Mixbus query...

[jeth]

Even after many months of reading in to a DIY mixer project I have a persistent doubt, as in so many cases the information available is conflicting.

The question is this.. what is generally preferable...

a) Keeping (virtual earth) mix buses as short as possible and extending the pre mix resistor wiring to the mix bus.

b) Longer mix buses that pass all sources keeping wiring between the source and the mix bus to the minimum....

???

I'd be pleased to clear this one up once and for all..but I suppose I may just hear the same conflicting views.
Just that in one place I'm reading that one of the main pros of virtual earth mixing is low noise, and in other places how susceptible the buses are to noise..??

 

[JohnRoberts]

[quote author="jeth"]Even after many months of reading in to a DIY mixer project I have a persistent doubt, as in so many cases the information available is conflicting.

The question is this.. what is generally preferable...

a) Keeping (virtual earth) mix buses as short as possible and extending the pre mix resistor wiring to the mix bus.

b) Longer mix buses that pass all sources keeping wiring between the source and the mix bus to the minimum....

???

I'd be pleased to clear this one up once and for all..but I suppose I may just hear the same conflicting views.
Just that in one place I'm reading that one of the main pros of virtual earth mixing is low noise, and in other places how susceptible the buses are to noise..??[/quote]

"A" is preferable (better electrically), "B" is more practical from a packaging perspective when you have a tens of input channels feeding multiple buses.

Having the bus be a long antenna with a bunch of stray capacitance is never a good thing, but there are ways to manage.

If you are just making a simple summer, move the resistors closer to the summing amp.

JR

PS virtual earth summing is not lower noise, it's just convenient because there is no varying insertion loss to make up for. The noise is a wash between passive combining with make up gain, and virtual earth.

 

[jeth]

Many thanks..this confirms my own personal feeling that it would make sense to keep mix buses short..just had read a few things which undermined my certainty..as usual..

 

[PRR]

Your "ground" is still a bus. Unless you go all-balanced.

 

[guy_4]

Hello,
Regarding Mix Bus, I have the project of upgrading the audio Mix Bus and output section of a "vintage " desk ( Trident 80B)
The idea is to replace the actual virtual earth mixing section with a design that would allow for the "best sound" quality.
I have already selected several possible designs ( Opamp or discreet ?, DC coupled ?, servo ?, i.e from Jensen and F.Forsell ) and I would love to hear any suggestion / ideas / recommendation from people with such experience of upgrades.
I was also thinking about the possibility of a modified "9K micpre" as mix amp. Would it be possible, and how ?
Thanks,
Guy

 

[JohnRoberts]

There is nothing inherently wrong with a virtual earth summing bus, as i said before the noise and distortion is generally a wash between most alternate* approaches. I am not familiar with details of 80B design but it it wasn't inherently flawed to begin with it may clean up nicely with a minimum of heavy lifting.

JR

* I have used an variant on virtual earth in some designs where the summing resistors are replaced by synthesized current sources. While these current sources have more noise than a simple resistor, their use reduces the noise, distortion, and phase shift normally caused by a summing amp running at high noise gain.