Caballero cab sim  [documentation]

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dylan159
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Post by dylan159 »

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):
celestion.png
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 background because it was scaled to the same axes:
jensen.png
We can summarize the main common frequency response characteristics then:
1.Bass roll-off under 100Hz at 10 (Celestion) to 18 (Jensen) dB/octave.
2.for Celestion: slight slope from 0dB (reference) at 100Hz to +5dB at 1kHz
3.for Jensen: a wide dip at about 400Hz, 5-10 dB less than 100 and 1k.
4.for Celestion: a narrow dip of -5dB at 1.5kHz
5.Both: a wide peak centered at 2.7-2.8kHz, about +12dB with respect to 100Hz, with Q of around 1.1
6.Very steep treble rolloff starting at about 5kHz. Seems to be about 46dB/octave on the Celestions and 16-28 on the Jensens.
7.High shelf above 10kHz at -15/-20dB
And here's my solution:
caballero1.2.png
Given the summary, it should be easy to follow because it's a logical and ordered implementation of it. The order of the stages isn't too critical, but I tried optimize headroom and noise both.
  • U1A is the input stage. Gives good input impedance and amplifies the signal a bit to give the right middle frequency response for points 2 and 3.
  • The mids control is a bridged T notch. Of course those are nothing new, but I liked this variable version found in the DSM cab sims because unlike others it can go from completely flat to standard notch while keeping the same center frequency. I iteratively changed the values to obtain a filter that had the right, close to 400hz center frequency while being narrow enough to only affect frequencies between the other filters (so between 100 and 1k). I found out that since notch depth increases together with bandwidth, using a stopper resistor with a deeper notch resulted in an overall wider bandwidth than a filter optimized for that notch depth. This takes care of point 3.
  • U1B is just a buffer to isolate the Sallen Key high pass filter that gives me the 12dB/oct cut under 100Hz as for point 1. This and the following low pass filters are something I liked from the Marshall circuits. I've tweaked the values for my desired cutoff of 96 Hz and a Q of 0.82. This with R13 shorted. When the switch is open, the filter becomes rather sloppy with a cutoff at 53 hz and Q of 0.45, but this is exactly what I needed to instead approximate the bass rolloff of the Celestion speakers. Other than that, I usually find cab sims to be a bit too bassy, also because the rolloff isn't as steep or even absent, and this gives me two good low frequency settings that are different enough.
  • The next two stages are two identical third-order Sallen Key low pass filters. These take care of point 6. The Marshall ones combined a second order with a third order, but I thought I could do a total sixth order rolloff since it sits nicely between the eighth-order-like Celestion and the fourth-order Jensen. The cutoff and Q have been again chosen iteratively to obtain the desider final response. Individually the filters have a cutoff around 5kHz and Q of about 1.14 but what matters is that they're stable and give the desired response. You can find alternative sets of values which will give a very similar results. I always try to use few and simple values, even if in this one you might find E12 resistors because I felt the extra accuracy was necessary. I hope that between the four alternatives you can find one that uses values you have at hand.
  • U2B is a gyrator based RLC series resonant filter. This one is responsible for the narrow notch at 1.5kHz at point 4.
  • U2D is a difference amplifier that does two things at once. It sums the output of the previous filters with the "shelf" path, taken directly from the first stage. This trick from the Blackstar cab sims lets the very high frequencies shelf at -22dB or so instead of descending into nothing. This will keep a very small amount of high frequencies, instead of the empty wasteland that would be there otherwise. This network feeds in the inverting input because the output of the three filters is overall inverted. The cutoff and gain are chosen so that this has very little effect on the rest of the response. Point 7 is dealt with.
    The second thing happening here is the gyrator based band boost in the feedback. This is responsible for the wide peak of point 5. I think this is a major and noticeable feature of the frequency response, and one that is often overlooked in other circuits, maybe implemented with a resonant low pass filter that gives a smaller and narrower peak. Since it's so important, I dedicated two controls to it. The peak one varies the center frequency, giving a wide range of sounds without invading the other filter ranges. The high control instead lowers the gain of the peak to almost the same as the middle frequencies.
A note I want to make is that the circuit is pretty flexible in the amount of controls you can have. you can exclude any of them and even make a version without controls! That's why for each I suggested a "stock" setting, but of course you can use any you prefer.
To show the results, I overlaid two preset responses on the previous graphs, so you can see how close or far they are from the target:
celestionsim.png
I think the resemblance with the celestions I chose is pretty good. The bass is on the loose setting, and I've shown the full sweep of the mids control so you can see that you can get everything in between.
jensensim.png
The Jensen similarity is still good, considering it was just an extra exercise for me to match two brands of speakers instead of one. Here the bass setting is tight and the mids are scooped. I think the main difference is that there's about 5dB more bass at 100Hz. I don't think I miss that, but if this is your main goal, I did have a gyrator band much like U2C but hooked up to the first op amp to boost that range in a previous version. I instead opted for the buffered mids control, but you can restore that and maybe delete U2B.
Then here's some nice measurements I made with ARTA. All is good while you're in simulation, but when you really enjoy your circuit you better check, or fool yourself you're hearing exactly what you wanted. I'll show the response of the controls that you can't see in the above graphs:
Bass. Loose and Tight (or Open and Closed. Do I mean the switch or the cabinet? Nobody will know)
bass.png
Peak extremes.
peak.png
High extremes.
high.png
In case you're wondering, the high frequency noise is from the messy breadboard being in between my monitor and PC.
Finally, let's talk about the headphone amplifier. One thing I liked about the astrosim is having one: it makes it a great portable practice tool for example, so I made my own. It's a pretty standard 5532 based headphone amplifier, using the second half as follower to boost the current output capability. It's more than adequate for this use, and misses being close to hi-fi mostly for being single supply and having a somewhat limited, by design, 15Hz-33kHz bandwidth. The maximum gain is 5.7 and I found it pretty loud, with a comfortable volume around half. Since you might be more confident with hearing damage or just have higher impedance headphones than mine, you can increase the gain by increasing R27, or decreasing R25 while increasing C22.

