[vman71]

This review is for the Music Reference 2 Input “Pot in a Box” Passive Preamp

I bought this little 2 input “pot in a box” passive preamp for a number of reasons that I’ll try and explain in this review.  I used this little passive preamp with my simple 2ch system that consists of the following:

EAD CD-1000mkIII (heavily modified)
Music Reference 2 input “pot in a box” Passive Preamp
Stan Warren custom chip amp
Buggtussel Circa 2-way monitor speakers

Okay, a few of the reasons why I chose this passive preamp vs. the others out there that are very similar:

1.   The outstanding reputation of Music Reference
2.   The simplicity and size (no power required)
3.   The fact that this one has 2 inputs and the ability to “mute”
4.   The vintage Noble volume pot

Connecting this passive preamp and finding a space for is very easy, as it doesn’t required it’s own shelf/slot and power connection.  Both are a very big plus for flexibility.

My thoughts and conclusion is that this passive preamp is completely transparent to the reproduction of sound being passed through.  It’s perfect when you want/need a passive preamp that offers flexibility, very high quality, and transparency.

[Roger A. Modjeski]

Thanks for sharing your thoughts in this forum. I have been using passive preamps for the last 15 years.

[pubul57]

Roger, can you share your theoretical thoughts on resistor passives versus TVC and AVC, an LDRs - Assume we agree that passive is the best approach with the right amp, cable, source sensitivities, impedances, etc - which form of passive should be best, and why?

[Roger A. Modjeski]

Roger, can you share your theoretical thoughts on resistor passives versus TVC and AVC, an LDRs - Assume we agree that passive is the best approach with the right amp, cable, source sensitivities, impedances, etc - which form of passive should be best, and why?

I still like a good quality pot for its transparency and simplicity. As long as the wiper is clean it has no distortion, good tracking, wide bandwidth and presents an input and output impedance that is independent of frequency. The pot we use in POT IN A BOX is very high quality, has a 10 finger wiper, is easy to clean if need be, though with frequent use I have not had to clean any that I have used consistently over 30 years. When used with a reasonably low capacitance cable it is flat from DC to over 20KHz, has no phase shift, no nonlinear distortions, no IM distortion.

The drawbacks of transformer controls are that they do not present a constant impedance over frequency, can load outputs, can overload at low frequencies, have a resonant peak somewhere, hopefully above 20 KHz. I think we are getting away with them as they are being driven by modern low impedance sources like CD players that have 10-100 ohm output impedance. Has anyone put 2 volts at 20 Hz into them from a 10 Kohm source, or at 20 KHz? Does anyone spec the IM distortion at 2 volts input from a CD player?

LDR's concern me because they are made of Cadmium Sulfide and who knows what that sounds like. With all the concern about the perfection of resistive materials, Caddock plate resistors and the like, who has listened to Cadmium Sulfide. I also wonder how good is the tracking between channels?

AVC in my book stands for Automatic Volume Control in an AM radio. What is it in audiophile-speak?

[poseidonsvoice]

I still like a good quality pot for its transparency and simplicity. As long as the wiper is clean it has no distortion, good tracking, wide bandwidth and presents an input and output impedance that is independent of frequency. The pot we use in POT IN A BOX is very high quality, has a 10 finger wiper, is easy to clean if need be, though with frequent use I have not had to clean any that I have used consistently over 30 years. When used with a reasonably low capacitance cable it is flat from DC to over 20KHz, has no phase shift, no nonlinear distortions, no IM distortion.

The drawbacks of transformer controls are that they do not present a constant impedance over frequency, can load outputs, can overload at low frequencies, have a resonant peak somewhere, hopefully above 20 KHz. I think we are getting away with them as they are being driven by modern low impedance sources like CD players that have 10-100 ohm output impedance. Has anyone put 2 volts at 20 Hz into them from a 10 Kohm source, or at 20 KHz? Does anyone spec the IM distortion at 2 volts input from a CD player?

LDR's concern me because they are made of Cadmium Sulfide and who knows what that sounds like. With all the concern about the perfection of resistive materials, Caddock plate resistors and the like, who has listened to Cadmium Sulfide. I also wonder how good is the tracking between channels?

AVC in my book stands for Automatic Volume Control in an AM radio. What is it in audiophile-speak?

AVC in this context is "Autoformer" volume Control to distinguish it from "Transformer" volume control. It has only one coil. They are typically less expensive as a result due to fewer windings and a smaller core (as long as we are talking about the same materials!). I anticipate, however, they have similar performance disadvantages like Roger described earlier, in that they don't have a constant impedance over frequency. However, I do believe they have a very flat frequency response (again compared to "Transfomers" of similar construction/quality). They cannot provide balanced lines or electrical isolation, compared to their "Transformer" cousins that have a primary & secondary winding.

I'm sure Roger can elaborate more.

Best,
Anand.

[Roger A. Modjeski]

AVC in this context is "Autoformer" volume Control to distinguish it from "Transformer" volume control. It has only one coil. They are typically less expensive as a result due to fewer windings and a smaller core (as long as we are talking about the same materials!). I anticipate, however, they have similar performance disadvantages like Roger described earlier, in that they don't have a constant impedance over frequency. However, I do believe they have a very flat frequency response (again compared to "Transfomers" of similar construction/quality). They cannot provide balanced lines or electrical isolation, compared to their "Transformer" cousins that have a primary & secondary winding.

I'm sure Roger can elaborate more.

Best,
Anand.

Thanks for the translation. Auto transformers do have certain advantages but if isolation or balanced to un-balanced is required a primary and secondary are the way to go. A properly designed tapped autotransformer will certainly have better high frequency behavior due to less leakage inductance and less shunt capacitance. As much as I like designing, making and using transformers in my amplifiers, I only do so when they are needed.

Do you have links to response curves, impedance curves, distortion vs level, frequency and drive impedance? These are what is needed to ascertain their performance. Of course one can listen, one can listen to anything and hear a difference when one wants to. Fuses come to mind currently.

[poseidonsvoice]

Thanks for the translation. Auto transformers do have certain advantages but if isolation or balanced to un-balanced is required a primary and secondary are the way to go. A properly designed tapped autotransformer will certainly have better high frequency behavior due to less leakage inductance and less shunt capacitance. As much as I like designing, making and using transformers in my amplifiers, I only do so when they are needed.

Do you have links to response curves, impedance curves, distortion vs level, frequency and drive impedance? These are what is needed to ascertain their performance. Of course one can listen, one can listen to anything and hear a difference when one wants to. Fuses come to mind currently.

Roger,

Yes, thanks for agreeing with me, I was hoping that I wasn't stating anything erroneous. This is EE 101 type of stuff, and I'm still in the 101 stage, compared to your PhD!

As to measurements (of autoformers), I don't have any personally. However, you could ask Dave Slagle of Intact Audio who has autoformer volume controls available.

http://www.intactaudio.com/forum/index.php?sid=ebaf8f591fe30294301262481d115569

As of this writing there are 21,000 registered users (who knows how many are active) so there is some traffic there.

All the best,
Anand.