Cgs synth

> I have built the kit but is not yet functioning 100% - more like 10%.
>
> An audio signal does come through each input, with a very low output,
> with the LP being the lowest (barely audible), BP the loudest. Freq
> pot has no effect. Res appears to be functioning. TL071 is doing it's
> job, there's just not much coming to it to actually amplify. I have
> double checked all component values and orientations, tracks, etc.
> Transistors check OK on diode tester. Note that removing the two
> non-output 2222 transistors has no effect on sound moving through the
> circuit, so I suspect that my problem is in that area.
>
> Any clues or ideas would be greatly appreciated! Thanks.

> the two non-output 2n2222s would be the differential pair. Those two
> transistors set the cutoff frequency. what happens if you connect a volt
> meter across the colloectors of these two transistors, then adjust the
> frequency pot. The voltage difference between the collectors (also
> across the diode string) should increase as the pot is increased.
>
> also, check the voltage at the center of the diode string. This should
> be around +6V and will only change a small amount as the cutoff
> frequency changes.
>
> check that the emitters of the 2n2222s (differential pair) are about
> -0.65V. make sure that the voltage at base of the 2n2222 on the left
> side increases a small amount as the frequency pot is increased.
>
> you can do all these tests with no audio input. These just check the
> biasing of the diode string. so, what results do you get?
>
> -ryan
>
>

> the two non-output 2n2222s would be the differential pair. Those two
> transistors set the cutoff frequency. what happens if you connect a volt
> meter across the colloectors of these two transistors, then adjust the
> frequency pot. The voltage difference between the collectors (also
> across the diode string) should increase as the pot is increased.
>
> also, check the voltage at the center of the diode string. This should
> be around +6V and will only change a small amount as the cutoff
> frequency changes.
>
> check that the emitters of the 2n2222s (differential pair) are about
> -0.65V. make sure that the voltage at base of the 2n2222 on the left
> side increases a small amount as the frequency pot is increased.
>
> you can do all these tests with no audio input. These just check the
> biasing of the diode string. so, what results do you get?
>
> -ryan
>
>
>
> betweenpie wrote:
> > I have built the kit but is not yet functioning 100% - more like 10%.
> >
> > An audio signal does come through each input, with a very low output,
> > with the LP being the lowest (barely audible), BP the loudest. Freq
> > pot has no effect. Res appears to be functioning. TL071 is doing it's
> > job, there's just not much coming to it to actually amplify. I have
> > double checked all component values and orientations, tracks, etc.
> > Transistors check OK on diode tester. Note that removing the two
> > non-output 2222 transistors has no effect on sound moving through the
> > circuit, so I suspect that my problem is in that area.
> >
> > Any clues or ideas would be greatly appreciated! Thanks.
>
>
> The CGS Modular Synth home page: http://www.cgs.synth.net/
>
>
>
>
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> Near as I can tell those all check out fine. These are the readings I
> got on the meter w/common connected to ground:
>
> Left transistor collector: 7.04 to 7.32 (freq pot low to high)
> Left transistor emitter: -.64
> Left transistor base: -22.6 to -.33

>
> Right transistor collector: 5.60 to 5.80
> Right transistor emitter: -.64

> Center diode string: 4.86 to 5.06

> Are you using metal can or plastic transistors? If plastic, they may be in
> backwards.
>
> As for the surge when you power down. It means absolutely nothing - the
> whole circuit is "unstable" at the point and anything could be happening.
>
> Ken
> _______________________________________________________________________
> Ken Stone sasami@... or sasami@...
> Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/
> <http://www.blaze.net.au/%7Esasami/synth/>>
> Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/
> <http://www.blaze.net.au/%7Esasami/>>
>
>

> Hi,
>
> please, note that I could be wrong about any of this, but if I had this
> problem, this is what I would be doing. I know troubleshooting is
> frustrating. Reccently, I spent a week searching a filter circuit for a
> very simple problem.
>
> I think we should look at this area a bit more. The reason, is that you
> get sort of the opposite results that I expect. If you are measuring
> left and right transistor as shown in the schematic, then the right side
> should be at a higher voltage than the left side as the pot is
> increased, for atleast most of the pot range. At minimum pot position,
> the left side might be a little higher than the right, depending on
> transistor matching. The diodes go from the right side to the left, to
> forward bias them, the voltage on the right has to be greater. This
> allows an adjustable current flow through the diodes, which causes them
> to act like variable resistors.
>
> I ran a simulation of the biasing section only. The results matched
> pretty well with my analysis on paper. here is some screen shots as the
> voltage 'V3' (basically pot setting) is changed from 0V to +15V:
>
> simulation circuit:
> http://www.sdiy.org/destrukto/temp/steiner-biasing.gif
>
> diode string voltage:
> http://www.sdiy.org/destrukto/temp/steiner-biasing_plot.gif
>
> left diff pair transistor base voltage:
> http://www.sdiy.org/destrukto/temp/steiner-base_voltage_plot.gif
>
> diode current (CV pot from 0 to +7.5V)
> http://www.sdiy.org/destrukto/temp/steiner-diode_current.gif
>
> that last plot shows the current through the diodes. only half the pot
> range is used, because beyond that, the biasing is no longer linear,
> which causes the diode current to no longer increase exponentially
> (exponential is what we want). On the diode string voltage plot; You can
> see the two voltage curves intersect, the point of intersection seems
> fairly dependent on the transistors and thier matching. There is a part
> where the diodes are not forward biased. My diode current plot shows
> that it still behaves mostly exponentially in that range, but I don't
> really understand how it does.
>
> now, on to your results. I didn't think before, but you measuring could
> effect the circuit behavior. not sure what the impedance of the volt
> meter is. anyway, I think there still might be a problem here.
>
> ken johnston wrote:
> > Near as I can tell those all check out fine. These are the readings I
> > got on the meter w/common connected to ground:
> >
> > Left transistor collector: 7.04 to 7.32 (freq pot low to high)
> > Left transistor emitter: -.64
> > Left transistor base: -22.6 to -.33
>
> the left transistor base doesn't seem right. the pot should go from 0V
> to +15V. My simulation shows that at 0V, the base voltage will be
> slightly negative, but for most of the pot range, it won't be. the
> collector voltage should be decreasing as the pot in increased.
>
> >
> > Right transistor collector: 5.60 to 5.80
> > Right transistor emitter: -.64
>
> this collector current increases as it should, but I expect a much
> larger change in the voltage. the emitter voltage looks good, so your
> transistors are turned on.
>
> > Center diode string: 4.86 to 5.06
>
> a little lower than I expect, but possibly there is a reason for that.
> maybe the volt meter effects it?
>
> ok, so make sure your CV pot is connected correctly (0V and +15v), and
> make sure the voltage at the pot wiper goes from 0 to 15v. sure that
> sound obvious, but I would double check it.
>
> I'll try to think of other possible problems, nothing comes to mind
> right now, but I'll think about it.
>
> -ryan