Newbie IC Help For DOD FX23

[rickortega]

Hello everyone!

I'm a newbie to this stuff and the forum. I checked the "search" section and did not find anything on the DOD FX23. I'd like to use the FX23 and some mods as a tinker pedal before I tackle my SD-1 mods.

Question: The DOD FX23 has 2 OPAMPs: an LM1458 (U1) and an LF353 (U2). Can I swap out these with more Tubescreamer-like OPAMPs (say, JRC4558DD, RC4558P or Burr Brown OPA2134PA ) to get some of the ugliness out of the distortion? Will there be any compatibility problems?

Any other mod recommendations that you may have for the FX23 would be greatly appreciated!!

Thanks,

Rick

[petemoore]

NTE says these OA's [you listed] are Duals, so Standard Duals should work...I'd socket them tho anyway...I'll do more checking and post back if I'm wrong....

[cd]

What is the FX23?  I've never heard of that before.  In any case, swapping out the opamps will not turn magically turn it into a super smooth tube pedal - it may even make the problem worse.

First of all, define "ugliness".  Is it a high end harshness/buzziness?  No punch, no low end?  A honky kind of sound?  Most of these are a function of EQ; that is, too much/not enough lows/mids/highs.  IME most DOD circuits are built around opamps with lots of pre/post EQ to get a certain sound.  Lots of opamps = higher noise, but easy to design around.  With such complex circuits, the opamp type doesn't have a large effect on the sound.

[Mark Hammer]

Swapping op-amps can or should be done for a number of reasons, such as improvements in noise specs or top end, or current consumption, or driving lower or higher impedances, etc.  In a few rare instances, such as the Tube Screamer, differences in the internal circuitry of the op-amp interacts with external components to produce some audible differences.  

What is critical here is that any improvements to be obtained by chip-swapping would depend entirely on the way the chip is used.  For instance, if clipping is achieved by diodes in the feedback loop, then MAYBE a chip swap would yield a difference the same way it appears to with the TS.  If it is NOT used in that manner,though, chip swapping may produce no such tonal change.  That is not to say that you couldn't find an op-amp with reduced current requirements for longer battery life, or better noise specs, but you may not get anything MORE than that out of the deal.

The LM1458, these days, is not really much cheaper than any of a whole spate of "high end" op-amps.  Certainly not cheap enough to make a difference in production costs if only one is used.  On the other hand, it does have some attributes which other op-amps do not have, and the FX-23 came into production when chip choices were not nearly as limited as they might have been in 1980.  That's my long way of saying it is probably supposed to be there.

The LF353 is a decent BiFET chip.  I don't think you will do dramatically better than it within the context of that circuit.

The FX-23 does not show up on the DOD site or in my Google searches.  What is it?

[rickortega]

Sorry guys for lack of pedal ID...and thanks very much for the feedback. The FX23 is supposed to be a "classic tube" sound; however, it's a buzzy, annoying sort of distortion and the idea of classic is way off base.

Here's a link to the schematic that I found for the FX23 http://www.diystompboxes.com/pedals/dodfx53.jpg

I thought that I'd practice on this one since I'll never use it with this sound. Any practice recommendaions are welcome.

Thanks,

Rick