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Old 27th October 2008, 10:49 AM   #1
nandax is offline nandax  Indonesia
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Default Help about OPAMP

i'd like to connect audio output to summing amplifier using opamp, and connect the output to speaker (8 ohms impedance)
my question is:
- is it ok directly connect the output of summing amplifier to the speaker? how do i measure the output impedance of summing amplifier?

i am planning to connect to voltage follower first before connect it to the speaker for impedance transformer, will it be ok?

thanks for the help
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Old 27th October 2008, 12:35 PM   #2
lineup is offline lineup  Sweden
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summing opamp

is this the same as an inverting mixer configuration?

Click the image to open in full size.
http://www.ecircuitcenter.com/Circuits/opsum/opsum.htm

the only little issue with is that it changes the phase of signal 180 degrees.
Can be overcome by adding a 2nd opamp, with inverting gain x1
to get it back to normal phase.

I can see no problems with using a power follower from the summing opamp to fead speaker.
Not if it is done in a good way.
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Old 27th October 2008, 12:35 PM   #3
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Have a look at the datasheet of your op amp. Usually op amps have output current in the range 20 mA or a bit more. Without transistors to help them they won't be able to drive a speaker to serious output levels.

Try the search function for projects with op amps.
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Old 27th October 2008, 05:24 PM   #4
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It's best to separate these two things. Build the summer, then put the output of the summer into a regular power amp to drive your speaker.
Depending on the topology, some power amps may be able to accept summing resistors but to have a separate opamp stage makes you much more flexible. The schematic above seems OK.

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Old 27th October 2008, 07:31 PM   #5
AndrewT is offline AndrewT  Scotland
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re post2:
the output is -Rf/R1*V1-Rf/R2*V2-Rf/R3*V3
If R1=R2=R3, the output Vout= -Rf/R1*(V1+V2+V3)
NB. the -(minus sign) indicates change of phase.

The non inverting pin can be connected direct to Audio Ground. To minimise output offset, a resistor between +in and ground should be inserted and it's value ~Rf//R1//R2//R3. If all four resistors are 12k then the grounding resistor =3k0.

Finally, The inverting input is a virtual ground. Each input sees an input impedance of R1, or R2, or R3.
The source equipment will determine the lowest value of these summing resistors and the gain will then determine the value of Rf.
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Old 28th October 2008, 11:33 AM   #6
nandax is offline nandax  Indonesia
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Quote:
Originally posted by lineup
summing opamp

is this the same as an inverting mixer configuration?

Click the image to open in full size.
http://www.ecircuitcenter.com/Circuits/opsum/opsum.htm

the only little issue with is that it changes the phase of signal 180 degrees.
Can be overcome by adding a 2nd opamp, with inverting gain x1
to get it back to normal phase.

I can see no problems with using a power follower from the summing opamp to fead speaker.
Not if it is done in a good way.
thanks for the reply..

i just figure out that i have to use speaker plus and minus as input because it is differential signal, so i'd like to use differential operational amplifier instead of summing amplifier to amplify the difference and connect the output to voltage follower to the speaker, would that be a problem?
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Old 28th October 2008, 11:55 AM   #7
lineup is offline lineup  Sweden
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no, problem

either you can use one 'normal' opamp, in differential mode
or use specially dedicated differential opamp

if the difference between input signals is very small
you may have to use more voltage gain, than for normal amp
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Old 28th October 2008, 01:41 PM   #8
nandax is offline nandax  Indonesia
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Quote:
Originally posted by lineup
no, problem

either you can use one 'normal' opamp, in differential mode
or use specially dedicated differential opamp

if the difference between input signals is very small
you may have to use more voltage gain, than for normal amp
thanks for the reply.
as u can see in the schematic i've made there are 2 inputs, speaker plus and minus with the level of
Vpp : nom : 2.1909
max: 3.156
and the voltage gain of OPAMP is 3
do u think the voltage gain would be big enough, for the output for the speaker?
and is the connection ok? for testing?
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Old 28th October 2008, 02:02 PM   #9
AndrewT is offline AndrewT  Scotland
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I doubt your speaker output is a balanced signal.
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Old 28th October 2008, 02:34 PM   #10
nandax is offline nandax  Indonesia
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Quote:
Originally posted by AndrewT
I doubt your speaker output is a balanced signal.

yes my speaker output is just 8 ohms load, and connect the minus connection to ground and plus to the output from opamp
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