Building a mix bus?

[Hiwatt]

How hard would it be to design a stereo mix bus not unlike the folcrum but with upwards of 40 stereo inputs?

[tINY]

If you are talking about a passive 40(stero) bus mixer, it would be pretty easy, if you didn't mind chasing levels around all day....



-tINY

[Hiwatt]

Well it dosn't have to be passive. The folcrum just happend to be the example I used. I take it using passive summong of 40+ channels is not a good idea.

[Geoff_T]

Originally posted by Hiwatt
Well it dosn't have to be passive. The folcrum just happend to be the example I used. I take it using passive summong of 40+ channels is not a good idea.

Hi

As a rough guide, 40 x 10Kohm mix resistors will give you

20 x log (40/10000) = -47.95880017dB

So you'll need about 50dB gain to get back to line levels, and you're 50dB nearer the noise floor.

[streetbeats]

Hey Geoff

Note that the original post specifies 40 stereo inputs

= 80 resistors

The calculations now make the reality seem almost certainly inpractical

[Geoff_T]

Originally posted by streetbeats
Hey Geoff

Note that the original post specifies 40 stereo inputs

= 80 resistors

The calculations now make the reality seem almost certainly inpractical

Hi

No, not unless you are talking about mono'ing all the signals together.

You would have two busses in the -50dBu region (you have to include the feed level in this equation) needing a ton of gain to bring back to line level.

Or, plan B, use an inverting amplifier to make a virtual earth mix. Then the gain, input to output, could be unity... though you'd need a stage or trick to flip the phase over.

[djui5]

Nice thread...

I'm new to all this......so why 10Kohm resistors?

[Geoff_T]

Originally posted by djui5
Nice thread...

I'm new to all this......so why 10Kohm resistors?

Hi

I just chose them as an arbitary value... they could be 15K or 1K but it depends on what load the source can drive.

Most Neve consoles were 15Kohm with a few 6800 and 7500 versions, especially on small consoles where it was essential to keep the bus impedance less than half the input impedance of the transformer in the mix amp.

As an aside, the first BCM10's were wrong and had 15K resistors which meant that the bus impedance was actually higher than the 10468's 1200 ohms. I later changed them to 6800 ohms.

[vince @ speck]

Originally posted by Geoff_T
Hi

No, not unless you are talking about mono'ing all the signals together.

You would have two busses in the -50dBu region (you have to include the feed level in this equation) needing a ton of gain to bring back to line level.

Or, plan B, use an inverting amplifier to make a virtual earth mix. Then the gain, input to output, could be unity... though you'd need a stage or trick to flip the phase over.

How about a 1:1 transformer at the output of the inverting amp wired with reverse polarity as a way to flip the phase.

I must say that a 40+ input passive mixer makes me a bit uncomfortable. Even if you can add an "ultra-quiet" 50dB makeup at the back-end, the potential for noise artifacts on the front-end due to the geography of laying out the connectors, switches, resistors, etc. is very high.

An "active" design might be a better idea for 40+ signals.

Regards,

Vince Poulos
Speck Electronics

[Geoff_T]

Originally posted by vince @ speck
How about a 1:1 transformer at the output of the inverting amp wired with reverse polarity as a way to flip the phase.

I must say that a 40+ input passive mixer makes me a bit uncomfortable. Even if you can add an "ultra-quiet" 50dB makeup at the back-end, the potential for noise artifacts on the front-end due to the geography of laying out the connectors, switches, resistors, etc. is very high.

An "active" design might be a better idea for 40+ signals.

Regards,

Vince Poulos
Speck Electronics

Hi Vince

Total agreement there! The concept of passive mixing might ring of "audiophile" quality but it's not very practical, especially on mixes in the region proposed.

[djui5]

Originally posted by Geoff_T
Hi

I just chose them as an arbitary value... they could be 15K or 1K but it depends on what load the source can drive.

Most Neve consoles were 15Kohm with a few 6800 and 7500 versions, especially on small consoles where it was essential to keep the bus impedance less than half the input impedance of the transformer in the mix amp.

As an aside, the first BCM10's were wrong and had 15K resistors which meant that the bus impedance was actually higher than the 10468's 1200 ohms. I later changed them to 6800 ohms.

I see...thanks Geoff...again!

[Geoff_T]

oops! posted my reply to the wrong post! Sorry!

[Geoff_T]

Hi

Blimey... we started off with a passive mix of 40 channels and now we have a modular system with faders and inserts!

You know, what you are trying to do is going to cost you a lot of $$$$$ and, in the end, you may not have achieved the specs and the sound that you really wanted. Then what will you do?

I'd give serious thought to buying an existing, proven, system than going down that road... the road to Hell is paved with good intentions!




PS This is supposed to be a reply to the post underneath! This forum is giving me a headache!

[Hiwatt]

Thanks to Geof T and Vince for the great replys. So an active mix bus looks much more practical eh? I picked 40 inputs thinking it was in the ballpark for an average mix these days. Say 20-30 tracks plus fx returns, group channels, panning etc = 40 stereo inputs. Of course I am just guessing here how mix busses are actually designed to work...

The reason I brought this up is because I am pondering getting a custom console built so everything could be modular. Basically the purpose of this console would be like the Lilo, to bring all your outboard pre's, eq, and comps together to have very customizable signal chains.

The channel strips would have multiple inputs and insert loops. Like being able to select the source from input A,B,or C and insert A,B,C etc...

The rest of the channel strip would have a post fader stereo output that I could patch into a dedicated summing device of choice.


It would kinda be like a very big signal router with level controls and panning if that makes any sense to anyone?

[Hiwatt]

He He, thanks for the warning Geoff. It's just this frankenstein console dreamt up by a guy (me) who understands signal flow pretty good, but very little about the inner workings of audio electronics.

[Hiwatt]

Although I could get by with a 24 channel 8bus line mixer... if there was one on the market for under 5 grand. I really like the lilo but I think I would miss the 8busses. I tend to use group channels quite a bit.

[streetbeats]

Originally posted by Geoff_T
Hi

No, not unless you are talking about mono'ing all the signals together.

You would have two busses in the -50dBu region (you have to include the feed level in this equation) needing a ton of gain to bring back to line level.

Or, plan B, use an inverting amplifier to make a virtual earth mix. Then the gain, input to output, could be unity... though you'd need a stage or trick to flip the phase over.

indeed. I was assuming a folcrom style implementation (my preferred implementation due to flexible choice of make up gain stage)

nice thread !