Mr. Marsh's neat MC preamplifier

[jackinnj]

RNM published this design in The Audio Amateur 1/1982 -- there were a couple corrections in 2/1982. It features very low input impedance and use of easily attainable components, no "impossible to source high gm P-jfets". I've changed the schematic to reflect the corrections, and updated to include the OnSemi devices which are readily available as well, and more easily attached to a heat sink:

[IMGDEAD]http://www.tech-diy.com/Preamplifiers/MarshMC/MarshPrePreMJE.gif[/IMGDEAD]

It was suggested in the "Bipolar Transistor Tester" thread that 2N4400/03 would be lower noise. Of course component values would have to change. Could also use the THATCorp low noise P and N matched transistors.

 

[scott wurcer]

Doesn't this circuit need to include the resistance of the cart, besides the cart is DC connected?

 

[rayfutrell]

The 2N5210 and 2N5087 aren't ideal because they have a base spreading resistance of around 150 Ohms, if I recall correctly. Quad used the Zetex ztx650 and 750 in their MC preamplifier in the QUAD 44 and 34 because of the very low base spreading resistance of these transistors. Toshiba made transistors with incredible low base spreading resistance that would be ideal for this application but they seem to be unobtainable now.

 

[Joachim Gerhard]

As far as i can tell, the current in the input stage is the base current of the output transistors. That can not be much so this circuit is far from optimum noise wise with low impedance cartridges connected.
I would run at least 5mA, better 10mA in the input stage and use low Rbb transistors.

 

[jackinnj]

Got me to think of how to measure Rbb directly -- Burkhard Vogel has a method in Chapter 5 of "The Sound of Silence" --

 

[rayfutrell]

You can make a good estimate of the Rbb by looking at the graph of voltage noise (nv rt Hz) vs collector current. Typically the noise decreases with collector current and bottoms out at some low value. Calculate the resistor value that gives that noise. Just remember, 1K resistor = 4 nv rt hz, 250 Ohm = 2 nv rt hz and 62 Ohms = 1 nv rt hz.

 

[scott wurcer]

Got me to think of how to measure Rbb directly -- Burkhard Vogel has a method in Chapter 5 of "The Sound of Silence" --

Just fit the input noise vs Ic to a plot of the root-sum squared re and rbb. The rbb that makes the best fit is a good guess. There are high current effects that can mess things up a little at large Ic so you might not get a perfect relationship but good enough.

There is a web site that uses RF power transistors that are even better (at least the npn) than the Toshiba devices were.

 

[Mark Johnson]

You can make a good estimate of the Rbb by looking at the graph of voltage noise (nv rt Hz) vs collector current. Typically the noise decreases with collector current and bottoms out at some low value. Calculate the resistor value that gives that noise. Just remember, 1K resistor = 4 nv rt hz, 250 Ohm = 2 nv rt hz and 62 Ohms = 1 nv rt hz.

Here's the MPSA18 datasheet http://www.onsemi.com/pub_link/Collateral/MPSA18-D.PDF

I guess you're talking about Figure 2. Looks like noise bottoms out at 3.6 nv rt hz which is equivalent to an 810 ohm resistor; is that your estimate of Rbb?

 

[rjm]

The figure you posted shows the noise voltage increasing with current rather than decreasing.

 

[Mark Johnson]

The figure you posted shows the noise voltage increasing with current rather than decreasing.

Oops, yes you're right! I posted Figure 2 of the MPSA18 datashseet, but Figure 3 might be closer to what member rayfutrell had in mind. Interestingly the "bottom out" point is about the same, approx 3.6 nv rt hz, leading to the same estimate of Rbb, 810 ohms. But 810 ohms seems awfully high.

 

[scott wurcer]

The MPSA18 is a very poor choice as an example, you shouls at least pick a transitor intended for ultra-low noise operation like the MAT-02 or the THAT Corp equivalents.

 

[rayfutrell]

This 2N5087 spec sheet gives a better example of what I am suggesting. See figure 12 of En vs Ic.
http://www.fairchildsemi.com/ds/MM/MMBT5087.pdf

I think the calculated MPSA18 Rbb is about right. I have some measured data I will try to find. I recall that the MPSA18 Rbb was not very low for a supposed low noise transistor.

 

[jackinnj]

FWIW, there's an error in equation 5.11 of "The Sound of Silence" -- Boltzmann's constant should be squared in the first term.

 

[Joachim Gerhard]

Here is a circuit that biases in input common base stage in a different way :
Patent US6069534 - Balance photo-receiver with complementary HBT common-base push pull pre ... - Google Patente
Here you can run the input transistors on more current then the base current of the second stage.

 

[Joachim Gerhard]

Here is another way to set the input idle independently.

 

[jackinnj]

I would run at least 5mA, better 10mA in the input stage and use low Rbb transistors.

This is just part of the equation -- summing square of kernel, current and thermal noise. So is there such a big difference between 5mA and 10mA?

[IMGDEAD]http://www.tech-diy.com/Preamplifiers/Phono_Noise/Noise_vs_rbb.gif[/IMGDEAD]

 

[Joachim Gerhard]

I have a cartridge with 4 Ohm impedance so yes, i say 10mA makes a difference.
It may not be big but even in your graph you can see the difference at lower impedances.
Is that 1 or 2dB ?
Also when you make an open loop phono stage the higher Gm on higher idle can be an advantage.

 

[rayfutrell]

I found the data on the MPSA18. It took awhile because I was looking for MPSA18 but the data is for both the 2N4101 and MPSA18 transistors. Apparently they are similar transistors. The measured Rbb is given as 900 Ohms.

 

[AndrewT]

and very close to

same estimate of Rbb, 810 ohms

 

[jackinnj]

I have a cartridge with 4 Ohm impedance so yes, i say 10mA makes a difference.
It may not be big but even in your graph you can see the difference at lower impedances.
Is that 1 or 2dB ?
Also when you make an open loop phono stage the higher Gm on higher idle can be an advantage.

If you optimize Noise with respect to I(collector) -- this visual should demonstrate how much there is (or is not) gained:

[IMGDEAD]http://www.tech-diy.com/Preamplifiers/Phono_Noise/Noise_vs_Ic.gif[/IMGDEAD]