Passive vs Active EQ on amps - what the pros and cons?

[Naetharu]

Hi folks,

I'm still looking for a new amp as I'm not too keen on the tone of my Ashdown in a band situation. One thing that is confusing me a little, however, is the difference between amps with Passive and amps with Active EQ options.

Is the differecen the same as with pre-amps built into a bass - i.e. the former reduction only while the latter +/- or is there something more going on here?

Further to this, is there a reason why passive EQs might be a good thing? I ask this as I have seen a few amps that advertise 'fully passive eq' as a key feature making me wonder why one might choose it over an active eq that, I presume, does the same thing only with a few more options.

All the best

James

[Tdw]

Yes passive eq offers only cut no boosting of frequencies. The only kind of passive eq i am familiar with is the fender/Marshall style tone stack (bass, mid, and treble controls often with a deep/bright switch) , its usually found on valve amps, especially for guitar.
The advantage of this (as far as i know anyway) is that because there are no amplifying elements and less parts in general in the eq the sound remains purer and on valve amps it means a pure valve signal path with no solid state amplification components at all can be practically attained. An active valve based eq would add a lot of cost, weight etc (the fender /Marshall tone stack is basically just made of resisters and capacitors). The disadvantage of this eq is that it only really offers various forms of mid cut and is not intuitive, the position of one control affects what the others are doing and the mid control can almost become another volume knob. However valve bass amps with this kind of eq such as the Trace Elliot v types often have a very high quality sound. So this kind of passive eq offers a purer signal path but active eqs usually offer the option of more kinds of eq shape.
Hope that helps

[LukeFRC]

Passive can only cut, active can boost. You can get not so good things happening if you boost- but having said that a well designed active EQ can sound nicer than a badly designed passive Eq.
Moral of the story is don't worry about it - if it sounds good and you like it - use it.

[tauzero]

Passive eq can boost and cut, depending on what it is. The James circuit was the passive predecessor of the active Baxandall circuit, and like the Baxandall, offers both boost and cut.

James:

[url="http://www.thermionic.info/james/James_SimpleToneControl.pdf"]http://www.thermionic.info/james/James_SimpleToneControl.pdf[/url]

Baxandall:

[url="http://www.thermionic.info/baxandall/Baxandall_NegativeFeedbackTone.pdf"]http://www.thermionic.info/baxandall/Baxandall_NegativeFeedbackTone.pdf[/url]

[ahpook]

I've just switched from using a preamp with an active EQ (an Ampeg) to a passive one (a Trace Elliot), and I'd say that the passive EQ doesn't give the instant gratification of an active one, in that sometimes you need to get a boost by cutting elsewhere and then turning up the gain, and that there's some interplay between the controls....

However, the passive EQ does seem more....I dunno, 'musical', for want of a better term... I'm still learning and haven't done any very loud stuff with the new preamp, but I'm warming to it.

If you're electronically minded then downloading [url="http://www.duncanamps.com/tsc/"]Duncan's Tone Stack Calculator [/url]and playing with the controls will give you an idea of how a Passive EQ functions. There's a few different designs, the 'Fender' model is the same as Trace Elliot and Alembic use in their preamps.

[tauzero]

[quote name='ahpook' timestamp='1443975606' post='2879126']
If you're electronically minded then downloading [url="http://www.duncanamps.com/tsc/"]Duncan's Tone Stack Calculator [/url]and playing with the controls will give you an idea of how a Passive EQ functions. There's a few different designs, the 'Fender' model is the same as Trace Elliot and Alembic use in their preamps.
[/quote]

And the James one illustrates boost and cut in a passive tone stack.

[ahpook]

[quote name='tauzero' timestamp='1443976524' post='2879133']
And the James one illustrates boost and cut in a passive tone stack.
[/quote]

A few of them boost if memory serves me.

[EliasMooseblaster]

Whilst I can't comment on James or Baxandall circuits, I did find this article extremely helpful in understanding Fender/Vox/Marshall-style passive tone stacks: http://pickroar.com/1003/the-tone-stack-explained-in-english-for-humans/

Both of my Ashdowns appear to have passive tone circuits in this family, and it's certainly true that they're not as intuitive as a nice, big, active, multi-band graphic EQ! They took some getting used to, especially as all the SS Laneys I'd used before had cut/boost controls, but I've come to rather like the simplicity they force upon you. There is a bit of an art to understanding them, but once you get past that you still, ultimately, have a three-band EQ with a lot of flexibility (even more so if the amp has deep/bright/mid-shift switches). On the other hand, if you're one of those players who's quite certain that "their sound" absolutely needs that specific boost at 800 Hz and a slight cut at 120 Hz then perhaps active will serve you better!

[Chienmortbb]

I don't belirve that Passive EQ can boost. With controls set flat it cuts everything by a certain amount but mimics boost by cutting some frequencies less than others. You need a lot of gain before or after passive eq to make up for the loss of signal. Passinwnd is the best guy to ask though as he designed the preamp that we are using with active bass and mid but a passive treble control.

The real answer though is to use your ears.

[Tdw]

I think it might be that some preamps have a mid cut built in as it were before the james tone stack so on flat it kind of has a mid cut (reducing the overall gain level), make it seem like the bass/treble is boosted when "fully up" but i dont think that anything is actually boosted, i think its all achieved with passive reduction of frequencies, although i am far from sure about all this.

[Passinwind]

[quote name='Chienmortbb' timestamp='1443997438' post='2879399']
I don't belirve that Passive EQ can boost. With controls set flat it cuts everything by a certain amount but mimics boost by cutting some frequencies less than others. You need a lot of gain before or after passive eq to make up for the loss of signal. Passinwnd is the best guy to ask though as he designed the preamp that we are using with active bass and mid but a passive treble control.
[/quote]

Boost and cut are always relative to something. I'd suggest that measuring at your speaker's output relative to your bass's signal level is by far the most relevant measure. Nothing wrong with looking at the system as a black box:

[quote name='Chienmortbb' timestamp='1443997438' post='2879399']The real answer though is to use your ears.[/quote]

And of course for those of us who aren't satisfied with just doing that, a deep rabbit hole awaits.

[tauzero]

[quote name='Chienmortbb' timestamp='1443997438' post='2879399']
I don't belirve that Passive EQ can boost. With controls set flat it cuts everything by a certain amount but mimics boost by cutting some frequencies less than others. You need a lot of gain before or after passive eq to make up for the loss of signal. Passinwnd is the best guy to ask though as he designed the preamp that we are using with active bass and mid but a passive treble control.

The real answer though is to use your ears.
[/quote]

I suppose it depends on what you count as boost. The Baxandall design makes up for the fact that the James network cuts the overall signal to about 1/3 of the input level so the whole lot needs more amplification afterwards. However, the tone controls don't behave as cut-only.