Parallel Looper Issues

[spacecommandant]

I've been working on a parallel mixer with optional tails switch. I modified the switch wiring from Induction's tails mod from the All-Star Digital Reverb, and incorporated the circuit grounding of the sends/bypass from Slacker's looper.
This version is currently working, although after plugging in power there is an initial large pop sound when either the On/Off switch or Tails switch is engaged. After waiting about 20 or 30 seconds after clicking either of these switches the first time, the switches no longer pop. I'm wondering if anyone has any ideas for eliminating this popping sound. Would it be worth trying JFET bypasses?
There is also a soft hiss in the background when engaged; what I'd describe as white noise. It's not super loud but it is noticeable at loud volumes. I'd like to reduce it as much as possible.
I'm also wondering if the 330k/47k resistors in the feedback loop of U2 are too large... those could likely be reduced, keeping the 7:1 ratio?
Any other insights and feedback for improvements would be greatly appreciated... thanks!

[Manfred]

This version is currently working, although after plugging in power there is an initial large pop sound when either the On/Off switch or Tails switch is engaged. After waiting about 20 or 30 seconds after clicking either of these switches the first time, the switches no longer pop. I'm wondering if anyone has any ideas for eliminating this popping sound. Would it be worth trying JFET bypasses?
There is also a soft hiss in the background when engaged; what I'd describe as white noise. It's not super loud but it is noticeable at loud volumes. I'd like to reduce it as much as possible.

I guess it's because C4 needs this time after power on to be charged via R5 to Vref.
Try putting the base of R5 on Vref instead of ground.
I don't know how to put it exactly, Is the hiss more of a constant noise or an alternating noise?

[spacecommandant]

Thanks so much for that idea.. I'll try it out tomorrow. The noise is a very steady white noise hiss. It's only moderately loud as I explained, but I don't recall hearing that type of background sound on any pedal I've ever built.

[ppluis0]

There is also a soft hiss in the background when engaged; what I'd describe as white noise. It's not super loud but it is noticeable at loud volumes. I'd like to reduce it as much as possible.

Hi commandant,

I think that you are experimenting a phenomenom called "Johnson Noise" that is related to circuits using high value resistors.

https://www.analog.com/en/analog-dialog ... ue-25.html

Also the kind of resistor material has its own noise properties: the carbon composition are particularly bad, following by carbon film, following by the most silent ones that are made of metal film.

My suggestion is that you can reduce all high value resistors by an order of magnitude (there are several places of 1Meg in your diagram that can be changed to 100K without damage) and apply in that positions resistors made of metal film if those were previously of carbon film.

Cheers,
Jose

[karul]

Also check those 100p at the feedback loops whether they are 100p or 10p. They are low pass filters. Lower the value, higher the cut-off freq. So instead 4.8kHz you end up with 48kHz.

[spacecommandant]

There is also a soft hiss in the background when engaged; what I'd describe as white noise. It's not super loud but it is noticeable at loud volumes. I'd like to reduce it as much as possible.

Hi commandant,

I think that you are experimenting a phenomenom called "Johnson Noise" that is related to circuits using high value resistors.

https://www.analog.com/en/analog-dialog ... ue-25.html

Also the kind of resistor material has its own noise properties: the carbon composition are particularly bad, following by carbon film, following by the most silent ones that are made of metal film.

My suggestion is that you can reduce all high value resistors by an order of magnitude (there are several places of 1Meg in your diagram that can be changed to 100K without damage) and apply in that positions resistors made of metal film if those were previously of carbon film.

Cheers,
Jose

I appreciate your response. I suspected that some of the resistors are too large but I didn't realize the 1Megs in their positions could also contribute to the noise... thanks for that tip.
I normally only use metal film resistors but I just remembered that some of the 1Megs are actually carbon film as I ran out of metal films.

[spacecommandant]

Also check those 100p at the feedback loops whether they are 100p or 10p. They are low pass filters. Lower the value, higher the cut-off freq. So instead 4.8kHz you end up with 48kHz.

