One Inverting stage in Bluesbreaker?

[Kindly Killer]

I am still grappling with the very basics of electronics, picking apart simple overdrive circuits, trying to get good and rolling my own.

http://www.generalguitargadgets.com/pdf/ggg_mbb_sc2.pdf

Looks like the second opamp is inverting, right? So does this pedal create an out-of-phase output? Why this design? Seems like a huge drawback with no payoff except maybe fewer components. Does the inverting variety sound fundamentally different or something?

I have noticed that the Xotic RC Booster (my second-favorite OD) uses two inverting stages. And every textbook example of a Baxandall tonestack I've seen uses the minus side as well. What is the reason for this? I'm sure I'm missing something really basic here.

I am going to fiddle with this circuit on the breadboard and make several versions, but I have to make a Mouser order and get my workshop organzied first. While that comes together, can anyone tell me what I'm missing here?

[Johan]

out of phace with what?...it's guitar=mono and single signal...take a 1/4 step back and your in phace again.
I can see two resons for the second opamp being inverting. one gainpot adjusts the gain of both stages and it reduces the risk of oscillation inside the chassis of the box
other than that, it's anyones guess..
J

[Mark Hammer]

There's an interesting article on page 8 here, regarding standards for effects: http://hammer.ampage.org/files/Device1-9.PDF

It was common, at that time, for pedals to produce an output that was phas-reversed from the input.  It is quite reasonable for Johan to ask "Out of phase with what?", since a prototypic setup where a guitar goes into a couple of pedals in series then into an amp (and speaker of unknown phase) produces nothing of note as result of phase relationships.

BUT.....

If anything is being run in stereo, or processed in parallel, or...
If anything is going into the board as well as into an amp, or....
If anything is being mic'd at the amp as well as plugged into the amp,

Then you need to assure "phase coherence" across all those signals, or else risk cancellations.  That's not to say that cancellations are always undesirable.  But if there is no big red or green sticker on the pedal that declares its outputs to be in vs out-of-phase with the input, then you'll have no way of knowing in advance what the phase relationship is between any two versions/copies of the signal, short of keeping all schematics on hand and following the signal path through each one

Having said that, Johan is correct in assuming that there can be certain design advantages to using an inverting stage.

[Kindly Killer]

Okay so it is out of phase. Yeah this is a huge problem for a guitar stompbox b/c what if you use the following extremely common recording setup:

                            bright, clean amp > microphone  > board channel 1
                         /
                        /
guitar > splitter
                       \
                         \
                            overdrive pedal  > slightly dirty amp > another microphone > board channel 2

i mean anyone would hear the wonkiness and hit the phase button on one channel and it's fixed, but still seems so odd...

i don't get how the gain pot controls the second stage. or does that just mean that the higher/lower output from the first stage controls how much the diodes will clip? it looks like the gain pot is isolated from the second feedback loop? or am i thinking of it wrong?

tell me if this is right: if you could assume a low impedance input, it would better have both stages inverting. or maybe have both stages inverting and - because it's for guitar which "likes" a high impedance input - use a separate input buffer? maybe the bluesbreaker started off that way but they changed it to cut out some extra parts? i have an Xotic RC Booster and it works that way (two inverting stages with a transistor input buffer).

[slacker]

The gain pot works like this. As you turn the gain up the resistance between pins 1 and 2 increases so the gain of the first stage increase. Have a look at R.G's tube screamer article for more info on how the first stage works http://www.geofex.com/Article_Folders/TStech/tsxtech.htm it's a similar arrangement.

The gain of the second stage is calculated as the resistance in the feedback loop/the series resistance connected to the inverting input. So in this case it's R10 divided by the resistance between pins 1 and 6, the fact that there's a couple of caps in there doesn't matter, so on minimum gain you've got  R10/(R15 + R8 + R7) = 220/109.4 = 2 and a bit. At maximum gain it's 220/9.4 = 23 and a bit. There's a bit more to it than that as the caps might add some tone shaping but that's the basic idea.
Diodes in the feedback loop of an inverting stage give different clipping than in the feedback loop of a non inverting stage so that might also be part of the reason for the design, but Marshall use the same building block in most of their dirt pedals so probably not.
You could probably make the first stage inverting as well to keep the overall phase the same, but I'm not sure how you'd do the tone shaping done by the stuff connected to pin 2. If you think it's important then it's probably easier just to add a another inverting stage on the end.
I don't personally think it matters, even in your example all you've got to do is move the mics and you can get what ever phase relationships between  the two amps that you want.

[Mark Hammer]

Also worth noting that a lot of mic preamps and mixers anticipate such common problems, and include phase-reverse switches.  Ultimately, it doesn't matter whether the phase inversion happens at the pedal, the speaker, or the mic, so long as two signals being combined in some fashion have the intended phase relationship with each other.

[Kindly Killer]

So in this case it's R10 divided by the resistance between pins 1 and 6, the fact that there's a couple of caps in there doesn't matter, so on minimum gain you've got  R10/(R15 + R8 + R7) = 220/109.4 = 2 and a bit. At maximum gain it's 220/9.4 = 23 and a bit.

Okay I get it - the resistance between 2 & 3 of the gain pot goes down as you turn it up, which figures into the ratio in the second stage. I couldn't see that in front of my nose. Thanks!

Mark I take your word for it since I am really new to the electronics side of things; LOL I am no position to criticize anyone's design. I like the suggestions in the article you linked to. Thanks!

Man all these links are great. It would be great to compile a bibliography on this site with as many things hosted here as possible (for posterity) and other stuff cataloged and linked. I am going to read the R.G. Keen article on set breaks tonite LOL

One thing I'm interested in doing over Christmas (nothing this weekend! woo-hoo!) is setting up a software oscilloscope and [whatever you call it] generator on my workshop computer. If I were up to speed on that I probably would have noticed the phase thing before I ever dug into the circuit... I bet y'all got some links on that subject?

[Johan]

most Marshalls are inverting, most fenders are not. people have been running dual amp setups with those two brands since the begining of time without problems. yes in theory it is a problem. in the real world it's not. if you run into trouble just place one amp ~15cm further back. ...dont worry so much