[SOLVED]: Buffer pedal adding distortion?

[poojalooba_cow]

I made a super-simple buffer pedal with the above schematic from Jack Orman's Basic Buffers page.  The only thing I changed was the cap in the voltage divider - it originally was a 10uF but I changed it to 47 because 1. - I ran out of 10 uF caps and 2. - to keep it in line with the Tube Screamer style of voltage dividers, which use 47 uF caps and the same configuration otherwise.

For the transistor, I'm using a 2N3904 and I added a dim LED circuit that turns on when you connect the input jack. it's a simple 9V-to-ground with a resistor in series LED circuit.  The whole buffer circuit works (even in such a tiny box - I made sure there is no shorting) except for a small volume drop - which I assume is normal, because Jack says on his page that transistor gain is only about 0.9.  However, there's also a tiny bit of distortion added - the signal with the buffer is noticeably dirtier than the signal without the buffer.  It's not ridiculous - it's just a tiny bit distorted, but it's still unacceptable for a circuit that's just supposed to buffer my input and nothing more.  I read somewhere also that unless the impedance of a buffer circuit matches the impedance of an amp, which is usually 1M ohms, then you're going to get undesirable effects like volume loss, etc.

So my questions are:

- Why am I getting a tiny bit of distortion from such a simple circuit?  Is it the LED circuit I added?  Is it because I used a 47uF cap instead of a 10?  Is my transistor leaking or otherwise bad?  I have plenty of spares of different makes but you can imagine I'm not too keen on replacing the transistor with such a tiny workable area.  I could always desolder the whole thing and start from scratch, but ugh...

- What can I do to eliminate the distortion and possibly the volume loss?  I would have liked to make an op-amp buffer but I have no acceptable op-amps at the time, but I've been eying the Neutron ECF/Auto-Wah on Geofex and I could get a bunch of spare parts when I order parts for that.

Any help would be much appreciated.

[samhay]

^I'm using a 2N3096

The schematic calls for an NPN transistor like a 2N3904. 2N3906 is PNP.

[poojalooba_cow]

Ha, sorry.  Yes, it's an NPN 3904.  I just remembered that it wasn't a 2N4401 and listed it off the top of my head.  I'll fix that.

[Seljer]

What kind of pickups have you got in your guitar? If you've got very high output humbuckers you can probably clip it on loud strums, such a buffer only gives 6V of clean output voltage swing. This issue can be improved by using two batteries in series for a 18V supply (just make the sure the capacitors voltage rating's are higher).

If it's clipping all the time you've probably got the transistor pinout wrong.

[poojalooba_cow]

What kind of pickups have you got in your guitar? If you've got very high output humbuckers you can probably clip it on loud strums, such a buffer only gives 6V of clean output voltage swing.

This could quite feasibly be the problem. Thanks for bringing this to my attention!

I've got a Godin/Richmond Dorchester with Lace Alumitone humbuckers - they're aluminum-based instead of copper-based.  I just looked at their website and this is a quote from them: "Result [of aluminum over copper]: less resistance, higher output coupled to a "current driven design" as opposed to conventional voltage based guitar pickups. The aluminum water jet cut exoskeleton is then matted to a micro winding using 90% less fine copper wire, a low impedance/high impedance pickup is then created."

I have an Ibanez guitar with P-90s that I can check to see if they clip also.  I don't actually own a guitar with normal single coils, so I have no "control" guitar to check with, ha ha.

If it's clipping all the time you've probably got the transistor pinout wrong.

I checked it three times before I soldered it down, but I could always check again...

[Seljer]

Don't exactly know what Lace is doing with the aluminum there, aluminum wire has 68% *more* resistance than copper for the same diameter  

edit: ah, now I remembered, those are the pickups with just one winding coupled with a transformer 

I checked it three times before I soldered it down, but I could always check again...

Sometimes the datasheets only give that 2D shot of the pinout where you can quickly mix up whether you're looking at it from above or from below  

[Mark Hammer]

1) The swap from 10uf to 47uf, in the voltage-divider circuit on the left that provides the Vref, should make no difference, unless perhaps you have the cap oriented wrong (+ going to ground), and even then I doubt it.  47uf is simply more capacitance than needed, but that doesn't hurt anything.

2) The circuit is a unity-gain buffer, so it does not increase the amplitude of the signal at its output.  However, it retains more of the original signal, and in some instances that can be audibly tantamount to nudging the gain just a little (i.e., sounds like it has a gain of 1.2x instead of 1x).  But that should not make the difference between clean and audible distortion.  It certainly shouldn't overdrive anything that comes after the buffer.

3) The suggestion that perhaps the limitation is the supply voltage used.  If you can rig it up, try a pair of 9v batteries in series (i.e., 18v supply).  If it IS a headroom issue, then the distortion should disappear.  If the distortion is still there, then the source of your undesired tonal change lies elsewhere.

[induction]

It's very difficult to see what's going on in those pictures. (For example, I don't see an input cap anywhere, and it's impossible to see what's connected to what.) Could you take some hi-res closeups of both sides of the board, the wiring connections, etc? Also, posting the layout would help.

As for the volume drop, a higher gain transistor will help, but an op-amp buffer will get you much closer to unity.

[duck_arse]

can you show us how and where you've wired your led, please?

[anotherjim]

I can't see the 0.1 input cap either. If that's missing, the guitar is pulling the input to a lower voltage than it should be.

Don't you have a meter?
There should be 1/2 battery voltage between Vr and battery neg - approx 4.5v
Voltage between Base and Emitter should be approx 0.6-0.7v
Emitter to Battery neg should be Vr minus the Base-Emitter voltage - approx 4.5v

[poojalooba_cow]

(For example, I don't see an input cap anywhere, and it's impossible to see what's connected to what.)

