Troubleshooting simple tape-saturation pedal

[Flynn]

I built a 2 channel version of the 15IPS "tape saturation" module; basically a distortion pedal, by the very cool company, DIYRE.

Schematic:  https://upverter.com/peterson.goodwyn/8c657127374e5ac9/15IPS-Colour/
Product page: https://www.diyrecordingequipment.com/collections/colour/products/15ips-colour

The only 'mod' i've done is to add a 'mix' control to the output (left lug to input jack, center lug to output jack, right lug to output (pin 7) of the NJM2068) so that i can blend between dry and wet.

A)  When I run audio through it I get full (dry) signal at fully counter-clockwise and just about the same at fully clockwise.  In the middle of the pot sweep it gets almost no signal.

B)  At no time do i hear any distortion (the effect).

I get the same results (Both A and B above) on both channels.

Power supply:  I am getting the same voltages whether 'under load' and not - +14.98v and -15.08v.

My PS's 0V line is connected to the circuit's ground.

Channel 1 AND Channel 2 NJM2068 (both measure the same):
Pin 1, 2,  3:  .02v
Pin 4: -15.08v
Pin 5, 6, 7:   .02v
Pin 8: +14.98

All diodes are showing around .02v.

Diodes are the only polarized components and they are arranged correctly.

All grounds are connected, as well as the pos and neg power lines.

Continuity tested the grounds, they're good.


I would greatly appreciate any suggestions on where to go from here with the troubleshooting!   

[PRR]

It is basically a 3V clipper. You need to hit it with signal MUCH higher than guitar level.

[robthequiet]

Put a clean boost in front of it, see if you get more dramatic colour. Seems a bit under powered for just a Guitar, as Paul says.

[stonerbox]

Put a clean boost in front of it, see if you get more dramatic colour. Seems a bit under powered for just a Guitar, as Paul says.

Flynn, or you could add a permanent adjustable gain stage up front since you seem to have plenty of space left on the board. Something like the Micro Amp from MXR would do the trick.
http://oi47.tinypic.com/1gpyxk.jpg

[reddesert]

Two issues: there are 5 diodes in series so they won't clip until a line-level voltage, although they are germanium diodes so it's more like 1.5 V than 3 V. A guitar signal or even a pedal output won't trigger clipping, as was said above.

Second, the simple passive blend pot often won't work well. It depends on the output impedances of the two signals you are trying to mix, and in the center position, you can load both of the sources such that the signal is attenuated a lot. See RG's article at http://www.geofex.com/Article_Folders/panner.pdf for example.  Also, I think the circuit you have is non-inverting, but of course if it were inverting then the blend would cancel.

[Flynn]

Thanks for the input everyone!

I'm not testing with a guitar, I'm running a drum loop out of my MOTU Traveler which are +4 outputs and back into PT to record it.

I will try removing the Mix pot's connection to the input (so it's just volume) and see where that lands me.

[robthequiet]

Ah, we have a tendency to sometimes assume it's going to be a guitar input. Have you tried other devices? Maybe the drum loop is a bit low as a signal? Just thinking out loud.

[Flynn]

No worries!  My mistake for not mentioning that i'm using it as a line-level FX unit and not a pedal.

The loop was super hot coming out of the card, and hot coming back in after the effect.

Drats!

[robthequiet]

Well, I'm not recommending surgery on the card itself, so thus the rec to put a booster in front, either embedded as Stonerbox said or a separate box. The other thing, re: surgery, might be to  put a resistor in the feedback loop on IC1A, although there may be unexpected effects due to the 60 Hz resonant filter. Not a strong suggestion, just a possibility. An external boosting box would be my first choice.

Edit: 2nd thought, possibly reduce the number of diode stages to reduce the hump the signal has to overcome.

[Ice-9]

The circuit in question is not really meant to produce much in the way of clipping as most of us guitar players see clipping in a distortion pedal, it is meant to simulate a small amount of tape saturation, what you could do is to remove one or two sets of diodes until you get closer to what you are looking for, or as mentioned above change the gain of the first op amp with a resistor or pot in the negative feedback loop.

EDIT-- Ha Rob beat to it, you must of posted your edit as I was typing. lol

[robthequiet]

hah, cheers, Mick, great minds... etc.

btw, any result from removing blend pot? reddesert had a good point about weak signal in the middle.

[Flynn]

Just desoldered them - will test and report back!!!

Thanks, gents!!!

[DavidRavenMoon]

I would think a software plugin would be a lot less hassle.


Sent from my iPhone using Tapatalk

[Flynn]

I would think a software plugin would be a lot less hassle.  - David Raven Moon

What fun is there in that?!   And what on earth is this forum for, then?!   

I say "put the mouse down...pick up the soldering iron!"

[Flynn]

removed the connection from the 'mix' pot to the input. 

no signal at all, even when swiped hard in either direction.  super odd!

IC's are getting power, etc.

going to do another 'once over' on my wiring tonight, especially with regards to the output /pin 7.

[robthequiet]

One strategy might be to wire a separate pot for each side of the mix.

On the other hand, if you had a 500K variable resistor in the feedback loop of the op amp, having a blend may be redundant, as you have a practically clean signal currently, then adding more gain would be a depth control. After all, when we recorded to tape back in the ice age, you had a channel going to tape, and set the level right on the verge of breakup, according to the VU meter. If your console and tape deck were out of alignment, you could easily blast a hole (figuratively) in the tape and not realize until playback. So much for the one take the Flügelhorn guy got it right. Sending a separate channel to a different track could be done, but the distortion would generally stand out in the mix.

Then again, maybe distorted Flügelhorn is a thing?

[Flynn]

I must sheepishly admit that I had one channel working (soldered up as you see in the photos) a few months back (using 2 9volts to create a dual supply) and the circuit worked. 

Then, magically, when i came back to work on it now after building the power supply for it, it doesn't work on the PS or on the 9volts.  Then, i connected the two channels together with regards to the power supply and ground.  Same results for both. 

I think i might try other IC's in there to see if I fried the 2068's.

[robthequiet]

Wait, is it possible that by having the input wired to the output, you have logically shorted  around the whole circuit? Not sure how your outboard components are wired.

[Flynn]

Well, i de-soldered the connection to the input from the left lug of the mix pot.  so now it's Main Output jack on the center lug and Pin7 (out of the IC) on the right lug. 

No sound at any position on the wiper.

[robthequiet]

Aha -- run "Out" to the top of the pot, pin 1, send pin 3 to ground, and take your output from the wiper, see if that changes anything.

Edit, ok I reread that. Main Output on center is correct, and with Pin 7 going to right lug, that leaves left lug to go to ground.