Impedence issue---probably a stupid question :-(

[rockgardenlove]

So I've heard somebody on here say that this shematic won't "load" a guitar pickup correctly because the impedence is too low.  What exactly does this mean, and how would I go about fixing that in the shematic?  A bit of an explanation would be greatly appreciated, I'd rather understand this than have to ask this question each time I want to do something like this.

This is the shematic:

So how would I go about making that suitable for guitar?

Thanks guys

[brett]

Hi.
Guitar pickups usually work best when they have a high impedance load.  At least 100 kohms, preferably 220kohms or more.  This circuit won't work well because the impedance is a bit too low.

The easiest way to increase input impedance in a circuit like this is to use a buffer consisting of an extra op-amp, at the front of the circuit.  Feed the pickup into the +ve input to the op-amp, and use large value resistors to make the voltage divider for Vbias.  In this case, that means changing the 2 x 10k to 2 x 470k or 2 x 1M.  Check out how this is done in the MXR distortion + or various op-amp "buffers".

good luck

[rockgardenlove]

What makes a circuit have "impedance" though...it's basically resistance right?  Is it resistance between the hot and ground?  Or what?  What does it mean to have a "load"?


Is there any way to get around this without having a buffer?  Could I make a transistor based buffer?

I also have a bunch of those LF351 opamps...if someone could draw some kind of shematic that might work with those to make a good buffer, that would help too...I must say I'm really confused.
Thanks

[bioroids]

Is there any way to get around this without having a buffer?  Could I make a transistor based buffer?

Yes, you can use a transistor (Fet, Bipolar, or Mosfet) buffer if you want instead of an opamp one.

About impedance, this is a must read: http://www.geofex.com/Article_Folders/impednc.htm from GEOFEX as usuall 

Luck!

Miguel

[rockgardenlove]

Great!  I was hoping there was a good read up, I was looking for one, but couldn't find one.  GEO is amazing.

[Transmogrifox]

I would like to add some of my own 2 cents to it.  RG does an excellent job explaining things in simple terms so that a person doesn't have to be an engineer or physicist to understand.

My comment is about what this "loading" physically means for a guitar with a passive pickup (or multiple).  The pickup has magnets to create a constant magnetic field near the guitar strings.  The guitar strings are metal objects moving in a magnetic field.  As the strings move, they effect the intensity of a magnetic field.  A changing magnetic field induces current in a wire loop.  These are the coils around the magnets in the pickups.  As the current increases in the coils, the coils in turn generate a magnetic field that opposes the magnetic field that  caused it.  This magnetic field generates a force on the guitar string.  This force opposes its motion, thus reducing its velocity.

If that wire loop (coil) is an open circuit, as in, not a loop, but just a coil open at both ends, then the magnetic field generates a voltage potential, but little (ideally none) current, thus the energy stays in the guitar string and it naturally decays in amplitude due to only mechanical effects such as friction at the nut and bridge, air resistance, and absorbtion from the guitar body.  I hope I haven't lost the reader at this point.

Basically what I'm saying, is that if that pickup coil is connected accross a low impedance, then more current will flow.  This will require more electrical energy.  More electrical energy does not come from the ether.  It comes directly from the energy in the guitar string, so the electrical impedance is not only loading the pickups electrically, but it's loading the guitar strings mechanically and it effectively reduces the sustain, since the loaded pickups damp your guitar strings.

If you have a high impedance on the pickups, less current will flow, and the voltage will not get above a certain potential (a few hundred millivolts).  Relatively constant voltage, and less currrent  means low power.  Low power electrically requires low power mechanically, thus the string can be left to resonate as if the pickup was not even there.

There are other factors that make it less desirable to have ultra high impedances like 10 Meg, but 100k to 1Meg are pretty common loading on the guitar pickup.

I just thought of that as a tidbit worth adding, since it is interesting to consider how the electrical loading directly effects the mechanical loading on the strings.  Somehow it seems more intuitive to understand mechanical loading.  It's easier to explain how climbing a lifting two people together takes more energy than lifting one person, than it is to explain how it takes more energy to supply 2 amps at 1 volt than 500mA at 1Volt.

[rockgardenlove]

^Wonderful explanation man...really appreciate that. 

So can I tweak that shematic to match the guitars pickups better?  Or is a buffer the only way?

[no one ever]

 

impedance isn't your only problem with that schem. 

[rockgardenlove]

I know the cap values are off...what else is wrong then?

[Transmogrifox]

^Wonderful explanation man...really appreciate that. 

So can I tweak that shematic to match the guitars pickups better?  Or is a buffer the only way?

