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Tube amp semiconductor (FET?) emulation
Tube amp semiconductor (FET?) emulation
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Old 21st May 2008, 08:15 PM   #1
danV is offline danV  Europe
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Default Tube amp semiconductor (FET?) emulation

Hi all, this is my 1st post here!

I'm a noob guitar player, and even noober in electronics (even if I've built a few effects pedals) ...

What I'd like to build is a versatile and portable guitar preamp, to link to a power amp + cabinet or a PA system.
It should have the following features:
- 2 or 3 channels (clean, crunch, lead)
- a power amp distortion simulator
- a cabinet simulator ...
... and it should be all solid state ! (I'd hate to mess up with high voltage and bulky transformers)

I would like to 'model' existing amplifiers, Fender, Vox, Marshall, Mesa or others, possibly using FETs. For now I'd like to stick to the preamp section.

The simplest method would be the one from Runoffgroove (or others before, I don't really know to whom give credit ), which I already used for other simulation attempts.
Example: http://www.runoffgroove.com/professor.html

However, something seems to be lacking in the sound ... So I've found on the net some more refined techniques to emulate preamps and power amps.
- http://www.desertcomb.com/gas/effect...ven_thesis.htm
- FET + BJT combinations at http://gabevee.tripod.com/sstube.html
- LXH2 simulators at http://home3.netcarrier.com/~lxh2/
- the patents by Pritchard ... (for ex. US5434356)

The last seems too complicated for me to fully understand, try out and implement.

LXH2 simulators sound very good, but I don't know how this approach could be easily applied to other simulations.

The 1st and 2nd ones seem promising, since they seem to provide a sistematic way to replace each triode.

BUT ! I didn't understand everithing in the articles, and I'm seeking for help. And here are my 1st (more will com for sure) questions about the 1rst article:

- I'd like to know how the values for the common source amplifier (Fig 3.6) are calculated. The source bypass capacitor and Rg seem the only common values with the original schematic (3.1). How is the rest related to it ?
- Is the lower half power frequency the lower -3dB cutoff frequency of the circuit ?

Finally, have you got other ideas ? Any comment on these general approaches ?



BTW: great forum, truly a gold mine!
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Old 25th May 2008, 06:12 PM   #2
Minion is offline Minion  Canada
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Well what you can do is try Linking a bunch if Pedal curcuits together to get the sound you want, with bypass switches to enable or disable parts of the curcuit?....

If you want a awesome sounding Mesa boogie emulation take a look at the "DR.Boogie" which is a great sounding pedal that uses Fets to emulate tubes....It also has a 3 band tone controll..


If you want a Good Marshal emulation which is extremely versitile try the Marshal Guvnor clone....


Generally the Power amp stage of an amp doesn"t add any Distortion , the preamp stage is allmost allways responcible for any distortion , even in tube amps...Many amps these days will use a Tube preamp with a solid state output stage like the Marshal valvestate series of amps....

There are a couple of cabnet simulators out there like the "Condor Cab Simulator"...it can simulate a fender type Cabnet and there are several mods available to emulate other Cabs like the Marshall...


I think if you do something like this you can probably get pretty close to what you are looking for....

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Old 25th May 2008, 06:22 PM   #3
unclejed613 is offline unclejed613  United States
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actually the way it is done in the newer amps is by duplicating an amp's transfer function with a DSP chip. only problem with this approach is if you copy a VOX AC-30, and use this model in the firmware of every amp you make, they ALL sound like THE SAME AC-30, and in reality, no two AC-30s sounded exactly alike.

i think it unfortunate that many modern amps have NO analog signal chain at all between input plug and power amp input stage. to "tweak" these amps (such as changing the tone control curves) requires the ability to "hack" firmware code, instead of the handful of caps and resistors that were useful in the past.
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Old 26th May 2008, 01:59 PM   #4
Nordic is offline Nordic  South Africa
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Yep, that is the game these days, microprocesor based effects....

You see, the sound of a grunting Meesa is not something you are going to reproduce with any amount of fets...

It is a compilation of many things happening at the same time...
Natural rolloff in driver do to frequency range it is usefull in.
Severe sagging of power supplies under load
High a-symetrical distortion way above the noise floor.

