Using transformers as buffers in virtual earth mixer

[Coldsnow]

HI,
Making a virtual earth mixer and needing aux sends, could I split the signal at the input with 3 x 1:1 600 ohm transformers 3 times to make 2 aux sends to accompany a main send instead of using opamps to buffer each channel. I know this is a more expensive way to go but would it work without sacrificing sound?

 

[analag]

This a question for the "Drawing Board"

analag

 

[CJ]

You can do it that way, a lot less wiring.

You lose the signal isolation between the three signals, there is galvic isolation, but if you short out the two aux windings, the short transfers to the pri and you have no signal.

A lesser version of that would be to hang a 0.1 cap off of the two aux windings, you will lose high end on your pri signal.

So if you do use the transformer only, just make sure that whatever you hang off the other two legs are not going to suck down the highs.

If the load on the two aux sends varies from day to day, then your pri signal will vary from day to day.

If you have a permanent load on the aux sends, and it sounds cool, then you are set.

 

[JohnRoberts]

[quote author="Coldsnow"]HI,
Making a virtual earth mixer and needing aux sends, could I split the signal at the input with 3 x 1:1 600 ohm transformers 3 times to make 2 aux sends to accompany a main send instead of using opamps to buffer each channel. I know this is a more expensive way to go but would it work without sacrificing sound?[/quote]

A transformer split could isolate the multiple feeds, but would not buffer anything, the loading at all three points would be reflected to transformer primary based on ratios, for 1:1 windings the load would be similar to hard wire common of all three together.

JR

 

[David Kulka]

Buffer usually refers to an active stage with a high impedance input and a lower impedance, higher current output. A transformer cannot do this.

 

[PRR]

> A lesser version of that would be to hang a 0.1 cap off of the two aux windings, you will lose high end on your pri signal.

Yet another way is a resistor. Put a separate 470 in series with each primary. Then if that one secondary is shorted, your source has to drive 470 ohms. If it can stand that OK, you're OK.

Altho, if you don't NEED 600 ohms in this day of everything-22K, don't use 600:600 iron, use 10K:10K iron. Now you can put 2K2 in front of each, and still have bass response. (A 600 winding fed with 2K2 may come up a bit bass-shy.) And if all three loads go short, the source only sees 2K2/3= 700 ohm load, which most sources are OK about.

(Of course if you have 3 loads and they all get shorted, the gig is ruined.... still it is wise to allow up to a third of your loads go gonzo and the others still working well enough to get paid. Different for live-on-the-air versus careful studio mixdown sessions, of course.)

A fairly standard 1-in many-out scheme is an 8-ohm amplifier and rows of 60 ohm resistors. (Duplicated for Balanced.) The loads get within 1dB of nominal level whether they be infinite, 22K, or true 600. Shorting 1, 2 or 7 loads does not blow the 8-ohm driver chip. It is realistic to hang 20 outputs on this, lines to faraway places with idiots at the patchbay, and expect all un-shorted loads to work fine.

With 600 ohm source and ten or twelve 22K loads I often use 1K5 or 2K2 resistors. I've had two shorted loads (feeding an OUT is essentially a short; and Mick should get new un-stomped patchcords someday) and it ran fine. At 2K2 you don't want to run for miles, but I felt no shame in driving Mick through 150 feet of cheap wire.