Monday, July 03, 2017

It's oh so... noisy

Annual Apology

13 years it's been since I started this blog. Many things have changed since then but my desire to have a Yamaha SS30 in a rack mount with MIDI is undimmed and just as subject to my whims.

Anyway, 9 months since the last post and 11 months since the last update on the rewiring; where was I?

Bring the noise (down)

Last night I worked on locating the source of noise when no notes are being played. To begin with I confirmed that there is still an unacceptable level of noise but gave up trying to locate it in one place. My scope might show a specific signal but my ears discerned general bleed-through from many tones. Maybe all of them!

After a bit of messing about and reacquainting myself with the boards (it has been a while) I decided to be bold and disconnect the outputs from the G boards until I found the source if the problem. Eventually I concluded that not only are there tones bleeding through every output on every G board - to a greater or lesser extent - but the noise is also present on the earth for each output too!

Once I had disconnected all the outputs and all the corresponding earths the output noise-floor dropped to a more acceptable level. I would like to able to put a figure on the noise before or after but I a) don't need to when I can hear it without any trouble and b) can't be bothered with setting up the extra cabling etc and trying to find an app to do it. As I'm in the garage, as far from my studio as possible on my property, it would have to be a phone or iPad app - but I digress. With the SS30 volume control at 50% and the mixer gain at 0 and the fader set to 0dB I can hear noise. Once the G boards were all disconnected it was not silent, but the kind of quiet I would expect given the SS30 is old and the mixer is no spring chicken either. 

Reconnecting the earths for each part of each G board in turn I could hear the various components of the noise return. Even in the same board the sound of each part varied. This means that as the noise is getting on to the ground track it doesn't come through as strongly at the far end as it does the nearest node.

All of which is to say I have a serious problem. But what is the source? What is the root cause? 

The blame game

My initial, gloomy diagnosis is that the vast number of grounding capacitors (four per tone) are all to some extent leaky, and all that is adding up and causing the noise. 
This is an irrational fear though. Why should they be leaky and why should that mean the ground is noisy? There is a bit of a myth in the synth community about re-capping. Sure, tanatlum caps in the power supply should be replaced in case they short. And electrolytics can dry out or leak and I have had to replace one on my Roland MKS-50. However these caps are low voltage and connected to ground. If they shorted out the worst that should happen is that the audio would go down the plug-hole run to ground or be attentuated. So, I should probably not assume the worst.

Therefore, my second thought was that blaming the caps might not quite work as an explanation because it's not a likely explanation. But, there is a more fundamental reason why blaming these caps makes no sense. To understand why,  you need to understand the whole method of generating keyed, string-like tones from square-waves. 

 The tones start as square-waves from clock dividers. That signal is then put through a series capacitor. This cap achieves two things. Firstly the signal is DC biased by whatever voltage is on the other side of it and secondly the wave is shaped into charging curves as the capacitance charges and discharges. This how the square-waves start to become the 'spikey' wave-forms needed for string sounds. 

Offset VCA

The DC voltage on the other side of the capacitor is -7V when the key is up. When the key is down it is 0V.  That means that when the key is up the signal is all offset to less than 0V and the current is all flowing - more or less, as it oscillates -  in the negative direction. When the key is up the signal is centered on 0V so the signal flows half in the positive direction and half in the negative.

 The next device in the circuit is a forward biased n-p diode. When the key is up nothing should get through that diode. With -7V offset on the signal, nothing can get through. Bit at 0V the positive half passes through the diode. In fact the amount can be between -7V and 0V as the attack and sustain controls set the raye of change. This means that the amount of signal gettig through the diode is controlled by the keying circuit effectively making it a VCA and explains why the SS30 is partialy polyphonic. Pretty neat huh?

This is just the first of two diodes in series, with a grounding capacitor after each. Even if the capacitors had become short circuit nothing should be getting through the first diode until a key is pressed and the DC bias voltage rises to 0V.

When I probed the output side of a the first diode on a circuit on G4 I saw plenty of noise, but it seemed to be unrelated to the signal on the input. If anything it seemed to be getting in through the capacitors or bakc through the diode. From where though is unclear. On the diode I probed, the signal on the input side seemed to be safely under 0V, but maybe others are not so well behaved. 

If we assume that somesignal is getting through the diode it's either becaise the diode is not functioning properly or the -7V is not low enopugh and some positiev current is getting through. On the other hand, if the diode is really blocking the noise it must be coming though from the capcitors via ground. But how does it get there? On the input side of the diode is the keyting cictcuit. The offset signal is heading straihgt back to the K boards. Is it entering ground there? The K boards down't even have a 0V connection until the key is pressed down and connected to earth. The attack and sustain controls do feed in some voltage but what that is I'm not sure. They could easily be disconnected to check.

Capacitors are supposed to pass some signals to earth but the issue seems to be that whereas the earth is supposed to absorb this unwanted signal, it is instead passing it on. Maybe the earth being used is not grounded properly.

Next time I hope to have some answers. 

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