Yes, and yes.

A buffer is the only practical way to tweak the circuit to match your guitar's pickups better.  

Multiple problems I can think of with matching the input impedance of the circuit to the pickups (and to the guitar for that matter):
Do you ALWAYS use the same guitar and never switch between the neck pickup, bridge, or in the case of strats and similar, the middle pickup?

Are you ALWAYS going to have the volume and tone pots on your guitar in the same position?

Are you ALWAYS going to have this circuit connected directly to your guitar?  Maybe you would want to EQ the outpu of a fuzz pedal.

The next issue is that you would need to model a pretty complex electrical/mechanical system to a fair approximation (get out the old physics book).  Since the integrity of this EQ counts on a low impedance voltage source (such as a buffer) driving it, you would have to take that into consideration to ensure that the EQ was actually doing the same thing.  And that's what would require you to ensure that you were always using the same pickup on the same guitar with the volume and tone knobs in the same position all the time.  It would all be very interactive.

So I think it's just a lot easier to add a JFET, 3 or 4 resistors, and a couple capacitors for a high impedance input buffer.

The other alternative would require a slough of inductors, capacitors, and maybe a few resistors to come up with an adequate input impedance for lossless matching.  It's an engineering task that is near tantamount to designing a front-end receiver for a spread spectrum radio.

What it comes down to, is that EQ was designed to be driven by a circuit (or device) with a low source impedance.  If it doesn't meet that criteria, then it will change the operation of the circuit...and by way of Murphy's law, it will probably change it in an undesirable way.

[rockgardenlove]

Hmm, alright.  Thanks for the reply.  The only IC's I have for now are the ones that I bought for use in that shematic though...can I make a buffer from them?  Can I make one from a 2n5088 or a 2n3904?  A shematic would be wonderful here...

Thanks so much, Im getting a grasp on this actually.

[Pushtone]

This looks like that "3 band trimmer EQ" circuit from a few weeks back. Glad to see someone is still with it. I think it would be a great FX building block. 

When I saw it in that thread, I thought (out loud of course) the GGG IC buffer would be a good match for this circuit.
However, If you use a JFET buffer... JFETS naturally have a high input impedance. So either one really.
For JFETS, I've been changing out the J201 for 2N5457 in my boosters. I think the 2N5457 sounds better when I want a clean unity gain preamp. (but I left the J201 in the AMZ Mini-Booster cause I like that one gritty)

Here's an old alternative, the Tillman preamp. Have you seen that?
http://www.till.com/articles/GuitarPreamp/

Pop one of these in front and breadboard it!

[rockgardenlove]

I'd rather not buy more IC's...any Bipolar solutions?

[Transmogrifox]

http://putfile.com/pic.php?pic=4/930114089.jpg&s=x11

Try that.  It's a simple enough BJT solution.  Don't use the guitar preamp linked above because the output impedance is significant compared to the input impedance of the EQ.  Remember, the EQ wants to be driven by a low source impedance.   This BJT buffer typically has about 30 ohms source impedance.You may need to make the output capacitor larger--like 47uF or so.

[rockgardenlove]

Awesome...thanks man.

Just one question, what transistor is that shematic biased for?

Thanks

[Transmogrifox]

I feared you would ask if I didn't note it .

It will work with about any functional BJT under the sun.  I didn't specify one because I  didn't want to leave you thinking that you had to use whatever particular transistor I may have put in there.  You can use 2N2222, 2N3904, 2N4124, MPSA18, 2N5089...probably most mosfets and some JFETs. 

So...whatever BJT you have, plug it in and it will work.  These transistor emitter followers are not terribly sensitive to component variations.  As a general rule, higher gain transistors are better, but the operation of this circuit does not depend on it.

[rockgardenlove]

Then a 2n5088 it is!


Thanks for all the help man, I really appreciate it. 

[brett]

Hi.

This BJT buffer typically has about 30 ohms source impedance.You may need to make the output capacitor larger--like 47uF or so.

Hmmm.....
The size of that cap is dependanr on 2 things: the lowest frequency you wish to pass, and the INPUT impedance of the next stage.

For a rolloff frequency (Fc) of around 30Hz, and input impedance of 10 kohms, a 0.47 uF capacitor is large enough.  Which is great, because you can use a greencap or film type instead of an electrolytic.  Also, because the input impedance of the buffer is high (>100k ohms) the input cap (between the guitar and base of the transistor) can be a 0.1uF greencap or metal film type.

cheers

[rockgardenlove]

Thanks for all the help guys.