DIY JHS Little Black Buffer - low output

[MrFrog]

Hi all,

I have recently tried to make my first DIY pedal. I chose the Little Black Buffer since it looked like it did not have too many parts and I wanted to make a buffer (actually if I can get this working I want to try to add another output and a mute switch to make it buffer with tuner out).

I put it together and the output from the pedal is much lower than if the guitar is plugged directly in to the amp. I know amp dial values are somewhat meaningless but to get a similar volume after the buffer I need the volume almost on 10 compared with 4 going direct from the passive stratocaster.

I was hoping someone could help me diagnose this (assuming a reduced output is not expected)

Link to the layout I was using:  https://tagboardeffects.blogspot.co.nz/2014/05/jhs-little-black-buffer.html

• I ran a knife along the seperating tracks on the veroboard
• I have checked continuity between the adjacent verotracks and the only ones 'connected' are the two with the jumper between them.
• I tried to check the values of the resistors (my multimeter can't do the 2.2M ones)
• I swapped the IC for a different one (I bought one locally and the other I had ordered from overseas) and I had the same issue.
• I made an audio probe and poked around to try to work out where the signal dropped off. I think my problem is something to do with the IC.
  .
  Assuming I have read the pinout correctly the non-inverted input is the pin I have marked 'ok'. For some reason when it 'exits' through the output it is quiet and then the 'inverted input' is also quiet.
  

I didn't 'take voltages' as I was not sure where I should be taking them from?

Photo of board (top):

Photo of board (bottom, not sure this is helpful but you can check out my sweet 'first-timer' soldering  ):


Thanks for any advice you might have!

[Quackzed]

looks alright at a once over, trace cuts look ok, grounds look ok, opamp looks oriented ok, cant see 1 smallest jumper under 470 side of board, but ill guess its under there... you can measure voltages at pin 4 (0 volts) pin 3 (4.5v) check ground is 0v and 9v is uhh oh yeah 9v pin 1 and 2 should be @ 4.5v

actually wait, theres no ground reference at the output and the 2nd opamp seems to be biased at 1/2 voltage but no signal going in or out of that second opamp?!? still shouldn't matter.. but thats a bit odd..

Csaba Dudás5 May 2014 at 20:22
Hm... What's the purpose of the rightmost two 2M2 resistors? As I see the complete right side of the opamp is doing nothing.

IvIark5 May 2014 at 20:55
The manufacturers recommend that to avoid noise from unused channels you should connect the non inverting input to a reference voltage, and link the inverting input and output. A much easier way to avoid noise from an unused channel is to use a single channel opamp )

well spotted csaba dudas5! i guess thats why... other than that i might suggest another 2.2M resistor from the output trace to ground, just to keep the output referenced to ground but that shouldn't change the behavior of it without that addition... it should work.
post the requested voltages, put the black probe on a ground point and use the red probe to the spot you want to measure, set the vom to the appropriate voltage setting (dont forget to hook up the battery!) and take your readings. i'll cross my fingers that in the process you magically fix it! 

[MrFrog]

Hi Quakzed,

Starting with the top left as pin1 then going anti clockwise

• 1.3
• 1.3
• 0.9
• 0 (ground! had me confused for a minute!)
• 2.2
• 4.4
• 4.4
• 8.7

Possibly poor soldering for the socket? I did have some problems getting all the pins to show through the board evenly since it was sitting on top of a wire.

[Quackzed]

thats possible, pins 1 and 2 should sit at half way btween 9v and ground. either the 2.2m that goes from 9v to pin 3 isnt making contact w that pin or some other accidental slight connection is pulling that point closer to ground... same thing seems to be happening to pin 5 (should be at half supply voltage, (4.5v) but is at 2.2v. before pulling the socket you could check continuity between the bare socket pin and the resistors/ board traces they should be touching, set your vom to the lowest resistance setting, usually 200 ohms or wahtever is lowest and touch the probes to the socket pin and the other to the trace or resistor its supposed to be connected to and you should get a low reading 10 ohms or less if they are indeed connected, might locate whats not touching... try the 2m2 with one end at 9v and see if both sides have continuity with their trace / socket pin /etc... might be a good idea to go over the spaces between traces and trace cuts to be sure they're not touching... try an exacto or sometimes even good folded paper to scrape btween traces... cleans up flux/solder residue and possibly tiny wire strands dust etc that is slightly 'shorting' out things ...

[MrFrog]

It works!

It was resistor at the end of pin 3.

Just in case another beginner comes across this this is what I did:

• I ran a knife through the gaps on the veroboard (I had done this before but figured it would not hurt to do it again).
• I tested the socket by doing the ohm test between the inside of the socket and the solder on the veroboard. These all worked fine.
• I checked each solder on the leg of the socket against one of the free holes on the veroboard and these all worked so socket was looking good.
• Then I checked each leg of the socket against the first component (as you suggested) and found that the circuit between 2.2M resistor and Pin 3 was giving a reading of 0.
• Looking really closely at  I can see that the solder did not go all the way to the board on  the resistor leg. Instead it had made a little blob just above the board.
• I slathered some more solder on the leg and it all works.

Thanks for the help!

I'm going to have a look at pin 5 as well, it does not matter for this circuit but I'm thinking I might try to use the other side of the TL072 to run a tuner so I'll need to get it working.

[Quackzed]

Alright!!!

good job sticking to it (bad pun groan)

glad you got it working!
  debugging is sometimes the thing that turns people off diying : you build something that doesnt work right away then try 10 things that dont fix it and jusst give up -it can be frustrating- so bravo, and hopefully many more successfull builds!!!