Question about EQ's that I never understood

Hey, what’s the logic behind building an analog tube equalizer?

Is the idea that the tubes bend and warp the shape of the bell curves (in a way that a non tube EQ can’t) so that the cut and boosts are more aesthetically pleasing? Or is it that we’re intentionally introducing artifacts into the frequency curve? What productive roll do the vacuums tubes actually play in the sound generation process?

I always thought tube EQ’s like the Millenia, Curve Bender, Pultec, Massive Passive and Hammer always sounded cool, but I never really understood what the tube does in this type of unit.

Great question. This question brings me back to when I bought my first guitar amp, the guy at GC said, “do you want a solid state or a tube amp?”. I diidn’t know what the difference was and he said that tube amps have a “warmer” sound to them. I’d love to know this answer too.

This reminds me of the CD vs Vinyl debates. Definitely interested in watching this topic.

I wouldn’t call it logic, I’d call it preference and history. Before transistors were widely used, the hardware EQ units probably used tubes. So there’s potentially that nostalgia around anything built with tubes, and if someone had used them for years they might want to continue to use them. So I think your question is kind of backwards … those could have been the first ones built. So until they go out of fashion someone will probably make them just because someone will buy them. :slightly_smiling_face:

I’d guess its all about ‘color’ and harmonics. Per your posts on another thread about harmonics, there are different harmonics with tube vs solid state. Many units may have a mix of both of these, so I don’t think its either/or, but as I understand it tubes generate more odd-order harmonics that may be perceived as ‘warmth’ … and solid state would have more of the even-order harmonics which could have more ‘bite’.

Ultimately it’s probably a choice and preference about sound coloration, workflow, “cool” factor, client appeal, etc.

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See my reply to Jonathan above. There is a whole cult mystique around tube guitar amps. It also doesn’t hurt the amp manufacturers that can sell tube amps for 3-4 times the cost of a solid state amp. :wink: GC probably doesn’t mind high dollar sales either. :smirk:

Ultimately it’s choice and taste. Dimebag Darrell (Pantera) used mainly Randall solid state amps for years, and people raved about his tone.

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Analog circuits use capacitors to filter certain frequencies, like a tone control on a guitar. The capacitor impedes low frequencies and allows high frequencies to pass through where they can then be sent to ground where you can’t hear them.

Vacuum tubes are used to amplify or rectify a current so they would be placed in an equalization circuit for that purpose and would then add distortion to the circuit giving a pleasing color to the sound.


I’ve also heard that the transformers do something special as well in conjunction with the tubes.

I think tubes are used in the pre and post stages of the eq, at least in most designs…but then considering that I’ve repaired a few tube amps in my day and messed with their eq settings by modding that path, there is usually one or two 12ax7 preamp tubes in the amplifier eq path, so I am sure there are also designs where the filtering is done on tubes.

OH jeez, bag o worms here.

I love tubes, love distortion, love vintage shit… most of that vintage gear was tubey, of course, but no matter, some was analog and fabulous.

I can only say for certainty, that I RARELY use a PARAMETRIC eq plugin now, or barely even a compressor plugin, maybe I dont like them that much, BUT the Scheps Omni Channel Strip was awesome, btw… I just didnt buy it, even @ $27 on sale! but probably still will someday, its that good.

I use mostly / exclusively TILT EQ’s and SATURATION plugins now, and emulations of old gear such as Waves Abbey Road Mastering.

Literally dont care about anything else, everything is BROAD strokes of the brush now, it just SOUNDS BETTER, maybe THAT is the key :sunglasses:

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My understanding is that Tube EQs just use the Tubes for amplification - basically make up gain.

Passive EQ circuits make signals quieter because the caps/ resistors load the signal down, so you need some form of amplifier to either buffer the signal before the EQ (which makes the signal more robust against being loaded down by passive components), or simply amplify the signal back to its normal level after it’s been loaded down by the EQ circuit (or both).

So the question can be more easily rendered as “What’s the value of Valve amplifier stages vs solid state amplifier stages”.

And the answer to that isn’t too tricky. Valve stages can add harmonics that are subjectively pleasing. They can change the shape of fast transients. Valve amplifiers often have transformers. Transformers do their own thing to transients, and can induce subtle distortions/ phase shifts. Same with components like coupling capacitors, carbon comp resistors with significant voltage drop across them… it’s not just that valves are “magical”, it’s the whole system.


That’s how I think of it. The tubes in a tube EQ are there because you had to have tubes to make an amplifier.

Different amplifier designs and tubes have different saturation characteristics. Combine that with the fact that where the distortion takes place in the signal path will also make it sound different, and since an EQ is adjusting the frequency response of a sound, it’s going to have a more dramatic effect on the distortion characteristics of the amplifier.

I personally don’t really care for putting distortion in my EQ, but I definitely understand why people like it.


So is it doing anything to the EQ part of the circuit? Like the curve slopes or the phase?

Or just adding distortion after the EQ’s already done its job?

Are you saying that an EQ like the massive passive could have been built without any tubes at all and still pretty much sound the same?

It shouldn’t really have a strong effect on the phase or anything.

or before. or during. depends on how the EQ is designed. Let’s take a really simple example, a bell boost.

Here’s what different order of operations does. I’m just using a hard clipper for the distortion so that the effect is more obvious.

  1. No distortion, no EQ.

  1. Boost. No distortion

  1. Clipper -> EQ.

  1. EQ -> Clipper

  1. Parallel EQ, Clipper-> Filter

  1. Parallel EQ Filter->Clipper

These are just 4 examples of clipping at different stages of the process. Combine this with the fact that you can have different flavors of distortion at each position, and also you can add distortion at any combination of locations along the path, you can make all sorts of different sounding EQ units where the EQ algorithm is standard.


That’s crazy. Considering the price of a tube EQ.

I love the effect of the Massive Passive on a stereo piano or the Curve Bender on a choir bus. But its good to keep in mind that there’s nothing really ‘magical’ going on there if I’m understanding you correctly.

Have you been paying attention to Plugin Alliance? They’ve released a bunch of plugins that do nothing but have tons of flavors of distortion. Like one box, that does nothing but emulate tubes infront of and behind a signal. Then they just made another one that emulates a bunch of different flavors of transformer heat. Interesting.

Sorry, that was sloppy writing on my part. It won’t change the phase or frequency response, at least not directly. But it can change the sound.