Simple 2 into 1 Stereo Mixer requirement - UK Vintage Radio Repair and Restoration Discussion Forum

yestertech · 1st Aug 2015, 11:24 pm

I'm currently homebrewing an audio switcher, using a 'reworked' unit made by a company called Audionics
this unit employs solid state switches , one for each leg of each of the stereo balance inputs
The prototype logic section seems to function OK as a breadboard layout, now I have to try and translate it to strip board .
One additional requirement we have is to add another pair of stereo signals to the stereo output of this unit. The signal is relative to standard zero level (line up ) and the output is electronically balanced
The additional input will be gated on and off by a computer play out system so it can be permanently mixed together with the output of the Switching unit.
I'd like to keep this a simple as possible.
I could do a balanced pair resistive pad of some sort, but then it has to be followed by an amp to restore the correct level.
Alternatively, I have to build a set of balanced input amps with some sort of combiner, followed by another balanced output amp.
Another possibility is some sort of hybrid transformer with 2 in and one out, but I suspect these are neither plentiful or cheap.
Does anyone have any other ideas ?

Andy

Diabolical Artificer · 2nd Aug 2015, 6:52 am

Andy, I may have some purpose built audio switches, will let you know later when I've had a rummage.

A.

mhennessy · 2nd Aug 2015, 6:46 pm

So you have a stereo, balanced output signal from the Audionics box? And you have a stereo, balanced signal from the computer?

If both signals are already balanced, there is no need to convert back into the unbalanced domain, only to convert back again to balanced.

Simply, 4 op-amp sections is all you need. The a pair 5532s will do the job. These will be Left Hot, Left Cold, Right Hot, and Right Cold.

Non-inverting inputs to ground.

10k resistors between output and inverting input.

10k resistors between Audionics and the inverting inputs.

Outputs of NE5532 sections to output connectors, via something like 51 ohms.

At this point, all you've done is inverted the outputs of the Audionics box. If absolute phase matters in this application, you can swap hot and cold signals at the output connectors.

Now, connect the computer playout system to the circuit - 10k resistors between that and the inverting inputs of the op-amps.

Of course, now we have a summing amplifier. Job done!

If the computer system is unbalanced, you can balance it by inverting each leg. Another NE5532. Very simple.

Mark

Diabolical Artificer · 3rd Aug 2015, 6:12 am

Boxes mentioned turned out to be speaker switching units Andy. Sorry my mistake, can't help, but Mark's put you on the right path. See project 94 on ESP project pages for more detail.

Andy.

yestertech · 3rd Aug 2015, 8:37 am

Quote:

Originally Posted by Dr Wobble

See project 94 on ESP project pages for more detail.

Thanks both
Andy, not sure to what you are referring above ?

A

Nickthedentist · 3rd Aug 2015, 8:55 am

Here, Andy:
http://sound.westhost.com/project94.htm

Diabolical Artificer · 3rd Aug 2015, 11:00 am

Thanks Nick, that's the one.

Andy.

1st Aug 2015, 11:24 pm	#1
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,130	Simple 2 into 1 Stereo Mixer requirement I'm currently homebrewing an audio switcher, using a 'reworked' unit made by a company called Audionics this unit employs solid state switches , one for each leg of each of the stereo balance inputs The prototype logic section seems to function OK as a breadboard layout, now I have to try and translate it to strip board . One additional requirement we have is to add another pair of stereo signals to the stereo output of this unit. The signal is relative to standard zero level (line up ) and the output is electronically balanced The additional input will be gated on and off by a computer play out system so it can be permanently mixed together with the output of the Switching unit. I'd like to keep this a simple as possible. I could do a balanced pair resistive pad of some sort, but then it has to be followed by an amp to restore the correct level. Alternatively, I have to build a set of balanced input amps with some sort of combiner, followed by another balanced output amp. Another possibility is some sort of hybrid transformer with 2 in and one out, but I suspect these are neither plentiful or cheap. Does anyone have any other ideas ? Andy Last edited by AC/HL; 2nd Aug 2015 at 12:33 am. Reason: As requested

2nd Aug 2015, 6:52 am	#2
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,658	Re: Simple 2 into 1 Stereo Mixer requirement Andy, I may have some purpose built audio switches, will let you know later when I've had a rummage. A. __________________ Curiosity hasn't killed this cat...so far.

2nd Aug 2015, 6:46 pm	#3
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Simple 2 into 1 Stereo Mixer requirement So you have a stereo, balanced output signal from the Audionics box? And you have a stereo, balanced signal from the computer? If both signals are already balanced, there is no need to convert back into the unbalanced domain, only to convert back again to balanced. Simply, 4 op-amp sections is all you need. The a pair 5532s will do the job. These will be Left Hot, Left Cold, Right Hot, and Right Cold. Non-inverting inputs to ground. 10k resistors between output and inverting input. 10k resistors between Audionics and the inverting inputs. Outputs of NE5532 sections to output connectors, via something like 51 ohms. At this point, all you've done is inverted the outputs of the Audionics box. If absolute phase matters in this application, you can swap hot and cold signals at the output connectors. Now, connect the computer playout system to the circuit - 10k resistors between that and the inverting inputs of the op-amps. Of course, now we have a summing amplifier. Job done! If the computer system is unbalanced, you can balance it by inverting each leg. Another NE5532. Very simple. Mark

3rd Aug 2015, 6:12 am	#4
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,658	Re: Simple 2 into 1 Stereo Mixer requirement Boxes mentioned turned out to be speaker switching units Andy. Sorry my mistake, can't help, but Mark's put you on the right path. See project 94 on ESP project pages for more detail. Andy. __________________ Curiosity hasn't killed this cat...so far.

3rd Aug 2015, 8:55 am	#6
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	Re: Simple 2 into 1 Stereo Mixer requirement Here, Andy: http://sound.westhost.com/project94.htm

3rd Aug 2015, 11:00 am	#7
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,658	Re: Simple 2 into 1 Stereo Mixer requirement Thanks Nick, that's the one. Andy. __________________ Curiosity hasn't killed this cat...so far.