Help needed understanding this prepreamplifier

[Hans Polak]

Looking again, I noticed 8 small rectangular components that are most likely transistors.
That means that the cicuit diagram with the 16 transistors is correct after all for Smabb’s amp.

Hans

[SMABB]

Hans,

It seems I am falling a bit behind...

So to catch up from beginning the story goes like this:
My Audio Research SP-11 mkll went into repair with noise on phono input. I borrowed a NOS Musical Fidelity Nu-Vista from my very good friend. The Nu-Vista only has MM, so he also lent me MC-2, as he also had this. That is were it all started; we had no info on the MC-2, and did not know how to match the MC-2 to Benz Ruby, so I went on here for help.

The reason why I would like to continue using the MC-2 is, that in my experience SP-11 needs at least 0,8 mV from a cartridge, otherwise noise will be too evidently and easily recognisable.

Moving on to the MC-2 I did check as requested. First of all it has to be noticed, that the stock transformer delivers 23,8 V into the MC-2. This is then reduced to 20,8 V on both rails after the 33 Ohm resistor.
The test with varying R1 and keeping R2 constant was just an initial test. I had no idea the sound of the MC-2 would change according to gain setting. I will order a large bunch of different resistors to make a correct test, where gain is kept constant for different values of R1 as you suggest.

I cannot follow your calculation on why you have to choose 490R instead of 470R for keeping gain constant at 10, when R1=47 Ohm. Could you perhaps explain in more detail.

Dagfinn showed us the table for the EC-1 MC from which you could set R1 and R2 using dip switches. Remember it is the actual same circuit board (labeled MC-2-3) as in the MC-2, and the highest values coincide with what was mentioned in the original manual for the MC-2. I have made a table and graphics based on these values for reference. What information do you get from this?

Steen

[Hans Polak]

Smabb,

Look at the image below with R1 =47.
What value should R2 have to get a gain of 10 ?
Well, the answer depends on the gain of the amp.
Let's say we have 1mV at the input and because of the wanted gain of 10, we want -10mV at the output.

1)Now suppose the amp has a gain of 5.
That means that to get -10mV at the output, we would need 10mV/5=2mV at the junction of R1 and R2.
But because there is only 1mV at the input, this will never happen because no matter how large R2, output cannot exceed -5V with 1mV at its input.

2)Now suppose Gain is 270
To get -10mV at the output, we need 10mV/270=37.7uV at the junction of R1 and R2.
Current through R1 is now (1mV-37.7uV)/47R=20.5uA.
R2 now has to be (10mV+33.7uV)/20.5uA = 490R.

It's obvious that you have the second situation, hope you can follow my calculation.

Hans

[SMABB]

Thanks again, Hans

Referring to your simple model it is quite clear to me now, how you calculate the 490R. Very nice of you taking the time to explain. Much appreciated as always

What do you (and other members) think of Electrocompaniet´s settings referred to in my previous post?
They are using a high gain of approx. 20 for cartridges loaded with resistance lower than or equal 10 Ohm, and a gain of approx. 13 for cartridges with load resistor higher than 20. Is there a natural explanation for these settings (assuming there is, I just don't now why myself...)?

[Hans Polak]

My personal opinion with the low open loop gain of this amp is that an overall gain of 10 should not be exceeded to keep the loop gain close to 30. Less loop gain has a negative effect on distortion reduction.

Hans

[Hans Polak]

P.s. The vision regarding a maximum gain of 10 with your MC2 was the reason to advise you to try a number of different gain settings with R2 whith the Cart directly connected without R1.
So learn to trust your ears and don’t rely too much on opinions from other people including myself.

Hans

[SMABB]

Hans,

Don't worry - for the past 35 years I have used my ears quite extensive. I know how live acoustical as well as amplified music sounds to my ears. Those are my references.

Starting from scratch I had interest in understanding this MC-2, as it does something that makes music flow in the room in front of me. Manuals were scarce with info, so I turned to this knowledgable forum for help.

Thanks to you and other contributors I was not disappointed. I have gained tremendous knowledge about the little black box.

I will revert with comments beginning of next year, when resistors have arrived and the MC-2 has been tested with different loadings but constant gain.

[PRR]

Originally Posted by SMABB

.... using a high gain of approx. 20 for cartridges loaded with resistance lower than or equal 10 Ohm, and a gain of approx. 13 for cartridges with load resistor higher than 20. Is there a natural explanation for these settings.....

Coil space/mass is limited. Few turns of fat wire makes a low voltage and impedance. More turns of skinny wire makes a higher voltage and impedance.

It is not universal or exact. Twice the turns is twice voltage but (in same space) four times the impedance. But the Designer can refine magnet strength, clearance, leverage for other results.

[Hans Polak]

The question concerned overall gain and why it should be higher with a lower combination of Rcart and Rload instead of using a constant gain independant of the used Cart since Rf can be selected accordingly.
The answer lies probably in the differing FR with low source resistance in combination with the 100nF input cap.
With a gain of 10 and a 10R source resistance, you will get a 4dB peak at ca. 2Mhz.
Increasing gain to 20, reduces this peak to 1.5dB.
With a higher source impedance like Smabb has, this peak is completely gone and gain can be lowered.

Hans

[SMABB]

Thanks Hans,

Your explanation makes perfectly sense, although I would never had thought of it myself.
That is why I highly appreciate the contributions from those in the know.