PI-1 Question

[Joe Johnson]

Hi,
I built the PI-1 using non-VCO option. Twice in the article, Carl mentions that with this option you can have a problem with current spikes from the speaker disrupting the front end. I am experiencing this problem. Carl says both times to delay the chopping signal out of the sample period. How do I do this? Is there a component value to change?
Thanks for any help,
JJ

[Carl]

Re: PI-1 Question

I believe the gating pulse I used for the chopper is sufficiently far from the sampling region. You could be having noise problems from the 7660 supply chip. I've just built my 4th PI-1 board, to test out the new PCB layout, and this one has a pretty noisy preamp signal, that I did not see on previous builds. Still trying to track it down.

One thing you can try, is syncing the 7660 off the 555 pulse generator. Take a series 10k resistor and 1nF cap from pin 3 of the 555 to pin 7 of the 7660. Now the 7660 will run at half the 555 frequency.

I tried this a long time ago, never could get it to work, then last week it dawned on me... incompatible voltage levels. So the cap is needed to decouple.

- Carl

[Joe Johnson]

Re: PI-1 Question

Thanks for the reply. I'm sorry but its still not clear. Isn't the second gating pulse to the integrator occuring at the same time as the chopping signal no matter what the value of R45 is? Shouldn't the chopping pulse be delayed after the second gating pulse? Sorry if I'm just being stupid here.

Regarding the 7660 being synced to the 555 timer, I have slowed the pulse rate to about 100hz. Is it still ok to sync them with this slow pulse rate? Will the 7660 still operate correctly at this slower speed?
Thanks,
JJ

[Carl]

Re: PI-1 Question

Well, I just probed my current board, and the chop pulse isn't what I thought it was. It's much worse... a wide pulse with one edge at the end of the first sample and one edge at the beginning of the secondary sample. I will have to look at this some more... possibly use the VCO 555 as a monostable, triggered off the secondary pulse.

100 Hz is probably too slow for the 7660. You can try it, but might need boost up C3. You can eliminate C2, turns out it's not needed for the voltage multiplier hookup on the 7660.

- Carl

[Joe Johnson]

Re: PI-1 Question

Exactly! This puts it right where you don't want it. It needs to feed another 74hc221 really. If you use the 555, wouldn't you really need 2 of them? Doesn't the first one stretch the pulse and the second one trigger off the high to low transition of the output of the first to give a pulse moved out in time by the width of the output of the first monostable? I'm unclear how one 555 will do it. I wish there was an easier way to move it than to add that much more circuitry...I'm out of space
Thanks,
JJ

[Carl-NC]

Re: PI-1 Question

No, I think it can be done with one. Trigger off the secondary sample pulse, and just use the 555 to set the pulse width. As long as it occurs after the secondary sample (i.e., falling edge), it should be OK.

Let me see what I can come up with. Right now, my proto board has zilch audio, trying to debug that. When I get it working with VCO, I'll switch to non-VCO and figure something out. Hopefully it can be done easily, with existing PCB components.

- Carl