Topic: fixing the impedance

[udovdh]

Hello,

What is the best way to fix the impedance that my mics see to say 47K? (independent from the recorder line in impedance)
On micbuilders I read about doing this by using a transistor on the batterybox and put it in between the signal path.
This sounds interesting but selection of the transistor for noise and distortion specs is not my kind of art.
Do people here have ideas or suggestions for fixing the impedance?

Udo

[udovdh]

The best way is to use what is called a buffer amp. What is the issue for your mics? a matching transformer might be a simpler option for you but your looking at $200 for a pair of good transformers Poolyconditioned has some transformers lying around from the AT preamps he is using to make battery boxes out of you might want to talk to him.

Stuff needs to be (very) small and not too expensive. Maybe even lo-tech, as it is currently. So it is either an opamp or a transistor?
Issue is slightly different tonal response of the mic plus bbox combo, impedance closer to manufacturers recommended specs thus guaranteeed performance, etc

[udovdh]

What mics do you have? lets see if I can help you also what recorder are you using?

The best way is to use what is called a buffer amp. What is the issue for your mics? a matching transformer might be a simpler option for you but your looking at $200 for a pair of good transformers Poolyconditioned has some transformers lying around from the AT preamps he is using to make battery boxes out of you might want to talk to him.

Stuff needs to be (very) small and not too expensive. Maybe even lo-tech, as it is currently. So it is either an opamp or a transistor?
Issue is slightly different tonal response of the mic plus bbox combo, impedance closer to manufacturers recommended specs thus guaranteeed performance, etc

Recorder can be the MT (5K3 line in impedance? was 47K at the Sony).
Mics are the Sennheisers in the mic area. They need 15K terminating impedance at least.
They work OK with less.
Current setup is the ultra-simple 2-wire batterybox which has given me solid performance.
In what way does this matter?

[udovdh]

In short: I do not need amplification.
So I do not need a pre-amp.
Stuff nees to be (very) small.
A buffering op-amp might fix stuff?
Suggestions?

[udovdh]

Yah tell me what mics you have and what recorder you are using and what you want to record loud music or speech.
A buffer amp is only one solution even if I build you a buffer amp it will be about the same size as my preamp you still need to power it with a 9 volt battery.

Mostly louder stuff.
MM-HLSC-1 and a MT.
Bbox with 9V battery generally works well.
Solution needs to be small as can be. 1 dual opamp with some resistors?

[udovdh]

So if what you have works well what is the problem you just need a bit more boost from time to time? I do not understand the impedance issue that you say you have. Why do you feel you have an impedance issue? with your set up as it is now

I don't need a boost. The recorder can do that, or I play with teh bbox resistors.
The mic needs to see a minimum impedance.
Currently it sees a lower impedance.
That needs to be fixed for optimal performance.

[Javier Cinakowski]

I have the Sennheiser MKE40's (mic madness) and Church's STC9000 Preamp.  The Church preamp has 10K input imdenance. The Senns supposedly need 15K impedance or greater.  I am concerned as well that I am not getting the best performance for my mics.  Also the line input on the Hi-MDs are 20K.  The output on the preamp is 10K.  I am concerned that is a problem too.   Do I need to get a new preamp?  Should I get a preamp that has 15K input and 20out?  While I am at it should get my mics converted to 3-wire?  (mke40's and mke2's)

[udovdh]

The slightly lower impedance is not a big issue but the performance will not be as designed by the manufacturer.

[Javier Cinakowski]

I was just concerned that I am not matching impedance and the quaility could be better.  I have not had any problems, but just want to get it right. 

[Javier Cinakowski]

Why would I be better with a 3-wire?  Im not sure I am convinced there are benefits to three-wire? 

Would the 3 wire be good for both the mke40's and mke2's?  I would make the upgrade if it would improve performance in any way.

I ripped the belt-clip of of my stc-9000 so it is not in mint condition.  I am assuming that you would not want to take back a non-mint unit.  Or can you just add the 3-wire to my current preamp?

