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Barton Guitar Input challenges.
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Barton Guitar Input challenges.
ApeIslandSound
Hello!

After building many of the great Barton circuits, I've run into one that is challenging my troubleshooting skills.

I've built the Guitar Input project. I've been bench-testing it by running an electric guitar into the input, and the audio output is going to my workshop mixer, with the input trim set to a moderate level (no overload lights or anything).

Whether I run from the audio out or the square out of the Barton, I'm still getting strange behaviour at the gain control. After about half-way up, the "fairly" clean signal turns into this odd squealy fizz. It certainly isn't anything like the video on the Barton page. Also, the pot's output range seems kind of scratchy at certain points, but I've tried two different pots, bench-tested before installation, and they both behave exactly the same way. I also tried replacing the IC's. There is the gain range switch, but this troubleshooting has all been on the quieter setting. Flick it to the higher gain, and you can forget it. Total fizz (not fuzz!).

I'm running a 15v system, and have made the appropriate changes to the two 1.2k resistor values.

I've carefully double-checked every component, and all appears to be well. No wrong components (I think!), all resistors were put on the meter before installation, it's a cleaned pcb with no solder bridges, all joints checked and/or reflowed, no reversed caps or IC's. Wiring "seems" fine (all grounds checked).

So I'm wondering if anyone can shed some light on this problem. Problems like this are almost always the result of something that I've screwed up, so don't be afraid to hurt my feelings! I can take it.

Thanks!
Escapegoat
Not familar with this circuit, but with the pot issue: scratchy pots are often indicative of a DC voltage getting on to a pot that normally carries only an AC signal. If this happens (through mistake of bad component), the tiny bits of crud on the pot's wiper move around and this creates tiny changes in the DC voltage that gets amplified as if it was part of the AC signal.
masterofstuff124
I use my guitar input from BMC all the time. The gain knob is almost too powerful. I never take it past 9o'clock. At just barely past zero it begins to breakup/fuzz. I usually have to play with the knob but it's usually set just barely past zero gain to get a clean guitar tone. I don't use the switch it just makes it even more gain heavy.
ApeIslandSound
Interesting stuff. Thanks for the replies. The DC/scratchy pot hypothesis makes me wonder if there's a DC offset removal cap causing a problem.

But, based on masterofstuff124's description, it sounds like his circuit is behaving in a similar manner to mine. Based on the video on the Barton project page, we both may have something wrong.....

Here's a link to the project description and schematic...

http://modularaddict.com/attachments/bartonguitarinput/bartonguitarinp ut.pdf

...and a link to the video....

https://youtu.be/xSnBP1HtVN8
basicbasic
Not sure if it's what you're referring too but I noticed mine has a tendency to oscillate/squeal if I crank the gain at all. Otherwise it seems to function as intended (with a shit-ton of gain to spare).

Does anyone know of any panels that have been done for this? My sharpie and DYMO jobby is a bit shithouse.
ApeIslandSound
basicbasic, I'm seeing a trend here. lol. Mine sizzles, spits, and squeals, even if I just try to get the gain more than a third of the way up.

I sent Michael an email, alerting him to this thread. Maybe he can help us figure this out.
CLee
The preamp has a gain of like 1000X with the 1meg pot. You could replace that with a 50K or 100K pot and see if you get a more manageable range.
diablojoy
Quote:
The preamp has a gain of like 1000X with the 1meg pot.


you could also try adding a 10-100pf cap across pins 1+2 of the pot.
masterofstuff124
any simple addition/trick to the A1M pot(gain) to get it to 100kish...
ApeIslandSound
Thanks CLee, but the problem is weirder than that. Even with a lower value pot, the behaviour is still odd. It is somewhat tamed, but still squeals and spits. It seems like there is still a problem elsewhere in the circuit.

masterofstuff124, test with a 100k resistor across the outer terminals of the pot. That will get you to 90k or so. The taper will get weird, but you will get a sense of the expected result.
basicbasic
I put a 100k resistor across pins 1 and 2 of the gain pot and that has brought the gain range down to something more useful (for me). Seems to have solved the oscillations too. applause
ApeIslandSound
basicbasic wrote:
I put a 100k resistor across pins 1 and 2 of the gain pot and that has brought the gain range down to something more useful (for me). Seems to have solved the oscillations too. applause


That does appear to have helped solve the range issue, but not the strange scratchy behaviour of the pot, at least in my case.
CLee
The cap that diablojoy recommends is also a good idea.

But you may have a bad ( scratchy) potentiometer. With the gain control pot in the feedback loop any scratchiness can cause the op amp to go open loop, and the output will swing to a rail, so it'll be very noticeable.
ApeIslandSound
Since the pad switch simply bypasses the 470k resistor at the input, has anyone tried adding a resistor in series with the switch?
masterofstuff124
what is the cap doing? the resistor across Pins 1 and 2 just brings the range of the Potentiometer down. what does the cap do? im debating which mods to do to my Guitar Input!
diablojoy
the opamp is providing quite a lot of gain
the cap in the feedback loop is just to help with stability
ie : prevent the opamp oscillating
controlFreak
Will report once mine is built
Dr. Sketch-n-Etch
What exactly do you mean by "no solder bridges?" How do you know? Solder bridges can be very elusive things to find, in my experience.
flab
i finished the barton guitar in yesterday! for the gain pot i used a 100k one instead of a 1M followed clee's advice above, also replaced the 500k pot for the sensitivity with 1M(that is what i had in hand). It feels that the envelope follower is not really picking up all the input signals from different sound sources in the same way, if using a bass i would have to play strong with the strings, guitar reacts better with it, audio sources like mp3s phones pcs are not really affecting it, i was planning to use it for lots of piezo sounds from surfaces ala microphonie and for my bass guitar or double bass- thoughts on the project? Miley Cyrus
Dolphinwolf
I built this circuit with the only modification being a 500K pot for input gain and all is well.
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