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Old 16th April 2005, 06:26 PM   #1
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Location: Toronto, Canada
Default Driving multiple amps simultaneously from a CD player

Hi there,

A friend wants to take the output of a CD player and drive
4 amps simultaneously. These would be small amps with their own
passive volume controls (either 4 GC's or 4 Sonic Impact T-amps)
In addition, he would like to add a passive 'master volume control'.

I wonder if something like this can work:

CD -> pot (100K?) -> op amp buffer (OPA2227?) -> 4 amps in parallel

In the final stage, the signal out of the buffer will simply be
split to the 4 amps, which in turn will drive 4 pairs of
ceiling speakers for background music.

I suspect a buffer is necessary to keep the input impendence
seen by the amps from being too high.

Does this have a chance of working? Will the volume/frequency
response of the other amps change appreciably if the
volume of one of them is adjusted?

If this doesn't work, can someone suggest an approach?

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Old 17th April 2005, 07:27 AM   #2
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Location: In the Wild, Wild West
Your approach sounds fine. Adjusting the volume on one amp should not affect the others. The only reason I can think of all hand is if the buffer op amp. sees too little impedance and the signal is dropped some due to loading.

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Old 17th April 2005, 08:11 AM   #3
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
opamp in follower mode will give a low output inpedance (a few 10's of ohms). add a 100R in series with output.
Make sure all the power amps have a DC blocking capacitor on their input. Otherwise the input bias of each amp will interact and screw up all the designers calcs.
regards Andrew T.
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Old 17th April 2005, 08:17 AM   #4
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Location: Chatham, England
AD811 works great for this application, or you could use the BUF634. Just make sure that you have lots of bypassing capacitance by the chip.
I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort
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Old 18th April 2005, 03:54 PM   #5
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Location: Toronto, Canada
Default Thanks!


Thank you for all your suggestions!

I'm curious if there's anything specific about the AD811
that may make it better than the BUF634 for this
application? (Or it is just that AD811 sounds better? )

One question regarding adding some resistence
to the output of the buffer. Is it to ensure that the
output impedence doesn't get too low relative to
the input impendence that follows?

For the input pot, how high can one realistically
go? I have a nice 250K stereo log pot but I
suspect that may be too high.

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Old 18th April 2005, 06:28 PM   #6
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
I know of 2 reasons for the inline resistor
1. to reduce the damaging effects of a short on the output, pulling plugs with gear live!
2. to isolate capacitance from the opamp output stage.

Another sugestion. Insert a high value (1M to 2.2M) resistor from live to ground at your output socket, to ensure 0Vdc at the output if this stage or the next stage has a leaky coupling capacitor.
regards Andrew T.
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