[sdiy] TB-303 humming noise problem

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Mon Feb 11 15:15:54 CET 2013

The TB-303 is an old analogue synth with digital control and does have 
quite a high output noise level.

This can be particularly troublesome if you're applying a lot of gain 
to distort the TB-303 output for creative purposes.

When my TB-303 is not actually playing, the noise floor at the line 
output socket consists of:

1. white noise,
2. LED multiplex feed-through (buzzing that changes in timbre depending 
on what panel LEDs are lit or flashing.)
3. Tempo clock noise (another faint low-pitch buzzing sound from the 
tempo oscillator that changes in pitch as the tempo is varied.)

All of these are quite noticeable if you turn up the gain of whatever 
the TB is being fed into.  Once the 303 starts playing there are also 
all manner of low-level clicks and thumps due to gate and CV feedthrough 
in the less-than-perfect design.  All of which arguably contribute to 
the character of the instrument's sound.

The problem is also not unique to the TB-303.  I have 808 samples, 909 
samples and TR-606 samples all which show plenty of low-level buzzing 
tones from internal digital signals present in the noise floor.  These 
days modern design practices go a long way to reducing this kind of 
problem by using seperate analogue and digital ground-planes, seperate 
analogue and digital supplies, screening cans, etc.  However, digital 
noise from USB signalling breaking into analogue audio is still a very 
common occurance with PC audio interfaces.


On 2013-02-11 10:58, cheater cheater wrote:
> Paul and Gordon,
> no disc tants that I can find. It may well be that it's simply a
> built-in error, so I'm trying to figure out what places would be good
> to add bypass caps. Any tips?
> Cheers,
> D.
> On Mon, Feb 11, 2013 at 9:09 AM, Gordon JC Pearce 
> <gordonjcp at gjcp.net> wrote:
>> On 11/02/13 07:45, Paul Anderson wrote:
>>> My suspicion is that the LED is being driven using pulse width
>>> modulation, and that's what you are hearing. Can you post a link to
>>> the schematic? There's probably a capacitor nearby that is in a bad
>>> mood.
>> Not PWM but multiplexing, and you are hearing switching hash getting 
>> back
>> into the audio (possibly because something is dragging the chopper 
>> PSU
>> down).
>> Time to check those little disc ceramics, very carefully - now those
>> capacitors *do* fail, frequently.
>> --
>> Gordonjcp MM0YEQ
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