Tube Preamp: EQ Section

The EQ is placed within the tube amplification signal chain which has a lot of harmonic distortion; as a result, it might seem that the EQ has some kind of "mild magic" in it. In reality, there's no real magic there; instead, there's complex interaction and very wide frequency bands. One of the key points is that the higher the gain is, the lower the EQ's impact becomes. To be more precise, the impact stays the same but the tube goes into clipping mode (which introduces harmonic distortion); from the frequency response curve, the EQ's influence may seem minor but the actual perceived impact is much greater. When you turn up the gain and boost a frequency band, you're mostly boosting harmonics, not the actual frequencies. The graph below shows different gain settings and EQ curves.

Another important thing to note is the interaction among various elements of the EQ section. All the tone controls (the GAIN, BASS, MIDDLE and TREBLE knobs along with the TOP switch) are mutually dependent. Try boosting the bass at min and max gain: the result will be very different. When the MIDDLE knob is fully clockwise, it pretty much eliminates the mid scoop (flattening out the frequency response curve in the process). This close interaction allows for a rich sonic palette. Analyzing endless frequency response curves doesn't sound like a great idea: double the incoming signal's loudness (boost it by 6dB), and all the curves stop mattering. Thus, we're going to try something different.

We're going to demonstrate the relative impact of each control; it's always there but it also interacts with the rest of the signal chain and gets altered by harmonic saturation. Let's start with the classic EQ position known as "everything at noon". The GAIN knob is turned fully down (to minimize harmonic distortion). The graph below is sure to send audiophiles into a state of shock! This graph will be our starting point for further measurements.

So how does one get a "perfectly flat response curve" from the Olympic MkIIIS (or the Twin MkIIIS for that matter)? That's easy: turn the MIDDLE knob to the max, then turn the BASS and TREBLE knobs fully down, and the response curve will be as neutral as it gets!

Don't be scared by the frequency cuts at the sides: they are part of the "magic". The slight low cut around 20Hz and the mild roll-off above 5kHz make the instrument sound better to the human ear; besides, such settings aren't commonly used in practice (although you can surely try that out). The two extra EQ knobs serve the exact purpose of compensating for the frequency cuts and highlighting the edges of the range. Let's return to the MIDDLE knob, though.

This knob's influence on the preamps' frequency response cannot be overstated. We prefer to keep it around 3 o'clock or fully clockwise; however, we can't stop those willing to start with "everything at noon". To be fair, "everything at noon" can be the right setting if you wish to give your sound some "body" and highlight the lows and highs. Let's look at the graphs.

Hilarious, isn't it? When everything is set at noon, you can't even flatten out the mids, let alone boost them; at the same time, the range of the sweep is rather wide and mostly focused on cutting the mids. To be more precise, the frequency band is around 500Hz. From 12 o'clock to the max, the knob provides a 4dB boost; from 12 o'clock to the min, it cuts 12dB (in a narrow band, which is once again part of the "magic"). At the same time, the overall gain across all frequencies has a 3–4dB range (in other words, the MIDDLE knob adds some gain). That's the secret to very mild and organic control over the mids. Wait, what about the BASS and TREBLE knobs — don't they have an impact? You bet they do! Let's look at the BASS knob know.

The BASS knob affects everything starting from the lows and all the way up to the mids — just like it should in an old-school preamp. In fact, this knob's impact spans beyond the 1kHz point. There aren't as many nuances as with the MIDDLE knob; still, there are some cool surprises that require a whole bunch of filters to be applied if done digitally — yet an analog device just does that on its own. What do the graphs say?

We've got the classic gentle +7dB boost and –8dB cut from the "everything at noon" position; however, the top graph barely shows any low cut. But wait! The low cut is very pronounced if you actually listen to the sound — particularly with the gain turned up. Now look closely at the graph once again: if you turn the BASS knob down to the minimum, you're actually giving a slight boost to the mids! Let's see what unconventional stuff the TREBLE knob can offer.

The highs are where all the wonders are. This knob can make your bass chime or meow, as well as freshen up your strings or help fit your instrument into the mix.

With that said, on to the TREBLE knob graphs!

Note the knob's influence on gain: not as pronounced as in the MIDDLE knob's case but still very characteristic. Also note the shift in the mid scoop that occurs when you turn the TREBLE knob. Another thing to consider is that the TREBLE knob boosts the 1kHz band when fully clockwise and the 300Hz band when fully counterclockwise. The knob's relative impact from 12 o'clock to the max is +5dB; from 12 o'clock to the min, it's –10dB at 20kHz. And yes: discounting the gain, the TREBLE knob affects everything in the 300 – 24000Hz range. Yet if you turn the knob and listen to the sound, you won't hear a really hard cut. That must be magic again (not really, but kind of). However, we've always felt that the earlier revisions of our preamps didn't allow enough control over the high frequencies, so we've designed a pretty awesome extra tool for the MkIII: the TOP switch. Let's take a look at that one.

Remember the magical BRIGHT switch found on Leo Fender's amps? Well, it didn't appeal to us that much. Push the gain over 50%, and that switch's impact goes down below 50%. Turn the gain to the max, and the BRIGHT switch stops affecting the signal whatsoever. To make matters worse, this doesn't work the other way round, so there's no way to tame an excessively bright instrument. In a word, that's not what we wanted for our preamps. We've come up with a solution that works both ways and provides smart tweaks where needed — that's not your usual "treble bleed"! Take a look at the graphs below.

What do we have here? That's "treble bleed"! Right, but it works both ways, and it sounds the way we wanted it to. So what does this switch offer, considering it affects the same frequencies as the TREBLE knob? It allows to expand the dynamic range of control over the highs, and it controls the amount of harmonic distortion in the high frequencies. It also lets you fine-tune the interaction between your instrument and the preamp: for instance, it can tame a bright Strat (using the Twin) or add some brightness to a passive bass with humbuckers (using the Olympic). Besides, the graphs show a considerable difference in the way the TREBLE knob and the TOP switch affect the same frequencies. Don't forget that when you turn up the gain, the TREBLE knob's impact diminishes but the influence of the TOP switch remains quite substantial. We've got one more graph for you: it shows the impact the TOP switch has when the response curve is set as flat as possible.

As you can see, even this setting alone creates three very different "presets" for bass. If you are a fan of "flat EQ", you can use those as starting points.

That's it when it comes to the EQ section. Don't forget that the GAIN knob adds even more nuances: it colors the sound, boosts harmonics and introduces even more drastic EQ "magic" than described in this article. Just look through the graphs for various Olympic MkIII setting where gain is turned up. The good news is, even the weirdest-looking settings are highly usable! That's what distinguishes the passive EQ found in our preamps from a mindless "±18dB on every knob" setup. Less tweaking, more playing — that's what we're aiming for with our devices!

Oh yeah, what happens if you turn every EQ knob fully counterclockwise? Here's another graph for an answer (normalization applied — in reality, you won't hear much at all):