Build Your Own Clone Message Board

Ok. Checked the component values again, Resoldered , Lifted the a leg of each diode. Still distorting sound. Still clean at IC1 pin 1. Any thoughts?

Please hold on - we're checking things out...

Thanks!

The brick has a 5v regulator on it, so you can rule that out as getting fried at 18v. Not possible.

So basically you've got the input and output op amps and the mixing op amps. If there is no distortion when the reverb mix is turned down, I would guess that IC2 is the issue. Or maybe Q1, but that seems a lot less likely.

Most dual op amps have an "absolute maximum" supply voltage rating of +/- 18v. I would guess that running the supply so close (or even a hair over) the max upper limit is what messed things up.

@Morgan. Have You figured out anything?


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Try replacing IC2 with the 4558 from your chorus pedal. Turn the reverb and dwell knobs all the way down and see if pin 7 is still distorted.

Also, does your guitar have passive pickups

Still distorted at pin 7 with the 4558. The 4558 worked fine again once back in the chorus. I am only using the passive pickups on my guitar.

Remove or lift one end of C5. That will take the brick out of the signal path and better isolate that op amp. Distortino still there?

To get C5 out I had to remove the brick. Any tests I should do with the brick out?

With the brick out there is clean sound out and no sound at IC2 pin7.

I don't have much technical advice at the moment but when I bought this kit when it was first offered I also had lots of problems at 18 volts. I removed the brick and checked and rechecked but found no problems? I put pedal back together and just stayed away from 18 volts. I ran pedal with a 9 and 15volt source with no proble. I unfortunately lost all my postings from the recent crash and can't refer to other things I might have changed. I do love the one mod that is super simple and added huge sound. I don't see it on PDF and my old PDF is at work but here it goes. R14 (33k) which is all the way at bottom right of board, add another 33k in parallel on a spdt switch, on/off. Put it right on top of it, solder one leg to the one underneath it and then the other leg to one side off switch the other side off switch goes to other leg on R14. Make sense? I have a pic and will post if interested, it does add boom to verb

Culturejam said something like "most op-amps are rated at a maximum supply of 18V and running at 18V could fry them". In fact the 4558 is rated at plus and minus 18Volts in a split rail configuration. The BYOC reverb is single rail and uses a two resistor divider to bias the inputs of the op-amps to half that of the supply. This is equivalent to running the op-amps from a split rail +9 and -9V supply, so they are well within their maximum rating.

If the circuit is configured correctly for single rail biasing then changing the supply from 9V to 18V should result in the op-amps bias point changing from 4.5V to 9V. All the op-amp output pins should be sitting at either 4.5 or 9V depending on which supply voltage is used. That does not happen - pin 7 of IC2b is at the right voltage for a 9V supply but swings up almost to the supply rail for 18V.

In a circuit designed for single rail operation using a resistive divider to establish a half the supply bias point, all the DC signal in and out paths should return to that point. I can see at least one place according to the circuit diagram where they do not. Pin 7 of IC2b connects to R17, the other end of R17 connects to one end of the tone control and the other end of the tone control connects to R18. The other end of R18 is shown as connected to circuit ground, not to the bias reference point. This forms a DC path to ground from the output of IC2b.

The other possible explanation for the output of IC2b not biasing to half of 18V is that the positive to negative input differential is becoming significantly positive (in fact looking at the measurements right on the pins of the op-amp this appears to be exactly what is happening!!!!!). Unlike the input to the reverb brick which is capacitively coupled with an external cap, the negative input of the IC2b circuit has two resistors connecting it to the output pin of the reverb brick. Now the spec for the Belton brick says it has internal coupling capacitors and external caps are not needed. However it does not say what type of caps these are or if they will tolerate being connected to external circuits operating at higher voltages than the brick.

The other possibility which I have already mentioned is that there are a lot of resistor capacitor networks hanging off the output of IC2b. It is possible that these are causing a borderline phase shift to the feedback loop of IC2b and that going to the higher supply voltage just tips it over into high frequency oscillation.

I need to find a school to explain to what you just said. I really would like to understand electronics on a deeper level, if I could find a school what would I be looking for? Electrical engineering? Electronics?

I pulled out old PDF and its not another 33k in parallel with R14, it's a 10k. What that does is drops value to 7.6Ohms and make pedal a boost with a little ambience

OK let me boil it down ...

1/. There is no problem in running these op-amps at 18 Volts. They are rated to twice that.

2/. With a 9V supply all the op-amp outputs should be at +4.5V (or close to). With an 18V supply the circuit should automatically adjust and all the op-amp outputs should be at +9V.

3/. I think that connecting R18 to 0V (AKA Ground) is wrong. I think it should be connected to the junction of R29 and R30 (or anywhere along the tracks that connect to that point).

