High Q or low DF Effects the same? ATTN NP & MJK - diyAudio
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Old 2nd March 2010, 01:18 AM   #1
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Default High Q or low DF Effects the same? ATTN NP & MJK

I have been musing a bit on Nelson Pass' work with transconductance amplifiers with high efficiency FR drivers. I am intrigued by the inter-relationship of driver Q and amplifier Zout. Mr. Pass' paper deals with full range drivers with strong motors relative to their moving mass (at least that is the way I interpret it). It seem that this also could apply then to low Q professional bass drivers as well.

This line of thinking brings me to Martin King's work with high Q helper woofers in open baffle systems (and this is how it relates to subwoofers). Lets look just at the open baffle situation for a moment. The highish Qt drivers that are indicated for this application arrive at their high Q I believe primarily due to their high Qes related to relative motor strength compared with cone mass. The high Zout amplifier is often analyzed as an increase in Qes brought on by a resistance in series with a voltage source amplifier.

Now if this is really an increase in Qes the question becomes is a low Q speaker with transconductance amp completely analogous to a high Qes speaker with a voltage amplifier? That is to say is the resulting Q the only factor or are there factors (other than cost) that would argue for the use of one approach or the other being preferred?
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Old 2nd March 2010, 12:43 PM   #2
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Quote:
Originally Posted by mashaffer View Post
...
The high Zout amplifier is often analyzed as an increase in Qes brought on by a resistance in series with a voltage source amplifier.
...
For the frequency range around fs i would consider both ways as
equivalent, as you described it.

In the range where VC inductivity comes into play, current lowers
with rising frequency, when a voltage conroled amp is used.

With a constant current amp there will be relatively more
output in the middle frequencies. That will IMO lead to different
alignments of crossover and correction networks.

In cases where the VC inductivity is equalized, there should be
indeed no difference between both approaches.

Kind Regards
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