Yamaha DI-01/Korg DST-1 Distortion

[Rob Strand]

Inspired by this thread:
https://www.diystompboxes.com/smfforum/index.php?topic=133014.0

This post documents the Yamaha DI-01/Korg DST-1 Distortion.

The factory schematic is available through the Korg DST-1 service manual.
The part values on the Yamaha DI-01 PCB appear to match the Korg DST-1 schematic.
I've checked a lot of values but not all.

The schematic is missing resistor R5 which is clearly 1k on the PCB.  I don't have a pic of the PCB traces to check where R5 connects.  I've marked up the schematic here:



The component overlay from the service manual:



Top side of the PCB:



The pedal uses a panasonic AN360 amplifier chip. 

The easy way out would be to buy that chip, if you can find a real one!
The AN360 device contains a few transistors.  We could try to build a clone using discrete transistors.

While the AN360 internal schematic is available the part values are not.    Another point of uncertainty is the datasheet gives some specs but it does not give the test circuit used to produce the specs table.

Here's a summary of known info on the AN360:



We don't know what voltages the pins bias at in order to help determine the internal part values.   However, the AN360 was used on the old radios and cassette units and fortunately the service manual for these gave some test voltages.   That helps pin things down a bit:



At least one other circuit example had Rpin3 at 120 ohm.
So massaging the above info my here's my best shot at the internal part values for the AN360,
[EDIT:  see below for an updated R3 value and better bias point estimates.]



If we use those internal part values on Yamaha/Korg circuit we can estimate the bias voltages:



It seems the 1k resistor on pin 3 causes the output bias point to be below Vcc/2.

Another interesting point is the radio circuit adds an extra diode across pins 2 and 5.  I wonder what that does to the sound.  Perhaps nothing - I haven't gone any further to check.

[Rob Strand]

I was perhaps a little hasty collating the info I have.

A couple of things came to mind later:

If you look at the circuit from the Senator CR2392u Radio:
- The presence of the diode between pins 2 and 5 affects the bias.
  While the forward bias is small we are dealing with small currents.
- There is a 1.8k resistor from pin5 to ground.
  That also affects the bias.

To get another test point I dug up another sample circuit with voltages from radio circuits.  The circuit has the diode but doesn't have the resistor on pin 5.

SABA RCR404:


So massaging the values to match both radio voltages and the brief datasheet info, I suggest we modify the R3 on the AN360 schematic to a value around 3.9k to 4.25k.

When this is done the output of the Yamaha/Korg circuit biases to about 4.5V to 5.0V, which is right on VCC/2 (a bit higher OK as the the emitter of Q3 isn't 0V).

If I get time I'll update the test ckts.   I need check a few different diode models to make sure the diode current, due to the spice model, doesn't affect things too much.
Update bias estimates for pedal: