[sdiy] TB-303 humming noise problem

[cheater cheater]

Hi Richie,
I don't really mind the white noise, but the buzzing is pretty bad.
How can I mod the TB-303 to lower the level? Do you think any digital
parts need bypassing?

Thanks,
D.

On Mon, Feb 11, 2013 at 3:15 PM,  <rburnett at richieburnett.co.uk> wrote:
> 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.
>
> -Richie,
>
>
>
> 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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