[Javier Cinakowski]

My microphones have not distorted.  I have recorded very loud concerts.  I would however like more low-end in my recordings, but I think the lack of bass is from the microphones frequency responce(roll-off at 100hz ) and not a 2 wire problem.   Microphone Madness claims these mics to have 138 max SPL with 3v PIP.

[udovdh]

Distorted bass at 4 feet of the stacks?

[Javier Cinakowski]

Distorted bass at 4 feet of the stacks?

Who/What are you talking about?

. Low end response is the hardest part of making a cardioid mic, if you want put your self on the church audio mic loner thread for my cardioid mic you will then have the opportunity to hear a good quality cardioid mic. Not that your mics suck but I think the bottom end of my mics is very good. If you listen to the sample recording I have made on a real stereo there is very good bottom end in the recording. I have not heard any recordings from the mics you have yet I am sure they are great, It would be impossible for me to say if my mics are better I guess that’s why I put them on the loaner program so others could hear them and compare them to what they have.
 

Chris Church

My microphones have not distorted.  I have recorded very loud concerts.  I would however like more low-end in my recordings, but I think the lack of bass is from the microphones frequency responce(roll-off at 100hz ) and not a 2 wire problem.

I will sign up for the loaner program and do a head to head compairison with the mke40's and the Church Audio mics.  Thanks for all of your help Chris, you are the man!!!  T+

[udovdh]

Distorted bass at 4 feet of the stacks?

Who/What are you talking about?

Recording loud volumes at close distance of the speakers?

[Javier Cinakowski]

Chris, who did you piss off?  You are getting T-'s like crazy.

[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...)

[Church-Audio]

Posted by: udovdh

" I want to  make a situation where the mic can perform within the manufacturer recommended specs.
A buffer transistor can help"

Good luck because half of the time the "manufacture specs" are total BS.

First of all measuring a microphone is not easy, sure we know how to measure speakers but mics is a whole different ball game.

We can do very accurate noise measurements, we can do very accurate (when we choose to) distortion measurements. But frequency response is a real problem for mic manufacturers.

Because what source do you use? How far? Two way or point source? What environment do you use anaholic or reflective? How far away should the mic be from the source speaker? what do we use as a test source? How do we do accurate low frequency measurements when using an Omni mic?

This is why trying to make a mic perform to manufacture standards is a waste of time. Unless we are talking about DPA or B&K and a few select others that do not lie about the specs of the mics they make. Its a waste of time to try and duplicate what is on a peace of paper.

First off how do you know your mic is with in specs of the data sheet? There are no guarantees. How do you know that your mics will be in spec once you’re done? You must have a known good sample to compare it to. If you do not then you are wasting your time.

A subjective test of improvements can not be made with out first having a known standard. You have to establish a base line for testing, then you can make a subjective analysis of improvements. IMHO

But if your quest is simply to make a buffer amp with the correct input impedance then that’s a whole different story, This can be easily done. But remember one thing the more shit between the mic capsule and the preamp the more you increase noise and decrease acoustical performance. It’s a law of electronics but the best law is the KISS law keep it simple stupid. If you need a boost from the mic capsule then by all means create one, but if you really don’t then. Leave it alone because the more you add to the chain the more noise you end up with especially when we are talking about gain stages.


Chris Church

[udovdh]

[...]
But if your quest is simply to make a buffer amp with the correct input impedance then that’s a whole different story, This can be easily done. But remember one thing the more shit between the mic capsule and the preamp the more you increase noise and decrease acoustical performance. It’s a law of electronics but the best law is the KISS law keep it simple stupid. If you need a boost from the mic capsule then by all means create one, but if you really don’t then. Leave it alone because the more you add to the chain the more noise you end up with especially when we are talking about gain stages.

Agreed.
It would at least be a nice exercise to learn a bit from the various aspects. It is very simple technology. Yet I can think of so many questions about this setup.
If someone could help me with the resistor I could learn how to calculate the current when there is no resistor.

