Analog combiner vs digital summing?

bcodemz

Member
2014-02-21 1:19 am
I need to mix 2 analog signals together. I can do that by using an analog splitter backwards and mix it like that. How is the quality of combining 2 signals together compared to spending $100 on a miniDSP and sum them together digitally?

[IMGDEAD]https://www.hometech2u.com.my/image/hometech/image/data/all_product_images/product-273/2-in-1-RCA-Male-to-2-RCA-Female-Y-Splitter-Audio-Adapter-hometech2u.jpg[/IMGDEAD]
 
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Then you can sum them by putting a resistor in series with each and joining the 'far' end of the resistors together. I'd suggest 2k2 resistors. If you use one of those splitters 'backwards' it will probably still work as normally audio outputs have series resistors, but the values will be a bit low (like 47ohms typically) so SQ might suffer.
 

bcodemz

Member
2014-02-21 1:19 am
Then you can sum them by putting a resistor in series with each and joining the 'far' end of the resistors together. I'd suggest 2k2 resistors. If you use one of those splitters 'backwards' it will probably still work as normally audio outputs have series resistors, but the values will be a bit low (like 47ohms typically) so SQ might suffer.

Is there a commercial product that I can buy that does what you said but with good SQ?
 
Well its not for sure that SQ is going to suffer if you just use a reverse splitter, but there is that potential. It will depend on how different the two signals are which you're summing. With a typical 47ohm series resistor, some significant current can flow between the outputs, beyond the current limit of your average opamp (which is in the region 10 to 20mA). Even below that limit, the extra strain on the opamps' power supplies due to the additional current demanded might be audible.

As a speaker designer presumably you know about passive crossovers - resistors get used in them typically. You just need smaller ones for this application as no significant power is being dissipated.
 

rayma

Member
2011-04-29 8:37 pm
For $70 + freight you could buy a real 2 input active mixer from some music supply. You'll probably also need two RCA female to 1/4 phone plug adapters from pamona.
For $100 you could get a 4 input mixer which might be useful in the future if you are recording with 2 area mikes and a close vocal mike.
I haven't seen any mixer kits I like. Last time I looked up the velleman kit it had 741 op amps, which are noisy & slow.
 
For reference see this RaneNote:

Why Not Wye?
Dennis Bohn, Rane Corporation
RaneNote 109 written 1991; last revised 4/04

Introduction
Wye-connectors (or "Y"-connectors, if you prefer) should never have been created.
Anything that can be hooked-up wrong, will be. You-know-who said that, and she was right. A wye-connector used to split a signal into two lines is being used properly; a wye-connector used to mix two signals into one is being abused and may even damage the equipment involved.
Here is the rule: Outputs are low impedance and must only be connected to high impedance inputs -- never, never tie two outputs directly together -- never. If you do, then each output tries to drive the very low impedance of the other, forcing both outputs into current-limit and possible damage. As a minimum, severe signal loss results.


Why Not Wye?
 
Or use an inverting opamp as a summing stage.

And if you want to low pass filter the summed channels, then use an MFB low pass active filter. It both sums the two (or more) channels and applies a 2pole active filter all from one opamp stage.
Duplicate R1 for each extra input channel.
Gain = R4/R1
for a two input version Vout = Vin*R4/R1 + Vin'*R4/R1' (potentially twice the individual channels).
https://en.wikipedia.org/wiki/Electronic_filter_topology
 

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As the junction of R1 and R4 is not a proper virtual ground, but part of an active filter, you can't simply duplicate R1 for each input channel. Each extra R1 effectively reduces the value of R1, so changing the Q. Instead, for three inputs use 3xR1 for each input etc. Then the filter circuit sees the same source impedance as for one channel.
 
Or use an inverting opamp as a summing stage.
It's a PIT* for a newby to lay that circuit out and etch boards. I've done it on perf board, also a real PIT* because of solder quality of dip IC to wire connections. The mcmelectronics dip project boards are a slight improvement, but their shipping minimum is $12. The Velleman kit sold everywhere including mcm has 741's, a a real **** op amp for audio. The guy selling boards from Dallas I knew of had no decoupling caps or anti-oscillation feedback cap locations on the board and the power supply was up to you.
anybody wants to recommend IC mixer kits with boards. components including decent pots, and some good sounding IC's like njm4560, njm4580, or st33078, jump in. The Behringer 2 input $70 mixer is about the cheapest solution for a complete newby without a $45 soldering iron and no $30 pound of solder.
 
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