John,
How about a remote controlled stepped resistance, wouldn't that be a viable option? You would have a much wider choice of your resistance elements. You could also circumvent the problems of a mechanical wiper by using high quality relays like one would use for an input attenuator or a source selector.
With 4 relays you could have 16 discrete values.
jd
It might be more economical to ship a tech and bag of resistors to solder in just the right value.
Scott and others, I have used continuously variable loading for the last 30 years. I now want to motorize it. I realize than many here have little or no idea WHY I might like continuously variable loading, but serious listeners are already paying attention to this effort. This is what separates hi end from mid fi. If you want only 2 loads, then the JC-3 phono preamp will be right for you. If you want continuous loading, then the motor driven pot scheme should give the best results. Of course, only serious cartridges need be loaded optimally. Below $2000 or so, I would not necessarily bother.
John,
I realize that it's a judgement call. But you'll probably find it difficult to get a low-value pot that would meet your standards. Continuous variability is nice, and may be a marketing point, but sound quality is more important.
But it's an interesting exercise for sure.
jd
John:
I use a similar approach in the NuForce preamp for the volume control, with an extra gang for measuring the pot tied to a micro back to a motor to adjust it. Works fine for that.
I think you need a hybrid or the crv will drive a customer nuts. (CRV= contact resistance variation, an important figure of merit). They will in effect be noisy as the wiper slides over the wires.
I'll measure a few 10 turn pots in my collection to see how much inductance (and shunt cap) they have.
I use a similar approach in the NuForce preamp for the volume control, with an extra gang for measuring the pot tied to a micro back to a motor to adjust it. Works fine for that.
I think you need a hybrid or the crv will drive a customer nuts. (CRV= contact resistance variation, an important figure of merit). They will in effect be noisy as the wiper slides over the wires.
I'll measure a few 10 turn pots in my collection to see how much inductance (and shunt cap) they have.
Don't worry to much, Demian, we are on it. ;-) Have you evaluated the 1/f potential in the light dependent alternative, yet? I must be noted that line level is not MC level by about 10,000.
I whould say when you do like to have all the variables, loading range should be variable between 5 Ohm and 47kOhm. There are advocates for both extrems and MCs vary today between 1 Ohm and 75 Ohm internal impedance and 0.015mV and 2mV output. I have put a small list on my MPP thread a while ago. Loading down an MC a lot, say to 5 Ohm brought a certain robust bass quality and louding them lightly made the sound more airy in my experiments.
I whould use a TKD motorpot for this. They make different varieties with prices up to 400,-$ a piece but you may know that already.
I tried out eveything from Wirwound, Cermet and conductive plastic and this one sounded by far the best in terms of transparency and dynamics without spit.
Operation is silky smooth and you could even use a stereo version because they have excellent tracking ( i measured 0.3dB) over a very wide range.
I am somewhat uncomfortable with wirewound for that pupose but i did not try precission types. I googled a bit and found wirewounds with a temco of better then 20ppm.
I whould use a TKD motorpot for this. They make different varieties with prices up to 400,-$ a piece but you may know that already.
I tried out eveything from Wirwound, Cermet and conductive plastic and this one sounded by far the best in terms of transparency and dynamics without spit.
Operation is silky smooth and you could even use a stereo version because they have excellent tracking ( i measured 0.3dB) over a very wide range.
I am somewhat uncomfortable with wirewound for that pupose but i did not try precission types. I googled a bit and found wirewounds with a temco of better then 20ppm.
Joachim what's the track material in that TDK motorpot? 0.3dB tracking is very good.
jd
even more; i'd like to know how they manufacture it.
assuming this is a "pot off the shelf"
mlloyd1
assuming this is a "pot off the shelf"
mlloyd1
Joachim, thanks for the input. Yes the TKD motor driven pot would work, but in THIS case, I think a 10 turn, really classy military-industrial hybrid pot would be best. Tracking is important, but I need a LINEAR pot, not an Audio taper one, (I think) so a wirewound becomes a natural. I spoke to your office here about the loading, and your business partner and associates recommended 1K and above for Lyra cartridges. I was rather surprised, but that is what they said. I have a call in to Dave Wilson, and I hope to have him get back to me this morning. He is a real expert in optimum loading and he has a number of World Class cartridges at his disposal. It is where the usual RANGE of loading is, today, that will determine the actual pot value. Outside the range, I can relay switch resistors in either series or parallel to extend the range to its extremes.
Joachim, I agree that the TKD is a darn good tracker, as good as I have ever measured. I would have used it in the Parasound JC-2, but we could not get cooperation from the company and we were forced to use Alps, which is, in my opinion, a lessor brand of pot.
I would think you'd like a log (or anti-log) one. You want the same ratio of change at the top as you have at the bottom. The rotation that changes, say 5k to 2k should be the same rotation you need to get from, say, 500 ohms to 200 ohms.
jd
Well, it has come down to this: A 1K or 2K wirewound hybrid linear 10 turn pot, with series/parallel switching up to 47K. Any ideas, anybody?
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I would think that pot position repeatability would be more important than extreme value resolution.
I think that the inputs have been satisfactory. Perhaps a more inclusive subject would be more interesting.
John,
I guess the listener would set this pot by ear, right? To whatever setting he think sounds best?
BTW What exactly is a 'hybrid' pot?
jd
Ouch, you're not gonna open THAT one again?
Wirewound with a carbon-filled plastic overcoat.
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