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69.136.121.232
Thanks for all the recommendations on phono stage options.
I just set this up today. I got this model because someday I will probably want to start experimenting with moving coil cartridges. But for now I'm using MM..
Runs very quiet however operates rather warm which is probably normal. The designer recommends keeping it powered up for one month at the start.
Thanks
Follow Ups:
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.
Of the many phono preamp spec I've read, I don't recall ever seeing that. Though, I don't read as many spec sheets for tube based phono preamps, so maybe I've missed tis trend. Who does spec imd for their phono preamps?
Maybe marketing departments don't want IMD measurements revealed. They're generally much worse numerically than THD. OTOH, Hafler used to specify the PH50 MM preamp at .04% IMD. I don't know if there are enough others such that a comparison could be made between those with FETs and those with other amplifying elements. I only know that my experience with the unit described above would make me extremely wary of any line stage unit or phono preamp employing FETs (JFETs to be more precise).
but Stereophile measurements for the phono stage peg the SP20 JFET hybrid at ~ -84 db (0.006%) with their test. Not as low as the line stage, but quite respectable considering it uses zero corrective feedback.
What do you want?
That's not very meaningful for purposes of this discussion. Unless you have a schematic, the topology of the phono stage is unknown, as are any remedial techniques that might have been applied by the manufacturer (such as feedback). Just as important, it's only one unit, and a statistical sampling from various manufacturers would be required to know whether FET-based preamps generally exhibit increased IM compared to other types. This one doesn't seem to, but again, we don't know for certain whether it qualifies under that description.I'll just add that it would be more assuring to see this specification published by the manufacturer, rather than an insulated third party with a vested interest in reporting good performance.
Edits: 10/25/24
That's not very meaningful for purposes of this discussion.
It demonstrates that JFETs can indeed produce good measured IMD results. Audio Research isn't exactly a newcomer to the world of excellent performance.
as are any remedial techniques that might have been applied by the manufacturer (such as feedback).
If you return to my first post, you'll find I've already answered that question.
I don't know why imd testing of phono stages isn't more common and less talked about. It seems too bad. The more info the better if you asked me. I did find a link re MP-7 (non B model) tested imd.
"I did find a link re MP-7 (non B model) tested imd."
I don't know how to compare that method of testing (18kHz/19kHz summed carriers) to my own. However, the 2nd order IM product at 1kHz was a whopping 3%.
Did you notice the huge discrepancy in the THD numbers between the manufacturer's spec and the test lab? Darlington Labs states <0.01%, but the lab measured <0.0006%. This makes me wonder if one of these used a reverse RIAA in taking the measurement, but the other didn't. That's one of the problems in measuring phono preamps, the EQ gets in the way.
Honestly, I don't know why any company would use FETs in a solid state phono preamp today, unless it's purely marketing. The noise floor and distortion of several modern operational amplifiers are so low as to be almost impossible to even measure. These are dedicated devices that have been refined for decades, and they require almost no secondary engineering on the part of the equipment manufacture to maintain the specs.
Whatever the case, I just stick to tubes and passive EQ, and call it good. That eliminates most of these gremlins. :)
For sure. It's like the wild west. That makes life very difficult for consumers trying to make meaningful measurement comparisons. I often see references to ancient IEC phono preamp measurement standards. Since those cost money, I may never get to read them. But do any modern manufacturers or reviewers even adhere to a set of standards when testing? It kind of seems like many just march to their own beat, maybe even make some stuff up and often failing to adequately reference the result, making it impossible figure out what it means.
Absolutely right, and all the junk coming from China has only made it worse. For my own gear, I either refurbish vintage equipment or build my own. Can't trust any of the manufacturers' claims now that the FTC has stopped regulating this industry. I suspect it will get a lot worse before it gets better.
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