Summing unit

[Tekay]

Hi!
Hope you're all well and kicking out there!

Been busy etching, drilling and soldering last couple of days and this is what I ended up with!

A 16 channel summing unit with two sidechains for parallell processing. First 4 channel pairs is switchable between stereo pairs or as dual mono, all channels also have a switchable -10 / -20dB pad. Lot easier to hock up outboards between the DAW and the unit if you can gain more from the DAW even if the signal is 20-30dB lower in the mix.

As you probably allready guessed this is based around the 1272 + some discrete amp boards for buffering and balanced sidechain returns/sends.

Still have some fine tuning to do but it's sound really FAT when I pass a cd through it. Buss is calculated for 32 channels and it's not noisy at all.
Original spec says 0.02% distortion @ 1kHz / 20dBm. I added a second stage as a buffer in the sidechain mixer section and measured 0.018% THD+Noise at +4dB and most of the distorsion is from the soundcard! Noise showes up between -110 and -120dB at the analyser.

Happy New Year

Btw: Don't forget to light a candle for people in the disaster!

 

[NewYorkDave]

That looks great, Tekay! :thumb:

 

[daArry]

it lives! :shock:

looks fantastic TK...congratz!

schem me up scotty!! :grin: anychance of like a block diagram (with mebbe a more detailed s/c section) :thumb: ... once u've done whatever further fine tuning ya mentioned of course :wink:

Best

D

 

[Scenaria]

thats pretty sweet...

I sure wouldnt mind a 40 or 48 channel summing box for the mix system

 

[dukasound]

Nice Thomas :thumb:
Duka

 

[MikoKensington]

:shock: Gorgeous!

 

[Kev]

" .. it lives "
indeed it does
:thumb:

excellent stuff TeKay

 

[Svart]

just TOO nice!

 

[Tekay]

A little better description will follow shortly!

Steve! This will probably work fine with 48 channels too!
With a add-on unit for total of 32 channel only 24 is connected to each L and R so to get down the summing loss to 39dB I had to put in a "slug" (dummy load ) over the primary of the input transformer, learned from Tanner that this was also done in the Neve consols as the minimum gain of a 1272 is 39dB.The loss for 24 channels is only 31,5dB.

 

[Infernal_Death]

Wow this really looks interesting. Thanks for every informations you can spare :thumb:
You really DO love Carnhill iron, don't you ? :green:

Flo

 

[Tekay]

I like the character from Carnhill / St'Ives transformers! haven't found any good substitutes yet! Don't so manny gapped transformers for transistor applications around these days!

 

[mhartung]

Tekay, what's a SLUG "across the primary"?

Very nice!

mike

 

[Tekay]

It's a dummy load resistor connected over the input transformers primary to lower the level. The 1272 minimum gain is 39dB so you need to take down the level through the buss. If you just trim down the level between the stages you'll loos headroom!

Read more here! http://lists.topica.com/lists/neve/read?sort=d&start=43

 

[thestudio]

How much headroom I/O????????

:guinness: :thumb:

 

[Tekay]

Max headroom thrue this unit is +26dB from buss in to main out, same as in the Neve 1272 summing amp!
Sidechain sends, max out is +20dB, sidechain returns. max +30dB

 

[Bauman]

Very cool!!
:thumb:

 

[Tekay]

Here's the blockdiagram and gainstructure for the unit!
http://www.vintagedesign.se/sv1/SU1block.pdf

 

[daArry]

Hey Tekay

Thanks for the PDF i've got a few q's tho:

1. What transformers have u used there? (they're not marked) Are they both the same Carnhill type?
2. What opamps have u used there? (also not marked) I'm assuming the neve amp board is the one on the far right?
3. What's the Mute/Cue Logic bit? Relays or a 3PDT switch? where's the 'logic' come in?
4. Whats the theory behind using sidechains in a app like this? (i've only used them on a comp before for the ducking effect) I read u said 'sidechains for parallell processing' but what does that equate to in a real-world instance?
5. Is the Headroom led meter one of those LM3915 chips?

It's starting to look more complicated now! :roll:

Cheers!

 

[Tekay]

Hi!
It's all based around the 1272, so the first and last stages are the same as on the BA283.
The other amp blocks are "home" brewed! Input stages from sidechains area 4 transistor class A design, biased to work with a single 24v.
The buffer and sidechain output driver is a modified version of a Neve BA226 unity gain stage.
The BA226 is almost the same a s the Quad Eight AM-4, or the amp stages in the Q8 MM-100 channel!

It's not a "real" logic, just a diod matrix, 3 position switches for cue or mute.

The headroom meter is a LM3915 with an input buffer!