The Power of Pre-EQ


Fractal Audio Systems
As outlined in the MIMIC white-paper the fundamental paradigm of distorted guitar tone is EQ -> Distortion -> EQ. For higher gain tones the post-EQ is typically the tone stack and the Presence and Depth controls, when available. Therefore the user has access to the post-EQ but no control over the pre-EQ. One notable exception to this rule is the Mesa Mark series of amplifiers where the tone stack is located before the distortion.

I suppose the ultimate amplifier would be one with dedicated pre-EQ and post-EQ controls although I can imagine many guitar players with looks of bewilderment when presented with such an amp. Indeed I believe there was an amp years ago that had separate input and output graphic EQs. I'm thinking it was made by Seymour Duncan but it was a long time ago so I'm not really sure. I don't believe it was terribly successful.

So as we delve into the realms of higher gain tones we are the mercy of the amp designer and his choice of pre-EQ. The standard practice is to cut the lows before the distortion stages. There are various approaches to this: small coupling caps, partially bypassed cathodes, etc. These are simple methods and given the relatively simple nature of tube amps all we can really expect. Other popular pre-EQ techniques are shelving filters, i.e. the Marshall 470K, 470pF network and networks which roll off highs.

The pre-EQ, along with the post-EQ, shapes the tone when the amount of distortion is low. As the distortion increases the tone becomes more dependent upon the post-EQ. Anyone who has adjusted a Mesa Mark series amp will attest to the seeming ineffectiveness of the tone controls at higher gains. They will also attest to the affect the tone controls have on the feel.

So... the pre-EQ is an important part of the overall tone equation. We guitarists tend to focus upon the post-EQ and put graphic or parametric EQs after the amp but we neglect the pre-EQ. Therefore it is worthwhile to experiment with pre-EQ. The simplest approach to start with is using a graphic EQ before the amp block. Note how boosting or cutting certain bands affects the tone and feel. Note how the effect changes as the gain is increased or decreased.

One popular studio technique in the 80's was to put a parametric EQ before an amp and boost a narrow band of frequencies. This gives a slight mid-emphasis to the sound and can be useful in helping the guitars stand out in a mix. This technique seems to have been lost over the years. Years ago Micheal Sweet from Stryper showed me the frequencies he used and IIRC he boosted around 800 Hz about 6 dB. I don't remember the Q but I would start around 1.4. Incidentally the frequencies he were boosting are just about the same as the frequencies that are cut by the tone stack. So when playing softly the net result is a flatter EQ. As you play harder the input EQ becomes less effective and the tone becomes more scooped.

Pre-EQ can make amps sound warmer, or tighten the bass. It can be used to increase brightness without becoming harsh. Pre-EQ is also very useful with amps on the verge of breakup or mildly overdriven. Experiment with boosting frequencies to give your leads a more vocal character or make them more unique.
I actually did something like this years ago. I had two mxr eqs, pancaked around a tube screamer. I heard people had done it in the past. It gave me a lot of control over tone shaping. I'll have to revisit this info.

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Pre-EQ makes me think about why Tube Screamers were so popular.

I love the color they give, i have a custom made dual drive TS like pedal tinkered to my specs and i have been using it for the past 8 years, it really defines and tightens the feel with a tube amp.

Extreme user friendly written, nice to read and helpful! Along with Amp X/Y, GEW&PEQ X/Y (on Axe XL ...grmpf...) capability shines in a new light ;) Thank you!

Now my perspective to all the EQs on Boosters, Overdrives, Distortion pedals change ;)
One notable exception to this rule is the Mesa Mark series of amplifiers where the tone stack is located before the distortion.

... and therefore they have the GEQ added to influence EQ after Preamp-stage?! aha .... i learned something ;)
... and therefore they have the GEQ added to influence EQ after Preamp-stage?! aha .... i learned something ;)

The GEQ was not on all MK-Series (MkII and later) Amps. It was an pre-sale option......I had one in my Mk3 back in 1997 ;) Not all MK-Amps had a post-gain graphic EQ....

Cliff's topic is great to explain the power of tone-forming which affects later overdrive circuits and the way they work! Very important topic ;)

But I have something else to say here (again):

One of the "other" amp (if you do it right) that "always" (!!!) uses Pre-Gain EQ on both channels is the mighty Dumble Overdrive Special! Our basic 213 ODS-100 Lead 1 is modeled with PAB on and the "internal" Post-Gain option called HRM (what stands, correct by Cliff, for"Hot Rodded Marshall"). HRM is a classic "James" Tonestack, as used in Marshall amps. Its pre-setted by internal trimmers. This option was done to "clear up" the signal especially when playing chords.
The other thing on this amp is: The PAB (PreAmpBoost) "partiell bypassed" the pre-gain tonestack which results in a Gain-Boost. But I always need to say this - this tonestack does NOT completely disappear in the circuit (in terms of MIMIC this could be important too ;) ) , it would if we do it as its done on certain MesaBoogie MK-Amp which features a push/pull Gain-Boost option with even greater impact. ;)

With a real d*mble voiced preamp, clipping starts only in V4 gain stage , therefore we "always" need a pre-gain tonestack and no PAB (!!!). The "magic" of the d*mble voicing is the sum of match of individually biased and voiced gain stages, the internal OD-trimmer which caused the fact only the V4 is clipping (without PAB!!!) in a four gain stage preamp, the pre-gain tonestack and last the preamp power supply circuit ( I called this the AC load line, but I'm not sure if this is the right word for it in english - I'll explain such things at the german axefest in april 2014).

The ODS-100 Lead 3 (non-HRM) is the only D*mble circuit which pulls out the smooth overdriven tones, we thinking of when speaking of d-style sound.

I agree with Cliff the Mesa Mk were special in this case (also because they fit into the AxeFx control concept), but the D*mble takes the meaning of PreGain Eq to a whole other level! :)
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I remember reading about Tom Sholtz's rig and philosophy way back in the day... he was always using pre and post eq and talking about it.

I never did embrace it fully though myself. Cool stuff.

What about a select box in the Amp block GEQ page to make the GEQ pre or post?
My first ever overdrive pedal by PEARL had a PEQ built in. That taught me so much about pre-EQ

Anyone remember The guy was, well, extreme, but he had the Pre-EQ/Distortion/Post-EQ subject well-covered.

In an extreme sense, a cocked-wah provides an example of what pre-EQ does.
Pre distortion eq is the most powerful tool we have.

Shaping the gain and voicing. I

have always used overqualified EQ in front of my amplifiers for high and low pass filtering and eq shaping. Once I had the axe I could all of that inside the box it was wonderful.

I agree the greatest amp would have pre and post distortion eq. I find it rediculous almost no one else has thought so. I have at least 10 pages in a sketch book of design ides based around this .

Who over mentioned the TS is correct we love those for the eq curve, not the added gain. These amps have more gains then. Necessary.
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