Author Topic: How do I modify a delay pedal's minimum delay time range?  (Read 421 times)

nashville

How do I modify a delay pedal's minimum delay time range?
« on: December 12, 2019, 09:06:46 AM »
Hello!

I have an analog BBD delay I like a lot, but about a third of the sweep is made up of delay times too short to be useful.

How would I modify the range of times to omit these shortest times so the entire sweep of the knob is reserved for more useful delay times?

Is it as simple as adding a fixed resistor to one of the pot lugs? How would I calculate the right value to use?

Thanks!

thetragichero

Re: How do I modify a delay pedal's minimum delay time range?
« Reply #1 on: December 12, 2019, 09:18:01 AM »
are you aware that any time longer than the current max delay time is highly likely to sound like doo doo because of the limitations of analog delay circuitry?
if you're happy with the current maximum delay time but want a more useful sweep a change in pot taper could possibly provide that. without a schematic we're guessing blindly here, though

j_flanders

Re: How do I modify a delay pedal's minimum delay time range?
« Reply #2 on: December 12, 2019, 10:09:22 AM »
Depends on the pedal. Delay time is often set with a resistor and a capacitor on the clock chip.
The minimum delay is often set by a fixed resistor.
In series with that resistor, there's a pot (set up as rheostat) to add additional resistance for longer delay times.
Minimum delay time = fixed resistor
Maximum delay time = fixed resistor + maximum resistance of the pot.
If you add a fix resistor to that you'll also add to the maximum delay time.
A larger fixed resistor and similarly smaller value pot would work.
You could add an additional fixed resistor in series and work out a resistor value to put in parallel to the pot (to reduce its maximum value and to make up for the extra fixed resistance).
Or just add an extra fixed resistor and don't use the longer delay times but you may find yourself with the opposite problem: some portion of the delay pot occupies unusably long delay times.
Or as stated above: use a different taper pot, to move out of the unusable range faster, to reduce the unusable range to a smaller range on the pot.
« Last Edit: December 12, 2019, 04:14:07 PM by j_flanders »

Mark Hammer

Re: How do I modify a delay pedal's minimum delay time range?
« Reply #3 on: December 12, 2019, 01:25:01 PM »
Like j_f said, it will depend on the particulars of the design.  For instance, here is the schematic of the Boss DM-3.  It's a reasonably "standard" sort of analog delay.  It uses an MN3205 and MN3102, rather than MN3005 and 3101, but that doesn't matter all that much.  Over on the upper right, we see the "Repeat Rate" control; what might be labelled Time or Delay in another different pedal.

VR3 sets the delay time, but it is shaped by two other components.  One is the trimmer RT3, that sets maximum delay time.  It would normally be adjusted with delay time set to maximum, and gradually trimmed back to the point where any clock noise is successfully managed by the circuit's lowpass filtering.  R53 is placed in parallel with one leg of VR3 to shape the taper and response of VR3.  Turn the wiper all the way over to the point they label as 17, and the 50k is placed in parallel with the 2k2 of R53 yielding a combined parallel resistance of 2k1.  The voltage fed through R55 is now divided by 2k2 (R55), and whatever the setting of RT3 is, plus 2k1.

Rotate the pot wiper fully in the other direction, towards point 18, and now R53 is bridged, such that the voltage coming from R55 is divided down by 2k2 (R55), plus 50k (VR3), over the value of RT3.

Following so far?  To reduce the range of adjustments that VR3 can produce, you might want to do any of a few things.  First, you might consider replacing VR3 with a 20-25k pot.  However, I understand that commercial analog delays might pose some difficulties in swapping pots, or simply finding one the same size.  So, alternatively, one can experiment with placing a fixed resistor in parallel with the wiper and point 17, to reduce its effective value.  And that might be supplemented by inserting a fixed resistor or trimmer between R55 and point 17.

This is simply one example.  I'm sure folks will come up with others.


ElectricDruid

Re: How do I modify a delay pedal's minimum delay time range?
« Reply #4 on: December 12, 2019, 04:01:12 PM »
Is it as simple as adding a fixed resistor to one of the pot lugs? How would I calculate the right value to use?

It's not quite that simple, but that's a good start.

If you need to eliminate a third of the pot's range, you need a value that is a third of the pot's value.

BUT (there's always a "but", right?)

Imagine the pot is 100K. So values below 33K don't really do much useful. So we stick a 33K resistor in series with the pot, and now the minimum value is 33K. Great! Success!...well, not quite. At the maximum value, we've now got 100K+33K = 133K, so the longest delay time got longer, and that probably means more clock noise, since the filters will be set for the original 100K value and we've increased that by 33%.

So the only way to do this without causing other changes would be to add a 33K resistor, but also swap the pot for a 66K pot (good luck finding one of those!). Hence the "not quite that simple".