Thanks for the tip. Yeah, they're 100p.

[karul]

Electrolytic cap at the output can be the reason for popping. You may try to ground the output of the looper when is off. Instead of DPDT for tails use 3PDT for additional switching.

Nope, that wont work, you need that output for the tails when looper is off. It's not simple true bypass.

[karul]

Do you really need that gain on return paths? Don't you have problems with clipping?

[karul]

If you use only one path only one jack is inserted, the other return (input) is generating noise. Use the return connectors as switches to ground them if no jack is inserted to lower the noise if you haven't already done that.

[spacecommandant]

Do you really need that gain on return paths? Don't you have problems with clipping?

I initially had different resistors in there and had clipping, so after fiddling with them I found the 330k/47k to work with no clipping. I still might adjust the ratio a little lower and lower their overall values... maybe something like 100k/18k. I want to have the ability to boost or totally attenuate the return paths.

If you use only one path only one jack is inserted, the other return (input) is generating noise. Use the return connectors as switches to ground them if no jack is inserted to lower the noise if you haven't already done that.

I'm using switching jacks for the sends and returns so if nothing is plugged in to them they're grounded.

[spacecommandant]

Do you really need that gain on return paths? Don't you have problems with clipping?

I forgot to mention that I'm also currently running it at 18v. There was too much clipping at 9v.

[karul]

I forgot to mention that I'm also currently running it at 18v. There was too much clipping at 9v.

OK. I saw 9V supply at the schematic and 8x gain - that's calling for trouble.

If this the All-Star Digital Delay that you are referring try he's method - make the switching before the capacitor, you may avoid the popping. Maybe is not best technical solution, but if solves your problem go for it. Or at least try it.

[karul]

If you want to have ability to boost or attenuate the return paths maybe variable gain stage is better solution than fixed gain and passive attenuation.

[spacecommandant]

I forgot to mention that I'm also currently running it at 18v. There was too much clipping at 9v.

OK. I saw 9V supply at the schematic and 8x gain - that's calling for trouble.

If this the All-Star Digital Delay that you are referring try he's method - make the switching before the capacitor, you may avoid the popping. Maybe is not best technical solution, but if solves your problem go for it. Or at least try it.

Thanks

[karul]

One more thing. This kind of loopers usually have one of these control sets:
1. blend & master volume
or
2. vol1 & vol2

You have all three. Having two separate vol controls aren't enough? You can avoid that blend/balance pot. There's no need for it. You are summing two signals with previously defined levels. Or use master volume and balance.

If you want to have ability to boost or attenuate the return paths maybe variable gain stage is better solution than fixed gain and passive attenuation.

Noise levels should be lower and there is no need for blend/balance pot. You can use less gain, you are loosing some dbs with that blend pot.

[karul]

Take a look at Chapter 6 and 9. Highly recommended book. viewtopic.php?p=287940&hilit=Small+Sign ... gn#p287940

[spacecommandant]

Take a look at Chapter 6 and 9. Highly recommended book. viewtopic.php?p=287940&hilit=Small+Sign ... gn#p287940

That looks like a great book... thanks again.

[spacecommandant]

If this the All-Star Digital Delay that you are referring try he's method - make the switching before the capacitor, you may avoid the popping. Maybe is not best technical solution, but if solves your problem go for it. Or at least try it.
[/quote]

I tried moving the wire from between R16 & R17 to before C4 and the pop wouldn't go away, not even after over a minute, so I moved it back. I'm not quite sure how to best move the wire from between R7 & R8 other than leaving it where it is and adding a 10u capacitor on either side of it, before R7 & R8.
I'll post a schematic of Slacker's looper that he designed to show you the bypass that I've tried to incorporate.

[spacecommandant]

Here is a schematic of the Tails / Bypass designed by Slacker that I attempted to incorporate. I changed R5 from 1k to 47k since in an earlier version of my circuit, it wouldn't work properly with 1k.