Page 1 of 1

2720-5 LFO and noise levels

Posted: Mon Jul 02, 2012 7:47 am
by crustypaul
I'm currently rebuilding an old Paia modular with a mixture of 4700 and 2700 modules and i'm having a bit of difficulty with the pair of 2720-5 control oscillator modules.

The LFO output on both were a little hit and miss until i replaced the 2N2712 in the LFO section of each with a 2SC945 and the 2N3391 in one of them with a BC548, accounting for the different pin out.
Now they both speed up and slow down okay, and the switching is working properly, but the peak to peak voltage changes from about 5v at the slowest speed to more like 7.5v at higher speeds. They both do the same thing even when i swapped out the transistors for originals from other modules so i'm assuming that this is normal?

The other issue is the noise sources. One of the modules wasn't outputting any noise and the other was working but the noise seems to be about 5v peak to peak. In the build guide it seems to say that the noise should be both 5v and 0.5v peak to peak at different points in the setup and testing section. I've tried swapping the noise transistor at Q3 in both modules, which got the dead one working again, but the noise level seems to vary wildly using different 2712 transistors let alone replacing it with a 2SC945. I've had anything from 0.5v to 8v peak to peak using different 2712's!

What kind of level should i actually be seeing at the noise output jack?
I'm assuming this must have something to do with the noise transistor gain, if so is there an ideal value?

Re: 2720-5 LFO and noise levels

Posted: Thu Jul 05, 2012 3:04 pm
by PAiA-Scott
The noise is usually about 0.5v and the control oscillator about 5v.

We used to grade the transistors supplied for the noise source. Some are hot, some are not, some are just right. They shouldn't have ticks and clicks in the sound.

It is normal the control oscillator amplitude changes some with frequency.

Re: 2720-5 LFO and noise levels

Posted: Tue Jul 10, 2012 6:09 am
by crustypaul
Many thanks for your reply.

I had assumed that the noise output should be about 0.5v to match the VCO output, but the variation in levels you get when changing noise transistor seems to be way bigger than i would expect.

Is it normal to see such a massive difference in output levels just from changing the noise transistor? A difference of 0.5v to 7-8v peak to peak, and anywhere in between, seems a bit excessive.

I guess if it is actually working properly its just going to be a case of trial and error to find a usable noise source.