Adding an fx loop to a circuit

[Mustachio]

Howdy fellas , So I'm still working on my turkey day project and I just wanted to pick a few brains to hopefully better understand things.

So I've been redrawing an envelope filter and making changes and everything is looking good. I replaced the ca3080 in my layout with an LM13700 and am only using half of it. So I have another half and the added buffers the chip as at the end. I was thinking it would be nice to add an fx loop into the circuit and I'm wondering where should I insert the loop.

My thoughts where this , should I take signal at the beginning of the circuit before it hits any components and have that as the send, Or should I take the send from the second half of the OTA. Maybe tying pin 5 to pin 7 and pin 8 to the send and the return coming back into pin 14 . Will the OTA share the signal across both half's without bridging any pins (internally) ?

I looked at the mutroxx digi made since he added an fx loop. His was off a TL082 though. Which I could replace the opamp in my project with a quad and try to insert the fx loop there. I may etch a prototype tonight and then just do trial and error as to where the best spot to put a send and return. Was just hoping there where some rules as to where is best to take and insert the fx back in. I'd imagine say you where inserting a fuzz or overdrive into an envelope filter, You may want the signal going into the fuzz untouched and then inserted back in before it is filtered (whomp) . Thats my idea just not sure yet.

Any help ideas or suggestions on this or what I can use the second half of the lm13700 for would be awesome! thanks guys !

[Ronan]

If a fuzz is placed before an envelope filter, it can compress the signal a lot, leaving not much envelope dynamics to drive the filter.

With an fx send/return, the idea is that the envelope follower gets to sample the raw guitar signal so it can drive the filter correctly.

If you are going to put an fx send/return in there, and you want the filter to work on the fuzz tones, do it like digi2t in the mutroxx, the send is clean guitar signal, and the return gets fed to the actual filter.

You could use the unused buffer on one half of the LM13700, as an output (send) buffer using pins 7 and 8, (or 10 and 9) or use an opamp like in the mutroxx, or google "b. blender" for ideas. An extra opamp is probably the preferred choice, but the LM13700 buffer "should" or "might" work fine, depending on the overall schematic. I've never tried it myself.

The 2 OTA's and 2 buffer arrangements in the LM13700 can all work independently of each other, but an OTA has a very high output impedance and won't really drive any useful load itself, hence the inclusion of the output buffers. The output buffers can be directly connected to the high output impedance of the OTA and give you a low impedance output (like in the datasheet examples).

> "what I can use the second half of the lm13700 for?"

If you are doing say a voltage-controlled filter for example, you could try running 2 identical filters in series to get a more dramatic effect, or run a low-pass with a switchable high-pass in series to make bandpass, or you could make a state-variable filter to get bandpass and low-pass filters. The datasheet gives examples. Snow white auto wah is an example of an LM13700 state variable filter running on 9V.

[Mustachio]

Thanks a ton Ian! What you said makes a lot of sense and helped me understand more of what I'm trying to do here! Gonna make a board tonight and test out a few things.

Thanks again!
Jim