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Old 15th December 2008, 06:05 PM   #1
jman 31 is offline jman 31  United States
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Default Latest guitar headphone amp...slight problem

I designed and built this guitar headphone amp by piecing together some schematics. I have it working on the bypass circuit, but when I change it over to the distortion circuit it plays for just a second and then cuts out. I can pull the wire off the headphone jack and put it back on and it will play for a second more and cut out.

I think I am close, but if anyone could find the time to look over this schematic that I drew of the circuit and tell me if they see any errors I would really appreciate it!

I think I should change r5 to a 1M resistor, but I'm not positive.

Thanks

Jman
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Old 15th December 2008, 06:12 PM   #2
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You could use a grid leak on IC1A, 1M's are good for R5 and there across the diodes.
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Old 15th December 2008, 07:06 PM   #3
jman 31 is offline jman 31  United States
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Quote:
Originally posted by scott wurcer
You could use a grid leak on IC1A, 1M's are good for R5 and there across the diodes.
I am very much a novice at this stuff and though I appreciate your comment You lost me. I am not sure what a grid leak is. and when you say across the diodes do you mean between them on the #3 input circuit?


Edit: I googled "grid leak" and I think I got that it is just a resistor to ground. Where would that be placed? Is that what you were talking about with the resistor between the diodes?
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Old 15th December 2008, 07:58 PM   #4
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Yes a 100K or 1M to ground across the diodes. Even though it's a FET amp it still needs a return for the bias current. You probably had the input creeping up or down to some DC voltage and 10X that saturated the output. Fix that first, not sure why just unplugging and plugging the output made it go again for a few seconds (might be something else wrong too).
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Old 15th December 2008, 08:33 PM   #5
lineup is offline lineup  Sweden
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Quote:
Originally posted by scott wurcer
You could use a grid leak on IC1A
Quote:
Originally posted by jman 31
Edit: I googled "grid leak" and I think I got that it is ...

I have no idea, too. What is 'grid leak'.




As I have remarked to you Scott, once before:
Using some technical mumbo jumbo engineering language
at a forum for 'normal people'
will not work.
It works when the audio electronics jetset have mutual backpatting gettogethers

Further diffciculty is at hand, as most of us forum members not even have english as our normal laguage.
To us this is a foreign speak.
.. we normally have only a slight idea what is a 'grid' or a 'leak'

Nelson Pass is much loved!
Part of it, in my opinion, is he uses words that are suitable and easily understandable to audience he talks with
.
And he surely could talk as an insider engineer in his papers, if he wanted to


Come down to us. Select your normal english words, Scott!
Because you are a very nice guy and have so many good knowledges to give us.
Remember: It Is Lonely at The Top
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Old 15th December 2008, 08:51 PM   #6
jman 31 is offline jman 31  United States
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I'm just happy to have the help at this point scott, so if its takes engineer speak to get it to me, keep it coming. I will figure it out eventually!

I now have this. I added a 100K resistor across the diodes. I didn't have any 1M's handy. It works if the volume knob on the guitar is all the way up, but it cuts off as the strings lessen in vibration if that makes any sense? So what that tells me I think is that I have a low output voltage somewhere? Does that sound right? Here is the revised schematic.

EDIT: Does it matter that I am going into IC1B before IC1A?
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Old 15th December 2008, 09:22 PM   #7
wxn is offline wxn  Lithuania
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Increase R5 to 500k-1M. ICA shouldn't differ from ICB. And no, "then it cuts off" doesn't make sense. Maybe you need more gain. I really don't see what could go wrong in such a simple circuit.
Measure DC voltages in a working unit between C6-R2, on diodes and on output, then post here
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Old 16th December 2008, 09:14 PM   #8
jman 31 is offline jman 31  United States
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1. I increased R5 to 1M. and increased R6 to 10K

2. What I mean about it cutting out is, I can hear the distorted guitar if the volume is between 6 and 10 on my guitar. If it is below that I don't get anything except some crackling if I strum real hard. If the volume is between 6 and 10 it will play a distorted signal until the string vibration lessens (I'm assuming the audio output level also lessens) and then at a certain point...silence. Not a gradual fade out, just immediate silence.

3. I measured the DC voltages you asked for wxn. They are as follows:
A. between R1 and C6 it is .45v
B. between C6 and R2 it is .08v
C. on diodes is .00v
D. on output before distortion circuit it is .24v
E. on output at the end of the distortion circuit it is .34v

I measured all of these from ground, I hope that is what you were after. I tried incresing the gain on IC1B, but I get distortion if I go much past a gain of 11.


Thanks for your time

Jman
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Old 16th December 2008, 09:42 PM   #9
wxn is offline wxn  Lithuania
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Everything looks fine.. Try the following one at a time:
1. remove C5
2. short circuit C6
3. remove diodes
4. remove everything between ICA and ICB and connect them directly. If that doesn't solve your problem, you have a wiring mistake or a faulty opamp.
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Old 17th December 2008, 01:06 AM   #10
jman 31 is offline jman 31  United States
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That was it! I removed C5 and no change, but when I shorted C6 it was working great! I only have one more question and then maybe I can leave you alone for awhile!

I had to drop R6 all the way to a 1K resistor to get rid of most of the distortion on the clean channel. When I turned the volume knob up on the guitar, anything past 4 was distorting. Does that make sense? That circuit can only handle a gain of 2 without distorting?

I noticed this also on the other amps I built also with the same schematic, but no distortion circuit. Any suggestions? I am using some really good Sony headphones so I know it's no them.

Thanks so much for all of you responses and helpfulness!!!!!!!!

Jman
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