Topic: fixing the impedance

[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