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Short question

Is the circuit functionally equal, with the yellow switches always in 'closed' state? (Each yellow switch is a 2DP switch together with the switch left of it)

Background

I'm creating a guitar pedal Fuzz effect, based on Randofo's Instructable, see the picture. https://www.instructables.com/id/The-Fuzz-of-1000-Faces/

The red cross and resistor on the right (marked in red) is an 'enhancement' how the circuit should be.

The original idea is to use for the selection of the 2 x 6 switches (the combination of the yellow marked and the ones left of it) to use 2P6P rotary switches.

I want to use (analog) multiplexers, to control them digitally.

Current, I'm using CD4052's to handle 4 transistors by one CD4052, where one channel is just for the yellow switch, and one for the switch left of the yellow.

However, I'm wondering if the yellow switches are needed at all. Reason is that it doesn't matter if current flows into the transistor's collector, because if the transistor's base (same 'switch signal') doesn't get any current, the collector's signal will not be forwarded to the emitter anyway.

So, is the circuit functionally equal, to remove the yellow switches? (and assume it's a continuous path) ?

(as a side remark, someone gave me a tip to use analog switch array's which I'm probably going to use). enter image description here

Analysis

Because of Andy Aka's useful comment to try Micro-cap 12 which I installed, I did some analysis, but I don't understand why the results are as they show.

I made the following (minimalistic) circuit:

enter image description here

I set V2 to DC 4.5 V, and amplitude 4.5V, to get values between 0-9V.

What can be seen is that the emitter of the transistor is 3.879V. In the Fuzz normally switch 2 and 4 are tied together (SPDT), similar as switch 3 and 5, but I want for analysis to control them separate.

When I however also connect switch 5, the result of the voltage of the transistor's emitters is still 3.879 V ... so far so good. And when I only select switch 5, I get 3.878 V.

So the result is that it doesn't matter if I remove switch 4 and 5.

However, when I tried NONE of the switches, so no collector is attached anywhere, it still shows 3.878 V.

enter image description here

So how is this possible? This would mean I don't have to connect anything at the collector? This seems strange to me.

Analysis 2

Now I simulated a full chain (using 1 capacitor, 1 transistor, 1 transistor, 1 capacitor).

The input I set to Amplitude 0.5V, DC 1V (to get a s ignal between 0.5 and 1.5V).

Transient Analysis

enter image description here

enter image description here

Dynamic DC

enter image description here

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    \$\begingroup\$ Have you got a simulator that you could try it out on Michel? \$\endgroup\$
    – Andy aka
    Apr 14, 2020 at 8:12
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    \$\begingroup\$ Take the plunge and download microcap 12 - it is now free and is superb and pi$$es on LTSpice IMHO. \$\endgroup\$
    – Andy aka
    Apr 14, 2020 at 8:39
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    \$\begingroup\$ There is a learning curve but, if there's anything that I would advise anyone in electronic design, it is to operate a decent simulator. \$\endgroup\$
    – Andy aka
    Apr 14, 2020 at 8:42
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    \$\begingroup\$ You need to be decoupling with a capacitor at the input. Just try the set-up in microcap with one transistor in each bank just to prove that you get signal continuity from left to right. No need to use switches. Run transient simulation or dynamic DC (I think you might be running dynamic DC at the moment). \$\endgroup\$
    – Andy aka
    Apr 15, 2020 at 8:34
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    \$\begingroup\$ Good luck - it is the right way to go (at least it's the way that I would go). \$\endgroup\$
    – Andy aka
    Apr 15, 2020 at 8:42

2 Answers 2

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What you should be getting when you simulate the basic circuit is this: -

enter image description here

I have just used a single resistor of 2 kohm at Q1's emitter and things look right. All the voltages and currents look good to me and I also did a quick check using a 1 kHz source at 10 mV peak and it's producing heavy distortion (as expected and required).

I think you didn't connect your 9 volt source.

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    \$\begingroup\$ It's so easy to change resistor values I just never use pots. See how quick I did it - you'll be at this speed in a few months. \$\endgroup\$
    – Andy aka
    Apr 16, 2020 at 18:44
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    \$\begingroup\$ One thing to note - my R2 value - I chose that so that the voltage at the collector of my_Q1 is at roughly half the supply rail. OK 6 volts is not quite middle but it's near enough. Stick with 2 kohm and see how much the DC output level changes when different transistors are used - it might cause asymmetrical clipping that might indeed be one of the "nice things" about this fuzz box i.e. it colours the sound differently. \$\endgroup\$
    – Andy aka
    Apr 16, 2020 at 18:54
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    \$\begingroup\$ Aha - I am a guitarist hence my knowledge of fuzz pedals and distortion. Hard to say if those differences matter realistically. \$\endgroup\$
    – Andy aka
    Apr 17, 2020 at 8:30
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    \$\begingroup\$ My site (original music): jamesoakwood.co.uk \$\endgroup\$
    – Andy aka
    Apr 17, 2020 at 9:54
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    \$\begingroup\$ Very nice music ... I'm listening while I'm working now :-) \$\endgroup\$ Apr 17, 2020 at 10:09
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Using Andy's proposed application MicroCap 12, I got to a few results:

  • In the last capacitor column, closing all switches after the capacitor does not make a change.
  • In the second transistor column, closing multiple collectors make a change, around 0.05V), but changing transistors also does not make a change, so I will not take that risk.
  • In the second transistor column, closing multiple gates make also a difference, in the same range as closing multiple collectors.
  • In the first capacitor group, closing all switches after the capacitor, change around 0.2V, so also no go.
  • In the first capacitor group, closing all switches before the capacitor, does not make a change.

Conclusion:

  • Transistors cannot be connected together (neither base nor collector).
  • Capacitors can, but only before the first capacitor column, and after the last capacitor column.

Some pictures how the eventual circuit will be when the first of each column is selected:

enter image description here enter image description here

Important:

The transient limits, I set to - Maximum Run Time: 4 ms - Maximum Time Step: 0.001 ms

If the Maximum Time Step is set longer, V12 will change in all cases.

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