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Constant input/output impednace attenuators (Ping: CheapJack)

Posted by Steve Eddy on October 28, 2007 at 23:15:06

Sorry, lost track of the thread below.

How can pi, T, H & O type attenuators offer constant I/P and O/P impedance?

Because of its topology. Here's a page which shows the various topologies along with a calculator you can use for plugging in your desired values:

Fixed Attenuators

While it says "Fixed attenuators" the topologies aren't limited to a single fixed attenuator.

Want to quote some brandname models?

Don't know off the top of my head anyone making them today. Company called Daven used to make them, mostly for professional audio applications. Here are some on the Surplus Sales of Nebraska website:

Audio Attenuators

Back to the original post, an advantage of a stepped attenuator over a volume pot is, IMO, the inherent noises produced by the rotation of the wiper shaft contacting the resistance film lining can be eliminated. The noise produced is proportional to the speed of rotation & the voltage across the swiper contact point to the grounding point of the point.

Yeah. Though I don't know many folks who listen to music while constantly changing the volume.

Also, it provides constant I/P impedance to the driving sources, e.g. CD player, preamp etc). Unlike inductive passive attenuators (using interstage transformers), where its I/P impedance & O/P impedance change bigtime with frequencies of the signals as looked upon by the driving source & receiving end (preamp or power amp).

Well, there are those who would argue that this isn't a problem as the more you attenuate the signal the more ideal the input and output impedances become.

The choice of I/P & O/P impedance depends on the O/P impedance of the driving soursce, say, a typical DVD player is 600R, & the O/P impedance of the power amp. Surely it's not simply "if you want" that easy.

Depends what your priorities are I suppose.

Anyway, I wasn't advocating any particular solution. Just pointing out that you can create an attenuator with constant input and output impedances.

se

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