Berglund Vari-Mu

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mjrippe

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Has anyone else read this article from 1997?  I found it in my archives.  It appears to be a hi-fi/audiophile magazine but he lays out a vari-mu circuit with a twist.
 

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mjrippe said:
Has anyone else read this article from 1997?  I found it in my archives.  It appears to be a hi-fi/audiophile magazine but he lays out a vari-mu circuit with a twist.
The claim for low distortion seems to lack substance. Indeed the push-pull arrangement does somewhat reduce 2nd-order distortion, but not more than any other push-pull arrangement, as found in many reputed compressors, such as Gates StaLevel, CBS Audimax, Collins 26, UA176, any Western Electric limiter, and indeed the famous fairchild 660/670.
The few non-push-pull implementations of Vari-mu exhibit not only 2nd-order distortion, but also control-voltage feedthrough that creates thumps in the audio.
In the proposed schematic, the side-chain rectification is only single-wave, which makes it prone to LF distortion under compression.
The fact that no measurements are published indicates that claims of superior performance are but wishful thinking.

The only noticeable thing in this schemo is how the ratio is increased by applying the control voltage to a LTP carrying signals of opposite phase. I would think it's not favourable to the noise performance, though.
 
Agree, I always suspected it to be some kind of a simulation exercise. When I tried quickly setting it up on perfboard, sound and performance was subjectively underwhelming. Don't take my word for it, it's a simple enough circuit with cheap parts - quick to wire up.

My main show stoppers, other than the subjective performance:

Very dependent on ECC189 tube selection, no trim for CV break-through

Output is post passive attenuator, 100K Ohms from a ECC88 cathode. Who would do that for audio?

/Jakob E.
 
gyraf said:
Agree, I always suspected it to be some kind of a simulation exercise.
Like it is very often with these articles. Nobody double-checks the content.

Very dependent on ECC189 tube selection, no trim for CV break-through
In order to make it acceptable, it would be necessary to add balance controls.

Output is post passive attenuator, 100K Ohms from a ECC88 cathode.
As you know, HiFi is a different world, 100mV 100kohms...
 

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