But how does it sound? Here's a quick demo of it. More details in the description.

Hope you enjoyed this. I'm always eager to know if someone tries the circuit and what thinks of it. Feel free to make a vero or even pcb layout for it under the CC license.

UPDATE: what I said above about moving the gyrator to better conform to the Jensen gave me an idea. Why accept compromises when you can have both? Think of this as a mod to the circuit: I'm simply doing what I suggested with a switch, moving the gyrator to do either the bass peak or the 1.5k notch. For reusing most of the existing gyrator the results are good.
caballero1.3.png
Here's the comparison with the Jensen speakers (again ignore the thin red curve of the base layer Celestion):
jennew.png
Anti-pop resistors are always good practice but they shouldn't be necessary since everything is sitting at Vref and there's practically no DC voltage potential that can develop across the caps.
Again, modularity is the word: if you don't want this switch, just wire it one way or the other. The values are standard either way.
Last edited by dylan159 on 09 Sep 2021, 18:57, edited 5 times in total.
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Post by Manfred »

Great Job! :thumbsup

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Post by asbestosaurus »

Awesome work, though I am a little confused by the high shelf returning to the inverting input at U2D

At what point between the output of U1A and the input of U2D is the signal polarity inverted?

It goes through an even number of non-inverting stages (1 buffer, 3 sallen-key). Is there just *that much* phase shift or have I missed something?

Love your work dylan159, Thanks for sharing!

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Post by dylan159 »

asbestosaurus wrote: 07 Sep 2021, 22:21 Awesome work, though I am a little confused by the high shelf returning to the inverting input at U2D

At what point between the output of U1A and the input of U2D is the signal polarity inverted?

It goes through an even number of non-inverting stages (1 buffer, 3 sallen-key). Is there just *that much* phase shift or have I missed something?

Love your work dylan159, Thanks for sharing!
Thanks for the kind words.
We usually think of SK filters (or passive RC filters for that matter) as noninverting because we're interested in the pass band, but each RC section can contribute up to -90° of phase shift, and in the end we're already at -180° at about 4k, again in phase at 7k or so and again -60° at 10k when we finally reach the frequency of interest. In practice this way works as intended, not affecting at all most of the rest of the response, and with very minimal ripple between 8k and 10k.
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Post by 287m »

NOOOO! Why now, after build dsm omni-ish in 1590B, you released this :|
don't think two quad fit in perf B case. Waiting verohero like fuzzhead or alex, and other.

how to add aux in and stereo output?

btw, great job.

