9v mod/removing dc offset

[scaesic]

hi!
ok, i've been looking at the mxr distortion 2 for quite a while, as you may have noticed. I've re-drawn the schematic and made it much more legible, removing all the parts i thought were unecessary.
here you should be able to see the re-drawn schem with some coloured pointers marking out points i have some questions for.

ok, first is the input. Originally i thought this was just a jack input, but then i noticed both legs of the "switch/jack" are grounded, and this doesn't make much sense to me, is it safe enough to just remove this and put a jack on the input?

secondly, i've been told it's necesary to remove a dc offset before the final output stage, would a capacitor on the right hand side (circled) do this job, and what values should i experiment with?

and thirdly, am i right in thinking i can use just one dpdt stomp for the entire circuit ?

if anyone can see any other flaws with the circuit please tell me!
thanks.

[Paul Perry (Frostwave)]

Maybe the input is arranged so that, when there is nothing plugged in, the input is grounded. That is often done on mixers, so that there is less noise picked up when there is no signal going to that input.

[scaesic]

that seems a strange idea for a pedal.

it'l be fine to just erase it and replace with a jack then?

anyone on the dc offset?

[Jeremy]

C16 is already filtering out the DC offset, unless I'm catching the wrong meaning for "DC offset". 

[R.G.]

Sometimes parts have non-obvious uses. That can make stripping out the "useless" parts a chancy business until you understand what they're for. (See "What are all those parts for?" at GEO)
Your schematic as shown will have bad DC offset popping. It comes not from the effect output, which is already DC blocked by C16 as was noted earlier, but from the "bypass" signal from the output of U2 pin 1. That point is sitting at Vs, so each time you push the stomp switch, a 4.5V transient is impressed on the output line. You need to DC -block the normal signal path, as well as pulling it to ground with a pull-down resistor to get rid of that.

Your circled capacitor does nothing to block DC offsets from the output. As shown, all it does is cause massive high frequency gain by shunting signal past R20 to ground - which is the same as Vs for AC signal purposes. If you're trying to block DC offsets from the input to the final output stage, you need to place a capacitor in series with R18.

As Paul correctly notes, the input jack wiring is set up to ground the input if there is no input cord. It's almost immaterial whether you do this or not in modern pedalboard use.

Finally, I don't see how you switch the 9V battery power. Is there another switch for that somewhere that's not shown?

[scaesic]

ok, doesnt the power need to be on whether in bypass or not in order to power the buffering op-amp?
the voltage from the battery/wall wart (wallwart only in my case), is to be completed when the input jack is plugged in.

i have added the rc configuration in the bypass line to remove the dc-offset there, finally, i added another capacitor in series with r18 to avoid putting it through the final op amp stage, is it worth adding this, or will it compromise too much low end signal?

i was just worried about headroom since the op amps were originally operating at 15V.

edit *i'll add the amended schem later*

[scaesic]

heres the ammended schem

[R.G.]

Better. There's still a couple of things to do.

The dry path is now OK, but you have introduced a problem in the effect path.

The output buffer with protection is OK, but it has to be properly biased as well. When the bypass switch is pointing to the dry signal (that is, the effect is bypassed) then you're OK, the output buffer is biased to Vs by the output of the opamp it connects to, and it's output has that same Vs DC level blocked by the output cap/resistor at the output jack.

The problem is that the effect signal path is already pulled to 0Vdc by C16/R22, so when you flip to effect, it pulls the input to the buffer stage to ground, and no signal will pass.

Do this: take C18/R23 and move them in front of the bypass switch. C18 should connect directly to pin 1 of U2, R23 to ground. The junction of C18/R23 connects to the bypass switch. Now C18 blocks pin 1's DC level, R23 pulls the DC level to 0V, and this matches the DC level that C16/R22 put out, so there's not a DC level pop when you change from one to the other.

The output buffer and protection stuff is OK, but probably not needed here.

[scaesic]

ahh, thanks for the response again. so you reckon its best to just shift the dc blocking cap after the output buffer to the front of the buffer? instead of having an rc combination before AND after the buffer?

what difference will the output buffer and protection make to the sound if i omitted it?

would a cap infront of r19 make any differnce to the headroom of the circuit?

[R.G.]

In a single-polarity power supply like this, you have to bias each opamp to about half the DC power supply at its output so it will work properly.

Most opamps, including the TL07x series, can only swing their outputs to within about 2V of either the positive or negative side of the available power supply voltage.

