Archtop bridge piezo with neck mounted pick up. Mixing passively????

[Vic Farrell]

Hi folks,

I am in the process of trying to blend a fishman archtop bridge piezo with a standard neck mounted pickup. Do any of you have knowledge as to the best value of pot to use and is it possible in the passive  mode. Also I am using a pickguard mounted pot. At present I am using one of the little ones that are glued to the underside of the pickguard and have the control wheel just sticking out the side. It is a 500k. I think that is too high a resistance. I am also limited as to placement. Any input will be appreciated.

Thanks Vic Farrell

[Ted Kritikos]

Does this help at all?

http://www.stewmac.com/tradesecrets/ts0135_activepassive.hzml?utm_s...

[Vic Farrell]

Yes and... No. This is not what I was trying to do and it uses a preamp for the piezo. I am doing a passive blend on the guitar with no preamp. It works but.... I am trying to find the best possible mix so pot value might have something to do with it. Thanks all the same. I have that artical and I like it. I might suggest it my costumer down the road.

 

Vic F.

[Rob Mercure]

Well, one of the real issues is that piezo pickups - generally being extremely high impedance, extremely high capacitance, extremely low current devices really don't "want" to blend with each other as the characteristics of the piezo signal output - in extremely poor electronics/physics terminology - are so "weak" that one pickup loads the second pickup, or is it the other way around - just a matter of perspective - that you have a situations of one "shorting out" the other!  'Tis the nature of the beast and there really isn't any way of getting around it  Even with two truly identical pickups the very fact that you are locating them in different places means that the signal vs frequency vs phase will be sufficiently different (the desirable bonus you're are looking for with other types of pickups) that a peak in response in one locations is nullified by a dip elsewhere.  Much like connecting very, very different speakers in parallel and listening to the results.

 

A good buffer is simply an amplifier with a flat, neutral, frequency response with a gain of 1 or slightly less to match source and local impedances - that's pretty close to the "official" electronics definition.  That's why this is more of less the ideal situation where a buffer serves to be useful as opposed to an amplifier or EQ.  And a buffer can be made with a very few low noise low wattage high value resistors, an electrolytic capacitor, and a battery - there are numerous examples on the net that a simply google search will reveal and these will create less change in the sound (assuming you can stand the original sound) of a piezo pickup that the unbuffered guitar cord it is fed by will.  The only thing you might add to each is a trim pot to balance the signal from one pickup to the next.  Oh, and 500K rather than being too high a resistance is too low but around a factor of 10-20!  Electromagnetic pickups these aint!

 

Rob

[Vic Farrell]

Thank you Rob. I thought as much But I wanted to make sure. I will try to sell the costumer on a preamp. I like the idea of a small one that can be put just inside the F hole. Truth be told I don't like the sound of the piezo pickup in the archtop bridge. It dose not sound natural. Even with a preamp. I am more partial to the k&k. So will a pot with higher resistance help at all? I am doubting it but I have to ask. Just a note I don't think this costumer wants to "spend" much. If you catch my meaning.

 

Thanks.

Vic F.

[Rob Mercure]

Yep, the higher the impedance you can feed the piezo into the better whatever tonality is posssible.  Ten mega-ohms is kinda the minimum athough there are lots of items on the market that use lower values.  And, again, if the customer doesn't want any preamplification keep throwing the word "buffer" at him/her as a well designed buffer won't change the tonal signature but instead make it's accessible.  As long as the amplification factor is "1" or less it's not technically a preamp although with any sort of tone controls that muddies up the definition.  If it weren't so technically difficult to wind very high impedance transformer winding (the cost would be prohibitively expensive for anything decent) you could get away with a tranny.  But to flip the issue on it's head you might explain that a low impedance microphone, until very recently, needed a transformer to bring the signal up to be useable and this is analogous.  If you or your customer is at all comfortable with soldering a buffer can be produced for less than $5 in parts on a piece of perf board barring any level potentiometer to adjust the output level.  Again, it's literally a few resistors,  a field effect transistor, and a blocking capacitor and the cap to plug the battery in with - if wired into the guitar there is no need for either an input of output jack (use the one that the piezo came with although it is nice to have one with a built in switch to disconnect the battery when unplugged.  With plugs and jacks the buffer can be built externally in a small tin such as those Altoids gum boxes with the battery taped on the outside.

 

Rob