Distortion on the first repeat in Deep Blue Delay Clone

[Rethfing]

Hi, i built a delay from this link: http://efekty-diy.pl/pdf/Deep%20Blue%20Delay.pdf
It works fine, but I hear distortion only at first repeat (it of course gets louder if I turn the mix knob up). The harder I hit strings the more distortion I get, but even when Im lightly picking there's always some unwanted distortion. Few months ago I built the same delay and it worked fine. I've checked my built twice and all components seem correct. Note that there is 33Ohm resistor along with 9V+, it reduces the voltage to 8V. Dont know what this is for, it sounded ok without the resistor (jumper instead), but I soldered it back in to see if the higher voltage is the cause of distortion - well it isnt.

7805:
In: 8V
Ground: 0V
Out: 5V

PT2399:
1. 5.04
2. 2.53
3. 0
4. 0
5. 2.85
6. 2.53
7. 4.45
8. 4.44
9. 2.53
10. 2.53
11. 2.53
12. 2.56
13. 2.53
14. 2.50
15. 2.53
16. 2.53

TL072:
1. 4.03
2. 4.03
3. 4.03
4. 0
5. 4.02
6. 4.03
7. 4.03
8. 8.01

[Mark Hammer]

What delay time did you have it set to?  I think you'll find that PT2399-based delays have a strong tendency to "grit up" when pushed beyond 500msec or so (though that will depend on the extent of lowpass filtering).

[Rethfing]

Yes Im aware of hum issue with PT2399. This is not the hum Im dealing with, its distortion of the first repeat of the sound Im playing. Its really distorted, like Tube Screamer with gain half way up (it depends on how hard I pluck strings though).

[midwayfair]

Yes Im aware of hum issue with PT2399. This is not the hum Im dealing with, its distortion of the first repeat of the sound Im playing. Its really distorted, like Tube Screamer with gain half way up (it depends on how hard I pluck strings though).

He's not talking about hum. It's harmonic distortion. The PT2399 will distort on longer delay times. Where do you have the delay knob set?

The DBD also uses the inverting op-amp input and will also distort if fed too much signal. What guitar are you using? What other pedals are in the chain?

[Rethfing]

As I wrote in the first post:
"Few months ago I built the same delay and it worked fine"

"The PT2399 will distort on longer delay times." - no it doesnt distort, it adds hum, noise, but not distortion. Im fully aware of PT2399 capabilities and Im telling you, that this issue is not what you think it is. The distortion im talking about appears no matter how I set the Time knob.

[Dasher]

Have you tried a different pt2399?

[Mark Hammer]

As I wrote in the first post:
"Few months ago I built the same delay and it worked fine"

"The PT2399 will distort on longer delay times." - no it doesnt distort, it adds hum, noise, but not distortion. Im fully aware of PT2399 capabilities and Im telling you, that this issue is not what you think it is. The distortion im talking about appears no matter how I set the Time knob.

Okay, now things are clearer.  It CAN be the case that the same person builds one PT2399 delay circuit and it "works fine", and then the 2nd or 3rd one, simply because of component tolerances, might push the delay time a little far with  insufficient lowpass filtering and...boom....audible distortion.

But this is clearly a different category of problem.  So, a few questions.

The biasing seems fine.  Are the bypass and effect output levels the same? (ignore the sound quality)

The 33R resistor is partly to help with power smoothing.  Along with C2, it provides for a lowpass filter on the supply line at 48hz.  The 47uf cap after the 5V regulator is there to do somethig similar.  Use your AC meter to look for ripple on the supply line at the +9v point, the Vref and on the output of the 7805.

[midwayfair]

To add to Mark's question about the bypass volume: Are the repeats unusually loud compared to the other DBD you built?

Can you take pics of both sides of your board?

[Rethfing]

Different PT2399 doesnt change a thing. Distortion occurs when I turn the Mix knob about 1/3+. Bypass and effect output levels are the same. The repeats are as loud as they should be, there is Mix knob to adjust them, but they are just the way they should be.

Im also e-mailing with guy who designed this PCB so maybe he will come up with something.

[Mark Hammer]

Is the problem something that only happens when the Mix knob is above 1/3, or is it something that only becomes too annoying when the Mix knob is turned up?

I am asking because if it is a problem that only begins to occur with higher Mix levels, then that suggests a potential source of the problem.

[Rethfing]

Im going to check that in the morning, but from what I've heard it doesnt happen with Mix knob lower than 1/3.

[Rethfing]

Ah its opposite, the distortion is there all the time, its just very hard to hear when the Mix knob is turned down.

[JebemMajke]

Try opa2134.

[Rethfing]

It worked with TL072 when I built it few months ago so I dont see point in changing opamps.

[Mark Hammer]

Ah its opposite, the distortion is there all the time, its just very hard to hear when the Mix knob is turned down.

Okay, that is useful information.  We now know it is not the mixing stage, but more likely:
a) the 2399 chip, or
b) the power and its regulation, or
c) a layout.grounding issue.

I know that sounds like a lot to consider, but at least it is NOT the op-amp chip itself.

[slacker]

I know you said they seem correct but double check all the resistor values around pins 15 and 16 of the PT2399, if they were wrong they could cause distortion.
The fact that only the first repeat is distorted makes it sound like the problem is after the mix control though. If it was PT2399 related then all the repeats would be distorted, as the later repeats are just copies of the first. I suppose the filtering and general degredation could be masking the distortion on later repeats though.

[R.G.]

Since the first repeat is the loudest, have you thought about the input level being too high for the 2399?

What happens if you pad down the audio level going to the 2399 input by 2:1?

[R O Tiree]

Pins 7 and 8 might be reading very high. Compare 4.45V here with the 0.7V posted by slacker on his Echo Base thread. Check to see if pins 7 and 8 have been shorted together?

[Rethfing]

Okay, that is useful information.  We now know it is not the mixing stage, but more likely:
a) the 2399 chip, or
b) the power and its regulation, or
c) a layout.grounding issue.

I know that sounds like a lot to consider, but at least it is NOT the op-amp chip itself.

a) I've tried another 2399 chip and it sounded the same
b) I dont think thats the issue, my power supply is professional and handmade, it powers other effects just fine
c) Well the PCB is made very well and the soldering is easy due to tinned pads. There are no shorts, no pcb trace is broken, everything looks nice and clean.

I know you said they seem correct but double check all the resistor values around pins 15 and 16 of the PT2399, if they were wrong they could cause distortion.

They are OK.

Since the first repeat is the loudest, have you thought about the input level being too high for the 2399?

What happens if you pad down the audio level going to the 2399 input by 2:1?

Well the problem dissapears if I roll the volume on my guitar almost half way down.

Pins 7 and 8 might be reading very high. Compare 4.45V here with the 0.7V posted by slacker on his Echo Base thread. Check to see if pins 7 and 8 have been shorted together?

They are not shorted. The PCB is very well made, it has tinned solder pads so the soldering is very easy and clean. I've even cleared it a while ago with alcohol to get rid of flux - everything is solder the way it should be.

[R O Tiree]

The gain on the input buffer is 2, at present. Try dropping R3 to 180k as well?

That will make things very quiet at the output, though, so you'll have to raise R5 to 22k to balance things out again.