Power Supply Modding
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Power Supply Modding

by Nordcore on Sun Jul 30, 2017 5:42 am

There was always a lot of talking about the "unstable" power supply of the 303 circuit design.
Some people even think that there is a magic component, the "sumida coil" in the original 303, which they believe as necessary to reproduce the original sound.

Doing some thinking about the circuit, some Google searching about what others had already found out, and some trials on my x0x, it gets pretty clear what is happening.

First we need the schematics:
psu.png (27.52 KiB) Viewed 227 times

The 9V goes into the voltage doubler, so we get round about 11V at C3 and about 24V at C5. As the current from the higher voltage is much lower (about 20mA) than from the first stage (some 200mA), this is not exactly doubled.

For the 5V and 6V we see pretty boring standard regulators.

For the analogue supply it gets more interesting:
First there is R1/C8 acting as low pass. Corner frequency is 16Hz. That is reasonably low to filter out any high frequency noise. Which *we* don't have here in our x0x-es, but which might be there in an original 303 supply, where this point is supplied by a switching DC/DC converter.

See 303 schematics:
303psu.png (50.95 KiB) Viewed 223 times

Within the 303 DC/DC converter there is a regulation, consisting of D42 and Q42. This regulates the voltage at R172 to about 15.6V.

Filtered by R1/C8 this 15.6V delivers the supply for IC16, an OP-Amp (ab)used as voltage regulator. First there are the 5.333V, used as virtual ground and as reference for the pitch CV. There is a proper reference diode (D43) and a trim (TM6) for the 5.333V, where this voltage is set to its exact value.
The 5.333V is than multiplied by (1+R178/RR179)=(1+2200/1800)=11.85V, giving the "12V" supply for the analogue voltage.

So, where is the difference?
Up to the description above the only difference is the height of the supply for IC16.
As the current draw from the analogue circuitry is about 10 to 20mA, there is 1 to 2V voltage drop across R172 and we have about 14V supply for IC16 within the 303 and about 22.5V within the x0x.
As the AN1458 (Note: AN6562 is the same, Panasonic has used both numbers for the same chip) does a pretty good job at regulation, there is no difference in the output voltage.

Until we consider component tolerances *and* the minimum difference IC16 needs between supply and output voltage.
a) The AN1458 needs 1.5V minimum voltage difference between supply and output, else the output follows the supply.
b) The 11.85V for the 12V are multiplied by 5% resistors from the (pretty exact) 5.333V.
c) the 15V Z-Diode D42 has another 5% (or so) Tolerance.
d) voltage drop across R172 is dependent of the current consumption by the analogue circuitry, and about 1 to 2V. The envelope generator draws a little more current during attack, as everything else is not fully constant - slightly more during a tone.

If, by chance of the tolerance of our assembled components in our specific 303, the 12V is a little on the high side, the 15V from D42 is a little on the low side, and the voltage drop on R172 is on the high side (during attack), than our 12V will see a small break in.

We will never see this on a x0x. Increasing R171 (to, as sometimes suggested, 220Ohms) does not change that, as there still is plenty of voltage to drop before IC16 might lose regulation.

To mimic this possible original behavior a different approach is needed:
a) add an adjustable regulator (e.g. LM317) between C5 and R1, so one could mimic any voltage tolerance from the original supply.
b) add a trim to R178/R179 divider, so the 12V could be adjusted.

I did that on my x0x.
One interesting point is, that adjusting the 12V in the range 5% resistors (all with regulation maintained) might give, there really is some change in the sound behavior of the x0x. So if you think "no two boxes sound the same" - here definitely is one of the sources.
The boring thing: most of that is "Accent Amount", if you adjust the settings carefully you could bring back a "higher 12V sound" back to the "nominal 18.85V" sound. All what really is left is an slightly increased setting range.

I'm still not sure about the 12V "dip" (regulation lost) sound. If overdone the attack gets weak, just add a little (which is a not very stable setting) and it might get a little more "living".
Last edited by Nordcore on Sun Jul 30, 2017 10:55 am, edited 4 times in total.

