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Alembic Club » Alembic Basses & Guitars » Archive through October 10, 2004 » Archive: 2003 » Archive through December 31, 2003 » Electronics 101 « Previous Next »

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Username: kr4bass

Post Number: 5
Registered: 1-2003
Posted on Saturday, December 20, 2003 - 8:34 pm:   Edit Post

I'll be the first to admit that I am NOT the sharpest knife in the drawer and now I'll prove it..... can someone please explain the q-switch and low-pass filter to me. I don't have 'em on my Epic (duh) but I'd like to what it is I'm reading about. I guess I just wanna be more informed. You've all been so helpful with my other questions so I figured I ask this one as well.
Looking forward to reading some informative responses.
Advanced Member
Username: davehouck

Post Number: 253
Registered: 5-2002
Posted on Sunday, December 21, 2003 - 12:36 am:   Edit Post

Q switches and low pass filters are not found on many basses so it is nothing out of the ordinary that you would not be familiar with their workings. I've copied the following from a discussion of a few months ago.

Basically, the filter control is a low pass filter; as you roll it back it lowers the point at which it passes low frequency signals and blocks high frequency signals. To quote Mica, "the range is from 350Hz-6KHz. Wherever you set the filter, only frequencies below that cutoff point are passed." The Q switch works in concert with the filter and provides an 8db boost at the cut off frequency you've set with the filter. When the switch is off you get a flat response up to the filter setting. Again to quote Mica, "the Q affects mainly the attack portion of the note".
Username: bob

Post Number: 95
Registered: 11-2002
Posted on Sunday, December 21, 2003 - 12:48 am:   Edit Post


We have discussed this a number of times, but it tends to come up in a variety of contexts, and I'm not even sure I could tell you how to do an effective search on the topic (though it's worth trying, because there are quite a few interesting descriptions). Someday, a few of us might get together and put together an FAQ list... but I don't think I've taken a stab at this one yet myself, so here goes.

A low-pass filter allows frequencies to "pass" (be heard), if they are "low"er than the selected filter frequency. The opposite example is a high-pass filter, which allows higher frequencies to be heard, and elminates those below the filter frequency.

On Alembic instruments, the low-pass filter frequency control has a range of 350 Hz to 6 kHz. That means if you leave it "wide open" (6 kHz), you'll basically allow all useful frequencies (for a bass) to pass, and it won't really act as a filter at all. On the other hand, if you set it to the minimum of 350 Hz, then you are starting to filter out some of the highest notes.

With standard tuning, the G string on a bass has a fundamental frequency of just under 98 Hz (cycles per second), so let's call it 100 for convenience. Playing an octave higher doubles the frequency, so at the 12th fret on the G the fundamental is 200, and at the 24th fret it's all the way up to 400 Hz - which is below the minimum filter frequency of 350.

Now, in addition to the fundamental, you also have to think about the frequencies of the harmonics, or "partials". I won't try to explain all that tonight (where is that book???), but when you pluck an open G at around 100, you also generate tones at various multiples of that, and these overtones have an important influence on how the note sounds to you. So even with much lower notes, setting the low pass filter to minimum is going to make those notes sound less bright, because you start filtering out some of the harmonics.

To a certain extent, this is similar to turning down the treble. However, a treble control typically takes effect at some specific frequency, and turning it down lowers everything above that by a fixed amount. A picture would help here, but most tone controls are "shelving", meaning that they have a gradual effect over a small range, and then plateau or flatten out for everything beyond that point. In contrast, a filter will usually roll off progressively, and I believe the standard low pass filters used here decrease at 12 dB per octave.

Another way of making that last point is to note that these are not "brick wall" filters: instead of totally eliminating any sound above the filter frequency, the output is gradually reduced, so that one octave higher it will be 12 dB lower, two octaves higher will be 24 dB quieter, and by then you can't really tell anyway.


So in at least two ways so far, the low pass filter is different than a typical treble control in that (a) you can specify the frequency at which it takes effect, and (b) it is a little more aggressive in that it continues to reduce higher frequencies by greater amounts.

Now we finally get to the Q control. If you've been reading the brochure or whatever, then you've noticed that you may have a 2 or 3 position Q toggle switch, or a "continuously variable Q" (CVQ) knob. They all do the same thing, but with varying flexibility.

Draw yourself a picture: a graph in which the x-axis is frequency (using a log scale), and the y-axis is output level in dB (think of it simply as volume). With a simple low pass filter, and no Q applied, you would draw a straight horizontal line from 0 Hz (on the x-axis) to the selected filter frequency, and then draw a diagonal line downward from where the filter kicks in. The output level is rolling off at a continous level above the filter frequency.

This is exactly how it would look if you had the Q-switch or knob in the "off" position. But as you increase the Q, then right at the corner where the line (output level) starts to drop down, you would instead see a rather sharp hump - with the output level increasing right at the point of the filter frequency.

Someone else might fill in the specific values for the different Q switch and knob options, but the point is that (for example) if you set the filter frequency to 350 Hz and crank up the Q, then for a fairly narrow range (say 330-370) you will see a boost in output, before everything starts to drop off.

