Electronics > Beginners
Just how bad is it? Audio mixer with headphone amp.
<< < (6/35) > >>
paulca:
Okay, so backing things down to being a bit more simple.

A single channel input pre-amp.
As per the annotation:  Gain of 0 - 3.128



I did consider messing around with a series resistor to try and get 1.000 gain in the middle of the pot, but I think it's a fools game.

When I can get this right I will move add one mono channels to the mix down amp and see if I can get that right.

I do like the idea of the inverting summing amp for mix down.  I like that the inputs will not interfere with each other, although now I have the volume pot before the pre-amp I'm not sure it matters.

I'll review the other comments before attempting the mix down, as I think my Tesco delivery has arrived :)
Bassman59:
Take a look at the schematic for the Rane MLM82S. Everything you want to implement, except for the headphone amp, is there.

One could argue that you should use THAT 124x differential receivers for the line-level inputs instead of the op-amps, and INA163 for mic-level inputs.

For the headphone amp, the OPA551 has enough drive to clean the wax outta your ears.
Audioguru:
Your idea of using an opamp for each input with a useless gain of only 3.1 makes no sense. You can barely hear a level change of only 3.1 times.
The input level control must be logarithmic, not linear so when it is turned down to half the input level is 1/10th and when it is turned to maximum the input level is maximum.
Since the input level control set to halfway cuts the level to 1/10th then you need gain of 10 following it. The gain of 10 can be in the mix opamp then the input level control can feed directly into the mix opamp through a series resistor that the inverting mixer needs.

Carvin and Rane use 3 opamps or an instrumentation amplifier because they have balanced mic inputs that you don't have. Rane even has balanced line inputs using a differential opamp but you do not have balanced line inputs. Then you do not need input opamps.

The 32 ohms headphones need about 2V RMS to produce 0.125W to be loud. The old NE5532 (about 41 years old) is spec'd to drive 10V RMS into 600 ohms which has a peak current of 23.6mA. With a peak current of 23.6mA the voltage in a 32 ohm earphone is 0.53V RMS producing 88mW which is loud but maybe not loud enough. Your circuit uses two NE5532 opamps each with a 47 ohm output resistor so the output power into a 32 ohm earphone is enough when  the power supply is 6V or more.

   
BrianHG:

--- Quote from: Audioguru on February 12, 2018, 10:10:43 pm ---Your idea of using an opamp for each input with a useless gain of only 3.1 makes no sense. You can barely hear a level change of only 3.1 times.
The input level control must be logarithmic, not linear so when it is turned down to half the input level is 1/10th and when it is turned to maximum the input level is maximum.
Since the input level control set to halfway cuts the level to 1/10th then you need gain of 10 following it. The gain of 10 can be in the mix opamp then the input level control can feed directly into the mix opamp through a series resistor that the inverting mixer needs.

Carvin and Rane use 3 opamps or an instrumentation amplifier because they have balanced mic inputs that you don't have. Rane even has balanced line inputs using a differential opamp but you do not have balanced line inputs. Then you do not need input opamps.

The 32 ohms headphones need about 2V RMS to produce 0.125W to be loud. The old NE5532 (about 41 years old) is spec'd to drive 10V RMS into 600 ohms which has a peak current of 23.6mA. With a peak current of 23.6mA the voltage in a 32 ohm earphone is 0.53V RMS producing 88mW which is loud but maybe not loud enough. Your circuit uses two NE5532 opamps each with a 47 ohm output resistor so the output power into a 32 ohm earphone is enough when  the power supply is 6V or more.

 

--- End quote ---
He is going from line level to line level at the next stage.  He is not amplifying a microphone, he is not looking for serious gain, nor does he want any of his stages to clip by turning up to source volume to the max.  If he wants minimal hiss and the best possible frequency response of the opamps, the minimal gain of 3 at that stage and another gain of 4, approx 10db at the mid stage isn't too terrible a choice.  Maybe to get a gain of 6db after the 3 source mix, he can choose a gain of 6, but going to a gain of 10, with a maximum line level source of +/-1.25v * 10, he hits his +/-12 supply rail, though, he can go to a +/-15v supply.  Placing some gain (maybe targeting to 6 instead of 3)  at the input stage, next the 1/3rd level loss due to the 3 way mixing, then adding some gain in that middle opamp as an additional boost, say x2 or x4, is a strategy where he can use with his 3 way all positive audio design mixer, generating some additional signal gain without any clipping risk anywhere.

Remember, if he was doing a negative mix in the middle feeding the -input, he would not need that extra voltage swing at the first opamp stage outputs.  But now, those opamps will need to invert the signal as well to make positive audio in the end.

Audioguru:
The original circuit has no overall gain, it has loss.
1) Input opamp gain= 2.
2) Passive mixer gain= no gain, the output is 0.33 times because a 47k mix resistor is loaded with two 47k resistors in parallel from the other two inputs.
3) Mix buffer, no gain but its output is divided to be 0.52 times.
4) Output buffer= no gain.
Then the output level is 2 x 0.33 x 0.52= 0.34 times with an input level control at maximum.

My idea has the mix opamp a true inverting active mixer with a gain of 10. The mix opamp feeds directly to the output with no loss and it also feeds the headphones amplifier.
The logarithmic input levels at halfway produce the same output level as an input level and the level control can boost a low level up to 10 times or reduce it to zero.

If true phase is important then an inverting opamp can be added to each side with no change of gain.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod