[sdiy] DIY Mixer - please help!

[Paul Dhillon Weber]

Many of the high end mixers, such as Harrison, achieve their sound and 
headroom through the use of audio transformers on the inputs/outputs.  
Originally, these were used to provide standardised impedance matching 
between equipment and for electrical isolation but have fallen out of 
favour in a lot of equipment mainly because of cost, bulkiness and 
weight.   Often designers used output transformers to achieve greater 
level and headroom without having to add additional, phase inducing and 
noisy amp stages. 

Companies like Neve, API, Harrison, Trident, Studer and now Manley, 
Chandler, Daking, etc. customarily use input and output transformers.   
Manufactures of quality transformers include St. Ives, Carnhill, 
Landahl, Jensen, Sowter and others which are available new, pulled and 
NOS.   They can range in price from about $30-$110 each. 

When considering designs, you might look at SSL (Solid State Logic) 
channel and summing circuits and Neve circuits.  SSL employed gates and 
compressors on each channel strip and/or buss and they used very high 
quality VCAs.   These circuits and clones are all over the internet.  
Typical (simple) signal flow for these kinds of mixers might look as 
follows:
input transformer -> buffer -> summing buss -> summing amplifier -> 
output driver -> output transformer. 

The sound quality imparted by excellent transformers is significantly 
different from electronically balanced equipment; greater weight of 
sound but that also means greater colouration.   Think about your 
favourite THICK recordings from the 70s.  That's the sound of transformers.

Of course, there are great transformerLESS designs out there too but 
they sound very different -- cleaner, more transparent if I may make a 
reckless generalisation.  

Are you going to use this a general synth module or as the final output 
stage for recording/monitoring? 

Paul Weber

cheater cheater wrote:
> Hi guys,
> It has come so far that I can't go on without a mixer, and, well,
> nothing I see on the market is satisfying to me right now. This can be
> of great use for modular synths as well since I would want it to be
> fully CV controllable. This is in part propelled by the fact that I
> have bought a TX816 which will need a proper mixer in order for me to
> be able to use it at all.
>
> I am currently collecting ideas as well as resources. It seems as
> though it's not so easy to find the schematics for most mixers that -
> at least I consider - are interesting. The first step is to come up
> with a good summing bus design. I think solid state is good for a
> start, but since it's all DIY, it'll be possible to swap it out for a
> tube stage if there's a need for that.
>
> I understand this list might not be the best forum to find out about
> this stuff - but I searched about for mixer/console DIY with people
> who actually *knew their stuff* when it comes to analogue music
> hardware and I simply couldn't find much interesting. If anyone has
> any recommendations, please do tell me. I trust the people here so I
> ask you guys first :)
>
> Here's a quick writeup of what I have in my brain right now. Most of
> it is probably wrong, so please correct me.
>
> What I am thinking of is a simple but effective modular approach which
> consists of:
>
> 1) multiple summing busses
> 2) multiple VCAs
>
> Each channel is one VCA that has CV inputs, a single input, and a
> single output to a single bus. Of course a stereo channel is that
> multiplied by 2. With a CV approach it's going to be very easy to
> control two channels with just the fader of the left channel, but
> that's a trivial comment.
> The bus sums and gives a single high-headroom output.
>
> The first thing to do is to make a good summing bus. I understand that
> to do this I will need to come up with a good op-amp design. What I am
> looking for is good headroom. Can someone recommend anything good?
> Supposedly the Harrison 4032c is pretty good. If you guys have heard
> the headroom in MJ's 'Thriller' - that's that. Has anyone got leads on
> that?
>
> As far as the mixer's format and general set of abilities, I am
> inspired by things like Manley mixers (EveAnna many positive
> affirmations to you if you can read this) and Chandler Limited (but a
> little less so because I haven't spoken to Mr Chandler or Mr Limited)
>
> I find that a fully voltage controllable design could be useful. For
> example a feedback compressor could be built which outputs CV that
> will be fed into the VCAs. Therefore the whole signal chain from the
> sound source to the recorder could be just two VCAs - I guess that's
> better if someone's looking for this sort of thing.
>
> Is there some interaction between the 'mixer strip' and the summing
> bus that I have left out? Or is my concept of how a mixer should be
> designed just plain wrong? :)
>
> Thanks guys
> Damian (probably wrong)
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