HeadAmp 5 to 4

Hi,

First, i'd like to present myself. My name is Thomas and i'm sound engineer in Paris. i've been reading topics on this forum for a whil now and i'd like to go for my own project.

So here it is:

I'd like to build a Headphone Amp for studio recording.
There would be 1 STEREO main input and 4 auxiliaries MONO input (may become stereo, depending on place inside the box and price).
4 headphone out with separated amp and volume control.
And the possibility for each output to be fed with any input (switch only, no mixing).
And also a possibility to mute L and R independently for each channels

So, i picked some schemes here and there and i came with the  schematics attached to this topic. (some will recognize Gyraf's headamp).

I'm not sure about the L/R mute part and the LED indication part. For the rest, i'm not sure either since i'm a total noob, but i think i have most of it.

Thanks in advance for your tips and advices.

Thomas

Your pic is a little hard to read, but if I understand well you hav econnected the LED's in series with the relay coils. What voltage/current are they rated? What is the supply voltage to the relay drivers? If the supply voltage is 5Vand the realays too, the voltage drop in the LED may be a problem; the coils would receive on 3.5V.
I see you have put capacitors before the potentiometers, which is fine, but they shouldhave pull-down resistors.
Just the same, you should have capacitors beteween the output of the THAT line receivers and the relays, with pull-down resistors.
You have put 100nF across the 220uF 'lytics. That's typical audiophoolery; can do no harm, but useless. OTOH, I don't see any power rail decoupling caps, which is much more important.
I can't see the reference of the counter IC's, but I think it needs at least a pull-down resistor on the CLK input; in fact, it probably needs an anti-bounce circuit. It may be as simple as a single cap across the CLK input, but may also be more complex.
Finally, if I had to do this, I wouldn't use electromechanical relays. I would use CMOS switches such as DG308. CMOS switches have been given a bad rep for the CD4066 used improperly in voltage mode, but DG308 and more modern variants operate beautifully in voltage mode.

Hi,

First, thanks for your answer, i already learned stuff.

So, i made a new schematics with your improvements.

It only represent 2 inputs and one output to be easier to read.

Now I'm using CMOS switches as you suggested (as i understood DG412 is a DG308 with even lower resistance).

Led indicators are now paralleled to avoid voltage leaks.

Also add coupling caps with  appropriate pull-down resistors where needed.

But, i'm not sure of where i should put decoupling caps.  As i understand, i added some 100nF caps between the supplies of the two NE5534. Am I right? Maybe i should also do the same for each DG412?

Thanks again,

Thomas

totoxraymond said:

Hi,

First, thanks for your answer, i already learned stuff.

So, i made a new schematics with your improvements.

It only represent 2 inputs and one output to be easier to read.

Click to expand...

Good. It is much clearer now.

Now I'm using CMOS switches as you suggested (as i understood DG412 is a DG308 with even lower resistance).

Click to expand...

Indeed, take advantage of new technology.

Also add coupling caps with  appropriate pull-down resistors where needed. 

Click to expand...

These resistors could/should be about 10x, in particular the ones after the CMOS switches and the ones after the volume control.
The 4k7 res in the non-inverting inputs of the 5534's have no reason to exist, but a small cap (about 100pF) to ground would be welcome.

But, i'm not sure of where i should put decoupling caps.  As i understand, i added some 100nF caps between the supplies of the two NE5534. Am I right?

Click to expand...

Yes, but you should add at least one pair of electrolytics (47-220uF). In terms of layout, the decoupling caps should be as close as reasonably possible to the supply pins and to the collectors of the output transistors. The load return current should be tied to the junction of the decoupling caps.
In fact, space permitting, I would advice to decouple each power amp with electrolytics in parallels with ceramics.

Maybe i should also do the same for each DG412?

Click to expand...

That is not really necessary. These CMOS switches do not draw any noticeable audio current; they are actually passive. Only the logic needs the supply. I've consistently used them with minimum decoupling i.e. one set of caps per board.

HI,

Thanks again for your answer.

I added your recommendations to the previous schematics and i end up with this. (I hope I understood everything in the right way.

Unless someone has any other suggestions, i intend to put this on a breadboard before i draw a PCB.

But before that, i need a PSU...



Thomas

Here is my first draw for the PSU.

Once again, i took pieces here and there.

I which to use only one transformer for both +/-15V and +5V but maybe it's not the good solution?

I'm  not quite sure about the values of decoupling caps either.