Strange Noise

[SLP]

I did a test some time ago:

I've completely removed every IC from the SID-PCB, disconnected the SID-PCB from the powersupply and the Core. It was basically just a PCB with resistors and caps that was connected to a little amp.

There was the noise.

It got louder as I moved the cable that was connected to the SID-pcb at the one side and open at the other side nearer to the PIC.

[TK.]

What happens when all ICs are removed, and pin 27 of the SID socket is connected to ground?

Best Regards, Thorsten.

[SLP]

ok.I've completely removed the whole SID-PCB, powered the core and connected a 10*10cm piece of aluminium-foil to the amp-input. guess what. right. noise.  :(

The question that I'm asking myself is how does changing the MIOS-app on the PIC (testtone-app -> SID V1.74685) make the Core send out electromagnetic waves in an audible range?

[TK.]

...this is what I'm currently try to find out with your help...

Propably you misunderstood me. With connecting to ground I meant to connect the Vs (0V) pin with Pin 27 of the SID socket with a cable when the SID is removed. Thats a very easy, but interesting check, as this would tell me if the problem is more located in the pre-amp stage, or of it is an issue with the SID itself.

Another test which would be interesting: stuff everything, make sure that the testtone application outputs a proper sound, upload the MBSID V1 application again and send CC#7 to the SID in order to vary the volume. Do you hear differences at low/high volume?

Best Regards, Thorsten.

[SLP]

OK. you're da boss  ;D

What happens when all ICs are removed, and pin 27 of the SID socket is connected to ground?

nothing. noise.  :-\

[btw. I'll be in berlin from 25-29 june '07]

[TK.]

Ok - to summarize the findings of the last days:

1) there is digital noise when the MBSID firmware is running

2) there is no digital noise when the testtone firmware is running

3) the digital noise sounds like register accesses to the 74HC595 (in my ears...)

4) you still hear the noise, when the audio output of the SID is shortened

So, the conclusion is clear: as previously assumed, there is a problem with the transistor amp circuit. Maybe some kind of crosstalk between the 74HC595 signals and the amp circuit.

You don't hear this noise with the testtone application, as register accesses are only done during the initialisation phase. The MBSID firmware can access the registers periodically.

I guess that you already checked the transistor circuit visually, and you are not able to find an error here.

So, my proposal: desolder the components of this circuit, clean the PCB, solder them again

Thats propably the fastest solution.

Best Regards, Thorsten.

[SLP]

There's absolutely nothing wrong with my amp.

The TDA7052 is probably my favourite IC and the proposed circut is really idiot-proved.

(attachment)

It's the same  with my soundcard.

The only solution I see is to use optocouplers to connect SID and CORE Module and to completely shield the Cores.  :-\

Unbenannt.JPG

[TK.]

So, you are not using the small transistor pre-amp of the MBHP_SID module before connecting it to your TDA7052 based amp?

This could be the reason!

Best Regards, Thorsten.

[SLP]

The function of the small transistor behind the SID Output isn't to amplify the signal, but to save the SID chip from any damage

of course I used the transistor which is on the pcb cause I didn't want to fry it.

I connected the amp to J3, the audio-out of the pcb.

[SLP]

Is it required to use the CAN-pullup-resistor if I'm using V1.7303 with the old PIC (462 I think) ?

[stryd_one]

452? No.