Building a Passive Audio Mixer

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I am not sure that I accept calling diodes passive devices, they are semiconductors, but some people do call them passive.

How important is actually being passive?

JR
 
8-in mixing is significant loss. Pan-pots more loss. Putting a master pot after that is really begging to be way down in Universal Hiss. We "usually" put a booster amp after the mix network before the next turn-down.

That "passive compressor" has VERY low output. It says 70mV, but I bet THD is very high before that. Also it is a HIGH impedance dingus: uncompressed signal comes through a half-Meg (470K) resistor! Anything less than a vacuum tube grid (or FET amp) will suck it down. That's also an awful high Noise Resistance for a thing that can't get over "70mV". Meanwhile the input throws a 100 Ohm load on the source on every peak. If this comes from eight 10K resistors, there is as much clipping as clean limiting happening.

Amplifiers are ESSENTIAL to audio. The telephone was hardly practical until the Carbon Mike was introduced. Get used to working with them. (And powering them.)

Assuming you are not doing open-heart surgery, 25V is generally considered safe. You can't even feel 9V on a dry finger. You can feel 9V on your tongue! (Burns like scalding coffee.) So don't lick power supplies. Solid-state preamp/line audio amplifiers work on voltages low enough they hardly tingle if you stick your hand on them. Which should not happen much.
 
I don't think you'll "achieve" any coloration/smoothness with passive summing and CineMag or Jensen (or Lundahl for that matter) transformers.  Well, maybe but I don't believe so. 
 
Re. the passive compressor:  I've been down that route - but as PRR mentions, that topology is really not worth the effort for anything real.

I ended up cheating and using sidechain power in my (G24) passive compressor. Audio path passive, but opto-based so relatively easy.

Jakob E.
 
I was advised I could do it without the amp, but there would be phase and frequency response issues.  I don't know how noticeable it would be, but there you go.  Any ideas how it could be done without power?
Adam
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It is possible to do it without an internal amp but you will need one somewhere to make up the losses in the pan and the mix bus. If you want the mixer istelf to be 100% passive you could use a couple of external mic pres to make up the gain. However, you will need to arrange it so that the losses are minimised else you will be building a noise box as PRR says and this may mean compromises in some areas. I think you need to drop the idea of the passive lim/comp as this will be very lossy. Alos I think you may need to arrange your inputs to be 600 ohm balanced rather than 10K balanced. This is a compromise but most modern equipment will drive 600 ohms without problem so it should not be a show stopper. The reason I suggest using 600 ohm inuts is that in a passive mixer you need to ensure that each stage do not present too low a load to the previous one or else it will affect its operation. So you typically start with a 600 ohm input followed by a 1K fader. So thefollowing pan pot does not load the fader, you design the pan pot so it presents at most a load of 5K to the fader. You should be able to do this so that the passive summing bus uses 10K resistor. With 8 channels the bus impedance will be 10K/8 = 1.25K. To use a 600:600 output transformer so you can feed this to an external mic pre you need to slug the bus so it looks like 200 ohms or so.

Taking these all together and assumimg you want 10dB in hand in the channel fader then the total loss will be:

10dB in the fader, 15dB in the pan pot and 34dB in the mix bus for a total of  59dB. Even the best mic pre in the world will only give you an output noise of around -70dBu with 59dB of gain so you can see how a paasive mixer can easily be a npoisy one. ou can improve matters by giving up the 10dB in hand on the channel faders (which is OK for an 8 channel line mixer) and improve the noise to -80dBu which will be fine for most real world situations.

The biggest attentuation is caused by the slugging of the mix bus. So an alternative would be to eliminate this and feed the bus into a 10K line input of an external amplifier. This drops the bus loss to 18dB so,without the 10dB in hand on the fader, the total loss becomes just  33dB which should improve the noise considerably.

Cheers

Ian
 
80hinhiding said:
Hi Ian,

Wow thank you :)  If I introduce power to this mixer, some safe level of it for a beginner, can it be less of a noise box?  I don't want to put a bunch of parts together and find the noise floor up under my noise.

I could compromise on the faders, and use a rotary pot if that would help.  I would also be willing to use a 3 position switch for panning, hard left, center, hard right.

Originally I hoped to get tubes into the mix too, and I see you work with them.  Very cool.

Adam

If you include amps in the design then noise performance becomes much less of an issue. You only need two of them for the stereo bus amps. This could be little more than a regular dual op amp and a handful of passive components. You could even make it battery powered if you wanted.

