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Help needed understanding this prepreamplifier
Help needed understanding this prepreamplifier
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Old 22nd November 2020, 09:47 AM   #1
SMABB is offline SMABB  Denmark
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Join Date: Oct 2012
Location: Denmark
Default Help needed understanding this prepreamplifier

Dear members,

I have recently acquired a fantastic sounding prepreamplifier from way back in the eighties; Electrocompaniet MC-2. An all class A device originally marketed as having a self-sensing system for detecting optimal load on cartridge based on current flow.
The idea was said to improve headroom and dynamics. The load and feedback was set through resistors (using dip-switches).

My unit is a later one with exchangeable resistors for optimising load and gain. My problem is, I simply do not understand how it works!
Ideally I want to be able to adjust MC-2 to any chosen cartridge. But how....?

I hope the community is able to shed some light on this

How is the load resistor correlated to the gain resistor?
Do the cartridge only see the load resistor value or is some of the circuit part of the total load seen by cartridge?

How to calculate and adjust gain using feedback resistor? I guess the feedback resistor value is dependant on the load resistor, but otherwise I have no clue.

I found a drawn schematics on the internet which correlates well with my unit except for trim resistor setup for feedback/gain. I have only a fixed resistor in mine.
This is for the first version with load resistor being (maybe?) 10 Ohm on input.
Feedback/gain reistors made up from trim resistor in series with 160 Ohm; both in parallel with 2.7 kOhm. This should result in values between 151 Ohm and 393 Ohm.
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Old 22nd November 2020, 09:58 AM   #2
MarcelvdG is online now MarcelvdG  Netherlands
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The schematic has a 10 ohm resistor to a virtual ground, so its input impedance is ideally 10 ohm - which is unusually low. Maybe they later changed to a non-inverting configuration, with the signal applied to the bases of the right two 2SB737s and the 10 ohm connected to ground?
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Old 22nd November 2020, 10:06 AM   #3
SMABB is offline SMABB  Denmark
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Join Date: Oct 2012
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Well - I am as surprised as you and wondering what is really going on here...

I found some guidance in the manual of the preamplifier which had the MC-2 build in..

Still left me wondering...
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Old 22nd November 2020, 10:12 AM   #4
MarcelvdG is online now MarcelvdG  Netherlands
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OK, so you have to change the resistor to the virtual ground point depending on the cartridge.
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Old 22nd November 2020, 10:20 AM   #5
SMABB is offline SMABB  Denmark
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Agree - but I simply have no clue what the cartridge see as a load...

How can I measure the total gain applied by the Circuit?
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Old 22nd November 2020, 10:24 AM   #6
MarcelvdG is online now MarcelvdG  Netherlands
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Assuming the amplifier has a decent amount of loop gain, which it probably has, the cartridge sees the input resistor as a load - so the 10 ohm, 33 ohm, 43 ohm or 100 ohm to the virtual ground point also sets the input impedance.
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Old 22nd November 2020, 10:39 AM   #7
SMABB is offline SMABB  Denmark
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Join Date: Oct 2012
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Allright - point taken. Glad you could help in making this crystal clear

Does anyone have an idea on how to calculate the gain resistor or maybe just measure gain from input to output?

Last edited by SMABB; 22nd November 2020 at 10:51 AM.
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Old 22nd November 2020, 10:56 AM   #8
MarcelvdG is online now MarcelvdG  Netherlands
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Join Date: Mar 2003
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The gain from the terminal voltage at the input to the output voltage is ideally -Rfb/R1, where R1 is that 10 ohm, 43 ohm or 100 ohm and Rfb the resistance of that circuit consisting of a 140 ohm resistor, a 300 ohm potmeter and a 2.7 kohm resistor.

The gain from the cartridge's open-circuit voltage (Thevenin voltage, EMF) to the output voltage is lower, ideally -Rfb/(R1 + Zcartridge), where Zcartridge is the impedance of the cartridge. You would get the same gain reduction with cartridge impedance with any amplifier having an input resistance of R1, it's just the voltage division between the amplifier's input resistance and the cartridge impedance.
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Old 22nd November 2020, 12:15 PM   #9
SMABB is offline SMABB  Denmark
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Wow - thanks Marcel
Now I'm finally getting somewhere in understanding this circuit.

Thanks a lot
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Old 22nd November 2020, 12:22 PM   #10
SMABB is offline SMABB  Denmark
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Join Date: Oct 2012
Location: Denmark
One last question for now
Is the circuit inverting the input signal?

Meaning I will have to alter the wiring connector pins on the cartridge?
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