DS-1 resistor R39?? What's the tone change it can do?

[markv]

I was wondering about this R39, (the most - not talked about) part of the DS-1.

Can anyone here tell me what is this for? Can this improve the tone when used via specific value of resistor?



Thanks in advance!!!   

[5thumbs]

I don't know...yet.  I'm working on mod that, amongst other things, replaces R39 with a jumper wire.

My gut feeling is that R39 and R40 are related to setting the feedback voltage for the op amps in the post-1994 DS-1.  The pre-1994 DS-1 does not have R39 or R40, but it has a different op amp than the post-1994 pedals.  These resistors are right next to the input/output of halves of the M5223AL dual op amp in the post-1994 DS-1.

Robert Keeley reduces R39 from 47K to 20K in his DS-1 mods, but offers no explanation as to the effect of that change.  While testing my mod, I plan to test with R39 removed, as well as with a pot wired as a variable resistor in R39 so i can tune in the values I want and also explain what this resistor does.  I also plan to do the same with R40.

If I don't post back here with the results, please send me a reminder in e-mail or PM.  Thanks!

[tcobretti]

I don't know anything about this pedal, but the easiest way to find out what it does is to sub in a trimpot with two to four times the value of the normal resistor.  Then tweak away, and once you have a sound you like, pull the trimpot and measure its resistance.  Then plug in a replacement resistor with the same value as the trimpot setting you liked.

[miqbal]

In visual term, you can simulate the circuit using LT SPICE:

http://www.linear.com/designtools/software/index.jsp

[5thumbs]

Now, for something completely different...a status update on my R39/R40 tests!

I had to pull R39 and R40 as a part of a DS-1 mod I'm working on.  I didn't analyze the results with anything other than my ears, but I was able to hear audible differences when putting a pot wired as a variable resistor in these positions.

I had a B100K pot wired in for R39 and found it to smooth out the upper mids a bit.  When zeroed out, there was a noticeable "yang"-ey characteristic to chords, particular A chords played in X-0-2-2-2-0 position.  Turning the pot up to 100K smoothed out these mids too much, causing a loss of articulation.  It was about half-travel on the pot (just about the 47K BOSS picked for the R39 resistor) that the balance of note articulation versus smoothness seemed to have the best balance.  To my ears, BOSS got that resistor value right on the money...47K in R39 sounded best to me.

As for R40, I did the same procedure with R39 in its original configuration.  At anything below 1K, the whole circuit would become inaudible.  (Not surprising, because you would have essentially opened a path between +9V and your audio path.)  Turning up the resistance above 1K didn't have much audible difference.  The biggest difference with R40 came with removing it completely.  With R40 removed, there was a drop in the midrange gain, or to put it differently, the clarity and midrange "fatness" dropped off considerably.  So once again, I think BOSS made the right call having R40 at 1K with the M5223AL chip.

Of course, your mileage may vary, but that's what I found out in my tests this morning.   

[anotherjim]

While looking into modding my own M5223 fitted MIT Ds-1, I came across the 1k R40 in the schematic and thought "WTF"?

Why would you want a pull-up on on op-amp output? The Mitsubishi data sheet for the 5223 has the answer. It's there to force the output stage of the amp into class-A operation, otherwise you get crossover distortion. So if you like x-over distortion take it out - but I don't think many people do (like it that is). The 5223 has some non-standard op-amp circuitry as it's intended for single supply and handle swings down to ground. If you were to replace the amp with a more common bi-polar amp such as the TL072, you should probably remove R40.

Oh and R39? I think power-up/down could cause a damaging negative voltage on the amp input. D8 limits that, but R39 then keeps the discharge current from C5 low. Where it is, R39 won't affect the tone at all.

[tubesimmer]

...With R40 removed, there was a drop in the midrange gain, or to put it differently, the clarity and midrange "fatness" dropped off considerably.  So once again, I think BOSS made the right call having R40 at 1K with the M5223AL chip.

Of course, your mileage may vary, but that's what I found out in my tests this morning.   

Old thread, I know; I wanted to tie the last 2 posts together though.

As anotherjim pointed out, the 5223 datasheet indeed indicates a pullup resistor of at least 3k at the O/P is needed to get the bias current up to class A.
The interesting thing is, the midrange is affected quite substantially with the pullup in the circuit, as 5thumbs found.

I found this to be true not having read this thread (or seen the datasheet) yet, using a 1MEG pot in place of R40.

The midrange warmth is most affected at lower distortion levels, and also changes the current through clippers D4 / D5.
Basically, less bias current results in less midrange warmth.

I put the 1k back in, as I liked the warm tone better - even though R40 sucks 3mA+ current.
(Looking at the datasheet graph, I suspect R40 @ 4.7K would work as well, and suck less current - did not test.)

[PRR]

http://home.hccnet.nl/w.geeraert/pdf515/M5223L.pdf  page 5

This is NOT an "audio" opamp. It is for motor control. It is more important (sometimes) to be able to swing a motor load all the way to common than to fret about a glitch on rapid zero-crossing.

The LM324 has the same feature/fault. In most low-Z work we "need" to add a resistor (pull-down was suggested in the original poof-sheet) or use a better chip. I was astonished that some pedals use '324 for audio without a resistor-- apparently the AB range covers typical pedal impedances.

The '324, and I suspect the 5223, were "popular" because they were cheaper than "better" opamps, back in the day when any opamp cost real cash.

[anotherjim]

I suppose R40 might change the tone a little due to it changing the source impedance to C9.

I am surprised R39 value should be so sensitive.
I would measure the 4.5v reference voltage at junction of R24 & R25 and then see what the first op-amp output voltage is (pin1) while adjusting R39 value. It should remain essentially the same as the 4.5v reference.

To be clear, this is the schematic?

[tubesimmer]

That's the schematic I worked from & matches my pedal version.
...but my DS1 is heavily modded.

Mods that affect U1B's tonal characteristics include:
1] C9 is 1.47uF to restore some bass post-5223 distortion.
2] 1nF cap across 100KB DIST POT
3]10K across R13 (C8 remains unchanged)
4] C10 = 12nF
5] R16 = 3.3k

The above changes were complete when the R40 test was done, with the results given in the previous post.

-NOTE-: these mods are a partial list in the context of many more elsewhere in the circuit.
I'm not recommending anyone try them outside of the context of the other mods not listed above.
Horrible, horrible noises that may scar the listener for life may result. Or not.

I should not be the one to do the R24/25 reference voltage test because I know a mod I have in place further upstream will affect Pin1 voltage.

[PRR]

> R40 might change the tone a little due to it changing the source impedance to C9

The opamp is <1 Ohm, except where crossover distortion upsets it. R14 at 2.2K will mask this; certainly 1K in parallel doesn't matter.

The crossover area may be very critical to the ear. The 1K R40 pull-up forces the output stage from class B (with ~~1.2V crossover glitch) into near-A (can pull up 3V perfectly clean).

We are in an under-documented corner of the chip's action. It might be interesting to just try a "good" opamp here.