DOD 250 more bass response.

[seanm]

I would like to mod a DOD 250 for more bass response. According to the simple mods page, the input and output caps should be increased. Is there any rule of thumb about what *too* large is. The mod says changing the input cap from .01 to .1 will help.

Is the output cap the cap attached to pin 6? It's already 10uf, should I go to 100uf?

What are the disadvantages of increasing the caps?

[the_badcliff]

Capacitors (somebody correct me if I am saying this wrong) are high impedance at low frequencies, and lower impedance at higher frequencies.  For any given capacitor there is a point where the higher frequencies will all pass, while the lower frequencies will be rolled off.  The higher the value of the input or output capacitor, the lower this cutoff frequency, and thus the more bass is allowed to pass.

There isn't a simple rule of thumb for what "too large" is, in your case just try different values until you find one you like.  Too big of an input cap may cause things to sound a bit muddy, but the circuit would still work.  I would recommend socketing the input capacitor.

The output cap is attached to pin 6 of the opamp, and in the GGG schematic is 4.7uF.    This is plenty large already and should pass all the frequencies a guitar will produce fairly equally.  In other words, I don't think you would notice a difference no matter how much larger you made it.

As far as disadvantages go, everything will work electrically, its all a matter of whether or not *you* like the sound.

[seanm]

the_badcliff thanks for the info! GGG?

I may try removing the 2 diodes and the cap, which should effectively give me a clean boost? Then I can try changing the input cap to see if I can get no bass loss. I know it can be done (Blackbox Cobalt when clean). Then add back in the distortion.

[Mojah63]

You can also put a larger cap in the distortion control feedback loop. Stock should be .05uf, .068uf should lower the corner hz of the distortion. Phillip does this in his Liquid drive. http://fuzzcentral.tripod.com/liquid.html

I've modded one of my 80's 250 using the liquid drive as a template. It does have more bass but I do find there is a little too much high hash. I put the small 25pf comp cap back across the ic.  I did end up buying a Burr Brown for it and it's a but darker than the stock ss crap ic. A 741 is kinda cool too... Lots of compressed mids.

P:)

[seanm]

You can also put a larger cap in the distortion control feedback loop. Stock should be .05uf, .068uf should lower the corner hz of the distortion. Phillip does this in his Liquid drive. http://fuzzcentral.tripod.com/liquid.html

I've modded one of my 80's 250 using the liquid drive as a template. It does have more bass but I do find there is a little too much high hash. I put the small 25pf comp cap back across the ic.  I did end up buying a Burr Brown for it and it's a but darker than the stock ss crap ic. A 741 is kinda cool too... Lots of compressed mids.

P:)

Thanks for the reference. The Liquid Drive looks interesting.

[Mark Hammer]

Hi Sean,

The deal with the DOD 250 (and the Dist+ as well) is that the pot for adjusting the gain in an op-amp also has the capacity to change the corner frequency at either the high end or low end.

If the pot/variable-resistance for altering gain is in the feedback loop (like the TS-9), increases in resistance both increase gain and decrease treble if the feedback cap is big enough (in some instances, you won't be able to hear any changes to high-end content if such changes are from, say, 60khz bandwidth down to 18khz bandwidth).  If the pot/resistance is in the ground leg of a non-inverting op-amp (as is true of the DOD 250 and Dist+) then gain is increased by decreases in pot resistance.  At the same time, this raises the low end cutoff such that there is less bass available at higher gain and more available at lower gain.  Again, how audible this is will depend on the actual cap value.  If the rolloff starts around 1hz at min gain and goes "up" to 40hz at highest gain, then clearly you won't notice it unless you're packing a 7-string long-scale bass.  The 250 uses a .047uf cap in this position, which, in combination with the gain control, shaves off a whole lot of bass at max gain.  My suggestion is to increase the cap to .22uf or more so that you can retain a fair amount of *usable* bass at higher gain.  This will likely make a bigger difference than changing input or output caps.

Was DOD or MXR stupid to have done it this way?  Not at all.  One of the interesting properties of the pot/cap interaction is that you get a more complex timbral change for free by having both the degree of clipping and bandwidth changing simultaneously.  As well, imagine the typical crappy hum-laden signal of your average 1978 unshielded single-coil player.  Is that the sort of signal where you want to boost 60-cycle hum by a factor of 100?  Not really.  Consequently, it was actually quite clever of them to roll off more bottom the more gain was applied.  Of course, nowadays, a great many more players are able to feed such boxes signals that have considerably less hum so this "safety feature" is not requied nearly as much.

P.S.: If you're in Ottawa, drop me an off-line note and we'll arrange to get together.