Caps in a differential speaker output

[jesup]

What would be the impact of caps on the output lines to a speaker if it's
being driven in differential mode? Say 10uf in series on each line, with
a 16ohm speaker? I can calculate a normal single-ended RC circuit, but
I'm not sure about this case. Would the low-frequency phase shifts match
or go in opposite directions (which would cause cancellation by the two
drivers at low frequencies)?

Thanks!

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[Phil Allison]

"jesup"

What would be the impact of caps on the output lines to a speaker if it's
being driven in differential mode? Say 10uf in series on each line, with
a 16ohm speaker? I can calculate a normal single-ended RC circuit, but
I'm not sure about this case. Would the low-frequency phase shifts match
or go in opposite directions (which would cause cancellation by the two
drivers at low frequencies)?

** What a dopey question.

Far as the speaker is concerned, there is only one drive signal - ie that
appearing between the power stages outputs.

Normal RC calcs apply.

The two 10uF caps are in series so equal to 5 uF in series with 16 ohms


..... Phil

 

[jesup]

jesup had written this in response to
http://www.electrondepot.com/electrodesign/Re-Caps-in-a-differential-speaker-output-545974-.htm
:
I know it's probably a silly question to an EE, but I'm a software guy,
and all the examples show no caps for a differential bridged headphone amp
- but the HW engineer likes caps, and dropped them in, and I'm trying to
debug a low-frequency (hard - 10+db/octave) rolloff.

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[legg]

What would be the impact of caps on the output lines to a speaker if it's
being driven in differential mode? Say 10uf in series on each line, with
a 16ohm speaker? I can calculate a normal single-ended RC circuit, but
I'm not sure about this case. Would the low-frequency phase shifts match
or go in opposite directions (which would cause cancellation by the two
drivers at low frequencies)?

Thanks!

Attempts to reply normally to this thread seem to bounce.

You should probably be measuring SPL, rather than voltage; but the
voltage will show low frequency reduction from ZRL/xc. A simple
LTSpice circuit is attached for a passive example of varying
inductance. A Dynamic driver is more complex.

Dynamic driver impedance may be complex at the low end; there's
usually a peak as the frequency reduces, before it begins to look
uncoupled. Low power dynamic transducers may be better controlled, as
the air volume moved is small, and efficiency is low.

Your hardware guy's insistence on caps in series with the output may
be misplaced, as these parts will not do much to protect the drivers
from a short circuit, or reduce audibility of start/stop thumps, in a
full bridge.

I assume you're expected to overcome the added loss, in software
equalization of the processed signal. In that case, you should point
out to the hardware guy that you can only do this over a fairly narrow
range of known load LF characteristics - and you that need to know
what they are. That's a hardware measurement issue. If he's been using
16R resistors for loads and measuring the voltage, it's time to get
real.

RL

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