Preout / buffer op-amp

[yakideo]

Hi everyone,

I'm currently building an tripath 2020 amplifier. I would like to add an preout (subwoofer) for my diy amp. I've been browsing around for a while and came to the conclusion that the best way to do that is to add an buffer op-amp that will create sort of a copy of the orginal input.

I was thinking of something like this:

input --> dual pot (stereo) ----- op-amp --> preout
********************* |
********************* |
**************** tripath amplifier

Can I just "branch" off the input and feed it to the op-amp?
If the preout is going to be mono, how do I mix down the input?
Is AD8616 a good choice?
TI has some Line Drivers (eg. DRV600), are they better suited for this kind of application?

Sorry for my bad asci art

[AndrewT]

Hi,
use an inverting opamp topology.
Instead of one input resistor, you fit two input resistors.
The inverting amplifier adds the signals it sees coming from the two resistors.

If you use an MFB filter, it is already inverting.
It can be used as an adder/summer to mono and as a low pass filter to feed the line to the bass amplifier.

The virtual earth at the input of an inverting opamp/amplifier reduces crosstalk between the stereo channels to inaudible proportions.

[yakideo]

Originally Posted by AndrewT

Hi,
use an inverting opamp topology.
Instead of one input resistor, you fit two input resistors.
The inverting amplifier adds the signals it sees coming from the two resistors.

If you use an MFB filter, it is already inverting.
It can be used as an adder/summer to mono and as a low pass filter to feed the line to the bass amplifier.

The virtual earth at the input of an inverting opamp/amplifier reduces crosstalk between the stereo channels to inaudible proportions.

What is an MFB filter? Are there any good resources for audio op-amps? I'm kind of new to the op-amp world...
Why is inverting op-amp topology better then the non-inverted (eg. input to +)?

[AndrewT]

non-inverting does not make a good adder/summer.
As soon as more than one channel is connected to the input the two channels get mixed, i.e. you lose the stereo.
An inverting operational amplifier (opamp) was used to add in an analogue computer. you are doing the same.
Your mono bass channel requires the addition of the two channels while preserving the separation (isolation) of the existing channels.

Stick MFB filter into search or stick it into google.

[theAnonymous1]

There is a summing amplifier example and explanation at this link......

Audio Designs With Opamps - 3

[AndrewT]

and Fig29 shows the MFB with R1 for one input channel.
Add in extra R1 for each addition channel you want to "add".