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Post by dylan159 »

287m wrote: 08 Sep 2021, 10:53 NOOOO! Why now, after build dsm omni-ish in 1590B, you released this :|
don't think two quad fit in perf B case. Waiting verohero like fuzzhead or alex, and other.

how to add aux in and stereo output?

btw, great job.
Ha! :D I'm sorry. I had this going for a month but I was still breadboarding the EVH and I wanted to get everything straight. I don't think the omnicab is perfect and neither is mine, but it's pretty good nonetheless.
I'm always a bit optimistic with enclosure sizes, and while I sometimes regret it when cramming everything inside, I still plan carefully so I haven't wasted enclosures at least. I think with a PCB it's possible to fit it in a B or 125B, even without SMT, but for vero it's probably unwise.
About the aux in: there's many ways to do this but better watch out. Summing where the shelf goes in the last op amp with a resistor and cap might work ok but surely won't give a flat response. A less elegant solution is to passively mix with two 10k resistors at the output and buffer with a bjt. If instead you're only interested in aux in for the headphones as I was when I added it to the astrosim, the best way is to just feed it to the input of the headphone amplifier with resistors and caps (also for stereo to mono).
About the stereo output: there's nothing to make the signal stereo by having two different signals, so stereo output doesn't make much sense here. You feed the signal to a mixer or interface and then pan or double track as you prefer. I guess you can make a "dual mono" output by feeding the output to both channels, like for the headphones.
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Post by dumitrescu.florin94 »

Hi there,

I've tried this circuit and I can't seem to get past an issue: It is very fuzz-ish, especially with the Distortion channel on my amp. It's even fuzzy with the guitar plugged straight into the cab sim. I've tried adding a pot in front of the input, but it's dulling out the sound and I have to turn that pot down quite alot before I get rid of the fuzz. It's almost like the input or one of the stages is clipping. I can confirm that the signal I am using is not too hot (i have another cabsim and that one is working flawlessly and also, clipping with straight passive HB signal?) What I've tried is lowering the first bias resistor R2 from 1M to 220k, no effect. I've added low pass filter before the input (a 47k resistor with an 820p cap to ground) and I've also added a 47pF cap across the 8.2k R6. Everything else in the circuit is as shown in this schematic (I've made the first one, not the second one, with the switches on the gyrator, but I don't think that makes any difference anyways).
Any idea what might cause this fizziness to the sound? I don't have any sound clips, but I can post some, if needed.

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Post by dylan159 »

dumitrescu.florin94 wrote: 16 Sep 2021, 10:07 Hi there,

I've tried this circuit and I can't seem to get past an issue: It is very fuzz-ish, especially with the Distortion channel on my amp. It's even fuzzy with the guitar plugged straight into the cab sim. I've tried adding a pot in front of the input, but it's dulling out the sound and I have to turn that pot down quite alot before I get rid of the fuzz. It's almost like the input or one of the stages is clipping. I can confirm that the signal I am using is not too hot (i have another cabsim and that one is working flawlessly and also, clipping with straight passive HB signal?) What I've tried is lowering the first bias resistor R2 from 1M to 220k, no effect. I've added low pass filter before the input (a 47k resistor with an 820p cap to ground) and I've also added a 47pF cap across the 8.2k R6. Everything else in the circuit is as shown in this schematic (I've made the first one, not the second one, with the switches on the gyrator, but I don't think that makes any difference anyways).
Any idea what might cause this fizziness to the sound? I don't have any sound clips, but I can post some, if needed.
Hi. Thanks for trying it. I can confirm that the circuit as shown works as intended, I've tested it, no need to do any change. The gain is also modest, 4 times at most in any part of the circuit, so it shouldn't clip with a guitar signal even at 9v. No need to change the input impedance, low pass the input with large resistors or add tiny bandwidth limiting capacitors, although this last one does no harm. My guess is that there's a build error somewhere causing the bias voltage to get lost, causing premature clipping. Check all the voltages and the paths to Vref.
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Post by dumitrescu.florin94 »

dylan159 wrote: 16 Sep 2021, 10:16
dumitrescu.florin94 wrote: 16 Sep 2021, 10:07 Hi there,

