Topic: fixing the impedance

[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