Another Passive Summing Mixer - need expert help

I've searched the hell out of this forum, scoured the web, and taken bits and pieces of what I found (mostly from NewYorkDave) and designed my own summing mixer...


Simply, I need an 8-channel mixer to monitor my outboard mic preamps as they are recorded by my DAW.

A typical channel signal flow would be this:
Transformer Balanced Mic Preamp Output -> Mixer IN (right away send signal thru to AD converter) -> Level -> Pan/Mute -> Mix Bus -> Line Amp -> OUT

Check my schematic attempt here:


Please take a look and let me know if it's on the right track. This is my first crack at designing something like this.

Thanks!

Basically it will work, but the Line In will unbalance your signal (the Through Out will not be balanced). This might not be a problem, but you could easily fix this by running a 10k resistor (which should be mached to the 10k pot) from the ring to ground.

Do you have a source for four position/two deck switches?

Samuel

> monitor my outboard mic preamps as they are recorded by my DAW.

Can't the DAW do that for you? (I don't use one.)

Adice from the unbalance issue, crosstalk is worse than -60dB unless sources are darn close to zero impedance output (and transformers never are). Plenty good enough for live rock and roll, but possibly a problem in some recording situations.

Output impedance is 4K8, and loss is 18dB, MUCH too high impedance and level to use mike-amps as output stages, yet maybe too much loss to use line-level amps.

Actually, with transformer outputs (not "balanced, but "floating"), you may not get any signal at all, except via stray capacitance. The idea to add a 10K load on the other side of the line may work better, but is still inelegant.

you may not get any signal at all

Click to expand...

If the Through Out is connected to the AD, he should, no?

What's your Line Amp? Thinking it over, I would say that an virtual earth mixer (an active mixer) is probably more suited to your application, as the number of routed channels may vary. With a passive mixer, you need to adjust the make up gain of the summing amp if you vary the number of sources. An active mixer automatically chooses the "right" gain for you.

You may read the Fred Frossel article about this (I'm sure you'll find the link in the MEAT).

Samuel

There's a big thread 'o links in the Meta on passive/active mixers.

Thanks so much for the discussion. Yep, I've already combed through the mixer META thread, read the Forsell article (not that I couldn't read it again).

I'll answer some of the questions:

- No my DAW cannot monitor without latency, thus this funky mixer.
- Haven't looked for a source for the switches yet...
- For my "Line Amps" I was hoping to use a ready-to-go PCB instead of building them from scratch. Something like a 325 or JLM 99V PCB (which I've built before).

Yes, a virtual earth (active) mixer seems like it'd be the way to go because of the flexibility, but could I still do a virtual earth mixer and use an off-the-shelf PCB like I'm planning?

I should add that I currently don't have a mixer.

To reiterate, I'm looking for this DIY mixer to:
- monitor my preamps on their way to my DAW
- accept my DAW L/R output
- sum the above signals, amplify appropriately and send to my power amp.


I just re-read the Forsell article and an active summing mixer seems like the way to go, BUT if I go "active" can I use off-the-shelf PCBs for my summing amps?

For the kind of "utility" device you seem to want, you're better off just buying something like a used Rane SM26. It'll do the job and you'll save time and money for other projects.

Oh, and if I hear "summing mixer" one more time, I'm going to scream. :wink: For our purposes (since this is not an RF forum and we're not designing demodulators) summing and mixing are one and the same.

[quote author="NewYorkDave"]For the kind of "utility" device you seem to want, you're better off just buying something like a used Rane SM26. It'll do the job and you'll save time and money for other projects.[/quote]

Thanks for the suggestion. I just checked, and the Rane SM26 easily goes for under $100 on ebay.
The Rane could work. All I need is a utility mixer that doesn't degrade the signals from my precious outboard preamps.

Looking on the Rane site though, it seems like the signal from the "thru" outputs still hit some ic opamps. If I go the Rane route should I consider splitting my preamp signal from a patchbay into the Rane (for monitoring) and DAW (for recording)?

[quote author="NewYorkDave"]Oh, and if I hear "summing mixer" one more time, I'm going to scream. :wink: For our purposes (since this is not an RF forum and we're not designing demodulators) summing and mixing are one and the same.[/quote]

Ha! C'mon, at least I qualified my thread title with "Another" - cause I've seen a zillion "s*mming mixer" threads in the archive.
I was going for something subtly toungue in cheek.... er something like that.

I went down this path with a passive mixer a while back, and if you split the signal, you will get crosstalk between channels. We (being me asking the questions, and Dave providing the answers :wink: ) modified the design a little to make the crosstalk less, but it was still there.

I believe the numbers using an active design improve, but there will still be some crosstalk.

Regards

ju

[quote author="fum"]I went down this path with a passive mixer a while back, and if you split the signal, you will get crosstalk between channels. We (being me asking the questions, and Dave providing the answers :wink: ) modified the design a little to make the crosstalk less, but it was still there.

I believe the numbers using an active design improve, but there will still be some crosstalk.

Regards

ju[/quote]

Yes, I read that thread about your mixer a few times. I think yours was balanced, right?

So, do you get crosstalk among your "thru" outputs? That's the more critical part in my applicaiton.

Yes, it was balanced, but your thru points will have crosstalk, and you'd have this whether it's balanced or wobbly.

I was after the same thing. A monitor mix without latency for tracking, with the thru's feeding the recorder.

I ended up finding a software control for the dAW hardware that gave me zero latency monitoring. Worked a treat, so I don't need the thru's anymore (use the summing for mixdown now =) )

Regards

ju

quote: "So, do you get crosstalk among your "thru" outputs? That's the more critical part in my applicaiton."

Your everyday sources are not going to have precisely zero output impedance. If you tie a couple of normal sources with output Z's anywhere from 50 ohms to 1k or so together, through (that is, mixing with) resistors, you will get some crosstalk. To avoid you either buffer each source with an amp and then sum those outputs with resistors, or use a feedback-based or other very-low-Z summing node (emitters of richly-biased common-base stages for example) with resistors T-ed in from each source.

If you passively sum with a resistor from each source to a small resistor, you trade off crosstalk for attentuation, which gets old in a hurry.

Sources can be made with very low output impedance but often get into trouble when driving reactive loads like cable capacitance---that's one reason why they have some impedance going to the outside world.

How little crosstalk do you really need?

[quote author="bcarso"]How little crosstalk do you really need?[/quote]

A little crosstalk is fine. Just fine. :grin:
I was just curious about the crosstalk on the thru outputs. I don't know enough to judge how much crosstalk happens there.

It seems more like a Rane or similar mixer with buffered inputs may be the way to go instead of the DIY mixer route... for now.

BUT

fum, Do you have a schematic of your mixer handy?

Not sure if it's been linked here before, but this document on from the John Hardy site has a schematic for an ACN:

http://www.johnhardyco.com/pdf/990.pdf