I've tried this circuit and I can't seem to get past an issue: It is very fuzz-ish, especially with the Distortion channel on my amp. It's even fuzzy with the guitar plugged straight into the cab sim. I've tried adding a pot in front of the input, but it's dulling out the sound and I have to turn that pot down quite alot before I get rid of the fuzz. It's almost like the input or one of the stages is clipping. I can confirm that the signal I am using is not too hot (i have another cabsim and that one is working flawlessly and also, clipping with straight passive HB signal?) What I've tried is lowering the first bias resistor R2 from 1M to 220k, no effect. I've added low pass filter before the input (a 47k resistor with an 820p cap to ground) and I've also added a 47pF cap across the 8.2k R6. Everything else in the circuit is as shown in this schematic (I've made the first one, not the second one, with the switches on the gyrator, but I don't think that makes any difference anyways).
Any idea what might cause this fizziness to the sound? I don't have any sound clips, but I can post some, if needed.
Hi. Thanks for trying it. I can confirm that the circuit as shown works as intended, I've tested it, no need to do any change. The gain is also modest, 4 times at most in any part of the circuit, so it shouldn't clip with a guitar signal even at 9v. No need to change the input impedance, low pass the input with large resistors or add tiny bandwidth limiting capacitors, although this last one does no harm. My guess is that there's a build error somewhere causing the bias voltage to get lost, causing premature clipping. Check all the voltages and the paths to Vref.

Hi Dylan,


It does sound really good (when it's not clipping ofc.) and I think it would be a great addition to my existing cabsim (basically, I've built a tube amp, realized that living in an apartment building and having a big a** cabinet won't work, so I've build a cabsim into the amp itself). That one is a bit more bass-y and dark and this one seems very bright and presence-y. I bet they would combine quite nice.

Thank you for the tips for looking around the circuit. I've retraced every component and it is in the right place, every bias point shows right, I've even tested it on an oscilloscope with a 1Khz 500mV signal and it doesn't show any clipping, but, as soon as I plug in my guitar, it starts clipping. It's happening more with bursts of volume (like the moment I strum or pick the strings), and it subsides when the note is sustaining. The only difference that I forgot to mention is that I am using an LM324N op-amp instead of a TL074 (didn't have any at hand and I have about 20 ish LM324Ns) but I kinda doubt that would be a reason for this clipping to happen.
After poking around with the oscilloscope a bit more, I found that the output of the last gyrator (the one with the Peak and High control) is outputting a very distorted signal. Is this normal behavior for this gyrator?
I've attached a Screenshot of the output below. I need to mention that this distortion disappears when I turn the input signal way down.

Thanks again for your feedback. I'll keep looking around this
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Post by dylan159 »

dumitrescu.florin94 wrote: 17 Sep 2021, 09:14 Hi Dylan,


It does sound really good (when it's not clipping ofc.) and I think it would be a great addition to my existing cabsim (basically, I've built a tube amp, realized that living in an apartment building and having a big a** cabinet won't work, so I've build a cabsim into the amp itself). That one is a bit more bass-y and dark and this one seems very bright and presence-y. I bet they would combine quite nice.

Thank you for the tips for looking around the circuit. I've retraced every component and it is in the right place, every bias point shows right, I've even tested it on an oscilloscope with a 1Khz 500mV signal and it doesn't show any clipping, but, as soon as I plug in my guitar, it starts clipping. It's happening more with bursts of volume (like the moment I strum or pick the strings), and it subsides when the note is sustaining. The only difference that I forgot to mention is that I am using an LM324N op-amp instead of a TL074 (didn't have any at hand and I have about 20 ish LM324Ns) but I kinda doubt that would be a reason for this clipping to happen.
After poking around with the oscilloscope a bit more, I found that the output of the last gyrator (the one with the Peak and High control) is outputting a very distorted signal. Is this normal behavior for this gyrator?
I've attached a Screenshot of the output below.
Hi. That output doesn't look good. maybe you've restricted your problem. Anyway it seems like a big peak is throwing off your bias, which shouldn't happen but has happened to me in the past when I had miswired something. Also worth checking for oscillation with the input grounded.
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Post by Brazdj »

@dylan159
Thank you so much for sharing such a detailed design. I am getting primed to breadboard this up.
Since you have a lot of experience in this realm of cab sims, what circuits, in your opinion, make good, clean preamps/amp-sims to place right in front of your condor (or other cab-sims)?
Thanks!