[Church-Audio]

try 1k and work your way up to 10k use metal film

[...]
But if your quest is simply to make a buffer amp with the correct input impedance then that’s a whole different story, This can be easily done. But remember one thing the more shit between the mic capsule and the preamp the more you increase noise and decrease acoustical performance. It’s a law of electronics but the best law is the KISS law keep it simple stupid. If you need a boost from the mic capsule then by all means create one, but if you really don’t then. Leave it alone because the more you add to the chain the more noise you end up with especially when we are talking about gain stages.

Agreed.
It would at least be a nice exercise to learn a bit from the various aspects. It is very simple technology. Yet I can think of so many questions about this setup.
If someone could help me with the resistor I could learn how to calculate the current when there is no resistor.

[udovdh]

try 1k and work your way up to 10k use metal film

How will I know I got the right value?
I got a spec sheet for the transistor, I'd like to use the info in there to at least get close to the right value of the resistor.
Any ideas?

[Church-Audio]

In order to know you need to look at noise floor and output level and distortion. This last one will be very hard for you to measure with out the right hardware/software.

Chris Church

try 1k and work your way up to 10k use metal film

How will I know I got the right value?
I got a spec sheet for the transistor, I'd like to use the info in there to at least get close to the right value of the resistor.
Any ideas?

[udovdh]

In order to know you need to look at noise floor and output level and distortion. This last one will be very hard for you to measure with out the right hardware/software.

The transistor datasheet have graphs for collector current versus source impedance.
The resistor is connected to the emitter, though.
So how do I influence the collector current? (I know the source impedance)
What is the relation between collector and emitter current?

[poorlyconditioned]

In order to know you need to look at noise floor and output level and distortion. This last one will be very hard for you to measure with out the right hardware/software.

The transistor datasheet have graphs for collector current versus source impedance.
The resistor is connected to the emitter, though.
So how do I influence the collector current? (I know the source impedance)
What is the relation between collector and emitter current?

Hey, I *really* appreciate your DIY attitude (and persistance ).

However, may I suggest two things:
- try experimenting with some cheaper capsules (like Panasonic, Transsound, primo, etc).  Don't mess up your Senns!  I would feel sad if someone messed up those beautiful mics.
- visit the Yahoo group "micbuilders".  Lots of tinkerers over there can help you out.

And if you do figure something out (like a cool mod or something), then you can bring it back here and teach us something.

  Richard

[poorlyconditioned]

In order to know you need to look at noise floor and output level and distortion. This last one will be very hard for you to measure with out the right hardware/software.

The transistor datasheet have graphs for collector current versus source impedance.
The resistor is connected to the emitter, though.
So how do I influence the collector current? (I know the source impedance)
What is the relation between collector and emitter current?

There are lots of online resources about BJT (standard transitors) and FETs.  I don't have specific references, but if you scan the previous messages at Micbuilders I think you can find something.

To answer your question, collector and emitter currents are *almost* the same.  Emitter current is roughly the sum of base and collector currents, but since base current is small relative to the others, you might as well call emitter and collector currents equal.

You choose collector (or emitter) current by "biasing" the transistor.  The bias is a voltage provided at the base that turns the transistor (partly) on.

I can't give any more details without refering to a specific circuit.  Again, it is best to read up a bit on this.

  Richard

[udovdh]

In order to know you need to look at noise floor and output level and distortion. This last one will be very hard for you to measure with out the right hardware/software.

The transistor datasheet have graphs for collector current versus source impedance.
The resistor is connected to the emitter, though.
So how do I influence the collector current? (I know the source impedance)
What is the relation between collector and emitter current?

To answer your question, collector and emitter currents are *almost* the same.  Emitter current is roughly the sum of base and collector currents, but since base current is small relative to the others, you might as well call emitter and collector currents equal.

Ah, OK, this helps.

You choose collector (or emitter) current by "biasing" the transistor.  The bias is a voltage provided at the base that turns the transistor (partly) on.

I can't give any more details without refering to a specific circuit.  Again, it is best to read up a bit on this.

The bias resistor value is (currently) dictated by the recommended resistor value to power the microphone.
Decoupling this voltage from the biasing voltage requires a capacitor I guess. (somethign I want to avoid).
So I can only play with the emitter resistor. I can make it bigger to get to the optimal collector current for my source impedance without other effects to my signal?

