EarthTonesAudio "Buster" (buffer and boost in-one)

[earthtonesaudio]

Put R4 to the output of U2.2 instead of the input.  Otherwise looks fine.

[waltk]

Thanks! OK.  Here's a corrected version.
There's a layout here: http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=39924&g2_serialNumber=2 and PCB image here http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=39927&g2_serialNumber=2 for anybody who is interested.  Unverified, so use at your own risk.

[Hupla]

This looks like an easy enough build. Think I'll give it a go on the breadboard later on and see how it goes.

[waltk]

Hi Alex,

Are you back?  I built the layout posted earlier, and everything's working great... with one exception.  There's a significant pop when switched.  I measured about .35V of DC on the output.  How would I get rid of this (without rolling off any audible frequency that would alter the tone)?  I looked around at other threads, and articles (http://www.geofex.com/Article_Folders/box_pop.htm) about reducing pop, but I couldn't figure out how to do it.  I think I'm some missing something basic in my understanding of how this works.  Is the DC there because Vr is a virtual ground above the true ground?

Thanks for your patience in helping me understand this...

[Thomeeque]

 Note, that:

.. I did not include pull-down resistors, as this is not intended to be true bypassed. ..

So, you need to put there pull-down resistors at very input and output, if you use bypass switch (cca 1M between A of C1 and ground and cca 100k between B of R6 and ground on your latest schemo)..

Or are you talking about switching K1?

T.

[earthtonesaudio]

Hi Walt,

Yep, I'm back.  You said you're getting 0.35V on the output?  If you're measuring after the output capacitor, that points to a leaky capacitor, and will cause pops if you are using true bypass switching, unless you add pull-downs as suggested.  I'd use 1M rather than 100k, but that's just personal preference and won't make an audible difference except in a few extreme cases.  I would actually recommend adding an output pull-down if you're using any true bypass pedals after this circuit.

If you're switching K1 and getting a pop, I can think of a couple things to try...

First would be to add a capacitor between the 1k resistor and +4.5V, as some have pointed out earlier.  If that fails, perhaps add a large value (but not leaky!) cap in series with K1, to block any DC offsets from causing problems.  In theory, there should be no DC potential between the output and the (-) input if the op-amp is working correctly, so I'm not sure if that's the problem.

[waltk]

Thanks for the quick answers guys.  The K1 switch just has some very small pops that are easy to ignore.  I tried putting 1M and 100K pull-down resistors on the input and output (as suggested by Thomeeque), and it seemed to help somewhat - there is no pop when I switch it off, but still some pop when I switch it on (it's a true bypass 3PDT stomp switch that switches both the signal and power).

About the leaky cap... I just tried measuring DC at the output again, and got 0 (even without the pulldowns).  I think I must have made a mistake in measuring it the first time.

I also noticed something else: while testing, I had the reverb on the amp cranked up.  So the pop was greatly exagerated (had a reverb tail).  With the reverb off, the pop is not so pronounced. 

I guess I'll add pull-down resistors to the layout.  What is the impact of using 1M vs. 100K pull-down resistors?  Is the reason for using 1M at the input so as not to lower the input impedance?

[Barcode80]

as good practice, you shouldn't switch power on the same switch as audio path. why are you switching power anyway?

putting power on the audio switch will almost always pop, regardless of the number of resistors you put as pulldowns.

if you insist on switching power with the same switch, try switching ground instead of the positive voltage, that should reduce it. however, best practice would probably be not to do it at all.

[waltk]

as good practice, you shouldn't switch power on the same switch as audio path. why are you switching power anyway?

putting power on the audio switch will almost always pop, regardless of the number of resistors you put as pulldowns.

if you insist on switching power with the same switch, try switching ground instead of the positive voltage, that should reduce it. however, best practice would probably be not to do it at all.

Thanks for the comments.

It's a 3PDT switch, so two poles are for switching the signal, and one for the power.  I didn't use the "ground switch with a stereo jack" method because I'm out of stereo jacks, and because I might want to leave this plugged in without draining the power.  If I'm using a separate pole on the switch, and not mixing signal with power in any way, can it still make a pop? (Why, inrush current?)

OK, just did a quick test.  Switching the ground instead of the positive voltage doesn't make a difference.  Taking the power off of the switch, and leaving it hardwired on seems to make a little difference, but not much.  Leaving the signal connected (hardwired), and just switching the power on and off does make a small pop.

[euronymous0001]

built the simple version of buster (v. 1) but replaced the 10k Pot with 50K to increase gain. It works fine until the gain pot reaches the 3 o'clock position, no sound would come out of the circuit.

if i want to have 25db maximum gain for this circuit, what do i need to modify to it.

TIA 

[earthtonesaudio]

If you put a meter on the output pin of the op-amp, my guess is that you'd see the output swing really high or really low as you increase the gain.

Add a 10u cap in series with the 1k gain-limiting resistor.  My best guess is DC imbalance + high gain results in the amp saturating.
More gain requires more care (and generally more parts) to keep it behaving how you want.