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Old 19th November 2005, 01:01 PM   #1
svokke is offline svokke  Belgium
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Default microphone preamp

I need a microphone preamp quick, so I looked at some commercial offerings. Below 100 things aren't looking great. I think it is better to make something by myself. I have some mic transformers, a few EF86, a lot of 12AX7(ecc83), 12AU7(ecc82), some tube types I don't know and a few power transformers in the house. I would like to use the preamp with balanced and unbalanced Low-Z dynamic microphones. What do you think of the schematic I made? I'm sure that it won't explode , but is it a good microphone preamp
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Old 19th November 2005, 03:42 PM   #2
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The schematic has errors. G1 of the EF86 gets the drive signal. G2 goes to the dropped B+. G3 is tied to the cathode. The volume control is in the WRONG place. Make R3 a 2.5 KOhm pot. and connect the wiper to g1 of the EF86. R2 should be a fixed 1 MOhm part. C1 should be 4.7 muF. to prevent low freq. losses. There's no forward bias on the IRF820's gate and it's an enhancement mode device. Also, a stopper resistor is strongly indicated at the FET's gate.

The FET bias problem is easily corrected. A 10 MOhm resistor connects the FET's gate to the + terminal of a 9 V. Lithium battery. The - terminal of the battery is connected to the FET's source.

I have a suggestion. Increase the B+ rail voltage. Increase the 2 load resistor values in a manner that keeps plate and drain current values unchanged. Eliminate the bypass cap. on the cathode resistor of EF86. Connect the screen grid bypass cap. to the cathode, not ground. While the EF86 is quite linear, local current NFB introduced by the unbypassed cathode resistor further linearizes the EF86. The loss in gain is made up for in the increased load resistance.
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Old 19th November 2005, 04:55 PM   #3
moamps is offline moamps  Croatia
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Default Re: microphone preamp

Quote:
Originally posted by svokke
...a few EF86, a lot of 12AX7(ecc83), 12AU7(ecc82)...
EF86 - Siemens V76
ECC83 - Elektor Feb.2003
ECC82 - Gyraf G9micpre (www.gyraf.dk)
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Old 19th November 2005, 05:05 PM   #4
svokke is offline svokke  Belgium
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I went back to the dawing board...
I'm unsure though why I should connect the screen grid bias smoothing cap to the cathode.
I'll also look for the schematics moamps posted
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Old 19th November 2005, 06:55 PM   #5
dhaen is offline dhaen  Europe
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Quote:
Originally posted by svokke
...
I'm unsure though why I should connect the screen grid bias smoothing cap to the cathode.
..
Because it's the screen to cathode voltage that matters. If it was connected to ground, there would be signal and PS noise superimposed on the screen. This is a very common configuration for the EF86, which was a pentode designed to be as low distortion as a triode.
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Old 19th November 2005, 10:44 PM   #6
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There's something I missed earlier. J2 needs to be a switching type that disconnects the secondary of the mike trafo from the "top" of the pot., when a plug is inserted. Otherwise, the trafo's secondary bypasses the volume control pot.

The EF86 is a small signal, not a power, pentode. The screen grid should be at a lower positive potential than the plate. Make the screen B+ dropping resistor 560 K or even 1 M.

I don't know if it's possible to DC couple an enhancement MOSFET to an "upstream" device. Put the coupling cap. back in and use a bias battery as I already indicated. Battery life in this application is YEARS. Use a 1 KOhm Carbon composition part as the gate stopper resistor. The body of the stopper resistor should be mounted as closely as is possible to the FET's gate terminal.
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Old 20th November 2005, 12:16 AM   #7
svokke is offline svokke  Belgium
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Quote:
Because it's the screen to cathode voltage that matters. If it was connected to ground, there would be signal and PS noise superimposed on the screen. This is a very common configuration for the EF86, which was a pentode designed to be as low distortion as a triode.
I think I should read the radiotron designers hanbook again. Then I might find out why it isn't always used that way.
Quote:
I don't know if it's possible to DC couple an enhancement MOSFET to an "upstream" device.
I've done this before in a guitar amplifier. It is possible, even without gate stopper resistor.
I did som work on the schematic again
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Old 20th November 2005, 02:12 AM   #8
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Quote:
I think I should read the radiotron designers hanbook again. Then I might find out why it isn't always used that way
The screen smoothing cap can be connected to ground if a cathode bypass cap is used (as in your circuit), because the cathode is at signal ground so it doesn't make any difference. However, if part or all of the cathode resistor is unbypassed, then the screen cap must be connected to cathode, so that the screen follows cathode voltage fluctuations with the signal.
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Old 24th November 2005, 08:50 PM   #9
svokke is offline svokke  Belgium
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I prototyped my mic preamp. It works, but it still needs some work done. The only thing I changed was a 220K anode resistor, I didn't have A 270K on stock. These are some readings:
Power supply voltage: 250V
Anode voltage= 76V
gain=100 measured with 0.1V input, 1KHz

There's very little noise and No hum at all. Now it sounds a bit on the "warm" side. Tomorrow I'll try fooling around with the cathode resistor. Maybe adding a resistor in series and bypassing it with a cap so I don't loose gain. Any thoughts?
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Old 25th November 2005, 05:16 PM   #10
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G2/screen voltage? Very important.

Quote:
Now it sounds a bit on the "warm" side.
That's from the high output impedance of the EF86 driving the high input capacitance of the directly coupled source follower i.e. gate to source, gate to drain. Not miller capacitance because of being in source follower mode. I have simmed it which shows a steep early HF roll-off starting around 10kHz.

Much better HF response can be had using a tube cathode follower. And also I think a HT of 400V is quite high and unnecessary for a mic-pre. 250-300V HT would be more desirable.

Wayne
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