I have listened to a few of the newer transtor based amps and have not realy been impressed, especially entry level marshall gear makes me wish the drivers had no basket so I could do a running dive through it...
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Old 26th May 2008, 04:35 PM   #5
danV is offline danV  Europe
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Hi all, thanks for the answers !

Basically what I'd like to do is a sort of unit, which can perform:
- a preamp emulation (and link it to a power amplifier + cab)
- a preamp + power amp + cab sim (for recording or linking to the PA).

The sounds should not be a perfect emulation, but something having the sound 'signature' of amplifiers. Jfet simulations can do that.

However the existing techniques (runoffgroove, the same one for the Dr.Boogie) do not satisfy me in terms of sound, therefore I was looking for something more refined, yet still simple to implement.

Anyway, any idea how to calculate those values ?


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Old 26th May 2008, 07:53 PM   #6
unclejed613 is offline unclejed613  United States
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you could simulate the sagging power supply with a 0.33 or 0.5 ohm resistor in series with the ps rails, and the asymmetrical distortion (as well as the tube-like soft clipping) with an op amp with resistor-diode networks in the feedback loop. as a matter of fact i have seen "tube sound" distortion boxes with just an op amp and resistor-diode combinations (piecewise linear modeling). the cabinet sound can be done with a comb filter (either switched capacitor or analog delay line) with the clock frequency at the resonant frequency of the cabinet being emulated. of course, to pull it off, your speakers should be acoustically neutral (no obvious resonances of their own). there are many analog tricks that can be useful to emulate other amps. Bob Carver had a line of amps that sounded exactly like other amps (one was a Mark Levinson, another was a McIntosh tube amp). he did this by using an acoustically neutral SS amp of his design, and the circuit elements needed to duplicate the other amp's transfer function in the feedback loop.
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Old 23rd June 2008, 02:30 PM   #7
lundborg is offline lundborg  Sweden
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I've used inverted CMOS logic, such as 4011UB quad NAND gates.
(You need circuits from the unbuffered (e.g. 40xxUB) series)
Just connect input to output on one of the gates with a few K
feedback resistor, and the proper size resistor on the input and there
you have a 'tube' emulator to put in your pre-amp chain.

At least it's a very simple and cheap way of doing it.
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Old 24th June 2008, 04:33 AM   #8
unclejed613 is offline unclejed613  United States
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i finally had time to look at the LXH2 page. the fender preamp looks a lot like piecewise linear modeling. shown below is a simple piecewise approximation of a tube stage. one set of diodes conducts first to make a lot of even harmonic, then the other set comes into play to add smaller amounts of odd harmonic. the + side begins conducting first, then the - side. as each diode reaches conduction, the circuit reduces gain a little bit, curving the output waveform.
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Old 24th June 2008, 02:57 PM   #9
danV is offline danV  Europe
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Thanks for the answers!

Right now I have found a good way in order to improve the ROG emulations (basically I use a different bias technique, by setting the correct idle drain voltage through the source resistor). The result is a more faithful harmonic content when clipping occurs, and less frequency response modifications due to the same drain/plate resistor.

However each JFET has still got to be biased individually, which is pretty annoying, even with some Excel calculation sheets I prepared. I'm still looking for some emulations which are more easily repeated, and the opamp one seems a good way (I had tried using different biasing, in order to have the right harmonic content, but it quickly becomes a pain .., the diodes + resistors seem a more efficient solution).
I'l also check the NAND gates, could be interesting!
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Old 24th June 2008, 06:46 PM   #10
AuroraB is offline AuroraB  Norway
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I've been digging around for these gadgets for quite some time now....
haven't tried to sim the circuits.

A couple of years ago, there was a guy who developed an AC-30 simulator, but that was taken off the net. A WayBack machine may retrieve the schematics, but important component values are lost
( maybe I should try to sim them....)

OTOH- have anybody really tried circuits like the "English channel" ( AC-30), Thunderchief or Thor from RunoffGroove, I'd really like to hear about the results...
It could be very interesting as a cheapie way of guitaramp DIY,- maybe even better than the the cheap opamp based commercial models??
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