The input cap is a tiny multilayer film cap.  It's hidden behind the wires.  It's definitely there.  I included some links to higher-res pictures below.



This is a literal schematic of how I've soldered everything together, including their relative positions on the board.  I retraced everything to make sure this was right.  
Hi-res front and back shots are here:

Front:
http://drive.google.com/uc?export=view&id=0B6iq6FA5mJa8QTdsTkFpaE1jdU0
(Input is on the bottom-left, output on the bottom-right.  +9V and Ground are top-left and top-right, respectively.)

Back:
http://drive.google.com/uc?export=view&id=0B6iq6FA5mJa8Y0pldi1SSC1yTnc
(+9V is on bottom-left, ground is on bottom-right.)

I checked the circuit with my guitar with P90s - there's no distortion and almost no volume loss.  It works perfectly for that guitar!  But my "main squeeze" is the guitar with the Alumitone humbuckers.  Those things are LOUD!  I had never done a comparison but at the same amp level, just unplugging one and plugging in the next guitar - those Alumitones blow me away.  They have a much larger output than my Ibanez.  

Don't you have a meter?
There should be 1/2 battery voltage between Vr and battery neg - approx 4.5v
Voltage between Base and Emitter should be approx 0.6-0.7v
Emitter to Battery neg should be Vr minus the Base-Emitter voltage - approx 4.5v

I do have a meter- I'm getting 8.95V between + and ground, 4.45V between + and voltage divider, and 4.40V from the voltage divider to ground.  I'm getting 0.47V between Base and Emitter but I'm only getting 0.09V between Emitter and ground... Hmmm... How would I fix this if this is the problem?

3) The suggestion that perhaps the limitation is the supply voltage used.  If you can rig it up, try a pair of 9v batteries in series (i.e., 18v supply).  If it IS a headroom issue, then the distortion should disappear.  If the distortion is still there, then the source of your undesired tonal change lies elsewhere.

This is where things get weird:  I tried hooking it up to two 9V in series, and the noise (static) level increased dramatically but the actual signal got muddied down.  The caps shouldn't be a problem, though, because they're all rated for 50V.  

This is where it gets even WEIRDER:  Certain batteries make different noise levels in the circuit!  The original Energizer I used adds no noise to the circuit.  A different Energizer I used adds a bit of high-end distortion.  A Duracell I used muddied everything down and caused tons of distortion.  Now, these are all brand-new fresh batteries.  They've never been used for anything before - I got them right out of the package.  I measured at least 9.3V on all three of them and 26 mA on all 3.  But somehow they're adding different amounts of noise to the circuit...  Really really weird... Any ideas on why?

So, I guess the circuit DOES work perfectly with my Ibanez P90s, but the Alumitones are too high-output for the small buffer voltage.  Is there another super-simple buffer circuit I could build that would handle my Alumitones better?  Maybe an IC/op-amp circuit that handles higher input from the guitar?  I'd still like it to be a super-simple circuit that I can preferably cram down inside this teeny tiny box I got for it.

[induction]

Transistor is backwards.

Edit: Also, looks like the input cap might be shorted.

[poojalooba_cow]

Transistor is backwards.

Not so!  I'm pretty sure... When looking at the flat side it's C - B - E left to right, correct?  I checked the pinouts online and everything and made sure to solder it in right.

Edit: Also, looks like the input cap might be shorted.

It's not.  It's just the angle of the photo that makes it look like that.

[induction]

[poojalooba_cow]

?????????

[anotherjim]

The usual plastic TO-92 package 2N3904 - with the flat side facing you and the pins pointing down, is EBC from left to right.
Other arrangements can exist - so the meter will help you confirm...
http://www.allaboutcircuits.com/vol_3/chpt_4/3.html

[poojalooba_cow]

UGH so that pinout picture is with the leads pointing up??? Dang it...

Now I have to go rewire my non-functional tube screamer also... Well at least I know what the problem is now!  Ha ha.  I'm used to using op-amps and PNP transistors so I thought the NPN transistors were just the same when I looked up the pinouts...

[induction]

UGH so that pinout picture is with the leads pointing up??? Dang it...

The pins are pointing out of the screen in your picture, if that's what you mean. Let us know if reversing the transistor sorts it.

[Seljer]

UGH so that pinout picture is with the leads pointing up??? Dang it...

Now I have to go rewire my non-functional tube screamer also... Well at least I know what the problem is now!  Ha ha.  I'm used to using op-amps and PNP transistors so I thought the NPN transistors were just the same when I looked up the pinouts...

This is how you learn debugging skills

[poojalooba_cow]

Transistor re-oriented, whole thing works like a charm.  With the P90 pickups as well as the Alumitones, even.  No distortion, no extra noise, and even a tiny bit of volume gain, probably from what Mark Hammer mentioned about the buffer pushing more of the overall guitar signal to the amp. 

I'm fairly new to the pedal-building bug, but so far I've built three Fuzz Faces and a John Hollis EasyVibe and they've all gone off without a hitch even with some basic mods added, and I've got a Tube Screamer I need to finish and parts for an R.G. Keen Neutron Auto-Wah in the mail.  So with all the pedals and cable length I'm building up, I needed a buffer and figured this should be an hour-long project at most.  What a pain just from flipping the transistor!

I'm kicking myself for overlooking such a simple problem.  What a ridiculous mess-up on such a tiny circuit.  I'm sure you guys have had tons of newbies here before who have totally reversed the transistors, though.  Thanks so much for helping me out, everybody!  I look forward to contributing and being a part of DIYstompboxes for a long time to come.