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Amp passive tone controls?

M

manchild

Jan 30, 2008
512
2
#1
Hi all,

Been looking for a better understanding of this but can't seem to find it...

I recently bought an Ashdown MIbass 2.0 head and cab. I had an email from them saying that the tone controls are passive with the following spec:

Bass: Shelving from 100hz and below
Middle around 800hz
Treble: Shelving at 4khz

Each with 20db range.

If it is a passive tone system (in that it is caps and resistors - thus not able to boost) is flat at 12 noon or with the tone controls maxed? If it is centered does that mean that the bass and treble do the same thing but in reverse order with the middle acting as a band pass filter from 800hz?

I find the tone controls to be quite subtle and transparent which I understand is due to being passive?

Can anyone explain how this works?

Thanks.
 
#2
20db doesn't sound very typically passive. Just being passive doesn't really tell you anything about what it does or how it sounds, there are loads of passive tons stack circuits that do various things. 20db is far from subtle. Where did you read it was passive? Sounds like might be confusing with one of the valve front end amps.
 
#4
Passive controls means that they are cut only, no boost. What Fender does to assure that there is no loss of volume is boost the entire signal after the tone controls with a gain recovery stage. If you take an amp like a Twin
Reverb, the tone controls are not at all subtle, in fact they're quite dramatic.

The big difference I hear with active vs. Passive is passive are more interactive, where active are more independent, meaning with passive if you crank up the mids you may also increase treble.
 
#6
BassmanPaul said:
Quoting Bob-i: Passive controls means that they are cut only, no boost.

Not true Bob. The Baxendale tone stack as used by Ampeg is cut and boost with flat being at the middle position. The Mid control, as used in the SVT amongst others, is fully active.
Click to expand...

Good point, that's the exception. The bax stack is still passive by definition and can boost and cut. I've never been much of a fan of the bax outside of the classic ampegs. Gimme the FMV anyday ;)
 
#7
Thanks for the replies guys - I emailed Ashdown and they gave me the above info in reply. I find the tones to be subtle in the sense that they only appear to make a difference if you crank them quite a lot - so with that said maybe the person was not qualified to talk about the products as I have used amps that make a big difference with only a light amount of movement.
 
#9
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#10
There are tons and tons of different kinds of passive EQ circuits an amp could employ. Some respond to controls rather logically: The control adjusts it's specific band of frequencies, range going from boosting to cutting, middle of the dial being closest to flat response at the given band. Some others are different beasts: some controls merely cut while some boost and cut, adjusting one control may adjust the range at which others work as well as the band of frequencies at which they operate on and the entire range of the said controls. Very interactive and illogical.

There is no way to tell how the tone controls in some particular amplifier work from control panel arrangement or from this, IMO, very limited information given by the manufacturer.

Being "passive" means zip. All it tells us is that its going to be more "lossy" and lower Signal-to-Noise-Ratio process than with active controls. Details of operation with both active and passive controls may wary widely as there are dozens of different types of tone controls circuits found from each category and the designer may always adjust them to work according to his preference.

Sometimes I hope manuals would display clear graphs of the EQ's effect on frequencies instead of Ashdown's facepalm-worthy solution to provide vital information to their customers. Quote from MiBass 2.0 manual concerning the EQ: "This passive EQ has, bass, middle and treble controls". That's all information concerning the EQ given by the user manual. All. Oh, thank you user manual for pointing the obvious. I clearly couldn't have simply glanced at the amplifier for one second to get this information.
 

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