For details see the elec-wire.pdf and the yahoo post (both linked earlier) plus the datasheet of the transistor.

Here's a quick schematic:

R1 is 8K2
Signal is output at the R2/transistor junction over a capacitor (not drawn).

[udovdh]

Found some info at http://ece-www.colorado.edu/~bart/book/book/chapter5/ch5_ex.htm about the relation between Ic and Ie etc.
In my case I see a fixed voltage over R2, independent of resistor value so I need to increase it to the value that gives the right Ic for low noise.
No bad influence because of the higher resistor values? (10+K)

[udovdh]

Also what SMALL type of capacitor would you suggest (1 - 2.2 uF) to go after the capacitor?
Wima's MKP is too big. Black Gate N/Nx looks interesting.
What is better and/or smaller?

[Church-Audio]

Also what SMALL type of capacitor would you suggest (1 - 2.2 uF) to go after the capacitor?
Wima's MKP is too big. Black Gate N/Nx looks interesting.
What is better and/or smaller?

I like 2.2 to 10uf for the cap alot of people are going to shoot me but I like Tantalum caps I have some really nice 2.2 20 volt caps I can send you if you want they are made in the USA and are very old. I like the way they sound they are about 2.5mm by 6mm long and are axial type.

[poorlyconditioned]

Also what SMALL type of capacitor would you suggest (1 - 2.2 uF) to go after the capacitor?
Wima's MKP is too big. Black Gate N/Nx looks interesting.
What is better and/or smaller?

I like 2.2 to 10uf for the cap alot of people are going to shoot me but I like Tantalum caps I have some really nice 2.2 20 volt caps I can send you if you want they are made in the USA and are very old. I like the way they sound the are about 2.5mm by 6mm long and are axial type.

I think there is something to that, Chris.

I've disassembled several Audio Technica power modules (AT8531, AT8533, etc) and found they use a single transistor to drive an isolation transformer, and they use a tantalum cap to couple it!  They could have used a 'lytic, but for some reason chose a tantalum instead.  I've asked several people, but noone can provide an explanation for this.

Oh yeah, I should also comment on the value of testing circuits out.  Earliier I talked about how the Edirol UA5 has significant harmonic distortion at just under 0dB.  I compared that to an Emagic EMI 6|2m (six inputs, two output 24 bit usb device) and that one had very little harmonic distortion.  But this is just an ADC, no preamp or gain control on the inputs.  Then I stuck my self-made preamp in front, ran a 20 second "sweep" (20Hz - 20kHz, -24dB down) from my NJB3.  I noticed a fair bit of drop off (maybe 6dB) at 20kHz.  I also noticed a strange peak at around 88Hz.  It turns out this is not my preamp, but the NJB3 output.  Anyway, it pays to check things out, and my hat's off to you Chris for doing this before you sell your gear!

  Richard

[poorlyconditioned]

Found some info at http://ece-www.colorado.edu/~bart/book/book/chapter5/ch5_ex.htm about the relation between Ic and Ie etc.
In my case I see a fixed voltage over R2, independent of resistor value so I need to increase it to the value that gives the right Ic for low noise.
No bad influence because of the higher resistor values? (10+K)

Another one of many resources:
  http://zebu.uoregon.edu/~rayfrey/431/AnalogNotes.pdf

  Richard

[Church-Audio]

The dip at 20k could be capacitance of the traces or wires in your circuit it does not take much to change the high end response I have found this out the hard way many times.

Chris Church

Also what SMALL type of capacitor would you suggest (1 - 2.2 uF) to go after the capacitor?
Wima's MKP is too big. Black Gate N/Nx looks interesting.
What is better and/or smaller?

I like 2.2 to 10uf for the cap alot of people are going to shoot me but I like Tantalum caps I have some really nice 2.2 20 volt caps I can send you if you want they are made in the USA and are very old. I like the way they sound the are about 2.5mm by 6mm long and are axial type.

I think there is something to that, Chris.

