[sdiy] TB303 / Polysix VCO current mirror thingy.

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Fri Apr 6 10:33:45 CEST 2018

> Is there any downside to this cheap-n-cheerful alternative, Richie?

Two that I can think of:

1. The trigger threshold of the thyristor has a temperature coefficient. 
  So the trip point will vary significantly with temperature, whereas a 
decent comparator combined with a precision voltage reference should do 
much better in this regard.  (However, the AN299 paper that Neil 
mentioned explains how this temperature coefficient was purposely used 
to cancel out the approximately equal and opposite temp coef of a 
particular type of capacitor used for the VCO integrator.  So I guess 
the temp coef of the SCR is not always a downside!)   I suspect the 
additional diode-connected transistor Q24 in the TB-303 schematic is an 
attempt to compensate for the temperature coefficient of the Q25/Q27 
thyristor in some way.

2. Thyristors (SCRs) have a holding current.  Once the thyristor fires 
and starts to conduct, this is the value below which the current must 
then fall before the thyristor un-latches and ceases conducting.  The 
problem here is that the current from the expo current source must stay 
below the holding current of the Thyristor.  Otherwise the Thyristor 
will fire, and then the charging current from the expo current source 
flowing through it will be sufficient to keep the thyristor latched 
permanently.  This imposes a limit to the upper operating frequency with 
a given integrator capacitor.  The TB-303's VCO simply stops oscillating 
if you try to drive it much above it's designed pitch range using an 
external CV.

Despite these potential disadvantages, I still think it's a neat bit of 
cost engineering.  Clever Japanese :-)


On 2018-04-05 23:01, Tom Wiltshire wrote:
> Lol! Exactly!
> Is there any downside to this cheap-n-cheerful alternative, Richie?
> T.
>> On 5 Apr 2018, at 19:09, Richie Burnett <rburnett at richieburnett.co.uk> 
>> wrote:
>> Yes. Cost reduction in line with the rest of the Tb-303 design ethos.
>> -Richie,
>> Sent from my Xperia SP on O2
>> ---- Tom Wiltshire wrote ----
>>> So this transistor arrangement replaces the comparator+hysteresis 
>>> that you’d see on a typical ramp osc?
>>>> On 5 Apr 2018, at 14:41, rburnett at richieburnett.co.uk wrote:
>>>> The back-to-back arrangement of NPN and PNP transistors in the 
>>>> TB-303 oscillator forms a Thyristor (SCR).  Once the voltage is 
>>>> sufficient to turn on one transistor, each of the transistors in 
>>>> this pair provides the base current for the other.  So the pair of 
>>>> transistors remain latched on, until the capacitor is recharged.  
>>>> Once it's recharged there is no current flow, so the SCR drops out 
>>>> of the latched state and the discharging ramp commences again.
>>>> -Richie,
>>>> On 2018-04-05 14:26, Gordonjcp wrote:
>>>>> If you look at the VCO in a TB303 or in a Korg Polysix they appear 
>>>>> to
>>>>> use a kind of current mirror arrangement with two NPN trannies, and 
>>>>> then
>>>>> a PNP tranny with its base wired to the collector of one and its
>>>>> collector to the collector of the mirror through a diode.
>>>>> There's a capacitor - to the "negative" rail in the 303 (really 
>>>>> ground
>>>>> but that's referenced to the +5.33V suppy) or to the "positive" 
>>>>> rail in
>>>>> the Polysix - which charges up and is discharged through the expo
>>>>> converter.
>>>>> At some point for a reason I can't entirely figure out, as the 
>>>>> capacitor
>>>>> discharges something in the current mirror conducts and it charges 
>>>>> again
>>>>> briefly, but I don't get why.
>>>>> Can anyone shed any light?
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