Topic: fixing the impedance

[Javier Cinakowski]

Distorted bass at 4 feet of the stacks?

Who/What are you talking about?

Recording loud volumes at close distance of the speakers?

Who is recording loud volumes at close distance?  Are you recording this or are you asking if we have. 

I have recorded loud volumes at close distance.  no dostortion

[udovdh]

Thanks for the explanation.

What would be a good lo-power buffer opamp? E.g. TI has a load of them but selecting the best is hard for an amateur.
Lo-power, CD-quality performance (96 dB SNR, THD etc), etc are what we need. TL062B?

[udovdh]

BTW: What is the influence of the batterybox resistors on the total impedance that the mic capsule sees?
Is it that line in impedance and batteyrbox resistor are in parallel?

[udovdh]

You can use a ne5532 opamp with a 33k feedback loop resistor or a burr brown opa2227p

Thanks for the tip.
They differ somewhat.
Which one to choose when needing high impedance toward the mic, about 5K+ for the recorder (MT)?
When at 9V single rail?
One is faster than the other. Noise is different? THD?
Any other implementation details? Decoupling after the opamp? Floating ground, etc... Pfff

[udovdh]

You sound like you know a few things about building circuits decoupling can be made via a 10uf caps for better bass response you can also use a voltage devider network and run the ne5532 from a single 9 volt supply the opa2227p needs a dual supply you can use a rail splitter chip to run it from a single 9 volt since you dont need gain your going for unity it should not be a problem
there are many designs out there using these chips take a look around.

Hello Chris,

Thanks for the info. Did you see myquestion about the influence of batterybox resistors? (bottom of page 2)
I will search for some 5532 examples. I hope I can avoid extra capacitors because they are relatively big.
Maybe a mod to increase the impedance of the MT line in is easiest (but I was told that would make the Mt noisier).
(still looking for MT modders... contact me please)

[udovdh]

A different, possibly smaller solution could be to use a J201 FET transistor to buffer the mic capsule. (suggested by Nick)
Does anybody here have experience with using a FET to do this?
Please have a look at the basic schematic for this setup.

Tips for optimizing the setup are welcome!

[Church-Audio]

That fet is used by Crown in a headset mic with a primo capsule in it. The only problem with using it is to get the best sound you need a 1giga ohm resistor they are very expensive around $5.00 each dale makes them this setup works well for capsules that do not have a fet already. if they do you are better off with a transistor like the 2n3904 easy to find schematics for, and J34 if you can find it.
I still do not to this day understand what problem you are having with your mics. Can you explain so we can try and help you.

Chris Church

A different, possibly smaller solution could be to use a J201 FET transistor to buffer the mic capsule. (suggested by Nick)
Does anybody here have experience with using a FET to do this?
Please have a look at the basic schematic for this setup.

Tips for optimizing the setup are welcome!

[poorlyconditioned]

That fet is used by Crown in a headset mic with a primo capsule in it. The only problem with using it is to get the best sound you need a 1giga ohm resistor they are very expensive around $5.00 each dale makes them this setup works well for capsules that do not have a fet already. if they do you are better off with a transistor like the 2n3904 easy to find schematics for, and J34 if you can find it.
I still do not to this day understand what problem you are having with your mics. Can you explain so we can try and help you.

Chris Church

A different, possibly smaller solution could be to use a J201 FET transistor to buffer the mic capsule. (suggested by Nick)
Does anybody here have experience with using a FET to do this?
Please have a look at the basic schematic for this setup.

Tips for optimizing the setup are welcome!

Exactly!  I don't see any need for fixing anything that aint broke.  Hell, most of the time I don't see any need to fix it if *is* broke .

  Richard

[udovdh]

That fet is used by Crown in a headset mic with a primo capsule in it. The only problem with using it is to get the best sound you need a 1giga ohm resistor they are very expensive around $5.00 each

http://www.till.com/articles/GuitarPreamp/ does not show GOhm resistors?

this setup works well for capsules that do not have a fet already.

The Microphone Madness types have a FET. What difference does that make?

if they do you are better off with a transistor like the 2n3904 easy to find schematics for, and J34 if you can find it.

J34 was not found in a quick search, not even a datasheet. 2n3904 can be had locally.
Differences?

I still do not to this day understand what problem you are having with your mics. Can you explain so we can try and help you.

Maybe not see it from the perspective of problem but the 'just because I can' approach?
Similar to using MKP instead of MKS capacitores when possible?

