me out and correct any flaws or errors?
I am using the LM833N. It is internally compensated for all closed loop gains. That is why there are no puffs on the feedback resistor. Thoughts?
MIC sensitivity is spec'd at -44 dB. I figure that means about 6 mv at 94 dB SPL where I do my speaker measurements. I designed the pre for 35 dB gain. That leaves me with 20 dB for feedback. Is 35 dB sufficient to feed one of these ECMs into consumer line in?
There will be .1u bypasses at the pins of the OPA.
How do my resistances look? Anything else?
I have all the parts so if it is OK, I'm good to go.
Also, how long a MIC cable can the FET in these ECMs handle before the cable drops the FR to -3 dB at say 60 KHz? I know the capsule won't go that high. But that is the frequency point that the cable will start taking signal down at 20 KHz.
All suggestions are welcome.
Thanks
Here it is:
There lots of WM 6x chips available right now that cannot be cut a la Linkwitz. I bought some off eBay. Ten of 'em for four bucks postage paid from Florida. Got them in three days. They are spec'd to be very flat and have specs similar to the old WM60A capsules. Here is a typical datasheet http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000CE11.pdf
So I drew up the following for them. Will you please check me out and correct any flaws or errors?
I am using the LM833N. It is internally compensated for all closed loop gains. That is why there are no puffs on the feedback resistor. Thoughts?
MIC sensitivity is spec'd at -44 dB. I figure that means about 6 mv at 94 dB SPL where I do my speaker measurements. I designed the pre for 35 dB gain. That leaves me with 20 dB for feedback. Is 35 dB sufficient to feed one of these ECMs into consumer line in?
There will be .1u bypasses at the pins of the OPA.
How do my resistances look? Anything else?
I have all the parts so if it is OK, I'm good to go.
Also, how long a MIC cable can the FET in these ECMs handle before the cable drops the FR to -3 dB at say 60 KHz? I know the capsule won't go that high. But that is the frequency point that the cable will start taking signal down at 20 KHz.
All suggestions are welcome.
Thanks
Here it is:
An externally hosted image should be here but it was not working when we last tested it.
Why use a virtual ground for the + input of the OPamp when you have a 'real' ground?
C3=10uF may be too low if you are looking to measure LF, response is 3dB down at 10Hz approx, and C4=2.2uF into a 10k pot combined with the input impedance of the card (probably 50k) will make this slightly worse.
C3=10uF may be too low if you are looking to measure LF, response is 3dB down at 10Hz approx, and C4=2.2uF into a 10k pot combined with the input impedance of the card (probably 50k) will make this slightly worse.
ilimzn,
Thanks for your reply.
The ground issue is my bad. I was working on a single supply version and decided to go with a split supply. I simply forgot to properly change the schema.
After thinking about it I decided to go to the mountain and use the Linkwitz circuit for this measurement mic preamp, but with a couple changes. Will you check me out?
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?
The Linkwitz circuit has a gain of about 10 dB. But the cut capsules are more efficient than uncut. 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? 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. What do you think?
And I can use any value for the Pot if it makes sense to have it at all. Suggestions?
As for the caps, I have a good selection of mylar and poly caps so I can pretty much use anything that will fit in my preamp box.
The resistances in the circuit were chosen by Linkwitz to keep noise down. Is there any good reason to increase them? Especially considering the uncut capsule interface with the circuit? I understand the uncut capsules like an input resistance of about 20k. 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?
I know these are a lot of questions. And I hope you will answer them all.
But I also hope there will be some discussion and others will chime in as well.
Thank you for whatever help you are able to give.
Here is the circuit:
Thanks for your reply.
The ground issue is my bad. I was working on a single supply version and decided to go with a split supply. I simply forgot to properly change the schema.
After thinking about it I decided to go to the mountain and use the Linkwitz circuit for this measurement mic preamp, but with a couple changes. Will you check me out?
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?
The Linkwitz circuit has a gain of about 10 dB. But the cut capsules are more efficient than uncut. 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? 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. What do you think?
And I can use any value for the Pot if it makes sense to have it at all. Suggestions?
As for the caps, I have a good selection of mylar and poly caps so I can pretty much use anything that will fit in my preamp box.
The resistances in the circuit were chosen by Linkwitz to keep noise down. Is there any good reason to increase them? Especially considering the uncut capsule interface with the circuit? I understand the uncut capsules like an input resistance of about 20k. 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?
I know these are a lot of questions. And I hope you will answer them all.
But I also hope there will be some discussion and others will chime in as well.
Thank you for whatever help you are able to give.
Here is the circuit:
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