Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on December 10, 2011, 11:44:49 am
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To be honest.....I really do not know what the difference is. My limited experience is that "active" involves some type of power, battery, amplification, etc.
But I have no idea.
What is the meaning of active and passive in the world of Tube Guitar Amps.?
Thank You
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What you said.
Active involves amplification (or amplification plus feedback -> zero net gain but changed operating characteristics). Passive involves everything else.
A "passive effects loop" might simply be jacks added at a point in an amp that is a convenient place to break the circuit and inject effects. An active loop might do the same, but then add return gain for level matching, or an active send stage for low output impedance.
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Active and passive may also refer to tone controls. 99% of tube amps use passive tone controls, in that they cut out or drop out certain frequencies only.
Active tone controls may actually boost or cut certain frequencies.... Each control would have it's own amplification stage allowing that, it's expensive to do with tubes so you will rarely ever see such a thing. It is much more common with solid state circuitry.
j.
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OK....Yeah.....The Solid Sate comment makes sense I guess. Seems like a lot of bass/bass amps have active controls.
It dawned on me that I hear/read the term all the time....but was not sure what it REALLY meant.
Thanks
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... Each control would have it's own amplification stage allowing that, it's expensive to do with tubes so you will rarely ever see such a thing. ...
Or the active tone controls could be within a feedback network (which is similar, but not same as what you said).
For example, there is an arrangement of tone circuit components to provide treble and bass controls. Signal level is cut to a middle value, and there is the appearance of a boost when either control is turned to provide less than mid-level loss. The midrange is indirectly control by the relative settings of the treble and bass controls.
The passive version of this circuit is called the "James circuit". The active version is the "Baxandall circuit". The difference is that the Baxandall uses extra tube stages and places the controls within a feedback loop made up of the extra stages.
The Baxandall can allow true boosting, as well as cutting. Does it matter? It seems like that should be a cool feature, until you realize that you will have to dedicate one or two triodes just to allowing the tone circuit to be active. Could you live with the cut-only action of the James circuit if you had 2 more tube stages to boost up the signal?
As BB said, it's much more common in solid state circuits, where you don't spend a lot of extra real estate or waste heat on amplification stages that don't do much to amplify the signal of interest. However, there are a lot of tube circuit out there that use them; they're simply not that common in guitar amp stuff.
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Interesting......
Thank You
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They are all "active" in some sense.
You first design an amp with just enough gain (because more cost is less profit).
Add tone control. Passive controls can not boost. Users want boost. Therefore you cut everything, and un-cut for "boost".
The amount of cut means you will need more gain. If the first design was minimal, you probably need another "active" stage.