DIY Factory

Green Mk II mic-pre Project
A project brought to you by Pete C.
Updated 21 Jun 2005

rev 1.0

For those that are interested to see where all this came from and enjoy the original just one more time
The original Green Mic-pre.

General Description

Text to come

Voltages

Supply : +14.8 V / 0 V / -15.2 V
(No Rotary Gain switch connected, no signal)

Setups

Setting CMRR (1k multiturn trimmer)
1. Find 2 100R resistors that are as matched closely as possible, ideally to 0.1 ohm.
2. Solder one end of each of the resistors together to form a 'V'.
3. Solder the un-attached legs of the resistors to the + & - inputs of the preamp.
4. Set the gain switch to the middle of it's rotation.
5. Set your signal generator to 100Hz and attach the positive lead to the joint of the 2 100R resistors. Other lead to GND. Amplitude 1v pk-pk.
6. On your scope, monitor the output of pin 1 of the middle 5532.
7. Trim the 1K trimmer until the signal on the scope is as low as possible. It should be possible to get it down to 1 or 2 mV.

Setting the Peak detector DC offset (20k Multiturn trimmer)
1. Set the rotary gain switch to minimum gain with no input signal.
2. Attach the multimeter + probe (set multimeter to DC mV) to the link behind the 10k Output Trim pot. The other multimeter lead goes to ground.
3. Trim the 20k trimmer to read as close to -3mV as possible.

You may have to repeat this step a couple of times to get it accurate. Let everything warm up first for a few minutes.
DO NOT rest your fingers on the diodes & resistors of the peak detector, those 100k & 200k resistors next to the TL074. This throws out the measurements by a large margin. Ask me, I know.......

Setting the LED meter level (100k trimmer)
I like to set my red LED to light at +20dB. Most modern balanced inputs will handle at least this level without overload. This level will give an extra element margin of overload if the input will handle +22dB for example.
The LED's are preset to light at -40dB, -20dB, 0dB & +20dB.

1. Use a 200 - 300 Hz signal.
2. Monitor the + signal output of the preamp with a multimeter (AC volts).
3. Turn up the output trim pot so no attenuation takes place (fully clockwise).
4. Turn up the output level of the signal generator until the multimeter reads 7.75v (+20dB). This is using a RMS reading multimeter. If a scope is used, the level should be set to 21.9 volts pk-pk.
5. Trim the 100k trimmer until the red LED just lights.

Where to connect the LED meter
There are 2 ways of connecting the LED meter. One method is to monitor the signal before the output trim pot (10k on front panel), the other way is after the output trim pot.
My preference is to monitor the signal before the output trim pot so that I know exactly what the level inside the preamp is.
To do this, a wire link must be soldered from the single hole above the 20k multiturn trimmer to the hole at the right end of the silkscreened link above & to the left of the 100k pot.
To monitor the signal after the output trim pot (10k on front panel), solder a wire to follow the silscreened link on the PCB, above & to the left of the 100k trimmer & ignore the single hole above the 20k multiturn trimmer.

Parts List

BOM Excel

Downloads

Schematic - GIF
Schamatic - PDF
Top View - JPG
Top View - BMP
Setups
PCB and Overlay - GIF
PCB and Overlay - PDF

Pics

ERRATA

So far so good.  Check back for any updates in this area.

Contact :

Kev or Pete if you have any questions

this page is still under development
and more will be added

brought to you by Pete and the DIY Team at :
www.diyfactory.com