Glide On Fade Awesome Mixer finally finished pics too!

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soundguy

Well-known member
Joined
Jun 4, 2004
Messages
2,041
Location
NYC, USA
Ok, after much ado, here are some photos of my mixer. Been working on this for the last two years and absolutely could not have done this without the people here, this is to date my most ambitious project and am beaming now that its working.

apimixerfront.jpg


It's an 8x2 mixer with a passive mix buss, the chasis is 3 RU. I drilled the chasis myself, turned out pretty good. Each input channel has a 312 type circuit, the makeup amplifiers are closer to a 325 type circuit. The buss is fed from one of the 2503 windings and each channel also has a direct 2503 output.

side.jpg


Each channel has an input impedance selector, buss mute, polarity invert, -20dB t pad, phantom power, pannner and input gain. The polarity, pad and input transformer impedance logic circuit are all relay controlled. The input relay board layout was done by justin at roll music. The board is set up to accomodate both the early 2622 input and the later 70's reichenbach input transformer used on the api consoles.

In addition to the input transformer selection, all the input channels are set up in pairs with diffferent cap selections for different tones. I built the box with tracking, not really mixing in mind, so there is a pretty wide pallette of tones availale across the 8 input channnels.

rear.jpg


All the hookup wire is silver teflon and the balanced hookup is mogami. The panner pots, gain pots, knobs, transformers and opamps were all taken from an api console, all the other components on the cards were restuffed to taste. The amps are all held in there with a screw down bracket I had made which mounts to the transformer, redco did the fabrication for the brackets. The dual concentric knob below the meters is the output fader.

NYDave designed for me a meter buffer circuit for use with the very old 500 ohm meters. They arent VU's, closer to PPM ballistis, very fast and very very cool. The meters are fed from the mix buss only, cant meter each individual input channel, but for what I designed this for, no biggie. The meter circuit is %100 isolated from the audio path.

overhead.jpg


Overall, Im totally psyched with the different tones I was able to tweak out of each of the amplifiers. Im also super happy about the sound of the makeup amps, the buss doesnt sound any different from the channel amps, its not noisy and doesnt feel like its working hard at all to makeup the 8 channels hanging on the buss. With the channels muted (and a loading resistor slugged) or feeding the buss actively, the tone and level of the buss doesnt change. My only complaint if I had to create one, is the crosstalk. If I was mixing a record solely on this, the crosstalk would be no good, but for how I intend to use it, its no problem for me right now at all.

What is the deal with cross talk? On a passive mix buss, is that a by product of a poor physical layout, or can cross talk be adjusted by changing the value of the resistor feeding the buss?

Im so happy this is finally done, all total with research and thinking, this took over two years to do, I absolutely cant wait to use it on my next session.

Big thank you to everyone who was patient and giving enough to help me along through my learning curve on this one!!!

dave
 
WOW!!!! WOW!!! WOW!!!!!

Congrats!

Question: How are your VU meters hooked up? Do you have a buffer circuit for it?

Is the unit powered by an external power supply?

Again, really great looking unit and great job.
 
Dave!
Absolutely stunning!!!!!!!!!!!!!!!!!!!!!!!!
:sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam: :sam:


Cheers,
Freddy G
 
thanks Im psyched.

the power supplies are external.

as for the meters:

NYDave designed for me a meter buffer circuit for use with the very old 500 ohm meters. They arent VU's, closer to PPM ballistis, very fast and very very cool. The meters are fed from the mix buss only, cant meter each individual input channel, but for what I designed this for, no biggie. The meter circuit is %100 isolated from the audio path.

I dont want to post the buffer without dave's permission. I also dont know if it will work with a regular VU type meter. You dont really need a buffer for a VU with a 2503 though, its got that fourth winding.

dave
 
That is incredible ! :shock: How much did the $ parts $ set you back?

After finishing this, the 1081 should be a piece of cake. :wink:

Z
 
I rarely comment on people's projects, but my goodness, that is impressive!

Well done, it looks superb.

I'd love to have the confidence to start as big a project as that.
 
I havent added the parts cost, I dont think I want to, I got some deals on some stuff, but this whole thing cost a little fortune. It sounds really good though, definitley way cooler sounding and more flexable than the current api 8 channel mixer thing, so thats all that matters to me.

If I hadnt done this slowly I would have been totally overwhelmed with it. I had no idea about passive mixing when I started this, so it was little by little. I have two more mixer projects planned now that this is done, both with neve amplifiers, I learned so much doing this, getting to those is less scary yet still a big bite to chew down on.

dave
 
Dave,

that one is probably one of the coolest thing I've seen here. :thumb: :thumb: :thumb:

Very nice looking job, lots of thought put in it. I hope it serves ya well!

:sam: :sam: :sam: :sam: :sam:
 
Hey Dave,

Your box looks fookin awesome. You've got what, 15 minutes of bench time puttin that together? :wink:

If I remember correctly, crosstalk on the passive mixbus is something that is there to stay. NYDave and I had a discussion about this at that other place, and we ended up adding a pair of resistors at the output that reduced overall crosstalk, but at the expense of additional loss in the buss ( which wasn't a big deal for me, as I had plenty of gain in the makeup stages).

I'll see if I can dig up the info on it (or NYDave may beat me to the punch and follow up here =) )

Regards

ju
 
Ok, after searching through the old place and coming up blank, turns out it was here that we talked about this:

[quote author="NYDave"]
No, don't do that; if it changes anything, it'll give you more crosstalk, since you'll be lowering the ratio of mix buss input resistance to source impedance. When considering a passive mix buss from the viewpoint of crosstalk, it's useful to simplify it as two voltage dividers in series. Say you want to figure out how much signal from channel 1 can leak through to channel 2. The first voltage divider is formed by channel 1's buss input resistor and the composite impedance of the mix buss. The second voltage divider is formed by channel 2's buss input resistor "looking into" the source impedance connected to channel 2.

The attenuation of the first voltage divider will be roughly 1/N (where N is the number of channels). In the case of your 16-channel mixer, it's -24dB. Then add to this the attenuation of the second voltage divider, which is completely dependent on how low your source impedance is compared to the mix buss input resistor.

Suppose, for the sake of argument, that you're using 10K buss resistors, and your source equipment is "prosumer" type with a 1K source impedance. That's only -20dB of additional attenuation, for a total of -44dB, which ain't fantastic by modern standards. Reducing your buss input resistors to 5k would, in this case, degrade this even further to -35dB total. On the other hand, if your source is a piece of "pro" gear with a source Z of 100 ohms or so, your total crosstalk (with 10K buss resistors) is -60dB, which is an order of magnitude better. All of this represents "typical" case and will vary somewhat depending on the settings of controls, etc. Nobody said passive mix buss design was easy! As you can see, its performance is fairly critical as regards circuit impedances, which is why it worked well in the days of "constant impedance" controls, but can be somewhat trickier to implement with lower-cost topologies.

If you can tolerate additional loss through the mixing network, one solution would be to shunt additional resistance across the buss to lower its impedance. The lower the impedance of the buss, the lower the crosstalk, but the greater the loss. You'll never eliminate crosstalk entirely. Even active summing has some crosstalk since an amplifier with infinite open-loop gain, and a perfectly conducting ground system, has not been developed yet Wink
[/quote]

The whole thread is here:
http://www.groupdiy.com/index.php?topic=1246

Regards

ju
 

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