Going this route means.

1. You can use a regular 10K:10K input tranformer.
2. You can use a regulr 10K fader.
3. You can have less loss in the pan pot
4. You can use a regular 600:600 outout transformer

Cheers

Ian
 
80hinhiding said:
Excellent.  I'll go that route Ian.  Any particular transformers you could recommend that are good but aren't really expensive?

For the input you can still use Cinemag or Jensen as you originally planned. For the output I would recommend Carnhill which are good value for money.
side: I tried wiring a single 1/4 jack for an insert test last night, and was just guessing at it.  Came close, but not quite there.  It's probably better to have two jacks anyway, but I am curious about the single jack configuration.

You can do an unbalanced insert on a single TRS jack but for a balanced on you need two jacks. Which do you want to do?

Cheers

Ian
 
80hinhiding said:
CineMag input transformers were priced a fair amount higher than the CineMag output transformers I was recommended.
As a rule, input transformers are more expensive than output ones because of the screening and the core material type. You can skimp on these but there will be a performance hit. There are cheaper types about that are reasonably good. There's lots of threads here abouts on that topic.
I would prefer balanced inserts, but would like to understand the wiring for both.  I'll continue testing tonight to see if I'll get it.  Any tips or reference material would be appreciated as always.

I am assuming you want the insert pre fader; in which case a balanced insert needs to be before the input transformer (on the balanced input). An unbalanced insert would be to be on the secondary side of the transformer (unbalanced). Here is a half normalled balanced insert wiring (courtesy of SOS magazine):

ConnectorsJFhalfnormal.jpg


The signal enters on the green wires (from your input XLR), the top jack is the send and the bottom one is the return  and the signal exits on the red wires (to your line input transformer primary).

For an unbalanced insert use a TRS with switched contacts as shown in 'D' below:

Phone_jack_symbols.png


The send goes to the tip, the return goes to the ring. Connect together the two switched contacs so that send gets connected to return when nothing is plugged in.
For pots and faders I was considering Bourns PTS series.  Is that overkill perhaps?

Adam

Loads of choices for pots. I Have not tried the Bourns faders but they look good. The ALPS K series are reasonably low cost also.

Cheers

ian
 
If you are trying this between two balanced  XLR connectors then you need two jack sockets. You cannot do a balanced insert with kust one jack socket.The jack you pictured is not suitable for either a balanced or unbalanced insert because it does not have any switched contacts. You need one that at least has switched tip and switched ring contacts.

Cheers

Ian
 
80hinhiding said:
There's a Tascam M224 mixer for sale locally for $100.  Would some of the components be good for this DIY build?  I don't know the difference between a Bourns PTS fader and one on that M224 board, as an example, because I haven't heard or worked with either.

Hard to say but my gut tells me no. More likely to have cheap faders than good quality ones. You might find the enclosure useful.

Cheers

Ian
 
80hinhiding said:
I'm discovering it's not so easy to find certain quality parts in low quantity. :S
This is an observation and gripe for probably 90% of the folks on this forum. You either gotta spend or compromise or get clever. Often there's a workaround but weather its comprising the quality or just as good (maybe even better) can be another obstacle. 
 
80hinhiding said:
Should I be considering only audio taper when it comes to potentiometers?  I see some linear sliders, but I don't know if they'd work well for a mixer.

Adam

For the faders you definitely need an audio taper. The linear faders are meant for DAWs.

Cheers

Ian
 
The NE5534 is a very good op amp for audio use.

Running from a 15V dc supply will be fine, you will just lose 6dB of headroom which for most purposes should not be a problem. I am not an op amp expert so I cannot comment on the 2K resistors to split the supply..

For the mix bus you will need to use an inverting configuration to create the virtual earth.

Cheers

Ian
 
Your effect can either be balanced or unbalanced. Some effects can be built completely balanced so they can be connected directly into a balanced system. Others are more conveniently built unbalanced. If they are to be used in a balanced system then they need to unbalanced their input, do their thing and re-balance their output in order to be used in a balanced system. If you wire an unbalanced effect into a balanced system the the whole connection becomes unbalanced.

Cheers

Ian
 
80hinhiding said:
Thanks. The like button seems to be gone, but your response confirms what I was guessing at based on a couple experiments.

I'll have to figure out how to unbalance and then re balance.

Just to add to my previous post, most mixers unbalance at the mic/line input; then they amplify, EQ, mix, solo etc etc all unbalanced and lastly balance again right at the outputs.

Cheers

ian
 


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