Dialing in Sound with SS Amp + Guitar Cab

kai_p

Member
Dear all,

I am using an FM3 with SS amp and Marshall 2x12 V30 guitar cab. I mostly use classical Marshall amp models without master volume, like Plexis, aiming for AC/DC-like sound. Typically, in my grid I will only have an amp block and no cab block, since I am using a physical guitar cab.

For a long time I had problems dialing in a good sound with this setup, most of the time the sound was either too bright/harsh or something was missing more to the low end. Since I am running my SS amp at lower volume levels than what the actual amps would achieve, I started to think that maybe the problem had to do with frequency-dependent perception, i.e., fletcher munson curves. As an experiment, I took two equal loudness curves for different volume levels and calculated the difference, yielding a "correction curve" corresponding to the difference in perception between my SS amp volume and the actual amp at the used settings. (Of course this is just an estimation.)

Using the obtained curve, I placed a graphic EQ behind the amp block and configured it correspondingly, so that the EQ would do the necessary correction. Indeed this seems to significantly improve things, the sound I get this way is much more credible and usable. In short, what I have done is simply something like an implementation of a "loudness button" one can find on hi-fi systems.

Now, I have some questions. First of all, are my considerations reasonable and would this be the way to go for getting more realistic sound? And furthermore: Doing these calculations and dialing in a graphical EQ by hand is error-prone and one might get better results when really knowing the right correction values and having a precise such EQ. Hence, I am asking myself if such a feature would be interesting to have in the FM3, i.e., some kind of "loudness block" in the grid to more easily and precisely get the necessary corrections.

I think my problems dialing in a good sound might not only apply to extreme cases like an incredibly loud Plexi vs. bearable volume levels but also for amps with master volume, since I have e.g. tried the JCM 800 model. For me, it is really hard to know which master setting in the FM3 combined with SS amp settings would be equivalent to the actual JCM 800.

I would also be interested in any comments from people who are using the FM3 with a SS amp + guitar cab on how they achieve a good sound.
 
  • amp block to SS power amp
  • split your signal to out 2 without a cab
  • level your signal correctly
  • out 2 knob fully clockwise
  • out 2 config to unity gain
  • amp block out eq (mark eq type) for fine tuning the lows and low mids
  • try out different impedance curves (I always love the LB uk curve)
  • focus on all the standard and additional parameters

I would ditch the EQ - I also started with it and ditched it to keep it simple.
I also had problems with the low end - but boosting some frequencies in the amp's out eq, it's fine.
I compared my settings with my real JCM and had to boost around 3-4 db 80 hz in the out eq to get similar results.
Except the mentioned parameters and the out EQ, I focus on all 'main' parameters - I don't go that deep I think it works.
 
@MrPond thanks for your hints! I will try this out. It sounds promising that you were able to get a realistic sound this way compared to your actual JCM. Yesterday I also found some better loudness correction curves which support your statements about the frequencies that need to be boosted.

I need to be able to play at different volume levels since I am playing in different contexts, so no "one level fits it all" approach is doable I think. However, if this is all about some frequency corrections, maybe I can create a few presets for low, medium and high volume and that's it.
 
When I play at low volumes, I just boost the highs a bit (treble, presence, ..) and that’s it.
 
I tried it out, and it seems simply boosting the low end already helps a lot. As in your case @MrPond it's not a huge boost that is needed, something like +3 dB. This makes the sound much more balanced and leads to way more settings for bass, middle, treble and presence usable. Very nice!
 
Thanks @Sean D. yes the impedance curve makes a big difference! I measured the curve of my cabinet some time ago with a small test circuit I built and an oscilloscope. Besides the resonant frequency this yields the full curve.

For those interested, the problem is that there is no means in FM3 Edit for directly entering curve data. However, I printed the curve on a sheet of paper, in scale matching the size of the curve's view in FM3 Edit on my computer monitor. If you place the sheet of paper on the monitor, you can slightly see the curve in the editor through the paper and play with the controls until the curves match. Might be a bit nerdy but works.
 
Just a final note, I measured the frequency response of my SS power amp and made a corresponding correction in the output EQ of the amp block. Even though the necessary corrections were relatively small (typically +/- 0.7 dB at most) it seems that this was the final step to getting a really good setup. All controls of the simulated amp work really well now, and it reacts very good to changes in input level (guitar volume pot). It really feels like a good actual amp now.

So I am using the measured impedance curve of my cabinet and the measured + corrected frequency response of my SS power amp, and now this sounds really good. So my impression is that the amp models in the FM3 are very accurate indeed!
 
I cannot post a link here, but you can google it, it's called "Thomann t.amp S-100 MK II", which I like because it does not have a fan and is hence completely quiet. It's a simple class AB 100 Watts power amp.
 
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