The output of the opamp will swing to wherever its inputs tell it to. The simplest way to do this is to put the appropriate DC voltage on the (+) input through a resistor and then to block DC from the (-) input with a capacitor. As an example, both halves of U2 are biased this way. In both cases, the (+) input is tied to Vs, in one case through a resistor, in the other by a wire - which is just a very low resistance.

Doing this will make the output sit within a few millivolts of the Vs voltage. It is OK to simply connect the (+) input of a second opamp to the output of a previous opamp if the previous opamp's output level is nearly Vs. This is happening with U3 pin 5. It gets its DC bias level from R16. R13, 14, and 15 are all blocked from other DC levels by series capacitors, so they cannot affect the DC level on pin 5.

So the opamps are happiest if their outputs are all floating at Vs. This gives you the maximum headroom for the opamp output to swing.

With a single-polarity power supply, you must block both the input and output DC levels of the effect from the external signal line. The external signal line has a 0.000000V DC level, while the input and output of the effect must sit at Vs for the effect to work. So you need a blocking capacitor at both input and output to keep this Vs-level DC away from the external lines. If you don't, you will cause problems with other things on the signal line, and probably cause big pops when you switch the effect.

If you block only one side of the signal bypass switch from Vs, then the effect tries to make the external DC level be 0V in one position, and Vs in the other position. This is heard as a huge pop when you flip the switch. Quiet switching makes it mandatory to have the same DC level before and after switching. It's required for quiet switching. What I've been getting at is that to get quiet switching, you must have the same DC level on the output jack in either position of the bypass switch. The existing circuit does this through C16/R22. The original circuit did not do it for the non-effect setting, but instead connected it directly to the output of U2, which sits at Vs.

All that's needed for (relatively) quiet switching is to DC block the path from U2 to the bypass switch. That leaves all pins of the bypass switch at zero volts DC in all cases, and you have a chance for quiet switching. The output buffer is OK, but probably not necessary in this application because the output signal comes either from the input buffer or from U3's output, and both of these are low impedance sources, suitable for driving the output cable with no treble loss.

[scaesic]

coooool, thanks for all that, you've been a lot of help and have obviously taken a lot of time to write all that out, and i appreciate it, thanks.

(and heres the but) however..... the output stage of u3 of the effect path seems to be biased via the inverting (-) leg. Is this wrong? it came from the original mxr schematic.

[R.G.]

the output stage of u3 of the effect path seems to be biased via the inverting (-) leg. Is this wrong? it came from the original mxr schematic.

You know, that does look odd. Where can I see the schematic you were working from?

If I had done that, I'd have biased it with a capacitor in the - input leg and a bias voltage to the + input.

[smank]

I suggest to remove C19 and to place a 2,2uF electrolitic cap (from 1uF to 4,7uF it's fine) from R20 to ground (the other leg of R20 is linked to opamp). In this way you can block the DC gain. You can choose also to increment the output level increasing a bit R21 (15K-22K).

[scaesic]

http://www.diystompboxes.com/pedals/dist2.GIF

heres the original.

smanks idea is basically what i have on the latest one, although i still don't get why the (-) input of the output part of u3 is biassed to Vs in the original.

what would happen if R20/Vs was removed entirely?

[slacker]

smanks idea is basically what i have on the latest one, although i still don't get why the (-) input of the output part of u3 is biassed to Vs in the original.

what would happen if R20/Vs was removed entirely?

U3 is set up as a non inverting amplifier, probably to make up for any loss of volume caused by the filter. The gain is set by R21 and R20, and in this case as they are both the same it has a gain of 2. In this configuration you can either connect R20 to ground via a large cap or to Vs, you get the same result but connecting it to Vs saves you a cap. This isn't biasing the opamp though, to bias it you either need to remove the cap between R18 and R19 so that it's biased to the DC level of pin 7 of U3. Or leave the cap in and add a large resistor 1M would do  from pin 3 to Vs.
If you remove R20 then then you just get a buffer.

[scaesic]

cool, just to note that cap in between r18 and r19 was only an idea.
the big question which im conserned with, is what difference will it make connecting r20 directly to Vs, or to ground via a cap.

Smank, did you try both configurations? i'll happily add in the cap to ground, but i'd quite like to keep as faithful to the original circuit as possible, seeing as it's already alterned to run on 9V instead of 15.

[slacker]

It won't make any difference, connecting it to Vs or to ground via a cap achieves the same thing.