Caballero cab sim

From the vendor

Caballero cab sim

Apr 20, 2022 8 min read

Updated: Aug 22, 2022

I have a long secret relationship with cab sims. I'm talking about analog cab sims, the ones that aren't anything more than a very specific equalizer. I know that digital and IR cab sims exist and are very popular, and can account for other characteristics of a guitar loudspeaker, but those are both outside my interests and my capabilities. I think a glorified filter can be very effective indeed for playing without a speaker and a microphone, quietly or not, and surely its hardware, being simple and made of common parts, is surely more within the capabilities of any DIY enthusiast.

I've collected and simulated many circuits, some quite impressive, some very simple, some like the astrosim that manage a complex frequency response all within the feedback loop of a single op amp, but I think whether to keep the circuit simple or not, they missed the mark a bit. My favorite is probably the series of Marshall cab sims found in the JTM and JMP1 amplifiers and then copied by Blackstar in the newer HT amps.

While the following circuit is completely original, I've taken inspiration from all those sources, keeping the features I liked more and implementing them in my circuit. There's no point denying the usefulness of the sources in making this.

So, where do I start to make something more to my taste? For a preliminary analysis, I've checked the frequency response charts published by both Celestion and Jensen on their site. While those surely have to be taken with a spoon of salt, because little is known of the test conditions, I think they still represent both a good starting point and a good goal to achieve.

Since I didn't want to emulate one particular speaker, and good luck with that, I put many of them together to get an average picture that hopefully also removes some inconsistencies, both from the same maker and between them. This is a collection of some more or less randomly selected 12" Celestion speakers (g12h, g12t75, g12m green, blue, seventy80, g12 EVH, g12 cream):

I made this myself with photoediting. Instead the Jensen website offers a comparison tool itself, even if I'm not sure the result is better. You can still see the g12h in the background because it was used to scale to the same axes:

We can summarize the main common frequency response characteristics then:

• Bass roll-off under 100Hz at 10 (Celestion) to 18 (Jensen) dB/octave.

• for Celestion: slight slope from 0dB (reference) at 100Hz to +5dB at 1kHz

• for Jensen: a wide notch at about 400Hz, 5-10 dB less than 100 and 1k.

• for Celestion: a narrow notch of -5dB at 1.5kHz

• Both: a wide peak centered at 2.7-2.8kHz, about +12dB with respect to 100Hz, with Q of around 1.1

• Very steep treble rolloff starting at about 5kHz. Seems to be about 46dB/octave on the Celestions and 16-28 on the Jensens.

• High shelf above 10kHz at -15/-20dB

And here's my solution:

Given the summary, it should be easy to follow b