Help simplifying a circuit

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Silesius
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Help simplifying a circuit

Post by Silesius » Sat Feb 08, 2020 4:13 am

Hi, I'm working on a Benjolin type toy, and the idea is to put a delay after it. I'm using the Scott Bernardi circuit but, since it doesn't have cv control over delay time, I have added this.
Pt2399 CV.jpeg
The problem is now the circuit needs 5 opamps instead of 4. I'm designing a pcb, and I would like to get by with only one tl074 if possible. So now I'm wondering how to get rid of one of the opamps in the Bernardi circuit, but I'm just a self taught hobbyist and this is beyond my level.
pt2399dly.png
As far as I can see, the two opamps in the left act as mixers, blending the dry signal with the delayed one. The top one is used to achieve feedback and the bottom one is used to balance the dry signal with the delay. What I haven't seen in other (simpler) circuits are the two opamps in the right side. Are they acting as some kind of recovery circuit after the rc filters? Are those filters really needed (there's several other pt2399 circuits that don't use them)?
Can I get rid of the voltage follower opamp after the first rc filter by connecting the 1k resistor (with the clipping diodes) to the 1uF cap? If this is not a good idea, what other approaches can be taken?
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guest
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Re: Help simplifying a circuit

Post by guest » Sat Feb 08, 2020 12:33 pm

yes, you can either get rid of or modify that first lowpass filter stage. the very first thing the signal hits in the PT2399 is an opamp acting as a lowpass filter. its a second order MFB lowpass filter, and you could add one stage of RC lowpass filtering before it if you wanted to (use 1k resistor and 1nF cap to start with, but perhaps modelling it to get the right response would be good).
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Synthiq
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Re: Help simplifying a circuit

Post by Synthiq » Sat Feb 08, 2020 7:44 pm

The purpose of the lowpass filter at the input of the delay is probably to remove higher harmonics when the signal is clipped to prevent aliasing in the clocked delay line. You can achieve similar, but not identical, results without the opamp driving the input of the delay line if you modify the component values in the passive lowpass filter after the diode clipper and the PT2399 input stage. Below are a couple of results when I compared the original circuit to a modified one. The top of the schematic is the modified circuit and the bottom the original. The components marked E1 to E3 are voltage controlled voltage sources and are used as opamps with a gain of 100,000.
Silesius1.gif
This is the frequency response of the two circuits and there are some differences but still much smaller than removing the filter altogether.
Silesius2.gif
This is the response for a 1kHz sinewave with 5V peak amplitude. I had to tweak a couple of capacitors (C12 & C13) to reduce the difference between the two waveforms.

The suggested values are just one set of components and there are many other combinations that can give similar or better matching to the original circuit so don't be afraid to experiment.

Edit: The 1uF coupling capacitor at the input of the PT2399 is still needed but wasn't included in my schematic as it doesn't affect the high frequency behavior.
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Silesius
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Re: Help simplifying a circuit

Post by Silesius » Mon Feb 10, 2020 3:49 am

Many thanks for your answers, Guest and Synthiq, and for the work Synthik has done with the simulation. I really should learn how to use this kind of software.
Now I'm trying to add the modified filter without the buffer (the top one in Synthik's schematic), but i'm having some trouble understanding it.
If I understand it correctly, I should add the 1uF cap after the 100k/220p node before the opamp in the original schematic (33k/2,7n in the modified one), am I right? And then I add the part inside the "delta t", which begins with the 22k resistor. I think I get it so far, but what confuses me the most is the voltage sources and that "out" label. Where am I supposed to connect pins 16 and 15 of the pt2399? My guess is I the "out" goes to pin 15 and pin 16 goes to the other side of the 560/220p cap, as per the original schematic. Sorry for the stupid questions, but I have never seen that voltage sources in schematics before... I just want to be sure because I have my pcb's almost finished and I want to do it right.

Synthiq
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Re: Help simplifying a circuit

Post by Synthiq » Mon Feb 10, 2020 10:53 am

Correct, you add the 1uF capacitor after the 33kohm/2.7nF filter and before the 22kohm resistor (R15). The components to the right replaces the components in the delta t block in your original schematic so 6.8nF cap becomes 2.7nF (C6), the 10kohm resistor to pin 16 becomes 18kohm, the 560pF capacitor between pin 15 and 16 becomes 220pF and the 22kohm feedback resistor from pin 15 becomes 68kohm. The component E12 represents the opamp inside the PT2399 with the inverting input connected to pin 16 and the output to pin 15 so this isn't a component you need to add.

I used the free LTSpice to simulate the circuit. If you have never used a circuit simulator it may be a bit of a threshold to learn how to use it, but after that it can save you a lot of time compared to built a real circuit and replacing components until you get the function you want.

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Re: Help simplifying a circuit

Post by mrand » Tue Feb 11, 2020 12:57 am

Just a quick note to say that Rod Elliott has some great coverage of the pt2399, including a much simplified version of the usual circuit.
https://sound-au.com/project26a.htm
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