What is a 3-band passive EQ?
- Thread starter Sky65
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Egads
One, Two, THIRTEEN!
A passive EQ only cuts (doesn't boost) the frequency range covered by the knob. Active EQs can also boost. There are also some more interactive tone stacks There's nothing wrong with a passive tone stack. Play it. If you like it, cool. One thing to keep in mind is that the design of the amp includes tones of choices that affect the tone you get out of it. You never really get just the tone of the guitar, but louder. Every amp colors the sound. Again, not a bad thing. This is what leads to choices and options.
Thanks. Question. If the EQ only cuts volume of the frequency range it is related to, wouldn't a full 10 adjustment {all in so to speak} on all EQ knobs give you an unmodified sound straight from the guitar?
Thanks
Tom
Tom, nearly every traditional guitar amp on the planet uses passive tone controls. In fact, some amps have a "tone control bypass switch" to do just what you're describing. My Two-Rock Custom Reverbs had one, and I think my Mesa Mark V had one though my memory is a little fuzzy on the Mesa.
Also, Egads forgot about passive EQs that can indeed boost frequencies. The Pultec EQ, a highly desirable piece of vintage gear in recording studios, uses legendary passive EQ filters that can both boost and cut. Here's how:
A passive EQ filter can be designed to boost frequencies if the filters themselves have a fixed amount of drop in level across all frequencies, and then can make use of a buffer circuit to return the boosted levels to their original volumes as the controls are turned to boost. In such a case, frequencies can have gain as the buffer circuit restores the original volume level in that part of the frequency spectrum that is boosted.
In the case of the Pultec, the buffer circuit is engaged with a switch labeled "boost", or "cut".
In the case of a passive EQ that can't boost, but can only cut, that doesn't incorporate the kind of circuit mentioned above, you have to consider the fact that the amplifier itself is probably not a straight wire with gain. Guitar amps have numerous design quirks that make them sound like guitar amps, and they are not linear devices. So if you have all your standard passive EQs set to 10, you will still not have a signal that sounds truly like your guitar unless you have a specially designed amp that is more of a hi fi amp than a guitar amp.
So a guitar amp can easily be designed so that with passive EQ it still sounds good set at 12 o'clock with the bass and treble being "cut only" EQs.
The other thing I'd mention is that there are amps with active controls, but the problem in a guitar amp is noise.
In other words, as a practical matter, it isn't important whether there are active or passive tone controls on an amp. In fact, it's largely irrelevant.
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Ok. Thanks. I don't know what all this actually means to me as far as selecting an amp. Just trying to learn. As you guys are leading me to the water I guess what really matters is getting the sound I am looking for. How I get there is irrelevant.
Thanks
Tom
dsenoj
Old dude in a PRS T-shirt
Without knowing better, I'd think "3-band passive EQ" is fancy ad-speak for treble, mid, and bass controls.
Yes. Standard guitar amp tone controls.
Egads
One, Two, THIRTEEN!
Hey Les. Always lots of good info.
To split a hair, wouldn't the buffer circuit make it an active system? Instead of taking a signal and rolling part of the frequency spectrum (like a typical guitar tone control), it sounds like that Pultec EQ is putting more signal back into the chain.
Keep in mind, I don't know what I'm talking about, and assume I'm wrong. I do like learnin' up myself!
To split a hair, wouldn't the buffer circuit make it an active system? Instead of taking a signal and rolling part of the frequency spectrum (like a typical guitar tone control), it sounds like that Pultec EQ is putting more signal back into the chain.
Keep in mind, I don't know what I'm talking about, and assume I'm wrong. I do like learnin' up myself!
No, the Pultec type circuit is uses purely passive components to do the EQing. The frequency boost comes from reducing the attenuation through the filter.
An active circuit uses the negative feedback loop of an amplifier to increase gain at various frequencies, not reduce attenuation.
Reducing attenuation is how the passive EQ boosts a frequency, and increasing gain is how an active EQ boosts a frequency. Each approach has its fans, but it's generally felt by the recording community that a passive EQ in the Pultec style is a very special sounding device.
In fact, Pultec has started to make their EQ again. Their only real competitors are Manley, who also make a Pultec derivation, and Tube Tech in Denmark. All are pretty expensive pieces of gear, because the circuit is expensive to do correctly. An active EQ is less expensive to create. The Manley Massive Passive is over 5 grand, and the others are well over 3 grand. Those are expensive EQs, but they sound fantastic.
Egads
One, Two, THIRTEEN!
Thanks for the education, Les (and sorry for the thread hijack, Sky!). I've not heard about that type of passive EQ before. Sounds cool. I'm going to read up some more on those products.
The Pultecs were developed in the very early 1950s. This design did - and still does - things that no other EQs do. First of all, the sound was, as Paul R. Smith would say, "musical." They add something to whatever you run through them, a beautiful sheen on the high end, and a nice fat bottom. Plus you can crank them and they still sound amazingly good, no shrillness, no nonsense.
But the most amazing thing about them is that you could boost and cut the same frequencies, and because the boosts and cuts had slightly different curves, you could do things with them that no other EQs could do. This is possible whether you're working with an original Pultec, or with the Manley or the various clones. Pulse Technologies has been resurrected, and now you can buy the Pultec EQs under the Pultec name, though I don't know how close the re-creation is, I've heard some good things.
I've used the Manley version enough to really love the design. Run a mix through one, and it just sounds...well...like a record is supposed to sound. Even the Universal Audio plugin version sounds very good, though the hardware really does take you that extra little bit into "wonderful."
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