Variable input impedance?

[PB+J]

I was listening to the podcast interview with Steve Rodby recently, and he went on and on about the importance of input impedance. It resonated (ha ha) with me because I've always noted how different pickups can sound dramatically different with different preamps, independent of tone controls. Rodby says he has a purpose built variable impedance circuit which he uses to match the pickup to the amp.

So... does anyone make such a thing? Radial Engineering makes a "drag control that varied impedance, but it's designed fro mag pickups and they say it will have no effect on piezos. ART sell one of their tube preamps with something they call "variable valve voicing," which I THINK might be variable input impedance, but they don't offer much actual detail and I'm skeptical about ART and the whole tube warmth thing.

Summit Audio makes a DI with variable input impedance--how hard it is to make a standalone circuit that would vary input impedance? (and of course, output impedance)

[Ric Vice]

Originally Posted by PB+J

Summit Audio makes a DI with variable input impedance--how hard it is to make a standalone circuit that would vary input impedance? (and of course, output impedance)

PBJ
Other than the Summit DI, the Headway EDB-1 has three different meg ohm settings, but there set at specific loads. 1 megohm , 5 meg ohm and 20 meg ohm. If would be really cool if someone like FDeck built that into one of his HPF/PRE amps, but he's already got a great design, and some very smart TB er's will say that 10 meg is really all you need. A Walter Woods Amp sounds great and is set at 1 meg ohm, so are Acousic Image, Euphonic Audio, Gallien Krueger, and Gentz Benz amps. Personally, I prefer the 5 megohm load with the Realist.



Ric

[drurb]

This thread is right on point.

[PB+J]

I use an AI clarus, and most of the time I use a schertler Dyn B. But I play some jazz guitar and I've noticed a big difference with different preamps. That Headway has been tempting for some time. Is it hard to make impedance infinitely variable (within a given range, I mean) I really have no idea

[robgrow]

A while back I built an FET buffer with a variable input impedance (between about 200K and 10 meg ohms). It didn't make as much difference as I thought it might, and I ended up setting it around 1 meg most of the time.

[drurb]

Originally Posted by robgrow

A while back I built an FET buffer with a variable input impedance (between about 200K and 10 meg ohms). It didn't make as much difference as I thought it might, and I ended up setting it around 1 meg most of the time.

As expected and thanks for sharing that experience. Gee, seems this thread could be merged with this one. What you report is exactly what I claimed to be the case.

[DoubleMIDI]

I experimented with piezo disks and PVDF foils (like in a Shadow NFX or Realist if I am informed correctly) and had problems with thin cables (2 to 2.5 mm diameter) that I needed for my Clevinger EUB for the new electronics I made. PVDF foils need a rather high input impedance, the smaller the area the larger the impedance, that can go up to 20 MOhms. (A smaller value may work with a bit of bass attanuation that need not always to be bad, thinking of the bass rolloff in a FDeck HPF pre, but in most cases it is bad).
With my thin cables that I neded from the pickup to the built-in electronics I found they are rather microphonic (the cable itself, not carrying vibration to the pickup along the cable, I clamped the cable between knocking point and pickup for the test). The reason is a rather soft dielectric in the cable that changes the capacitance and so generates noise. It generates more noise if the input impedance is higher. So it makes sense to keep the input impedance as low as possible, but not lower than that. Making the input impedance adjustable on a preamp/impedance converter looks like a good idea. It doesn't need to be continuously variable, steps by a factor of 1.5 to 2 should be OK. And there is no need to change the impedance once you found the best setting (the lowest impedance that gives enough bass response) for a certain setup of pickup and preamp.
BTW if you want to check your cables just put it in the preamp, leave the other end open, turn on the volume at your amp and knock at the cable. (After this test you can connect the pickup and try again.) Even with less than 10cm (4inch) of a thin soft coax cable (connector on one end, open at the other) I got a lot of noise (like on damped contact microphone and indeed this cable acts almost exactly like a bad contact microphone).
For these experiments I used a slightly modified version if FDecks simple impedance converter with a single FET and 10 MOhms input impedance.
I also experimented with cable, that I used for microphones and that works fine with them, for the piezo. The result was not as bad as with the thin cable, but bad enough that I got a better cable for a direct piezo to preamp connection for my acoustic instrument (a rather stiff Sommer cable, can't remember the exact name). But this cable is only needed if I forget to take my impedance converter with me and need to plug in the amp directly.
The best thing is to keep the impedance converter ("preamp") close to the pickup and use a good short cable between them. After the impedance converter almost any shielded cable should work.