Magnetic pick-up into a Fishman Isys+ ?

erky32

Before I start cutting wires and soldering, I wonder if anyone could advise if my question is possible without additional mods. The UST is presently directly wired into the Isys+ preamp so I would have to cut & solder a replacement mag pup. I'm aware there is a big output impedance difference between UST & coil, so would I need some kind of buffer to raise the impedance that the preamp sees ? .....what would consequence be ? ....is there a diy-able buffer if required? 

ICBM

Do you mean in addition to the UST, or to replace it?

If it's to replace, I've done a couple like this - not on Fishman systems though - which worked perfectly with the magnetic just connected in place of the UST. The impedance mismatch is fine that way round, it's the other way which would cause trouble.

If it's in addition, that's more difficult because the UST will see the magnetic as a very low impedance load and basically not work.

erky32

ICBM said:

Do you mean in addition to the UST, or to replace it?

If it's to replace, I've done a couple like this - not on Fishman systems though - which worked perfectly with the magnetic just connected in place of the UST. The impedance mismatch is fine that way round, it's the other way which would cause trouble.

If it's in addition, that's more difficult because the UST will see the magnetic as a very low impedance load and basically not work.

Thank you @ICBM , thats a good "go ahead" advice (as always ) ....and yes it's a replacement idea. So I'll crack on with that! 

erky32

hello @!CBM ....didn't work! ....replaced UST for HB mag coil, & not a glimmer of a signal, checked all conns etc to be good, but totally mute with HB, tested HB ok separately, and then reinstated UST and Isys+ worked ok again. One thing interesting maybe (?) was inbuilt tuner didnt work either with HB. Any thoughts if I may trouble you? Maybe the Isys sees the HB as a short? Do you think I should try a small Capacitor( 0.22mF or something ?)

ICBM

What type of UST is it? Is it a piezo pickup or one like a B-Band which acts more like a condenser mic?

I'm not familiar with the Isys+ at all unfortunately. I've done it successfully with a couple of standard piezo-based systems, but this must be different...

Try a cap, I have no idea what value!

erky32

It's a piezo coax wire, just a coax ,I think, with some sensitive centre core sheath. I'll try a small (.2 to 1mF) cap anyway - see if it jumps into life!!

erky32

Sorted! ....soldered a 0.1mF cap in line and it jumped into life! ...anyway, @ICBM  thanks for sharing & helping once again!

ICBM

erky32 said:

Sorted! ....soldered a 0.1mF cap in line and it jumped into life! ...anyway, @ICBM  thanks for sharing & helping once again!

No problem, I've learned something useful too . As you said, it sounds like it's a capacitive system like the B-Band so didn't like being loaded with what is more or less a short.

I don't know whether the cap value would make any difference to the sound, but if it's not a massive faff to try different values it could be worth experimenting... although if it's already large enough to have a cut-off frequency below the audio range then it won't matter.

erky32

I've already tucked all the wires and cap' neat inside the body, so I'll leave it now. The one I used was a clumpy 630v 0.1mF (only suitable one lying around!) which I assumed would offer no resistance to a signal, didnt notice any downside, maybe a smaller one would work. Maybe a small electrolytic would work? ....positive side into preamp? Worth trying , but doubt I'll go back in again! - Cheers, Eric

ICBM

No, don't use an electrolytic unless you can't get a big enough value any other way. They're prone to degradation over time and have poor HF response.

Unless you're hearing a lack of low-end, the 0.1uF is fine.

crunchman

If my understanding is correct, a smaller value capacitor would start to roll off lower frequency signals.  I think the coupling caps in some valve amps are 0.022uF so your 0.1uF probably isn't in the range where it would block low frequencies from a guitar.  There is a formula you can use to work it out if you are really worried about it.

Blocking very low frequencies might not be a bad thing.   A lot of sound desks have a high pass filter button on their channels to filter out frequencies below a certain frequency.  That's normally around 80 or 100Hz.

Generally, electrolytics aren't regarded as good in audio circuits.  They are used where large values are required but smaller values are normally ceramic or metal film.

erky32

Thanks guys, - yes I think the 0.1 is fairly transparent as a coupler. Ive got a load of electrolytics but was struggling with my ceramic/metal film collection - think I need a selection box to replenish stocks! (I didnt know about that HF compromise with electrolytes - thanks!)   Really appreciate all your helps