Summing opamp + Headphone amp question

[juan_felt]

Hi!

I'm working on a project where I want to build a practice headphone amp, where I can plug a bass and a Line in.

The bass output will be buffered, and I want to work with this headphone amp


The question is how can I split the Mono signal of the bass into two mono ones, and mix them with each stereo signal from the line in, before going into the amp, and without causing any issues.
I was thinking of using this to mix each signal, but I don't know how to properly split the mono one.



Thanks a lot!

[PRR]

You "split" a signal by just typing both inputs to the one output.

Your power amp plan has no connection for DC power to come in. But it looks like single-supply. Your mixer plan seems to be dual-supply. Do you know what you are doing?

Both plans are unity-gain. It will take a big signal, or more gain, to work with ordinary sources.

[juan_felt]

Hi, first thank you for your reply.

You "split" a signal by just typing both inputs to the one output.

But if "split" the mono output and feed each new mono signal to each channel of the stereo signal coming from the line in, won't create a connection between both stereo ones?

Your power amp plan has no connection for DC power to come in. But it looks like single-supply. Your mixer plan seems to be dual-supply. Do you know what you are doing?

Both plans are unity-gain. It will take a big signal, or more gain, to work with ordinary sources.

I have an idea on what to do, why would the mixer be a dual supply? I was planning on biasing the DC power from the 9v battery and then using it to power it up.

Regarding the gain, my idea was to change it.

Thanks again!

[PRR]

[ElectricDruid]

Hi, first thank you for your reply.

You "split" a signal by just typing both inputs to the one output.

But if "split" the mono output and feed each new mono signal to each channel of the stereo signal coming from the line in, won't create a connection between both stereo ones?

No, because the "join" is at a virtual ground, and there's no signal at a virtual ground. That's the clever thing about active mixers. See PRR's diagram, where he helpfully indicated this.

I have an idea on what to do, why would the mixer be a dual supply? I was planning on biasing the DC power from the 9v battery and then using it to power it up.

Regarding the gain, my idea was to change it.

The mixer looks like dual supply because the +ve inputs are tied to ground. They should be at whatever the mid-point voltage is, so if that's ground, that implies a bipolar supply.

You'd need to make a Vb midpoint voltage to tie those to, but aside from that, powering the op-amp from 9V should be fine.

And yes, tweaking the gain should be easy enough. You can adjust the values of the input resistors to balance the levels better between the bass and the line input, and you can change the mixer's feedback resistor to change the overall gain in that stage. And that's before we even get to the headphone amp!!

HTH,
Tom

[juan_felt]

Thanks a lot!

Hi, first thank you for your reply.

You "split" a signal by just typing both inputs to the one output.

But if "split" the mono output and feed each new mono signal to each channel of the stereo signal coming from the line in, won't create a connection between both stereo ones?

No, because the "join" is at a virtual ground, and there's no signal at a virtual ground. That's the clever thing about active mixers. See PRR's diagram, where he helpfully indicated this.

I have an idea on what to do, why would the mixer be a dual supply? I was planning on biasing the DC power from the 9v battery and then using it to power it up.

Regarding the gain, my idea was to change it.

The mixer looks like dual supply because the +ve inputs are tied to ground. They should be at whatever the mid-point voltage is, so if that's ground, that implies a bipolar supply.

You'd need to make a Vb midpoint voltage to tie those to, but aside from that, powering the op-amp from 9V should be fine.

And yes, tweaking the gain should be easy enough. You can adjust the values of the input resistors to balance the levels better between the bass and the line input, and you can change the mixer's feedback resistor to change the overall gain in that stage. And that's before we even get to the headphone amp!!

HTH,
Tom

And thanks as well!!

Always a great place to find great people and learn. Priceless..

Have a nice weekend! I'll get back when I finish the project!

[juan_felt]

Hi!

I tried everything, the headphone amp sounds AMAZING, but I have an issue with the summing amplifier. I tried the following:

I started with all 10k resistors, but I get really low volume and noise. If I reduce the resistors to really low values, I get better balance between the two signals, but the volume is very low (the noise dissapears).
Is there a better way to blend an instrument signal and a line level one (I'm using a mac as the source).

Thanks!

[ElectricDruid]

The gain for each channel is the ratio of the op-amps feedback resistor (from the output to the -ve input) to the input's resistor (from input to the -ve input of the op-amp). so:

gain = Rf / Rin

You can change the gain for each channel by changing the input resistor for that channel - the bass input's resistors don't need to be (probably shouldn't be) the same value as the line input resistors. Smaller resistors give more gain.
You can boost the overall gain by increasing the feedback resistor Rf - bigger values gives more gain.

HTH,
Tom