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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: O.K. ... fine ... posted by Mart on May 18, 1999 at 06:21:21:
***if they aren't current-sources & no self-respecting E.E. has ever referred to these devices as simply tubes in a schematic? What do you call them?
FWIW, a lot of A.E.'s have the very same problem with the S.S. pseudo current-sourcing, because of the Qes problem.***Thus, if S.S.'s aren't current-sources & tubes aren't either, how do they make so many current-source devices?
Tubes, transistors, MOSFET's, etc. are just amplification devices. What you do with them in a circuit is totally different matter. You can build a voltage source or a current curcuit using any of the above.
HOWEVER! All audio amplifiers ARE SUPPOSED to be voltage sources, not current sources. Why? History, simply put. Voltage interface was created before current interface and it is much easier to understand. Therefor, almost all audio components use voltage as interface signal.
Your speakers are too designed to work properly when driven only with constant voltage source, not current source, in order to reproduce flat response. It is presumed that the voltage source will have enough current capability to maintain its voltage into variable load - but that doesn't make it "current source" or "current-mode amp", it is simply a part of a voltage source definition. Strictly speaking, you get the closer to the voltage source model the lower is your output resistance.
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Back to the subject. Do you know why low-Z output desensitizes cables?Output resistance (and impedance) of any amplifier will work together with its load parameters to create a frequency-dependent network. That may or may not be important. In a power amp with say, 1 Ohm output resistance there will be some frequency response fluctuations caused by the speaker's impedance curve.
The situation is similar in case of preamp output and interconnects. There you usually have to consider the cable/power amp input capacitance. For example, a pream with a 1k output resistance will create a high frequency roll-off with a -3dB point at 318kHz when combined with a cable with 500pF capacitance.
So yes, reducing output resistance of your voltage amplifier (either a preamp or power amp) reduces its sensitivity to load variations. How low it should be - that is not possible to answer quickly. Depends...
I have seen some attempts at representing a weak point in amplifier design (its high output resistance) as something positive, by claiming "a more proper current source operation". This is just balloney, plain and simple. Unless some day someone creates a speaker that requires a current source (totally possible, but it will not work with today's amps, unfortunately), any amps with high output resistance shall remain somewhat deficient. Deficient enough? That is a different question. Some other virtues of such designs might make them still desirable, but certainly not their incorrectly supposed "current source" operation.
Hope that helps,
Victor.
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PS: What's with the anger?
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Follow Ups
- Re: O.K. ... fine ... - Victor Khomenko 06:58:29 05/18/99 (0)
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