Hey guys,
Still monkeying around with the Dist. +/DOD250 type circuits. I've made some changes to the basic circuit including having a fixed resistance to ground from inverting input, and the gain control is now in feedback loop. This works better for my purposes - I don't like the "brighter as it gets gainier" nature of the original version. I've done some monkeying with capacitor values to voice it more to my setup as well as increase the output volume to a more useable 50K. I've socketed the diodes and messed around with all types but keep coming back to a pair of Ge's - they're just the ticket for me.
I've used 741's and TL071's - I notice a certain "tightness" in the TL071 compared to the 741, but they're no quieter in the "background noise" department than the 741's...to my volume-addled ear anyhow.
Oh, I've also lowered the gain quite a bit too - I have a 100K volume pot in the feedback loop and a max gain of less than 50.
I know this circuit has a reputation for being noisy, but is there anyway to eliminate some of the background "ocean" sounds? Will metal film resistors make that much difference? (I'm presently using 1/2W carbon films.) Would it make sense to use a dual op amp and try to make some kind of hiss filtering circuit to follow?
I'm very happy with the tone as it stands and I can live with the racket - just wondering if there's something I can play around with?
Thanks!
Jim
I think metal film resistors would help. Also metal film or polyfilm caps will help too.
Quotejust wondering if there's something I can play around with
There's one you haven't mentioned.
Diode clippers in a feedback loop have a slightly different sound at times depending on whether you drive the noninverting input or the inverting input through a resistor. You might try it both ways.
Back to back diodes also sound different in the feedback loop of an opamp and as external diodes to ground, fed through some limiting impedance.
the dod-250 was 1 of my fisrt builds and i had the same problms. I tried all the same things, mods, others opinions-everything i can find here and realized there was really nothing that can be done-at least what i found anyway. You can roll back the volume /gain a little and that helps but thats about it. I like it for what it is i decided. Even thought about modding it to the ynewie model--never did. It was just time for a new build. Oh, my boss noise gate helped a little. Thats about all..A few threads if u search.
Good luck! Oh..I decided i liked this effect better w leds instead of the 1n914's.
Two places to start with noise:
1. make a good clean power supply section. One good supply conditioning section can be found in the BOSS blues driver circuit.
2. Noise generated at the input.
--Short the input to ground and listen to the circuit to verify that the noise is being generated by the circuit and not your guitar, or by the circuit's interaction with your guitar.
--Add a small capacitor (22-68 pF) in shunt to the op amp input. Distortion pedals can be used as AM demodulators, so you may be demodulating RF noise into the audio range.
--if the V/2 bias reference of the op-amp is very large, reduce it to about 10k and use an input buffer to compensate (I can't remember the schematics of those type, so this only applies if this is not in place)
--add a 47 ish pF capacitor in parallel with the clipping diodes to further roll off RF noise.
If this doesn't help... find somebody to re-design the circuit.
You will note that the Dist+ lacks any sort of high-frequency limiting cap in the feedback loop of the op-amp.
Ask yourself this: Do you REALLY need bandwidth beyond 8khz in this pedal?
Quote from: Jim JonesI've made some changes to the basic circuit including having a fixed resistance to ground from inverting input, and the gain control is now in feedback loop. This works better for my purposes - I don't like the "brighter as it gets gainier" nature of the original version
I mention this now and again but nobody ever seems to pick up on it! With circuits that limit bass as gain is increased you can disconnect the diodes then reconnect them via a series cap. If you select a value which has a slight reactance to the very lowest strings of the guitar there should be no discernable increase/decrease in low end as you turn the gain pot, but you get all the benefits of low intermodulation distortion associated with limiting bass in diode clipping circuits .
At low gain the diodes don’t/barely conduct so the cap doesn’t do anything, but as gain increases the diodes conduct more and more and the reactance of the cap starts to kick in, passing a small percentage of the lows onto the output unclipped thus compensating for the limiting action of the circuit.
Although it cleans up the low end a bit you can compensate with a few tweaks here and there. It also helps to retain some of the natural character of the guitar when heavily clipped.
I used to do this all the time and it works really well. If you’re canny you can incorporate a pot into the scheme of things to dial in the amount of low end (probably the best option as different guitars will give different results according to pickup type).
Hi Gez,
That's interesting stuff - I'll trying playing with that cap when I get a chance. I have to say I have no complaints about overall tone for my tastes with the gain pot relocated to the feedback loop and the cap-to- ground from the non-inverting input upped to 0.47uF.
Mark,
I have a 220p cap parallel with the 100K gain pot in the op amp feedback loop - I forgot to mention that earlier.
Transmogrifox,
I've tried many of the things you mentioned in your post, although I haven't shorted out the input like you suggested. I'll give that a try but I'm pretty sure it's just a noisy mother and that's just the way it is. :)
Thanks all!
Jim