Dry/Clean Blend EQD Sea Machine

[lars-musik]

Hi!
Again a bass-related dry/clean-blend question.

Brian Madbean drew the nice schematic for the Gravity Wave/Sea Machine Chorus below.

Now I would like to build one that is also suited for bass. First idea was to implement one of the standard jfet blend circuits (you helped me greatly with adding one of those to the Zendrive).

But looking at the schematic I have the feeling the "Depth" potentiometer does exactly that and the buffered input signal (via IC1_A) has no real chance of losing bass frequencies whilst passing through the other two (why two?) opamps. Am I right?
   

[PRR]

> why two?

Preserves phase. The mixer "wants" to be inverter connection, so another inverter gets back to input phase. There's other ways to do this, but this is fine.

I see the direct path as flawless 2Hz to 160KHz, which is beyond any bass speaker.

[lars-musik]

Thanks very much, Paul. That is exactly what I was hoping for.

Of course, I would like to understand where you pinned the 2Hz. I cannot see (or calculate for that matter) the high-pass filter(s) involved. Could you point them out to me? That would be great.

[duck_arse]

R1 in parallel with R2 gives 500k, with the 100nF gives ~3.2Hz high pass, and C13//R18 gives another high pass at 1.6Hz, so Paul will need to say if he was rounding up or down (or that my calcs are carp). C12 across R17 means 1/2 the gain at 159ish kHz, low pass.

[antonis]

R1 in parallel with R2 gives 500k, with the 100nF gives ~3.2Hz high pass

It doesn't mind Duckie, but omitting R1 gives a HP at 1.59Hz..

Actually, R1, C1, R2 form a Pi (Π) network but you can safely disregard of R1 at your calculations..
(exactlly as in FW rectification filtering with reversed caps & resistors placing..)

If you like, you may consider R1 as the lower impedance of a voltage divider with IN impedance and then proceed with C1 & R2 HP filter..

[midwayfair]

Vary R14 to change the gain of the dry path, which is presently at unity. The depth is already the effect level. There's also already a depth control.

Duck, R1 is not preceded by a capacitor. It appears in parallel with R1 for the input impedance but not as part of an RC filter.

[PRR]

> R1 in parallel with R2

R1 does not load the cap.

I see 100nFd against R2 1Meg. I know that 0.01u (10n) against 1Meg is under 20Hz, so I said 2Hz. Yes, 1.59Hz, but as this is way beyond what we would need I didn't bother getting exact.

Some correction if source impedance is not real low, and ideally we'd account the >100Meg into the opamp pin, but again it didn't seem remotely relevant.

[lars-musik]

Thanks all for your help. Slowly, slowly my knowledge builds up due to your patient explanations.

[ElectricDruid]

In my head, C12 could even be larger, and not just for Bass. At the moment, there's gain applied for frequencies we *don't* want (like anything between 20KHz and the current rolloff around 150KHz - almost three octaves).

I'd increase the cap to roll the gain off above the audio range, and for Bass I might go to 1nF, which gives 15.9KHz top end. That's only the -3dB point of course, not a brick wall by any stretch. You could probably put 2n2 in without really hurting the top-end much, but personally I'd stay at 1nF max.

HTH,
Tom

[midwayfair]

At the moment, there's gain applied for frequencies we *don't* want

There's no gain. It's inverting at unity gain. The cap is only there in an abundance of caution.

In fact, all the op amp stages are unity gain in this. I'm not totally sure why Brian wanted the extra stage, but at some point I think he had a PT2399 design that was all non-inverting and he ended up with a signal increase.