DC at output

[swever]

I have this "Tube Reamer" from "Advanced DIY effect pedals" by Indyguitarist on a breadboard now.

I get 4.5V DC at output. A cap at the end did the trick, but I would guess it does change the tone.

Is the DC supposed to be there or is it that I made a mistake? What is the right way to remove the DC without a capacitor?

[bluebunny]

I would say it's a mistake.  You need a cap.

[swever]

I would say it's a mistake.  You need a cap.

Do you mean its a mistake in the schematic?

[bluebunny]

Yep.  The way it's drawn, you can't avoid the DC you've measured.  If you look at pretty much any schematic for anything audio powered by a 9V battery, you'll find an output cap.  Your signal has to wiggle somewhere.  That "somewhere" has to be within your power supply lines.  So that you don't run out of headroom one way or the other, the middle of this "somewhere" needs to be roughly in the centre of your power supply.  This is why you see 4.5V - it's the point around which your signal wiggles.

[balkanizeyou]

put the cap before the volume control. If it alters the tone, change the value of the cap for a bigger one until it doesn't, but I think that 100nF should do the trick

[swever]

Thanks for the info, peeps!

What would be the difference between cap before volume pot vs after?

[wavley]

Thanks for the info, peeps!

What would be the difference between cap before volume pot vs after?

Before, you want to keep the DC off that pot so it doesn't crackle when you turn it.

[PRR]

> A cap .... I would guess it does change the tone.

Audio is FULL of caps.

Audio is the intersection of small interesting signals and big boring batteries. We modulate the boring battery with the interesting small signal to make an interesting big signal. We do NOT want that boring battery power to reach the speaker, not even the next stage. So there's caps like EVERYWHERE.

One more won't "change tone".

Yes a wrong-value cap will cut bass. But we can design the cap for the thing it drives so the bass loss is imperceptible down to 50Hz, 20Hz, 1Hz, or lower if we think best. 20Hz is usually plenty good (especially for guitar: it and its speakers only go to 82Hz).

If the load to be driven is 1Meg, use a 0.01uFd cap to reach 20Hz.

If the load is smaller than 1Meg, use a proportionally bigger cap.

Here we see a 100K pot for load. (Agree we keep DC off pots when possible.) Since that is 10 times smaller than 1Meg, pencil 10 times larger than 0.01uFd, or 0.1uFd.

BUT there's always a next stage. If it is a 1Meg input, no big change. (100K||1Meg= 91K.) If it is a 100K input, 100K||100K= 50K. In guitar-cord work, inputs tend to be 100K to 1Meg. So the worst-case load is 50K. Use a 0.2uFd cap.

(Or, knowing that excess bass in a fuzz/distort makes things muddy, shorten the cap some to shave some bass. Also reduces cost. A 0.1uFd is very reasonable here and could be a nickle cheaper and slimmer than 0.2uFd.)

(You will also see 5uFd-10uFd electrolytic caps here because a 10uFd e-cap may be even cheaper than a 0.2uFd film, and 0.3Hz bass isn't usually a Problem.)

[anotherjim]

I'd put caps before volume if I can. Another advantage is that the pot serves as the anti-pop pull down resistor for the cap - saves adding a resistor to do that.