How to tame this Marshall JCM cab sim

[mthibeau]

I built the JCM cab sim last night. It sounds great with good clean headroom (more than ROG Condor), but it's so loud... just a single coil going into the circuit caused my mixer to clip, even with the -20db pad engaged and the gain on zero.

Here is the schematic:
http://www.diystompboxes.com/pedals/schems/msim.jpg

I built it from the Sabrotone vero layout:
http://www.sabrotone.com/?p=3899

The schematic mentions to use an XLR out. Does this just mean wiring it up to an XLR jack, or run through a transformer (I saw that somewhere on here), or some kind of XLR buffer to tame the volume and make it balanced?

- Or should I just stick a volume pot on the output of this thing and call it a day?

[PRR]

Something may be wired wrong.

The first stage has gain a little under 2.

The second stage is most of the tone-shaping, and wobbles a lot, but between 3 in midrange and 7 at the extremes of the guitar band.

The last two stages are basically unity-gain.  

So overall gain is mostly 2*3 or 6, or 16dB. You should not need a 20dB pad into a Line Input. (Into a MIC input, yes, it is way-hot.)

Check your wiring. The maze around the second stage is busy. Also many chances to grab a wrong value (3.3K instead of 33K would be my most-likely goof).

For Line Input, increase the 27K input resistor as needed.

If you must go MIC In, use a 10K, 5K, even 1K volume pot across the output.

[mthibeau]

crap, I realized I did have it in a Mic input. I switched to a line input on my interface and the signal is manageable. I like this much more than the ROG Condor I built with the Marshall mod.

I'll mess with the resistor (or a pot) at the input as it's a little to loud for what I wanted to use it for, and probably add a 5k on the output just in case it's needed down the line.

I plan to use it mid-way in my direct-only pedal board, after all the pre-amps /gain, and before the time based and additional modulation FX.

Thanks for all your help.

[samhay]

If you want level control, you could replace the 47k in the feedback loop of the first op-amp with an e.g. A100k pot. This will not affect the frequency response. I don't like the low impedance of this design though, and you could fix this by increasing the 27k to the first op-amp to e.g. 330k or 470k and use an A1M pot instead.
The sabrotone layout uses a 4.7n reistor on the output. If you put a smallish pot/resistor on the output, this will shave off a lot of bass - I would use 0.1-10u here instead.

[mthibeau]

Thanks for the tips... I do want level control on input and output... and the low impedance bugs be too...

OK, making sure i have this straight before I try it tonight:

- Replace the 47k resistor at the first op-amp with a A1M pot
- Replace the 27k resistor at the input with 330-470k
- Replace the 4.7n capacitor at the output with a 100n+ capacitor

- I already added a 5k pot on the output (it did sap a bit of low)

- Thanks again!

[samhay]

Correct on all counts.
If you add the gain control at the front (1M pot) then a volume pot at the output is unnecessary.

[Willybomb]

Thanks for th reply - I requested the layout at sabrotone and had the same problem when I built it.

I've followed your suggestions, more or less.  I've replaced the 27k with a 470k, but I didn't have a 1meg pot around but used a 500k instead as I have an oversupply of those!  I've noticed that the level control works really well now, but there's not much travel before it gets too loud again.  What difference would the 1meg pot make?

[samhay]

willybomb - the gain of the first stage is R(pot)/470k. If you use a 500k pot, then max gain is ~1, which is probably enough by the sound of things.
I'm guessing you are using a linear 500k pot? A log or audio pot would probably work much better here.

[Willybomb]

Nah, I've got a log pot in there.

[nocentelli]

500k linear might be better: I've noticed linear often works better than log when used as an input level control.

[mthibeau]

I did the mods on my build, they work great.

I used a 470k for R1 (replacing the 27k), and an A1M pot to replace R2 (47k at first op amp).

This worked great as an input level control, the sweep of the pot felt fine for my needs.

The odd thing is that this circuit didn't play nice with anything after it, every true bypass pedal started to pop loudly, and my EHX Cathedral digital reverb stopped passing signal when engaged (it's true bypass so it worked when off, but with a loud pop like the others). I found using a buffer after the cab sim fixed all of that mess, so I built a Klon buffer inside the box with the Cab sim. While it may go over my head, any idea why something like this happens? Right before the speaker sim is a DOD FX-17 volume/wah (as a volume pedal)

Works very well on my board, been playing with it all night. It's definately replacing my Award matchbox MB10 cab sim, which sounds more like a 4 x 10. which is cool and all, but I grew up with 12" speakers and heavy metal, even though I like much more mellow (clean) stuff now, I still like the sound of it through a 4 x 12 or 2 x 12 instead of 10" speakers.

Thanks everyone for the help.

[samhay]

^The odd thing is that this circuit didn't play nice with anything after it
Having taken another look, I'm not surprised. I thought Harald had used a 4n7 ouput cap, but was reading the layout wrong. He hasn't added an output cap! Try adding an output cap and pulldown resistor - 1u and 100k should work nicely. There isn't much room on the layout, so you may need to use flying components or a small daughter board.

[hab]

Sorry guys. I did this layout and was a bit quick about it. I wouldn't have known of any gain issues, but I should have caught the missing output cap.

Anyway, I did a new layout with your modification suggestions. Should be up now: http://www.sabrotone.com/?p=3899
Hope this one works.

[samhay]

Nice one Harald.

[samhay]

Sorry for the thread resurrection, but this has finally floated back near the top of my pile.
Took another look at the design of this today and realised that a significant amount of the high pass filtering in this design comes from the first stage. The mods described here to increase input impedance didn't account for this. If the 27k input resistor is increase to e.g. 470k, then the input cap should be decreased a similar amount - in this case to 2n7. I doubt you could hear the difference if you used a 2n2 or 3n3.

[Willybomb]

C1 being the input cap?

[samhay]

In the sabrotone layout, yes C1 is the input cap.