Differences between Helix Amps and Preamps


I've been using @James Freeman 's techniques to try and match the vitriol preamp to the vitrol amp, and actually had pretty good luck.

The preamp has channel volume, Post Gain, Master Volume, and Sag, but no presence, bias, biasx or depth

I was able to set the preamp as close as I could to the amp, and then match EQ for the rest of it, and I can't honestly ABX between them now. Am I just not knowing what to listen for? Gain seems the same, noise is the same, tone is the same, dynamics seem the same.
The Preamps are taken right before the phase inverter but also have a power supply that sags, the only purpose of the Master control in the Preamp only model is to control the amount of Sag.
In a full amp the most amount of current is taken by the power tubes when the Master volume is set high, concordantly the voltage feed (B+) of the Preamp also sags, so that emulates that without the power section.
With Preamp models only, if you turn Sag to zero the Master is just another Channel volume.

Here's some very useful posts by Ben Adrian on TGP regarding Preamp models;

I'll quote the posts here if you don't want to visit TheOtherPlace™.

benadrian said:
The Master Volume, generally, is in between the pre-amp and the power amp. The phase inverter, for all practical purposes, operates as part of the power amp as a whole. This is because amps with negative feedback insert that signal into the phase inverter, so the phase inverter tube and the power tubes operate in a state of equilibrium.

Saying the master "Adjusts the amount of power amp distortion" is a bit clunky, yes. It might be more clear if it said something like, "the master volume reduces the signal level going into the power amp circuit. This matches the actual Master Volume of the modeled amps when those amps have a master volume control. If an amp does not have a master volume control, then it reduces the signal level between the preamp circuit and the power amp circuit. Setting it at 10 is no level reduction and matches the non-master volume amp in the real world."

So, in short, you turn an amp up loud and the preamp and the power amp will distort. If you want to quiet the amp down or reduce power amp distortion, you turn down the master volume control. The amp model should behave in the exact same way.

Sometimes I like to think of a master volume as a ratio control for preamp and power amp distortion. When an amp is turned up, everything distorts. When the master volume is at maximum, the power amp will distort when pused by the preamp, and this is the maximum power amp to preamp distortion ratio. as the master is reduced, the ratio will decrease. As very low levels of the master, the preamp can be distorting a whole lot, and the power amp to be clean. I don't know if this will make sense to everyone, but it's how my brain sometimes works.

Another good thing to remember, we matched the knob positions in the amp models. If anyone here has used a Deluxe Reverb you know that after about 4-5, the amp stops getting louder. Once the amp goes past 7-8 it can get pretty ugly. The model behaves the same. Once the drive passes 40% or so, it'll never be a clean amp. Cranking the drive will never give a tight distortion, it'll blow out the power amp. Some think this sounds awesome, some think it sounds ugly. That's totally subjective. But if you are using a model and you want more drive, think of how that model would sound when cranked. Sometimes it sounds a lot better to put a drive pedal in front of an amp than to push an amp to its limits

Oh, and one case where the master volume is different is the Hiwatt. That amp has an additional gain stage between the master volume and the phase inverter. The model matches this trait.

Wow, I just kept writing. Hope you all got through this. Cheers!

benadrian said:
There is a secret behind the preamps.

When we model the amps, we never turn anything down. The internal, digital signal is 32 bit floating point, so we don't have to worry about clipping anything internally. This allows us to model everything and keep the same level that is happening in the physical, analog preamp.

Now, if we fed this signal with no level padding into the D to A converters, it would probably clip when getting converted back to 24bit. So, we decided not level the preamps. Instead, we just turn them down by 24dB.

So, if you boost the signal coming out of the Helix by 24dB and then feed it into a power amp, it will be the exact level as if you were able to plug the preamp out of the actual amp into that power amp.

So, the differences in levels of the Helix preamps directly relate to the differences in the levels of the real-world, analog preamps of the amps that were modeled. And this is taking the level right at the input of the phase inverters of the power amps. If an amp had a preamp out on the back panel, we did not use that jack. There is no industry-accepted level adjusting for preamp outs. Some amps let it out full blast, some amps pad it 36dB or more.

Theoretically, a user could put two gain blocks after a preamp model and boost them both by 12dB, but that would most likely clip the output circuity of the Helix.

We never really made this public. It turned out that a lot of people just didn't want to mess with the preamps. However, the gain blocks in Helix use almost no DSP, so there was no harm in leaving the preamp block at their "accurate minus 24dB" level and letting users tweak them to whatever level they desire.


benadrian said:
Here's the scoop. As one might imagine, we model the preamp and the power amp sections of amps. What is not readily apparent to most users is that we are also modeling the power supply. The audio in the amp moves through preamp and into the power amp, as we all know. The power supply feeds the amp in reverse. It delivers power to the output transformer first, then the power tube screens, then the phase inverter, then the preamp from back to front. This means that preamp sag depends on how the power amp is performing.

In a single amp model, we can have bi-directional communication. In a preamp only, we have a behind the scenes trick to maintain the sag as if it was hooked up to a power amp, without using the DSP resources of having a power amp model. There's currently no way in our architecture to have a power amp model talk in an audio-backwards direction to a preamp model. Also, if there are two preamp models and two power amp models, there's has to be a bit of code to untangle the logic of which preamp is controlled by which power amp. In addition to that, every preamp is different, so there has to be some kind of logic to untangle what to do when a preamp that needs four nodes of power supply is only hooked up to a power amp with three nodes of power supply.

So it's not impossible, and it's definitely on our list of stuff to do, but it's much more complicated than most people think to mush two amp halves together and have the new system perform realistically.

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this is an area where i should spend more time. i briefly tried both a while back and thought the amps sounded better but i bet if i spent some more time i could get the preamps to sound just as good and then save the DSP too.
Personally, I just like having Presence available, and sometimes I'm also making use of a cranked Master.
In addition, ever since I downgraded to a Stomp, I prefer using Amp&Cab blocks to save a block.
There’s a frying eggs with Pam sound in the Panama that I do t get with Vitriol. I actually really like it for tracks that need to poke through, but for when I don’t want it, I can’t get rid of it. Is this part of the presence or something else in the power amp?
so i briefly tried switching to preamps last night before rehearsal. sounded pretty good in terms of overall tone, attack, and tightness but i couldn't get the same chunk or heft in the chugs and palm mutes that i am used to with the amp blocks. it was close, its not like it was far off. and i didn't really mess with it for too long bc everyone else was ready to get started. i think i could probably dial them in better next time and will keep working at it. i think the invective preamp block is better than what i remember when i tried this a long time ago with the revv red.
It trips me out that I am having so much trouble getting the panama to sound even remotely like the vitriol...I can even get the spectrums to match up perfectly, but they act so differently, even with the gain/distortion levels matched as close as I can.

Are there other helix models that are similar to the vitriol or 5150? Its crazy that I can match Revalver's 5150 so close to the vitriol/Bias 5150, but can't seem to get the helix one there. I would just use the vitriol, but eat eats a lot more DSP