BTW:

Please have a look at http://groups.yahoo.com/group/micbuilders/message/3552.
It describes an idea for a simple PNP follower attached directly to the mic capsule (no capacitor).
It uses even fewer parts. How well would this work?

Could I use a FET for that setup instead of a PNP transistor?

Udo

[udovdh]

So I built a one channel version of the source follower idea as a proof of concept.
It appears to work so far that audio reaches my recorder through the transistor.
It doesn't sound bad but I cannot measure details/differences.

I was wondering about how to properly size the resistor between emitter and 9V. (see the yahoo link)
I mean, does it matter? I guess it influences noise and distortion to a certain level? I would like to minimise noise and distortion...


Also:

Can I make the transistor attenuate stuff by putting a resistor between the collector and ground?

[udovdh]

Nobody? 

[Church-Audio]

In order to say what resistor you need we need to know three things. 1- Transistor type 2- desired gain 3-source impedance. Since we already know the supply voltage. One thing I will say is the cable fet idea works great with high impedance loads like a guitar anywhere from 8k to 1meg but not well with low impedance of 1k typical of a electret mic capsule. You might want to rethink using this as a means of getting more gain I think it will just increase your noise floor.

Chris Church

But what do I know.

Nobody? 

[udovdh]

Hello,

In order to say what resistor you need we need to know three things. 1- Transistor type 2- desired gain 3-source impedance. Since we already know the supply voltage. One thing I will say is the cable fet idea works great with high impedance loads like a guitar anywhere from 8k to 1meg but not well with low impedance of 1k typical of a electret mic capsule. You might want to rethink using this as a means of getting more gain I think it will just increase your noise floor.

It is a 2N5087, gain is 1 or less, source impedance is 3K. Less than one gain requires extra resistor to GND I assume.
Thanks for the info about the FET case. Won't need to try that one for my mic.

[poorlyconditioned]

Hello,

In order to say what resistor you need we need to know three things. 1- Transistor type 2- desired gain 3-source impedance. Since we already know the supply voltage. One thing I will say is the cable fet idea works great with high impedance loads like a guitar anywhere from 8k to 1meg but not well with low impedance of 1k typical of a electret mic capsule. You might want to rethink using this as a means of getting more gain I think it will just increase your noise floor.

It is a 2N5087, gain is 1 or less, source impedance is 3K. Less than one gain requires extra resistor to GND I assume.
Thanks for the info about the FET case. Won't need to try that one for my mic.

My first impression is *why* are you doing this?  Most standard mics have a FET in them, and this is sufficient to drive a short cable to a recorder.

But there are a few reasons for putting a transistor "buffer" in between the FET and the mic cable.  One is to drive long cables, and possibly low impedance inputs, like transformer input preamps, or any professional preamp for that matter. (eg., the Presonus Firepod has an input impedance of 1.7k or something).  Driving a low impedance input from a FET will still work, but you might lose some of the signal (maybe 10dB, I don't know).  Another reason is to drive a balanced cable.  If you're interested in this, check out the balanced line driver (by Scott Helmke, adapted from the original Scheops design) at the "Micbuilders" Yahoo group.  This circuit actually uses Phantom power to supply the FET, so no battery box is needed at all.  Finally, another use for buffering might be to drive a transformer (low impedance) to step up the voltage without using a preamp.

Anyway, electronics is like programming: You should think of what you're trying to do before starting.  What is the problem right now?  How do you plan to fix it?  And finally, the kicker: how can you judge/measure that your fix has actually worked!

  Richard

[udovdh]

Hello,

In order to say what resistor you need we need to know three things. 1- Transistor type 2- desired gain 3-source impedance. Since we already know the supply voltage. One thing I will say is the cable fet idea works great with high impedance loads like a guitar anywhere from 8k to 1meg but not well with low impedance of 1k typical of a electret mic capsule. You might want to rethink using this as a means of getting more gain I think it will just increase your noise floor.

It is a 2N5087, gain is 1 or less, source impedance is 3K. Less than one gain requires extra resistor to GND I assume.
Thanks for the info about the FET case. Won't need to try that one for my mic.

My first impression is *why* are you doing this?  Most standard mics have a FET in them, and this is sufficient to drive a short cable to a recorder.

Yes. Stuff sounds slightly different with the low impedance line ins and it is not just the different bass-rolloff  due to the lower impedance.
I want to  make a situation where the mic can perform within the manufacturer recommended specs.
A buffer transistor can help.
Maybe it can even work as an attenuator? (catch 2 flies...)