High Gain Circuit Problems

[skelts]

Hi all,

First time poster here. I recently purchased a bluesbreaker/transparent overdrive built kit from a company called Arcadia Electronics. Putting the thing together went fine and the pedal works great except for the "high gain" switch which is supposed to engage two red LED's (D6 and D7) for extra crunch. The problem is with the switch engaged (P1 on the schematic), about 90% of the volume drops out and one LED stays permanently on (pretty sure they are supposed to flicker with the guitar signal). With the help of some clips I have tried lifting the connection that takes these two LED's to ground and moving it around schematic but with no good results. Currently I am out of ideas and hoping there is someone on here who smart and helpful enough to suggest anything I might have missed If you haven't guessed already, this is my first dive into pedal building.

Here is the schematic from Arcadia

Any help would be massively appreciated

https://i.postimg.cc/Y2Xbh3SV/Untitled.jpg

[duck_arse]

welcome to the forum.

I think your circuit has an error, and you have found it. because the opamp is biased at its input to VB [will be about 4V5], the output also sits at 4V5. the two clipping leds are shown connecting to ground, which means they will have +4V5 across them at all times, which will turn one on. if the leds are taken to VB instead of ground, it should be good to go, as one end of the leds will be at 4V5 and the other end will be at 4V5.

otherwise, insert a cap where it says P1 Mod, this will break the DC to ground and the leds will then only react to the AC signal.

[antonis]

Exactly what Stephen said..!! 

PS1
Welcome also..

P.S.2
Better insert the cap mentioned by Stephen between pin7 & R7 to also prevent Tone & Volume pots crackle..

[skelts]

Thank you both for the warm welcome and super quick response!

duck_arse, what you are saying makes sense. Unfortunately I just tried this and had no luck. Lifting the connection to VB meant that the switch now has no effect and its like its always in the "off" position regardless of whether it is "off" or "on", meaning that the pedals normal sound is always in play. I will try the cap idea as soon as I can get hold of some. Does the value matter hugely for this particular use?

Antonis, I'm assuming you mean to move the cap to the position you mention. Not to add a second one here?

Cheers guys,

Sam

[antonis]

Antonis, I'm assuming you mean to move the cap to the position you mention. Not to add a second one here?

Yeapp..




P.S.
I presume you didn't precisely follow Stephens suggestion..
> if the leds are taken to VB instead of ground <

[skelts]

Thanks again, and for the stated value. Most helpful

I think I've followed Stephens guidance. I've lifted one leg of each LED off the PCB to remove the ground link, soldered a jumper wire between to keep them in parallel and then joined these to VB. Essentially created what's in the picture above. Hopefully the cap method works out.

[duck_arse]

the next troubleshooting aid is photos of what you've built, so's we can see your method.


and antonis, please follow the DC path from pin 7 to output, at your leisure.

[11-90-an]

the next troubleshooting aid is photos of what you've built, so's we can see your method.

+1 on photos...

[antonis]

and antonis, please follow the DC path from pin 7 to output, at your leisure.

In you mean on original circuit posted by OP, there IS a DC path from pin7 to GND through R7 - VR4 - R11 - VR3 - R4 ..
(unless you are sure for pin7 - VB absolutely zero difference..) 

[skelts]

Greetings again...so unfortunately the cap has not done the trick I made up some extra test hooks and placed the cap between position 2 of the mod section and the LED clipping. Tried various caps of differing values and connecting the LED circuit to ground and to VB with the cap in place. So I guess it is time for some photos...as always any ideas and input is greatly appriciated.



The two red wires coming off to the left connect the switch to the mod section of the schematic. I have lifted one of the wires and used test hooks to try the cap Stephen suggested. The purple wire coming off to the right is connected in parallel to the LED's after lifting them both from ground. It its currently connected to +ve side of C2 to connect it to VB. I have also tried it connected to ground. Both scenarios with the cap cause the switch to have no effect on the sound (standard pedal overdrive sound).

















Still need to try Antonis' suggestion of putting the cap between the OP amp and R7 but need to lift R7 to do this. Will try this tomorrow thanks again for the help so far!

[skelts]

For anyone who might be reading this in the future with a similar issue, I've finally had some luck with the solution Antonis posted by wiring a 100nf Cap between leg 7 of the op amp and R7. Seems like the mod works now and you can hear extra clipping when the switch is engaged. Although the effect is not as pronounced as I was anticipating, it is still a useful sound to have. I am going to experiment with how I can get some extra gain out of this mod now and maybe cut some of the high end off the pedals sound as a whole before it goes on my board. Going to make a new post about this though.

Thanks to everyone for your input

[Mark Hammer]

If there were no LEDs to be concerned about, C11, near the output, does all the DC-blocking you need, such that nothing after the pedal will be subjected to an unwanted DC voltage at its input.

The problem is that, in the absence of anything between U1b's output and R7, the bias voltage at pin 5 (Vb) makes its way merrily from pin 5 to the output.  Small wonder the LED was lighting up.

[antonis]

I am going to experiment with how I can get some extra gain out of this mod now and maybe cut some of the high end off the pedals sound as a whole before it goes on my board.

You can tweak R3 value (lower it for "less soft" clipping) and/or R5 value (raise it for more gain)..
Also, try a larger value cap in place of 100nF between pin7 & R7..

P.S.
IMHO, C8 & C12 values should differ by about an order of magnitude, but let it be..

[skelts]

Mark, your explanation makes perfect sense. Seems so obvious when it is put like that!

Antonis, definitely going to play with those R values like you say. That sounds like it could be interesting. What would increasing the cap value before R7 achieve? Some of the schematics for similar pedals I have seen online have C8 and C12 both at 10nf so I'm naturally curious as to why you might change them. Although I'm also inclined to let it be for now....baby steps

[antonis]

What would increasing the cap value before R7 achieve?

To not interact with tonestack following..
(more bass to pass and let tonestack, which actually is s 2 pole LPF, to handle them..)

I have seen online have C8 and C12 both at 10nf so I'm naturally curious as to why you might change them.

It's not easy to calculate (not even to estimate) individual LPF single pole when reactive elements values aren't far apart..
Second filter significaly loads first one - or - is driven by high impedance source (you can think of it as you like..)

[teemuk]

Also, clipping thresholds of two series diodes plus series resistor vs. LED probably aren't strikingly different so I'm not surprised you are not hearing a huge difference when engaging the LED clipper.

[skelts]

Thanks teemuk. I was under the impression that the diodes used in the first clipping stage (1N4148's) clipped a lot softer than the LED's. But I'm guessing this is what the effect of R3 is though like Antonis has already said. Is there anyway I could make the LED's clip harder so they stand out more?

[antonis]

Is there anyway I could make the LED's clip harder so they stand out more?

Strictly speakig, No..!! 

Red LEDs exhibit the lower forward voltage drop so you can't replace them with other LEDs of even lower voltage drop..

It might worth to try replacing them with a pair of Si diodes, taking into account the signal lower level on Output..

[skelts]

Just when I thought I was out of the woods... Turns out only one LED is clipping (darn it!). My research tells me that both LED's should be flickering and not just one which is currently what is happening. Just to check this I set up a switch and A/B'd with a single LED to see if it sounded any different, which it did not.

Back to the drawing board...

[aron]

Not sure how you did it, but a "trick" is to solder the two LEDs to each other and use the set of legs from one of the LEDs. So basically it's like soldering in one LED. I use this trick sometimes.