Magnetic + Piezo Active Pickup Combinations

[knathsif]

Hi!

An intro's probably called for, since this is my first post here. I've been mucking about with guitar electronics for quite a number of years now. This was mainly kicked off by the QTB mods on the old Guitar Nuts page. Since then, I've become quite handy with guitar electronics and have built a few effects pedals as well.

Now... I've gone and taken up a friendly challenge from a friend of mine which I need a little help on.

Guitar: Godin with H-S-H passive pickups and an LR Baggs X-Bridge with some sort of pre-amp. 1 Mag Vol, 1 Mag Tone, 1 Piezo Vol. (I can't imagine why people would want this without some sort of Mag/Piezo switch...)

Desired Outcome: 3-way toggle switch to switch between 1) Passive (Mag) pickups, 2) Mag + Piezo pickups with blend control, 3) Piezo pickups.

I've decided to try and use the "Active Blender / Volume Module" design as seen here: guitarnuts2.proboards.com/index.cgi?board=schem&action=display&thread=4595. I couldn't for the life of me figure out a way to use a toggle switch to achieve the switching so I resorted to a 3-position rotary switch instead. Here's what I've come up with.



Please excuse the horrible picture. I'm using my lunch break at work to do this so I don't have access to my usual image gear.

SW-1 is the 3 position rotary. Position 1 (dotted) will give the mag signal bypassing the blend unit, Position 2 will give the mag + piezo into the blend unit, Position 3 will give the piezo signal bypassing the blend unit.

The boxed "V"s in the picture refer to the JFET buffer from the Active Blend Module but without the pot. The mag pickup signal will come straight from the pickups (as if the volume pot was at 100% and no load) while the piezo pickup will come through a trim pot replacing the current piezo volume pot (for equalising mag and piezo volumes later).

The boxed "B" refers to the Active Blend Module in its entirety.

I just realised that I forgot to draw in the master volume. It'll go immediately after the Blend Module. Again, I'll be using JohnH's design (for Vol control) in its entirety as well. (Thanks, JohnH et al!! You da man(s)!!!)

I have a few questions relating to this design. I'm not that experienced with active electronics in guitars so please pardon my ignorance.

1) Is there a need for so many buffers or can I just do away with the two buffers before the Blend Module? After all, there'll be another buffer/volume unit after the Blend Module serving as a master volume. Since the piezo has an active pre-amp on it but the mag pickups are passive, I was worried about differing impedances going into the Blend Module.

2) Regarding the Blend Module, I designed the switching arrangement as I would an effects pedal. Reason being I was afraid of some sort of high end loss if the Blend Module was only partially by-passed. Was I correct?

3) Can anyone see a way to do this using a 3-way toggle? That'll be ace!

Thank you all in advance!! ;D

Regards,
Shaun

[newey]

Knathsif-

Hello and Welcome!

JohnH can give you more particulars on this, but off the cuff, I don't see why you would need so many buffers. The existing piezo preamp is, presumably, already buffering that signal.

If you haven't already done so, click through on the link in John's post to the original discussion with Treguiers about this project. In that thread, John points out that the buffer module you are going to use is actually one section of a larger circuit he used, to both buffer and boost a passive magnetic pickup so as to match up well with a piezo's active circuit.

Treguier's project already had an active mag pickup, and so didn't need any further boosting.

[JohnH]

I think it would be a good idea to put a buffer after the magnetic pickup, just after its own volume control. That will give a good low impedance output from the mag pickups that will not suffer tone loss.

The Piezo system, as newey points out, should be fine without an extra buffer.

Then wire the piezo preamp and the new mag buffer directly to the inputs of the blender module - no switching is needed. Now you can install a three position toggle. A standard Gibson/switchcraft is not suitable. What you need is a real three-way selector that selects three separate connections. You can make this with a dpdt on/on/on mini toggle, to make an effective 1p3t toggle (see if you can work it out, otherwise, I think Ive seen it on Wolfs site). This selector simply chooses the mag, the blend or the piezo. it does not matter that the blend module is permanently connected to the other two, because it is all buffered.

