Geofex Adjusticator Layout Question

[MasaRGR]

Hi,

I'm trying to prepare a layout for the Adjusticator schematic at GEOFEX http://www.geofex.com/circuits/Adjusticator.gif and I have a few questions regarding what sections can be eliminated by using both halves of a dual opamp. I apologize in advance for my lack of knowledge as I've only tackled a handful of simple schematics so far.

I am attempting to build a loop level adjuster to adjust my effects loop from my amp (line level) down to instrument level for using stompboxes and then back up to line level to return to the amp. I have downloaded the data sheet for the opamp I will be using and have started to plan out a layout for the circuit.

Looking at the Adjusticator schematic at the top of the page, it looks like some sections could be used for both the front and the back end of the circuit? Will any section of either stage become redundant? Or will I just have to build the two separate stages in their entirety and just have a common power source and ground for the opamp?

Thanks,
Masa

[R.G.]

For a fixed application like you propose, there are some meta-issues to be solved first before going to the circuit.

First, does the output from your amp even need to be buffered? In many cases, the preamp from an amp can drive a pot to an effect input with minimal losses. So you might be able to replace the first opamp section with a capacitor to a 100K pot to ground to lower the level, especially since for a guitar level loop, you always want to lower the level, never raise it.

Next, does your amp provide +/- voltages that you can make into +/- 12V or so for the remaining gain section? If so, the power supply for the receiving section can be simplified, and you won't need the 10K/10K bias voltage section; you can use system ground for that.

Finally, some effects provide HUGE swings on their output, as big as +/- 4V with a 9V supply. Hot rodded effects with bigger supply voltages are even bigger. If you have those, you will need the wide range of -20db to +20db range on the Adjusticator. But maybe not.

Finally, if you go for the circuit as is, yes, you can combine the bias voltage generation sections into one, but you'll still need most of the rest of the circuit because you need to block DC in and out.

[MasaRGR]

R.G., thank you sir.

This really helps a lot. I will try the cap-pot-ground first and see how well it works in reducing the level before the input of the effect pedal. If that doesn't work, then I'll try to use both stages with the combined bias voltage generator section.

I should have mentioned that I intended to house the adjusticator circuit(s) in an external enclosure that I could loop a delay pedal through and not modify the amp itself... although now that you mention it, that is an intriguing possibility.

Thanks again!

[RomanMtz]

For a fixed application like you propose, there are some meta-issues to be solved first before going to the circuit.

First, does the output from your amp even need to be buffered? In many cases, the preamp from an amp can drive a pot to an effect input with minimal losses. So you might be able to replace the first opamp section with a capacitor to a 100K pot to ground to lower the level, especially since for a guitar level loop, you always want to lower the level, never raise it.

Next, does your amp provide +/- voltages that you can make into +/- 12V or so for the remaining gain section? If so, the power supply for the receiving section can be simplified, and you won't need the 10K/10K bias voltage section; you can use system ground for that.

Finally, some effects provide HUGE swings on their output, as big as +/- 4V with a 9V supply. Hot rodded effects with bigger supply voltages are even bigger. If you have those, you will need the wide range of -20db to +20db range on the Adjusticator. But maybe not.

Finally, if you go for the circuit as is, yes, you can combine the bias voltage generation sections into one, but you'll still need most of the rest of the circuit because you need to block DC in and out.

Hello! Sorry to revive this thread but I'm in the process of doing something similar for the effects loop of my Randall RM50. The serial effects loop send is too hot for my Zoom G3 effects pedal and when I try to use it in the loop, it sounds horrible, like when you clip your PC audio interface when recording.

So I was thinking of making a dual adjusticator as well... one stage to lower the output of the send to instrument level (or acceptable for the G3) and then amplify it again to go back into the amp through the effects return.

What I gather from R.G.'s comment though is that the first stage (attenuating stage I guess I can call it) is not needed? The send signal can be directly attenuated by the cap-pot-ground input to the "Adjusticator line level shifter" I guess I can call it and then just reamplify it after it gets effected in the G3?

Does the signal from the G3 need to go into a regular or High Z adjusticator circuit in order to have the best performance?

Thanks!

[RomanMtz]

R.G.? Anyone?