How to simulate an interesting RC network in the Randall Brahma preamp

[Chris Hurley]

This isn't exactly a "wish", so I posted it in the general discussion...

I've recently been enjoying the Randall Brahma preamp. It has a really interesting tone and feel that I've not experienced before. Supposedly, it was designed to replicate George Lynch's old Plexi, but it doesn't exactly behave in a way that I recognize from other british-style preamps and digital models.

In looking at the module, there is an interesting RC network at the input of the preamp module. (The modular preamps have a full-range tube stage in the dock which is a full-bandwidth buffer stage, followed by the circuit in the preamp module which can vary widely.)

I've replicated this network on other modules and found that the magic is largely in that network. On other modules, you have a small input capacitor that limits bass response and tightens the attack or a larger capacitor which makes the attack software and flubbier.

This network seems to give a blend- you get a nice tight attack but with some lower end clutter and complexity that is interesting. I suspect that this may be an effort to replicate the bright and normal channels of a jumpered plexi but I'm not sure and as I mentioned, I don't really recognize the feel and sound of it.

One take on the schematic is here: http://faqload.com/faqs/music/randall-mts-brahma-module

Does anyone have any thoughts on how you might replicate this sort of thing in the Axe-FX III? I imagine you could do it with one of the many EQ blocks in front of an existing module but I'm not sure how to go about it.

 

[FractalAudio]

Two things:
1. That input circuit is nothing special. It's a "treble peaker". It's got an extra peaking section. You see these frequently in tube amps.
2. Your gain control network doesn't look right. I would double-check your tracing.

The low end clutter and complexity is unlikely due to this network. It's more likely due to the stage right before the cathode follower. That stage is biased pretty cold and has a cathode bypass cap. This typically causes that murky sound. I personally don't like that sound because it reduces string separation but I know some people like it. You see this in European amps a lot. American and British amps usually bias this stage towards the middle. American and British amps will typically bias an earlier stage cold with the last stage biased close to "neutral".

For example a JCM800 biases the previous stage cold with a 10K cathode resistor but the last stage has an 820 ohm and no cathode cap. The SLO100 (and Mesa Rectifier rip-off) bias the previous stage really cold with a 39K and then the last stage is 1.8K.

Another source of "clutter" is bias excursion. I'd have to run the numbers on that design to see what the excursion is but I suspect it's the bias more than the excursion that is causing the clutter.

I think the reason some people like this clutter is because it covers up your playing. It causes a lot of intermodulation distortion in the audible spectrum which masks poor technique. It also increases pick attack and perceived low end. In a dense mix, though, the guitar can get lost and require lots of low cut. It's enjoyable when playing alone but in a mix it's too messy.

If you want to reproduce this clutter experiment with the Bias and Bias Excursion parameters in the Preamp section of the Amp block.

Also, your bass and midrange pot values look wrong. I suspect they're backwards. And the bass pot looks like it is connected to the wrong pin on the midrange pot.

 

[Adman103]

Two things:
1. That input circuit is nothing special. It's a "treble peaker". It's got an extra peaking section. You see these frequently in tube amps.
2. Your gain control network doesn't look right. I would double-check your tracing.

The low end clutter and complexity is unlikely due to this network. It's more likely due to the stage right before the cathode follower. That stage is biased pretty cold and has a cathode bypass cap. This typically causes that murky sound. I personally don't like that sound because it reduces string separation but I know some people like it. You see this in European amps a lot. American and British amps usually bias this stage towards the middle. American and British amps will typically bias an earlier stage cold with the last stage biased close to "neutral".

For example a JCM800 biases the previous stage cold with a 10K cathode resistor but the last stage has an 820 ohm and no cathode cap. The SLO100 (and Mesa Rectifier rip-off) bias the previous stage really cold with a 39K and then the last stage is 1.8K.

Another source of "clutter" is bias excursion. I'd have to run the numbers on that design to see what the excursion is but I suspect it's the bias more than the excursion that is causing the clutter.

I think the reason some people like this clutter is because it covers up your playing. It causes a lot of intermodulation distortion in the audible spectrum which masks poor technique. It also increases pick attack and perceived low end. In a dense mix, though, the guitar can get lost and require lots of low cut. It's enjoyable when playing alone but in a mix it's too messy.

If you want to reproduce this clutter experiment with the Bias and Bias Excursion parameters in the Preamp section of the Amp block.

Also, you bass and midrange pot values look wrong. I suspect they're backwards. And the bass pot looks like it is connected to the wrong pin on the midrange pot.

I have learned so much more about amps since giving up amps and going Fractal.

 

[Chris Hurley]

Two things:
1. That input circuit is nothing special. It's a "treble peaker". It's got an extra peaking section. You see these frequently in tube amps.
2. Your gain control network doesn't look right. I would double-check your tracing.

The low end clutter and complexity is unlikely due to this network. It's more likely due to the stage right before the cathode follower. That stage is biased pretty cold and has a cathode bypass cap. This typically causes that murky sound. I personally don't like that sound because it reduces string separation but I know some people like it. You see this in European amps a lot. American and British amps usually bias this stage towards the middle. American and British amps will typically bias an earlier stage cold with the last stage biased close to "neutral".

For example a JCM800 biases the previous stage cold with a 10K cathode resistor but the last stage has an 820 ohm and no cathode cap. The SLO100 (and Mesa Rectifier rip-off) bias the previous stage really cold with a 39K and then the last stage is 1.8K.

Another source of "clutter" is bias excursion. I'd have to run the numbers on that design to see what the excursion is but I suspect it's the bias more than the excursion that is causing the clutter.

I think the reason some people like this clutter is because it covers up your playing. It causes a lot of intermodulation distortion in the audible spectrum which masks poor technique. It also increases pick attack and perceived low end. In a dense mix, though, the guitar can get lost and require lots of low cut. It's enjoyable when playing alone but in a mix it's too messy.

If you want to reproduce this clutter experiment with the Bias and Bias Excursion parameters in the Preamp section of the Amp block.

Also, your bass and midrange pot values look wrong. I suspect they're backwards. And the bass pot looks like it is connected to the wrong pin on the midrange pot.

Thanks for the comments. Its not my schematic- the RC network doesn't exactly match what is on my factory Brahma or the Brahma modification that Dave Friedman did for me. The one that I built yesterday is a little different still. I used 1000pf, 470k and 330k because that's what I had handy.

When I added just this network to an existing preamp (something along the lines of a Friedman BE), it added this particular flavor that wasn't there before. Maybe it reveals the other items you mentioned later in the circuit.

I'll agree that it doesn't lend itself to every situation. I will probably put it on a switch on my physical preamp for times when I'm just playing to enjoy an amp and not concerned about whether or not I'm covering up my playing or goofing up a mix that doesn't exist in that context.

I'll give the Bias and Bias Excursion parameters a spin.

 

[Chris Hurley]

I'm not sure about that second .68uf/2k7 cathode arrangement and the schematic either. On a lot of schematics for various Friedman amps, I'll often see .68/2k7 on the first two stages but the stage before the cathode follower is generally the common 820ohms.

That bright cap switching doesn't make sense to me- this schematic looks to be all kinds of screwed up.

I know the preamp that I added the RC network to is the 820ohm setup. I bet thats an error on the schematic too but I don't have the brahma preamp in front of me to look at.

Thanks again for the comments. Certainly nothing earth shattering.