Re: Audio Amplifier Question


"BobW" <nimby_NEEDSPAM@xxxxxxxxxxxxxx> wrote in message
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"Geoff" <gszczypi@xxxxxxxxxxxxxx> wrote in message
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Hi,
I am working on an audio project with split +/-12V supplies and now have
a
line level output that I would like to connect to a power amplifier like
the
TDA8552 to drive a couple of 8R 1Wspeaker. The AMP has an input rated
for -0.3V to VDD+0.3V with a set gain of 20dB so I was going to
attenuate
the signal enough so it doesn't overdrive the speakers at max volume.
Im
guessing that since it's line level and could be 2.82Vrms(4Vp-p) MAX and
I
need 2.82Vrms across the 8R speaker to get the maximum 1W power I want,
I
should attenuate the signal by 20dB? Am I on the right track?

Also, how does the amplifier treat the negative part of the AC signal?
Does
it get amplified or does it need to be above ground level or removed? I
am
a bit confused about how to interface this -/+ audio signal to this
signal
supply AMP.

Any comments greatly appreciated
Regards,
Geoff.


Geoff,

According to the data***, this part runs on 5.5V max (not 24 volts like
you have).

The music's signal level will vary according to the source material, so
you
need to have a volume control somewhere in the system. The 8552 has an
internal digitally-controlled volume control. Its default state is set
to -40dB, so if you've set the 8552's maximum gain to +20dB then the net
gain will be -20dB at power up. That's not what you've anticipated, I
assume.

If you don't have a 5V supply available, and you don't want to deal with
the
8552's digital volume control, then I suggest you pick a different
amplifier.

Regarding how the 8552 deals with the "negative part" of the signal -- the
8552 biases its input to Vdd/2, so assuming it's on a 5V supply then
anything above 2.5V will drive the output positive and anything below 2.5V
will drive the output negative. Since most users will not be able to
create
a signal with it centered around the 8552's Vdd/2, typically what is done
is
to capacitively couple the input signal from its source. Since the 8552
has
a built in resistive load to Vdd/2 (on its input) then the net affect is
that the signal's average level (its zero) is translated up to the 8552's
Vdd/2 (2.5V) level. This technique is probably what you'll use for
whatever
amp you select.

Regardless of what amp you select, you need to read and understand its
data***.

Bob


Thanks heaps you're your time Bob.



I should have mentioned that I have a 5V supply for the 8552 and didn't plan
to run it on the +/-12V (: I also understand that it will originally
attenuate the signal on start-up so did plan to use the volume controls. I
also made a mistake earlier, 4Vp-p is not 2.82Vrms, its 1.41Vrms, this is
what I have been told line levels can peak too. So if I have a potential
1.41Vrms input and I need 2.82Vrms across the speaker for max power, I only
need 6dB gain instead of max 20db that the amplifier offers to get this
drive so I need to attenuate by 14dB, or 5 fold so that clipping doesn't
occur at the maximum input voltage. Sound right?



That's very interesting about the input being held at VDD/2 and obviously
answers all my questions in that department. Are most Amp's like this? I
had a good look through he LM386 and TDS8552 datasheets and couldn't find
any mention.. No wonder I was confused.



Thanks again.



Regards,





Geoff


.