Build Your Own Clone Message Board

I've made just over a dozen pedals and this is the one I want to work the most. So naturally it's giving me much grief. The issue I am having is that the circuit produces a massive amount of gain, no matter the trimpot setting or external pot settings (bias trimpot typically left at 4.5v). Myself and my teacher tested the circuit on a champ on low volume, and you would think it was a deluxe on max volume. Along with the gain, and probably because of the gain, there's an immense amount of distortion. We did a number of tests on the circuit, including one that Keith (BYOC) recommended, and we're still having the issue. All voltages seem to check out.

Things we tried:

Pulling the 3207 chip (Keith's initial troubleshoot recommendation) to see if the dry signal was still distorted: Same deal. Dry signal without the 3207 is extreme gain and massive distortion.

We tested voltage oscillation at Q4 base: This tested and oscillated as we had hoped.

We had additional 4558 IC's on hand, so we went ahead and swapped one with what was given in the kit and no difference. We don't think it's the 4558.


If anyone at all has the slightest recommendation on this one, please please please, I would really like to hear this one sing!

Are you easily able to inspect the signal (via oscilloscope, or by an audible method) at intermediate points in the circuit? It seems that if either amplifier built around one half of the 4558 has far more gain than intended it could explain what you are seeing/hearing. If you can't inspect in those ways, and if there is nothing obvious from visual inspection (such as the wrong resistors in place) and you are not in the mood to desolder so that resistances can be checked out of the circuit, one test you might try is to carefully and temporarily place an additional feedback resistance in parallel with one or the other of the feedback paths already there, in order to reduce the gain. That is, connect a resistor between the op amp output pin and the corresponding inverting input pin (so, pins 1 and 2 for the amp early in the chain, or pins 6 and 7 for the later one). This is more a diagnostic than a cure. I might try 22k or 10k or perhaps even a bit lower. Since those are pairs of neighboring pins on the IC you would need to be very careful as you touch the bent leads of a resistor to the pins so as not make some other additional connection that could be unhealthy to the IC.

If this sort of thing is the culprit and the dry signal is already distorted it would suggest that the problem is in the components related to the pins 1-2-3 section of the 4558.

I had the same basic thought when reading the first post, namely that it seems very possible that there's too much resistance or a break (i.e. infinite resistance) in the feedback loop of one side or the other of the 4558 dual op amp. Take a look at the schematic HERE and you'll see that each of the op amp feedback loops contains a 47K resistor (R6 and R24), which serves to moderate the gain of those stages. The higher the resistance through the feedback loop, the lower the gain moderation, so if there's a much higher value resistor or a break in that part of the circuit, you'll have much higher signal gain being put out.

Try measuring the resistance between pins 1 & 2 and between 6 & 7 on the 4558. On my BYOC Analog Chorus, which has the same op amp circuit structure as the Bass Chorus, I see about 25 Kohms in both places (you don't see the 47K value because of the presence of parallel circuit paths in the pedal, which results in lower measured values). See how the resistance measures in these two locations on your pedal.

_________________
“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

You guys nailed it! The resistance was 15.4M between 6 & 7. I reflowed and added solder to those 3 components, did some rewiring (unrelated), and now it works as I had hoped!

Thanks for the support!

That's great news! Enjoy the pedal!

_________________
“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2