4/. If the difference between pins 5 and 6 of IC2b results in pin 5 being significantly positive of pin 6 (anything more that a few millivolts) pin 7 will go hard positive. The output of the reverb brick may be causing pin 6 to go negative of 5 when an 18V supply is used.

5/. IC2b may go unstable at 18V.

6/. Changing R14 to a 10K (or wiring a 10K in parallel with R14) seems to cure the 18V problem. This may be because in reducing the gain of this stage it stops instability (if that is the problem) or reduces the effect of a small difference between the inputs which is causing the output to go hard positive (if that is the problem).

If you are serious about studying you should look for a course in basic electronics - an introduction to electronics - electronics for the hobbyist etc. There are books - the 'for dummies' series is usually quite good so probably 'Electronics for dummies' (no slur on you intended, that is just what the books are called, I own several of the series myself).

This is not "wrong", as this tonestack is based off the big muff tonestack that was developed in the early 70's and has been successfully implemented in thousands of pedals since. That old tonestack has it's issues with impedance and signal loss, but Keith found it to be a good match in the circuit (probably why the gain of IC2b was bumped up a bit). However, when running the pedal at 18 volts, referencing R18 to 1/2v instead of ground, as you suggest, is probably an excellent idea.

Just to boil it down further, this circuit was designed to operated at 9 volts. It was thought that running it at 18 volts would be fine, for reasons that have already been outlined, but it's looking like there are issues with running the circuit at that voltage. It was my understanding that there were plenty of these kits being run at 18 volts over the last couple of years, and that we were seeing something new in the latest batch of kits with these 3 recent builds that were having problems at 18 volts; but it's looking like my understanding was incorrect as we've had 2 moderators verify the same issues on their builds at 18 volts. HOWEVER, just to try and keep this on track, this particular thread is really about the OP's issue with distortion in the wet signal path, and more specifically on the output of IC1b. The OP should be getting a new brick from BYOC soon, as we think the problem may be in his particular brick, since IC1b cleaned up when the brick was removed. (some of this stuff was handled offline)

I have had mine for a long time. It works fine at 18v, however, I see absolutely no benefit to doing so. I hear no difference between 9v and 18v, so I don't really see the need to do it.
-P

Yeah, it's looking like reducing the value of R14 let's it operate at 18 volts, probably because reducing the gain of that stage stops instability, like tark said. IIRC, you did the R14 mod, right?

I did, and I have it on a toggle to swap between stock and Mod. I gave it a whirl at 18v with the stock setting and I end up with a bit more oscillation when the dwell knob is up past noon, and can get it to do some crazy glitched out oscillation with the Dwell and Verb knobs maxed and running at 18v. No such sounds at 9v. With the Modded R14, there is no crazy sounds when the Dwell and Verb are maxed, just tons of reverb.
-P

Hi Morgan,

Well my longer post was more circumspect about the tone stack, I guess my re-phrasing sounded too aggressive.

Anyway you got me curious about the Big Muff tone stack so I had a look at some of the circuits. I take your point about the tone stack being used in a lot of pedals but even in the 4 transistor versions of the Big Muff you can see that the two transistor clipping stages are pretty much identical.

While the first stage is AC coupled into the second, the second clipping stage is not (in fact every stage BUT that one is AC coupled). It goes straight in to that tone stack placing about a 140K resistive load DC coupled to the 12K collector load (or 22K, it seems values for the two clipper stage may differ depending on which production version). Not enough to matter very much, but it would pull down that stages collector voltage a little bit compared to the previous stage. Looks like when they went to the IC version all they did was substitute op-amp gain stages for the transistors and the tone stack remained exactly the same.

I wonder if it would sound any different if you AC coupled the tone stack (or in the op-amp version referred the tone stack to 1/2Vs). Probably not.

Got a new Brick. Put it in. Same problem.
Any ideas?

I'm really not trying to be snarky, but, if it works fine at 9v, why not just use it at 9v? Mine works with both 9v and 18v and there is no difference in the quality/tone of the pedal. I just don't think it is worth all of this fuss to get it to run at 18v. That is all my opinion.
-P

It doesn't work fine at 9 volts, the OP has a distorting sound we're trying to trouble shoot...

Not at the moment. I really thought that if lifting the diodes didn't clear it up, replacing the brick would. I'll seek further support...

Let's refresh things a bit. The issue seems to be boiling down to distortion at the input of the brick (pin 4). You have clean signal at the output of IC1a (pin 1). You already lifted the diodes and that did not do anything to alleviate the distortion. Where do you first detect the distortion in the signal path between the output of IC1a and the input of the brick? Using your signal tester, that signal path is as follows:

IC1 pin 1 > R6 > IC1 pin 7 > C5 (probably can't probe this) > brick pin 4

And to be clear - we're just trying to get this to work at 9 volts at the moment. Don't mess around with 18 volts until we get it working stock.