Input impedence-How high is high?

[Phil Redman]

I'd like to get an amp that I can run without a separate preamp. Seems like all of the manufacturors of piezo pickups recommend an amp or preamp with "high" input impedence. Some of the amps designed specifially for this have a 10 megohm input. I'm thinking of the Acoutic Image and Raven Labs pre for example. I've seen other amps (KG MB150 etc.) that are around 1 megohm for the high impedence input. My question is: Is there a significant difference in performance between 1 megohm and 10 megohm input impedence with a piezo pickup? I'm thinking of the Fishman Full Circle for the pickup.

Thanks,
Phil

[AMJBASS]

With the full circle you are on the safe side. You don't really need a preamp with it. A 1 Meg input would be great. When you start getting into the K&K pickups(especially the Double Big Twin), the Fishman BP-100, and underwoods it makes a huge difference having that 10 Meg input like the AI amps have.

[Aleph5]

I use the Full Circle with a MB-150S and I don't notice any lack of low end. But in theory every doubling of input impedance cuts the low frequncy rolloff point in half. Maybe 1Meg with the FC already has a corner freqency below the lowest URB freqency (low E at about 42 Hz), which would mean higher impedances wouldn't yield any improvement. Don't really know where the fc is for the FC into 1Meg, though.

[B String]

A friend of mine was happy with his GK amp and full circle.
He was getting everything he wanted. I had him borrow a
10 meg ohm pre Rick Turner made me. No knobs or anything,
just an inline box. My friend used it for a few gigs, said he
really didn't hear much difference. He did the same gig the
next night without the pre, called me during the gig to tell me
what a huge difference it was without it. He just got used to
the fuller, more balanced sound with the pre.
My Acoustic Image Focus has a 10 meg input already. Simple.

[PB+J]

I've used both a full circle and bass max with an avalon U5, 3 megohms input impedance. I like the sound of the avalon better than the sound of the raven labs PMB I also own, with either pickup.

[fdeck]

In theory, a higher input impedance can result in a lower noise floor, but you might not notice the difference.

[PB+J]

More details--just now I did a comparison of the Avalon U5 (3 megohm input impedance) and the raven labs PMB I (10 megohm impedance)

other equipment: ply engle, stewart power amp, ampeg portabass 1x10. Tried both preamps with the eq flat, and both with a slight mid cut and a high ennd roll off.

In all cases the Avalon sounded better--warmer, deeper, less piezo-ey, with lots of top end but no harshness, which is typical of the sound of the U5. The U5 is a great sounding unit

So I can report that with the full circle, the difference between 3 megohms and ten megohms is negligible. Don't know if this applies to other pickups

[robgrow]

Originally Posted by PB+J

More details--just now I did a comparison of the Avalon U5 (3 megohm input impedance) and the raven labs PMB I (10 megohm impedance)

other equipment: ply engle, stewart power amp, ampeg portabass 1x10. Tried both preamps with the eq flat, and both with a slight mid cut and a high ennd roll off.

In all cases the Avalon sounded better--warmer, deeper, less piezo-ey, with lots of top end but no harshness, which is typical of the sound of the U5. The U5 is a great sounding unit

So I can report that with the full circle, the difference between 3 megohms and ten megohms is negligible. Don't know if this applies to other pickups

PB+J,

In my mind, this is an apples an oranges comparison. While the Raven Labs PMB is very nice, Avalon gear is in a whole other league as far as design, construction and sonic quality. I think your experiment simply underscores that fact. The difference in sound is primarily between the 2 units themselves, not so much the difference in their input impedances.

I do think, however, that you're on to something in that, with at least some pickups, there is a certain point where increasing the input impedance any more only has a minimal effect on sound quality.

-bob

[PB+J]

bob:

I agree--they are every different beasts. I just wanted to find out if I could hear any difference because of input impedance. The results are inconclusive at best

[Rick Turner]

Please don't confuse the effect of input impedance with the effect of different electronic design. In the recording world, folks spend thousands of bucks on mic preamps because they hear the difference...not among impedance specs but among different approaches to circuit topology and parts quality. For some reason, nobody seems to think there's much of a difference with instrument preamp stages; of course that is totally bogus...the differences are just as dramatic, though few have heard really high end bass front ends.

If you want to hear the difference, if there is one with that pickup, then listen to the 10 meg Raven, and then put a 5 meg resistor across hot and ground on the input jack inside the unit. That will get you close enough to 3 meg to really hear a loading difference without being confused by the circuit design. Then you're apples to apples.

[PB+J]

Thanks for your time and wisdom rick

[Phil Redman]

Thanks for the thoughtful responses folks.

Phil