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Re: Power Supply Modding

by antto on Sun Jul 30, 2017 6:42 am

i don't know where the difference is, probably the current flows in different patterns and creates different kind of noise
after all, the PSU circuits are different

but i think the more important question is whether that matters at all

the 303 doesn't have a dedicated digital 5V, and this is one difference
since the LEDs are driven by a matrix, which frequently turns them on and off one by one (or in groups, doesn't matter)
i've heard that the overal noise floor of the x0x is lower than on the 303, and i think that's a good thing
but some folks think that the exact behaviour and level of the noise floor in the 303 is a desirable component

tbh i'm sick of argueing with those people, so i'll try to stay at a distance
beware that some of them may understand that this is all bullsh*t but might have interest in keepying the myth alive for profit

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Re: Power Supply Modding

by antto on Sun Jul 30, 2017 6:58 am

if we look at this differently

the PSU provides a few voltage sources to the rest of the circuit

so for each of those voltages, it has a given amount of current that it can provide, and this might also change under actual stress (usage)
additionally, it will have some noise, which may get worse under stress

now, the 303 analog section draws a little current normally, so things are quite calm
except when notes are played.. charging (and maybe also uncharging) the envelopes happens in a short time frame during gate-on and gate-off
but there are reservoir capacitors pretty much everywhere including in the envelope sections
in any case, this causes stress, specifically the gate-on trigger, since it's audible afterwards

the filter is vulnerable to stress on the 5.333V line (and maybe on the 12V line also) because it has a low-freq resonance point (about 8Hz) besides the normal variable cutoff frequency
so when you cause stress to the supply voltages, you excite (more or less) the low-freq instability of the filter (this is only valid when the resonance is actually in effect)
as a result, the 5.333V line begins to resonate

thing is, in some units, this resonation is small and decays out quickly
while in other units it's much bigger and may not even decay down but grow
when that happens not only the main cutoff freq of the filter starts "warbling" but also the VCO pitch starts going up and down

the instability can be adjusted by adjusting (via modification) the cutoff frequencies of the 4 or 5 highpass filters (capacitors) inside the VCF feedback path
so the people who think this "warbling" is a desirable effect - they could play with the caps and get their filter unstable
i wouldn't
in fact, my second x0xb0x started warbling (as well as having all kinds of other nasty side effects) after i (stupidly) poured nail-polish-remover all over it
the warbling means that your filter is getting unstable.. if you like it - good

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Re: Power Supply Modding

by Nordcore on Sun Jul 30, 2017 1:45 pm

Neither the 303 nor the x0x-es I had here did have any noise or any other strange supply behavior.
The supplies where always clean.

What I described in the the first post are two relative simple influences:

One, which every 303 and every x0x has:

That is the absolute height of the 12V power supply.
That has as an astonishing large influence on the sound. Nothing magic: just the modulation amounts gets larger.
That might make up for a very impressive sound difference between two boxes, if one box is on the upper end, the other on the lower end, of what ist to be expected from 5% tolerance resistors.
This could be explored very easily by adding a trimm to R178/R179.
(I just used a 50kohms 20 turn trim parallel to the voltage divider. Not fool proof, but works well if you adjust the trim middle before soldering and than only turn it a little with the DMM connected. )

The other sound influence is "starving the regulator", which can't happen in a x0x but *might* happen in some 303.
That requires the supply of IC23 being lower than 1.5V above the "12V" output voltage. As the voltage drop at R1 is about 1 to 2 V that leads to 11.85V+1.5+1...2=14,35V-15.35V at the C5 side of R1.
If there is more voltage, as in the x0x, the starvation will never happen.
If there is less voltage, the regulation would not work at all, leading to unusable behavior.

The sound effect could be easily tested by supplying an adjustable voltage to R1. As stated above I used a simple LM317 for that.

What you have observed on the 5.333V is interesting, let me think about that.
(For the first time in engineering we are trying to make a regulation less stable... )

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Re: Power Supply Modding

by antto on Sun Jul 30, 2017 3:13 pm

Nordcore wrote:(For the first time in engineering we are trying to make a regulation less stable... )

i'm not

but you're certainly not alone

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Please be positive and constructive with your questions and comments.