Passive audio input combiner

[samchurchill]

Hi, I'm building a rack that includes a video switcher with analog L+R audio inputs.  Normally these will be fed via Dante from an AVIO adapter, but I want to add analog XLR inputs onto the back of the rack in case we're running a simpler setup where we don't have Dante.

As there should never be a time when there's sound coming in through both Dante and the rack's analog inputs, I'm looking to combine them in the simplest way possible.  Ideally I don't even want to need to switch anything or twist any knobs.

If I just combine them with no electronics, would a line level signal on the rack's analog inputs have the potential to damage the AVIO adapter?  If so, are there any small passive combiners that any of you can recommend?  Or, if I choose to build something myself, would it just be a matter of adding a couple of diodes to stop the flow of any current back to the AVIO adapter?

Thanks in advance!

[dbuckley]

Have a look here:  https://www.ranecommercial.com/kb_article.php?article=2106

You'll need to scroll down!

[samchurchill]

Great, thanks very much - looks like I need to make a balanced summing box.

I guess this is what something like https://www.studiospares.com/Microphones/Splitter-Combiners/plc21-passive-line-combiner-by-lambden-audio-458220.htm is doing? I’ll be able to make two circuits in a single small box, which is great. 

[dbuckley]

No, no, no, no - that box is far more upmarket.  And either really awful, or incredible value for money.

Quote

All inputs and outputs are fully isolated from each other via a transformer.

A quality transformer would cost far more than they are selling the complete assembly for!  Which is why I'm suspicious of how good a transformer that costs next to nothing will sound.

[sunray]

2 hours ago, dbuckley said:

No, no, no, no - that box is far more upmarket.  And either really awful, or incredible value for money.

A quality transformer would cost far more than they are selling the complete assembly for!  Which is why I'm suspicious of how good a transformer that costs next to nothing will sound.

BBC liked doing this with a Sowter 600Ω:600Ω:600Ω transformer, when I made enquiries some number of years ago and this would likely have been somewhere around 2005, they were £80. I was lucky I managed to obtain an Eddystone box containing a pair (with 6 pigtails to convert a stereo tape deck to mono) from a recycling bin and I wished to replicate it.

Not being prepared to spend £200 for an ocassional use item I sourced some transformers which appeared to have the same spec at £18 each and turned out to be rubbish, infact  at £5 had a better frequency response and they were designed for car stereo sort of situations.

 

In use the Sowter and the cheap version had a very noticeable different sound.

 

However getting back to OP, just using a 3 winding transformer and nothing else to combine 2 outputs together is not a good idea as that is effectively coupling 2 outputs directly together and that can create some potentially damaging high currents. (I picked up a scrapped soundcraft desk for this very reason).

Personally I'd opt for sockets with break contacts as a much simpler option.

[samchurchill]

Ah, thanks, sockets with break contacts would be a great low-tech solution. My Google powers seem to be deserting me - any idea where I’d find such a thing?

[sandall]

AFAIK you won't find any inline sockets with them, but most decent panel jack sockets come with or without break contacts.

[themadhippy]

Quote

sockets with break contacts would be a great low-tech solution. My Google powers seem to be deserting me - any idea where I’d find such a thing?

You'll find  XLRs with built in contacts at the left end of the rainbow,just beside the pile unicorn manure. Time to add them to the wish they made thread I thinks.

[samchurchill]

I went almost to the end of the rainbow and found https://www.neutrik.com/en/product/nc3fbv2-sw - shame there’s only one set of NC contacts!

Unless anyone has any better ideas, I guess I’m looking at the above with a relay (frustrating as I was hoping to stay passive) or building the balanced combiner circuit.

[sandall]

Going from XLR to 3-pole jack & back again might be easier/cheaper.

[sunray]

The annoying thing is I stripped some kit to repurpose it (Ironically I built a changeover box) and the existing XLR sockets had 3 changeover contacts and used for the same purpose to some din sockets.

I was in 2 minds to replace them with standard sockets but didn't think the effort was worth it.

Can I find a similar product now? ...     Not a chance

Do I now wish I'd drilled out the pop rivets?  Yeah sorry.🫣

 

Right now I think Sandall has the right idea.

Edited November 2, 2023 by sunray

[samchurchill]

Thanks for the advice and info everyone. I think, at least temporarily, I’m going to go with just keeping the AVIO plugged into the rack’s analog inputs, which will go into the video switcher’s inputs. It’s not quite as tidy as keeping it all inside the rack, but will do the job.

This might seem a bit mad, as using 3-pole switched jacks is the “correct” way to do it (given XLRs aren’t available). However, we don’t have XLR-to-Jack cables anywhere else in our setup, and I’ve been caught before with them going missing if they’re not used on a regular basis.

[dbuckley]

On 11/2/2023 at 2:06 PM, sunray said:

I sourced some transformers which appeared to have the same spec at £18 each and turned out to be rubbish, infact 

Colour me surprised.  Transformers are one of those things where you do appear to get exactly what you pay for.  I'll take a cheap active transformerless DI over something with a mediocre transformer every day of the week.  Strangely, or perhaps not, transformers that sound awful on critical listening are often entirely adequate used sniffing the speaker (obviously with a resistor inline) on the back of an overdriven guitar amplifier....

[sunray]

7 hours ago, dbuckley said:

Colour me surprised.  Transformers are one of those things where you do appear to get exactly what you pay for.

I absolutely agtree and have a fair rew Sowters in various applications, however generally I've found the GLI's pictured earlier to be quite good in the right situation, ie not for a DI situation.

7 hours ago, dbuckley said:

 I'll take a cheap active transformerless DI over something with a mediocre transformer every day of the week.  Strangely, or perhaps not, transformers that sound awful on critical listening are often entirely adequate used sniffing the speaker (obviously with a resistor inline) on the back of an overdriven guitar amplifier....

Yes and to be fair such a transformer it probably wasted in that situation, as it happens I've found myself using a 24V mains transformer in exactly that situation for bass guitar, I imagine it would not be suitable for other guitars though.

As they say 'Any port in a storm'.

[sunray]

On 11/1/2023 at 5:21 PM, samchurchill said:

Hi, I'm building a rack that includes a video switcher with analog L+R audio inputs.  Normally these will be fed via Dante from an AVIO adapter, but I want to add analog XLR inputs onto the back of the rack in case we're running a simpler setup where we don't have Dante.

As there should never be a time when there's sound coming in through both Dante and the rack's analog inputs, I'm looking to combine them in the simplest way possible.  Ideally I don't even want to need to switch anything or twist any knobs.

If I just combine them with no electronics, would a line level signal on the rack's analog inputs have the potential to damage the AVIO adapter?  If so, are there any small passive combiners that any of you can recommend?  Or, if I choose to build something myself, would it just be a matter of adding a couple of diodes to stop the flow of any current back to the AVIO adapter?

Thanks in advance!

I've been assuming this is all balanced, if it happens to be unbalanced only one contact is required.