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IM Distortion?

Posted by Triode_Kingdom (A) on October 20, 2024 at 16:57:45

In Reply to: Darlington Labs MP7B phono stage posted by bob24 on October 19, 2024 at 12:06:30:

Is it coincidence that the manufacturers of FET-based products like this don't publish IMD specifications? I recently refurbished a vintage full-feature preamp to use as backup while my current tube-based preamp is under construction. The preamp is FET-based, and it met all the factory specs when I tested it on the bench. However, when I installed it in the system, I was immediately aware that something was wrong. There was a level of "grit" to the music reminiscent of early solid-state, and I couldn't bear to listen to it for more than a few minutes at a time.

When I put the preamp back on the bench for more exhaustive testing, I discovered the reason. Although THD of the line stage was less than .005% at full output (which met the manufacturer's spec), IMD was around 0.1%. There was no spec for IMD, and more investigation revealed that none of the other models from the same manufacturer - all using similar FET topology - included this specification.

I eventually stripped out all the original circuitry and replaced it with custom circuits built on prototype boards. The new circuitry uses low-noise, low-distortion operational amplifiers. Many people say there's an unpleasant characteristic sound to opamps, and I don't disagree. However, this is much less invasive that what I was hearing previously with the FETs. Distortion fell drastically following the changes, approaching the limits of my measuring system, which means the actual numbers are even lower than than what I see. Here are the measured results:

Original: THD ≤ 0.005% IMD ≤ 0.1%
Modified: THD 0.0003% IMD 0.0027%

In addition to the improvement in numbers, the "grit" heard previously is entirely absent. The preamp is listenable, and I am using it now as a temporary solution.

I don't know why the original FETs behave the way they do, but this experience has reinforced my determination to never add anything into my system that isn't based on tubes. The tube gear I buy and build never interjects an overriding characteristic like this, and it never produces a result that audibly degrades the source.

I'm only mentioning this as food for thought. If you ever decide the Darlington Labs isn't to your liking, I would urge you to compare it with a good tube-base design. I think you'll be pleasantly surprised.

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