Now the master volume - you should find that just a passive 100k pot after the blender module is OK into a guitar amp. But if you want to drive a line input on a mixer desk (which can have much lower impedance), then the active volume module is better.

An option withe the active volume module is to give it a bit of added gain - it needs a different JFET circuit however- any interest in that?

good luck!

John

[knathsif]

Woohooo, it makes sense now!

newey: Hello to you too and thanks for the input! I did have a read through the thread you're referring to. Again, my grasp of active electronics isn't the best but the thread did give me the impression that I was over-doing the buffers somewhat.

JohnH: Many thanks to you too for your advice! As mentioned to newey, I had my suspicions about them buffers. I still don't fully understand the why's but I intend to, slowly but surely. For now, though, I'll take your suggestions as is. Awesome thing is I finally figured out the DPDT into 1P3T thing. Since you said it was possible, I knew I had to be missing something. After some thought and a whole lot of beer, I finally hit on it! ;D

Here's the revised design:



And now it's 5am here and my eyes are falling out...

Thanks again, everyone! This place rocks! I think 'exalts' are in order for you guys. (Is that how things work here?)

Oh, and the extra gain sounds interesting from a purely electronics view point but I can't quite see how that would be a useful thing for guitars. Off the top of my beer addled head, a higher output pickup (eg DiMarzio Super Distortion) would just end up clipping everything after your guitar, right? Cool if you're pushing amps into overdrive permanently but not so cool if you're playing a mix of cleans and overdrives. Or am I missing something? Again, this forum is awesome..

[ashcatlt]

Yay! Glad to see we've got another one around here who thinks better with a drink in his hand.

What you've got there looks good to me. Please ask specific questions on the points you don't understand and we'll do our best to clarify.

As for the extra gain: It's my understanding that the idea is to increase the output from the mag pickup to help it match the higher output of the piezo pre-amp. You could just as easily pad down the piezo output (or use the volume control that's already there) or, I guess, actually use the blender pot.

[knathsif]

ashcatlt: Hi! Well, my intended solution to the louder piezo volume was to convert the existing piezo volume pot into a trim pot to lower the piezo volume to match the mag volume at 100% Master Vol. Boosting the mag volume is something to consider, though.

[JohnH]

I think your diagram looks fine. With just the passive master volume pot, it keeps the circuit clean and simple. It will work well into a guitar amp, modeller or stompbox. If you ever want to plug direct to a PA, to get the zingiest piezo sound through full range speakers, just plug it through a tuner pedal or some other non tru-bypass stompbox.

Just to relate how I work my system. The piezo I added to a Strat has no volume control of its own, but I did tweak its preamp to balance it with the mag. I have a blend control that works as this one, going from full piezo to full mag, but I have no master volume. Instead, the mag has its own volume control, at the end of the passive mag circuit. I find I can get any setting I want with this - but what you show should also achieve that too.

john

[knathsif]

Awesome help! Thanks!

Well, my friend and I found the need, actually, to have the ability to switch quickly from mag to piezo and back mid-song, hence the toggle switch. But, yeah, like you say, we should be able to wring a whole bunch o' tones from the arrangement.

I'll be a while, though, before starting work on it (such is the life of a full time aircraft engineer) but I'll keep this thread updated on the progress. ;D

Now, what should I do with the H-S-H? Hee hee.. The possibilities are endless with this one!

Thanks again for all the help!

[JohnH]

Good - and Im glad you are an engineer (me too) - this bodes well.

HSH? - yes too many possibilities! Actually, i think the classic wiring of Bridge humbucker, bridge inner+middle in parallel, middle, neck inner+Middle, and Neck humbucker, as provided by a 5-way is a really good simple option, which the guitar may already have. The next simple thing to add is a toggle or push pull that cuts one or both humbuckers to single coil.

This one is in the works:
guitarnuts2.proboards.com/index.cgi?board=wiring&action=display&thread=4762&page=3

see diagram at reply 32 - it provides 15 sounds based on pairs of coils in series or parallel, or single coils. the two p/p switches could be replaced by one three position toggle

John