[RJason22]

Hi. I'm a new poster, long time fan of this forum. I have a question that I haven't been able to wrap my mind around after the usual google research. Impedance matching when using studio amps (QSC, Crown, Behringer, etc) in a home audio setup.

Component Chain

Technics turntable [RCA] MF V-LPS phono pre [RCA] TC-716 source selector [RCA or RCA-to-XLR cable] rack mount EQ [XLR] rack mount Crown XLS 402 amp [12awg] speakers

Consumer Components:

I have two sources. A phono preamp that outputs 1.8V with an output impedance of 1K ohms. And a FiiO DAC that ouptuts 1.5V and doesn't specify the output impedance, but I'm guessing it's also 1K ohms, which seems to be a standard. I'll run these through a TCC TC-716 passive source selector rather than a preamp. All using RCA jacks.

Studio Equipment:

ART EQ341 has RCA jack inputs on it (input impedance 15K ohms). There is an option for XLR inputs on this EQ as well with a higher impedance of 20K ohms. This then hooks up to a Crown amp that only has XLR inputs (balanced 20K ohms, unbalanced 10K ohms) with an input sensitivity of 1.25 Volts


I'm very confused at how all this works. Looking at home audio equipment, I see input impedance of 47K ohms on a lot of it, this pro equipment seems to present 10-20K input impedance. I don't know whether to use the RCA inputs on the EQ or get a cable that converts RCA to XLR so I can present a 20K ohm input to the source equipment. I'm also not sure if I need to buy something along the lines of one of these ( http://www.bhphotovideo.com/c/product/748845-REG/Sescom_SES_OTL_UB_SES_OTL_UB_2_Channel_RCA_to.html )


Then there's some things I'm seeing about -10dBV and +4dBV, which seems to be related. Of course the goal is to maximize the matching and get the most out of the amp and source equipment. Any links to good tutorials or advice would be greatly appreciated. Believe it or not, I studied engineering many moons ago, but I haven't been able to find a good website to figure this out yet. THANKS!

 

[Selden Ball]

Using a simple cable adapter won't change the relevant impedance values at all. At audio frequencies (which are below 20KHz) you don't need to consider the impedance of the cable or impedance matching. Those factors are only relevant at much higher frequencies, like those used by analog and digital video signals and digital audio signals, which have frequencies well above a MegaHertz.


At the low frequencies used by analog audio signals, the impedances which matter are the serial impedance of the output circuit and the impedance to ground of the input circuit that it's driving. The output impedance should be very low compared to the input impedance: the lower, the better. Essentially, the relative signal seen by the input amplifier is proportional to the receiving circuit's input impedance divided by the driving circuit's output impedance. (I'm ignoring the effects of capacitance in very long cables.)


A few relevant articles are
http://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/imped.html
http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm

 

[RJason22]

Thank you for the information and links. The sound-on-sound article is great for my current level of understanding. I guess I'm good with the setup as it is then. I know it "works" (no hum), but wanted to make sure I was taking advantage of each component (ie... not over-driving a phono preamp or under-utilizing the amps power). I thought all the loads in question were pretty reasonable anyway. 1K ohms phono pre output into 15K ohms EQ input is the step from consumer to studio, but that does follow the "10 times" rule I've read about. I have no idea if the ART EQ converts the RCA inputs to a balanced signal when I hook the XLRs to the Crown Amp, but I'm guessing it remains unbalanced, but through XLR connections. Everything sounds great, but we're always trying to improve!

 

[Selden Ball]

You're very welcome. Enjoyable sound (and imagery) is the goal of this hobby, after all.