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evengravy

JFET Mic Pre Advise Please

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I know it has been a while since I posted on this circuit but I have been flat out with other projects until now,

so, I have modified the circuit to include diodes between the base of Q4 and ground as you suggested and this has reduced the current to just below 6ma,

I'm not sure how to simulate for output impedence, It will need to drive a 600ohm load max (i think around 75Ω @1kHz)

how would I go about simulating for this, I have been searching online for answers but I can't seem to find any relative info.

any help is greatly appreciated,

also the ac coupling hasn't been added as I intend to incorporate a transformer on the input, I will get to changing this and the bias resistor values soon.

post-38176-14279144128555_thumb.jpg

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Origionally, the circuit had a high output current so it could drive a fairly low inpedance load. Now its current is much less so it cannot drive a fairly low impedance load.

Its input impedance is 10M ohms so why ruin it with an input transformer?

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Origionally, the circuit had a high output current so it could drive a fairly low inpedance load. Now its current is much less so it cannot drive a fairly low impedance load.


Originally the circuit had such a high bias current that Q4 was overheating, now the bias current is lower so Q4 will no longer overheat.

To drive a lower impedance load a class AB stage should be added. It's a silly idea to use a class A amplifier like this to drive a low impedance load.

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so, if i understand what you're saying correctly I need to increase current through q4 such that it can drive the low impedance load but such that it is low enough so as it wont overheat. Is this correct?

I have no idea how to go about running output impedance simulation, I am using multisim.

also in regard to the input transformer, I understand that the input impedance is high with the jfet input, but i'm unsure how to add phantom power using dc blocking capacitors, this is why I thought a transformer would be easier, but i'll scratch that idea.

thanks guys.

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I think I may have been wrong with the output impedance values that I posted, basically the circuit (well two in series will make up the mic pre) will need to be suitable for connection to a line level mixer, after some more reading it seems as if "The impedance of the line input is high -- about 10K to 1 Meg ohms" http://www.tape.com/resource/impedance.html

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audioguru: "its input impedance is 10M ohms so why ruin it with an input transformer"

I really need to make this ciruit balanced and add to add phantom power, which is why I thought of adding a transformer, is there another simple way of achieving these functions without the input transformer?

I know that it is possible to add dc blocking caps to allow phantom power to be blocked from entering the input, but is there a way to work with the balanced input that emulates the transformer function.

even.

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I don't see why it needs to be balanced.

AC coupling is normally used to block the DC as you correctly suggested.

A transformer will just ruin it, as mentioned above.

Ballancing is normally done usign a differential amplifier which would need a matched pair of FETs in this case as you don't want to use an op-amp.

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"A mic preamp used an input transformer about 30 years ago but now they use an electronic circuit."

Yea, I understand what you mean but there is a certain tonal advantage to using a transformer also, I see what you are saying about the impedance, adding a transformer would be wasting the high input impedance of the Jfet,

If I were to choose a transformer however what specs would be suitable? impedance in/out and ratio

however, in order to achieve the electrical balancing, could I use similar spec matched jfet pair (2SK-369) and the same circuit doubled, one circuit for positive input and one for the negative? then merely add phantom power and dc blocking caps to the input?

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Most modern condenser mics have a balanced 600 ohms output so using fets at the preamp is useless. An input transformer in the preamp might pickup hum and has poor low and high frequency response. A transformer saturates at high levels. Input transformers might not be available anymore (remember Jensen?).

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