(I could be off on the numbers here, maybe the peak of the boost is earlier than the filter frequency, I forget. I have an ump-teenth generation xerox of actual measurements from a Series II with various settings of both frequency and Q, but it's too blurred to read the values - which is why the charts haven't been posted here yet).

Looking at your drawing again, this peak has both height and width. The height is measured in dB, and in more general audio terms, the width (or perhaps ratio of height to width?) is measured or referred to as "Q", with a tall narrow peak being considered a higher Q, and a more gradual, rolling hill sort of thing being a lower Q.

There is some terminology which confuses me here. I'm pretty sure that Q is normally used to characterize the height/width ratio of the peak, but Alembic describes their Q switch positions simply as dB (which would be height). Also, when you start looking at something like the SF-2, you learn that a higher "damping ratio" increases the "resonance", or gives you a higher and sharper peak, but that's more electronics than most of us need to understand (though it is consistent with my understanding of higher Q).

There is quite a bit of interesting research, on a variety of acoustic instruments, which suggests that similar resonant peaks (at frequencies specific to particular instruments) are largely responsible for determining the nature of the instrument's sound. This is a pretty complicated subject, but I believe there is some validity to the claim that the filter/Q approach is a more "natural" approach to tailoring sound, than the traditional bass/treble knobs.

Whatever. The bottom line is that you can turn down the frequency on the low pass filter to cut out the highs, and then also turn up the Q to give you a fairly dramatic boost right at the point where you've chosen to start rolling things off. It takes a little getting used to, and most people tend to over-do the Q at first, but it turns out to be incredibly flexible.

Seemingly simple question, long-winded answer. A graph or two would be really useful, but I don't want to make up the values, and besides - I'd rather go play for a while. Hope this helps.
Advanced Member
Username: davehouck

Post Number: 255
Registered: 5-2002
Posted on Sunday, December 21, 2003 - 8:05 am:   Edit Post

Excellent description, Bob!
Advanced Member
Username: bigredbass

Post Number: 211
Registered: 9-2002
Posted on Sunday, December 21, 2003 - 10:21 pm:   Edit Post


You won't find a better explanation than Bob's reply.

I thought I would supply what this is like to use in the real world. I'll assume you have the 'standard' Epic circuit with vol/pan/bass+-/treble+-.

I play a Spoiler with the following layout: vol/tone/2-position QSwitch/pickup selector switch.

I came to this bass after several 'active basses' with the same layout as your Epic. I had two issues with this layout:

1) Of course, the EQ cut/boost points were fixed.
Sometimes they were right, sometimes the 100hz
bass freq was too fat, the treble freq too
tinny, and there was no way to change them.

2) I found that with active boost on the bass, and
active boost possible at the amp, I'd wind up
with really outrageous tones from all this
boost happening. It was hard to get
an 'honest' tone through all of this
gain/boost, and the whatever noise was present
rose along with all this. It was an EMG P/J
with their BTS circuit. Of non-ALEMBIC
electronics, I appreciate EMG a lot, but it
was just m i s s i n g something. . . .

When I got my Spoiler, I was at first wondering just what would I do with just 1 tone control and this damn switch would make the tone control sound like a wah wah . . . OK, own up you other guys, we all did this at first. And it seemed a little strange as well that here were active pickups with a conventional tone control . . .

But one thing was immediately apparent: these were the cleanest, richest pickups I'd ever heard.
It was like the difference in a $50 'bingo caller' mic and a $2000 ribbon mic. And this was just with the QSwitch off, where the circuit acts like a (very good) conventional tone circuit.
HERE was that firm, honest sound I'd never found anywhere else, and I've played a LOT of basses in my time.

This is why I agree COMPLETELY with Mica that while there are well built basses by other small luthier companies, it's amazing that so many of them sound ALIKE since they are all using the same EMGs, Barts, bridges, etc. I was stunned one time to try a VERY expensive Fodera only to find it sounded exactly like a $300 Japanese bass I'd hot-rodded with the same EMG setup!

The 'Q' took a while to filter into my brain. Basically it opens up a whole 'nother palette of tones. Roll of all the highs, and you get a huge, fat, organ-pedal bass type of sound. Open it just a pinch up from there, add chorus, and you ease off into synth-bass, MiniMoog bass types of sounds. Open it WAY up, and you get impossibly bright, clavinet type tones, which get huge with a deep bass boost at the amp. And as Bob mentioned, the 'Q' deals its own boost around these frequencies which makes it REAL interesting.
Yet you throw one switch, and you're back to a conventional tone control.

My favorite tip: For a real John Entwistle / Chris Squire type of tone, use only the neck pickup, 'Q' on and more open than closed, and use a pick over the bridge pickup. With a big amp on songs where this tone is appropriate, you'll OWN the song and the stage.

It's amazing how many sounds I've found with 1 tone control and this damn switch that makes the tone control sound like a wah wah . . . . !

It's a different way of hearing things. For a lot of people, the bass/treble set-up may be better. But to truly hear the bass, ALEMBIC's uncolored pickups run through this uncommon tone circuit is unmatched for true clarity, to my ears.

J o e y

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