Pulldown Resistor Question

[pryde]

I am building a Solasound Mkii and have populated my board. It has a PDR option and I currently have a 1M in there but still get a significant POP when engaging the pedal.

How large a value can be used as a PDR? I am assuming by increasing the value that the pop will hopefully diminish? Thanks for any advice with this.

[GrindCustoms]

I usually use 2.2M PDR's...........always get the job done

[stecykmi]

a pulldown resistor is there to keep the input at 0v when it might otherwise be floating (ie when the effect is switched off). the value is not critical, and increasing or decreasing the value doesn't necessarily correspond to an improvement in noise.

i would make sure you're using a switching scheme that grounds the input when the effect is switched off.

[pryde]

Thanks. So can I measure voltage that might be collecting at the beginning of the circuit?

One thing that is strange is that all the MKii schematics have the input cap (4.7uf) with the positive side facing toward the input but on the breadboard and my pcb the effect would not work unless I turned the cap around (negative side facing input). Is it possible I have a bad electro? Could this be causing the pop?

[stecykmi]

Thanks. So can I measure voltage that might be collecting at the beginning of the circuit?

One thing that is strange is that all the MKii schematics have the input cap (4.7uf) with the positive side facing toward the input but on the breadboard and my pcb the effect would not work unless I turned the cap around (negative side facing input). Is it possible I have a bad electro? Could this be causing the pop?

you can try, it may not get a very accurate measurement. it doesn't take very much voltage to cause a pop (consider that a guitar only has an output of about 100mV), so set the voltmeter to an appropriate setting.

it's possible that it's the electro. try swapping it if you still can't solve it. try a non-polarized poly cap of some type, even temporarily just to debug.

[pryde]

I removed and tried a few 4.7uf input caps and none worked according to the schematic. They all worked fine when I flipped them, very strange?

I put a 2.2M PDR and it seams to have made it better but still a bit of a pop that I think I can live with. I threw together a road rage and powered it with -9v (it is a PNP ge circuit) and it still works just fine.

The circuit sounds really quite good, Q3 C biased to ~4.5v. I am just a bit baffled that it won't work with the cap inserted with "correct" polarity per the schematic?

This occured on the breadboard as well as the pcb?

[Jack Deville]

Okay.
I'll bite. There are many sources of switching noise in mechanical systems.
On DC bleed: as a general stompbos rule of thumb, a voltage offset in excess of 5mV will often cause a "switch to pop."  Most notably if there is no limit to how much/quickly the current is transmitted.
I'm writing a tech article about this right now. There will be a whole lot more detail in that publication.
I'll post a URL when it is released.

[pryde]

Okay.
I'll bite. There are many sources of switching noise in mechanical systems.
On DC bleed: as a general stompbos rule of thumb, a voltage offset in excess of 5mV will often cause a "switch to pop."  Most notably if there is no limit to how much/quickly the current is transmitted.
I'm writing a tech article about this right now. There will be a whole lot more detail in that publication.
I'll post a URL when it is released.

Great timing, thank you 

Any insight on the input cap polarity? Why does mine only work "backward" ?

[oldhousescott]

If your effect is truly positive ground/PNP transistor style, then the positive terminal of the cap should go toward the input jack and the negative should go to the base of Q1. However, the voltage across the cap is so small, it may very well work reversed (for a while, until the electrolyte breaks down and the capacitor becomes a short circuit).

I can't say why it's not working for you in the correct orientation. Check that your 100k resistor to Q1's base is present and the correct value.

Actually, measure the voltage across the cap in the normal orientation. It should be below a tenth of a volt.

[midwayfair]

If your effect is truly positive ground/PNP transistor style, then the positive terminal of the cap should go toward the input jack and the negative should go to the base of Q1. However, the voltage across the cap is so small, it may very well work reversed (for a while, until the electrolyte breaks down and the capacitor becomes a short circuit).

I can't say why it's not working for you in the correct orientation. Check that your 100k resistor to Q1's base is present and the correct value.

Actually, measure the voltage across the cap in the normal orientation. It should be below a tenth of a volt.

Pryde, it's like animal farm up in your circuit. Some parts of your circuit are more positive than others.

Oldhousescott brings up a good point, and you might give serious consideration to using a non-polarized electrolytic there to avoid just burning out the cap.

There are times when you need to REDUCE the pull-down. I couldn't get a 1M to work on CJ's Neptune (output pulldown) and had to go all the way down to 750K before it stopped. But have you tried any of the tricks to slow down the LED turning on to be sure that's not the cause?

[pryde]

If your effect is truly positive ground/PNP transistor style, then the positive terminal of the cap should go toward the input jack and the negative should go to the base of Q1. However, the voltage across the cap is so small, it may very well work reversed (for a while, until the electrolyte breaks down and the capacitor becomes a short circuit).

I can't say why it's not working for you in the correct orientation. Check that your 100k resistor to Q1's base is present and the correct value.

Actually, measure the voltage across the cap in the normal orientation. It should be below a tenth of a volt.

Pryde, it's like animal farm up in your circuit. Some parts of your circuit are more positive than others.

Oldhousescott brings up a good point, and you might give serious consideration to using a non-polarized electrolytic there to avoid just burning out the cap.

There are times when you need to REDUCE the pull-down. I couldn't get a 1M to work on CJ's Neptune (output pulldown) and had to go all the way down to 750K before it stopped. But have you tried any of the tricks to slow down the LED turning on to be sure that's not the cause?

Oldhhouse,
First, the transistors are matched PNP OC75's from SB. OK, with the 4.7u cap oriented wrong (negative toward input) I am getting .06v on the positive side of the cap and the effects sounds/works good.
With the cap correct (positive toward input) I am getting .35v on both ends and the effect does not work? The 100k resistor at Q1 base is correct. I am very baffled  

Midway,
The pop issue has basically been resolved with a 2.2M PDR. I have the circuit board populated but hooked to the breadboard for testing. The only LED is in the breakout box. Also, I do not have a non-polarized 4.7uf to try on hand. I would like to figure out what the issue might be first.

Any thoughts from here?

[Jack Deville]

Good read here:
http://mrblackpedals.com/blogs/straight-jive

No discussion about pull-down resistor values. That's another discussion entirely.
There is some useful information about switch pop however.

[GrindCustoms]

Great article Jack, never again i'll suggest a 2.2M resistor.......Hahahaha!

Lots of unicorn just died there.....

[sgmezei]

Good read here:
http://mrblackpedals.com/blogs/straight-jive

No discussion about pull-down resistor values. That's another discussion entirely.
There is some useful information about switch pop however.

Jack that article is great. Thank you. Getting a refill on my coffee and going to read it again.

[pryde]

Good read here:
http://mrblackpedals.com/blogs/straight-jive

No discussion about pull-down resistor values. That's another discussion entirely.
There is some useful information about switch pop however.

Great info. thanks. It seems like trying different input caps could make a difference as well (less leaky=less pop)

On another note my problem circuit is now working with the input cap oriented correctly (+ toward input). I am not sure how or why. I first stuch a non-polarized cap in and it worked. Removed it and put the original polarized one back in and it works  I still have the 2.2M PDR but maybe need to rethink that based on the article.