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Post by dylan159 »

Brazdj wrote: 19 Sep 2021, 18:42 @dylan159
Thank you so much for sharing such a detailed design. I am getting primed to breadboard this up.
Since you have a lot of experience in this realm of cab sims, what circuits, in your opinion, make good, clean preamps/amp-sims to place right in front of your condor (or other cab-sims)?
Thanks!
Thanks. Good luck with breading, I guarantee it fits even on a smaller one!
I don't have that much experience, I've just looked around a bit. Condor is the simple ROG cabsim by the way. This is the Cab-allero.
About what to put before it, I like the idea of the core shaper as general EQ (it can do a FMV-like response and much more), with another pedal in front as distortion source when you want that. Or just take one of those pedals with a FMV in them if you want something that gives both the response of a guitar preamp and the bonus distortion. Or just try what you like best! It doesn't take much to get a good sound with this as starting point.
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Post by Lani »

PCB is verified by Dylan159. Attached you will find transfers as well as gerbers. All should be good and ready to go. Ordered a few boards Saturday so I will report back once I get mine up and running. Enjoy!! :thumbsup
CABALLERO v1.3 - 3DPCB - 08-22.png
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Post by dylan159 »

Here's a couple of pictures of the build. Working with a double sided DIY pcb can be adventurous and not always pretty, but what matters is that it works and it fits in a 1590B, even in this state, which is reassuring. I had to cut a riser frame to make it fit, but the use of board-mounted pots and switches or smaller caps should make that unnecessary. Or just go for a 125B, it's still smaller than a speaker cabinet.
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Post by jhergonz »

Thank you for sharing. Great timing, because I'm looking for a nice sounding simple cabsim circuit.

I will definitely build this ASAP before I build an stereo version of Omni cab. Ofcourse that headphone output will be a great add on. and I think Aux input with also be a great additional feature, with summing amplifier for the cabsim and AUX input, before the headphones amp circuit.

I will post an update(with video) in here once I finished building this.

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Post by jhergonz »

it is fine to use transistors for the gyrators, right?

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Post by dylan159 »

jhergonz wrote: 13 Nov 2022, 10:39 it is fine to use transistors for the gyrators, right?
It's possible, but the higher output impedance of an emitter follower will affect the frequency response when the series resistance required is low, if you don't account for it. I'd simulate both filters and decrease the series 470R resistor to match where the difference is relevant.
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Post by jhergonz »

dylan159 wrote: 13 Nov 2022, 12:47
jhergonz wrote: 13 Nov 2022, 10:39 it is fine to use transistors for the gyrators, right?
It's possible, but the higher output impedance of an emitter follower will affect the frequency response when the series resistance required is low, if you don't account for it. I'd simulate both filters and decrease the series 470R resistor to match where the difference is relevant.
What a relief, I was simulating the circuit before I ask the question. thank you for your response.

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Post by jhergonz »

update, I simulate the gyrator using 2N3904, and the response changed. I should stick with TL072.

but I also change the Sealed/Open switch with a potentiometer. Maybe I will call it Cab size? I still don't know.


Once I tested it, I will record a short demo and share the schematics here.

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dylan159
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Post by dylan159 »

jhergonz wrote: 16 Nov 2022, 05:58 update, I simulate the gyrator using 2N3904, and the response changed. I should stick with TL072.

but I also change the Sealed/Open switch with a potentiometer. Maybe I will call it Cab size? I still don't know.


Once I tested it, I will record a short demo and share the schematics here.
Of course it changed, but can you compensate for it reducing the 470r, without occurring into distortion?
For me the switch is subtle enough that it's not worth having settings in between. If you want to share the schematic remember that mine is ShareAlike for derivatives, thanks (also in your best interests).
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