Parametric Mids Frequencies?

[milothicus]

I'm hoping to recreate the tone stack out of an SWR working man's 15 amp because I liked the flexibility of it when i had one a few years ago. The amp was just too heavy to carry around for rehearsals, so i moved on. I'm now considering using the SWR circuit as the basis for a preamp pedal, and I also just want to learn.

The mids portion of the tone stack is shown here: https://imgur.com/a/KcjzPBt

I used circuitlab to model the entire tone stack, and the frequency response shows the mid peak moving from about 750Hz to 1kHz with the frequency knob.

The label on the original amp indicates a sweep from 200 to 800Hz.

It's a big difference that I can't explain. Undocumented component changes? Interaction with other parts of the circuit? Fudging of values on the panel?

Any thoughts?

[pdf64]

Perhaps the front panel was designed and committed to before the amp circuit was quite finished?
A massive mid sweep seems great in theory, but perhaps in practice it was excessive and became a hair trigger when trying to find the sweet spot.

[eschertron]

I had a Hartke Kickback bass combo for a while, There was a notch filter that swept at least from 200Hz to 800Hz. I usually set it somewhere between 300Hz and 500Hz to clean up the muddy low-mid freqs. I'd think 750Hz to 1kHz would be an unattractive range for a mid control on a bass, so I believe 200Hz to 800Hz (-ish) seems reasonable.

The input and output impedances will have a a great deal of effect over the tone stack's operation. Did you take that into account when modeling the cicuit?

[milothicus]

Thanks for the responses. here's my project on circuitlab: https://www.circuitlab.com/circuit/h...r-mid-control/

I tried to have the model include enough components, but maybe i need to include more of the circuit? To my eye, the frequency control is fairly well isolated from the rest of the circuit.

[vintagekiki]

Your problem was solved by Trace Elliot
https://www.basschat.co.uk/topic/294481-what-does-the-trace-elliot-pre-shape-button-do/

https://www.basschat.co.uk/topic/65862-trace-elliot-mid-pre-shape-eq-debate/

http://hpbimg.someinfos.de/diy/allgemein/_schematics/amp-schem/trace-elliot-GP7-100w.pdf

[uneumann]

Thanks for the responses. here's my project on circuitlab: https://www.circuitlab.com/circuit/h...r-mid-control/

I tried to have the model include enough components, but maybe i need to include more of the circuit? To my eye, the frequency control is fairly well isolated from the rest of the circuit.

Interesting circuit. In my test the range is around 200-800Hz. See the curves below for 4 steps of boost and 5 steps of freq control. It appears that Q varies with the freq setting.

[milothicus]

Interesting. So it looks like more of a circuitlab issue, or something in my model. Is yours done in some version of spice?

[vintagekiki]

About SWR Parametric EQ

https://el34world.com/charts/Schematics/Files/Swr/Swr_Schematics.htm

https://www.schematicsunlimited.com/s/swr

https://elektrotanya.com/showresult?what=SWR&kategoria=&kat2=all&page=1

https://www.talkbass.com/attachments/swr-studio-220-pre-pdf.379952/

[uneumann]

Interesting. So it looks like more of a circuitlab issue, or something in my model. Is yours done in some version of spice?

Yes - LTSpiceXVII. I just copied your circuit.

Edit - I just noticed that you have the top op amp + and - inputs reversed. Strange, the simulation works either way, but I don't think the real circuit will work the way it's drawn.