I've disassembled several Audio Technica power modules (AT8531, AT8533, etc) and found they use a single transistor to drive an isolation transformer, and they use a tantalum cap to couple it!  They could have used a 'lytic, but for some reason chose a tantalum instead.  I've asked several people, but noone can provide an explanation for this.

Oh yeah, I should also comment on the value of testing circuits out.  Earliier I talked about how the Edirol UA5 has significant harmonic distortion at just under 0dB.  I compared that to an Emagic EMI 6|2m (six inputs, two output 24 bit usb device) and that one had very little harmonic distortion.  But this is just an ADC, no preamp or gain control on the inputs.  Then I stuck my self-made preamp in front, ran a 20 second "sweep" (20Hz - 20kHz, -24dB down) from my NJB3.  I noticed a fair bit of drop off (maybe 6dB) at 20kHz.  I also noticed a strange peak at around 88Hz.  It turns out this is not my preamp, but the NJB3 output.  Anyway, it pays to check things out, and my hat's off to you Chris for doing this before you sell your gear!

  Richard

[udovdh]

Found some info at http://ece-www.colorado.edu/~bart/book/book/chapter5/ch5_ex.htm about the relation between Ic and Ie etc.
In my case I see a fixed voltage over R2, independent of resistor value so I need to increase it to the value that gives the right Ic for low noise.
No bad influence because of the higher resistor values? (10+K)

Another one of many resources:
  http://zebu.uoregon.edu/~rayfrey/431/AnalogNotes.pdf

Thanks!
Interesting read, especially chapter 5.3 and on.

I did not find the answer to the question about the influence of higher emitter resistor values, though.

On the other hand I did see examples of how to properly implement the transistor with just a few extra resistors.
Because of the double function of the 8k2 (feeds the mic and biases transistor) I do not know if I can do the trick of unloading the transistor as explained in chapter 5.6.
I do not want to add an extra capacitor nor can I change the way that the mic is powered. (?)

I do want to look into the amplifier possibility of chapter 5.5 but use it as attenuator, if possible without touching the problem form my previous parapgraph.

BTW: why choose tantalum capacitors?
I haven't learned this area of expertise but read stuff like http://waltjung.org/PDFs/Picking_Capacitors_1.pdf and http://waltjung.org/PDFs/Picking_Capacitors_2.pdf; they pointed me towards the polypropylene metal film types. (Maybe this info is getting old?)
Now I can choose electrolytical yet `unpoliarized` types from various vendors for size reasons.

[udovdh]

OK, so I changed R2 in my schematic to 11K4:


I now see the voltage over R2 drop slightly to 1.35V from 1.4V with smaller values of R2. (3K3 or so)
Also the voltage over R1 drops slightly to 1.92V from 2V after the change of R2.
The current through R2 is now 0.12 mA and close to where I want it. (Assuming a Fairchild 2N5087, correct datasheet and valid info for this circuit; i.e.: info in Rsource versus Icollector graphs is relevant for an emitter source follower)
(9.34V from + to GND, 0.56V over base-emitter, 7.41V over base to ground)


If I build a stereo version of this circuit, how much is the leakage of audio over the resistors?
The battery is low impedance but need I take extra measures to prevent crosstalk?

Assuming that turning the emitter follower into an amplifier/attenuator would introduce too many extra parts or issues (see my previous question) I will do some tests to see how the circuit performs. What is the clipping point for the signal in this transistor setup?

[Church-Audio]

Most of these questions can not be answered 100% with out building the circuit and having the same mics you’re using. Electronics is part theory and part Voodoo. So build it and listen to it that is the best way. The problem with measuring distortion is even a scope is not good enough you need to get some good software with a good sound card that is the cheapest method for measuring anything now a days. Check out winaudiomls I have the lab version it’s very expensive but the regular version can be had for $79.00 when he lists it on eBay.

Chris Church

OK, so I changed R2 in my schematic to 11K4:


I now see the voltage over R2 drop slightly to 1.35V from 1.4V with smaller values of R2. (3K3 or so)
Also the voltage over R1 drops slightly to 1.92V from 2V after the change of R2.
The current through R2 is now 0.12 mA and close to where I want it. (Assuming a Fairchild 2N5087, correct datasheet and valid info for this circuit; i.e.: info in Rsource versus Icollector graphs is relevant for an emitter source follower)
(9.34V from + to GND, 0.56V over base-emitter, 7.41V over base to ground)


If I build a stereo version of this circuit, how much is the leakage of audio over the resistors?
The battery is low impedance but need I take extra measures to prevent crosstalk?

Assuming that turning the emitter follower into an amplifier/attenuator would introduce too many extra parts or issues (see my previous question) I will do some tests to see how the circuit performs. What is the clipping point for the signal in this transistor setup?

[udovdh]

Most of these questions can not be answered 100% with out building the circuit and having the same mics you’re using. Electronics is part theory and part Voodoo. So build it and listen to it that is the best way. The problem with measuring distortion is even a scope is not good enough you need to get some good software with a good sound card that is the cheapest method for measuring anything now a days. Check out winaudiomls I have the lab version it’s very expensive but the regular version can be had for $79.00 when he lists it on eBay.

No real answers, so I will do a side by side test.
I will do some recording tests with a MT, one mic in one channel via the old battery box and one in the other channel via the proof of concept thingie I now have.
Then I crank up the stereo and listen for differences in the recording.
I cannot make it loud enough for stack situations.

The idea now is that I will at most see gain differences and that I will need some attenuation.
Best would be to attenuate before the transistor but that would interfere with the bias voltage or the impedance we want for the mic to see?

[udovdh]

Did a small recording test with a plain old channel and one with the transistor.
Sounds good enough on my computer speakers, should check with headphones.

Signal of the transistor version is a tad louder. About 2dB
Assuming the transistor does not add gain (I read that the output could be slightly lower in amplitude?) this is an effect of the higher impedance?
(how high for a 2N5087?)

[poorlyconditioned]

Did a small recording test with a plain old channel and one with the transistor.
Sounds good enough on my computer speakers, should check with headphones.

Signal of the transistor version is a tad louder. About 2dB
Assuming the transistor does not add gain (I read that the output could be slightly lower in amplitude?) this is an effect of the higher impedance?
(how high for a 2N5087?)

So, this probably confirms:
1. your circuit OK (but you need waveform analysis to be sure).
2. you've lowered the output impedance, and this gained you about 2dB over the FET alone (driving a middle level impedance input, probably mic or line in on a minidisc or iRiver, right?)

So, is it really worth it?   I would say no, in this case.  Because you've added stuff that may add a little bit of distortion, and you've just gained 2dB, which is not much.

Now, it might be usefull in other conditions, eg., to drive transformers for voltage gain.  But not for this application.

  Richard

[udovdh]

Did a small recording test with a plain old channel and one with the transistor.
Sounds good enough on my computer speakers, should check with headphones.

Signal of the transistor version is a tad louder. About 2dB
Assuming the transistor does not add gain (I read that the output could be slightly lower in amplitude?) this is an effect of the higher impedance?
(how high for a 2N5087?)

So, this probably confirms:
1. your circuit OK (but you need waveform analysis to be sure).

I did look at them. At first sight they look similar.

2. you've lowered the output impedance, and this gained you about 2dB over the FET alone (driving a middle level impedance input, probably mic or line in on a minidisc or iRiver, right?)

The >5K3 line input of a Microtrack. 2K2 of the Iriver H120 should be more or of a challenge? (requires 1/8"...)

So, is it really worth it?   I would say no, in this case.  Because you've added stuff that may add a little bit of distortion, and you've just gained 2dB, which is not much.

I am not in it for the gain. I got plenty of signal. I just need a happy mic plus FET. I also need attenuation.
So I will need to test with the iRiver as well. It needs an 1/8" jack. Impedance is even lower.

Now, it might be usefull in other conditions, eg., to drive transformers for voltage gain.  But not for this application.

We'll see. I did not yet reach loud enough levels as in a 'rock' concert.
Need to push some more. Should clip the MT before reaching the levels I will hear at a show. Worthwhile or not, at the edges of performance the difference should be clearest?

Kind regards,
Udo