Zobel compensation for XLR and RCA (AES/EBU and SPDIF) - diyAudio
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Old 1st October 2009, 02:10 PM   #1
NeoY2k is offline NeoY2k  France
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Default Zobel compensation for XLR and RCA (AES/EBU and SPDIF)

Hello,
Could someone shed some light on connector compensation?

RCA plugs are between 35 and 50R. I wonder what are the RCA sockets (pcb mount, adding extra solder joints can only make things more ugly).

What does the RCA plug+RCA socket impedance mismatch looks like?

I heard that "The discontinuity from RCA plugs looks more like a lumped sum (capacitor) than an transmission line impedance mismatch and can be fixed pretty well with a zoble network (about 8 turns of wire round a 1/4 watt 75 resistor) in series right at the RCA jack or plug" (Justin_case, Audiocircle).

Well, that would work for a driver end, you don't want no 75R in series at the receiver end lol. But thats' not the problem, you can translate it to parallell. The question is: what L is 8 turns of wire round a 1/4W R, and why this value?

I'd be interested in XLR compensation too, they are not real 110R. Due to their physical dimensions, they will probably look like a small L in series, but... how much? How to compensate?

With XLR, you can't avoid solder joints etc, AES/EBU is crap. RCA is far from perfect, but at least you can use compression/crimp plugs to reduce the impedance mismatch length. Canare RCAP are good, and inexpensive (5-6$ a connector shouldn't worry you too much... We pay more for Neutrik XLRs in France!!!!).

What's hard, is to evaluate it. I would need a TDR or VNA, and you can't breadboard at these frequencies, and expect the same result in final design. It means you have to make as many boards as combinations you want to try (well, you can rework it too). SMD, of course.

So if someone could come up with even gross ideas, measurements, explanations, schematics... He would really be welcome!

Because that's an area not often looked at. Providing a 75R load/source already seems hard enough, but more care is welcome. Especially for those XLRs...

Thank you,
Nicolas
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Old 1st October 2009, 08:52 PM   #2
remi207 is offline remi207  France
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Join Date: Mar 2008
Default forget RCA plug, use BNC connector

Hello Nicolas

The RCA is a bad solution for digital transfert. So, the BNC 75 is the best solution for SPDIF, cheaper than XRL for same performance. My module SPDIF output have a bandwith higer than 150 Mhz.

The common mode rejection ration is close for XRL and BNC scheme, the RG179 cable is perfect at low cost ; about 5 €?

Rémi
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Old 5th October 2009, 09:59 AM   #3
NeoY2k is offline NeoY2k  France
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Join Date: Dec 2007
Yep, but that's not the point. Debate is not XLR/RCA over BNC. We all know XLR is crap, RCA not good. I'll have BNC input too, as it is defined as AES3.

AES/EBU and SPDIF are more standard. We just can't ignore them for anything but a fixed setup.
I need to connect "legacy" formats, without modding the boxes, on the go (I'm a sound engineer). The challenge is then to juice as much performance from these flawed interconnects as possible.

Yep, try to deal with that...

Any news about these zobels? These won't be easy to find by myself... I can use a TDR, but if I have to rent it for a long time, to swap caps/inductors and see if I can get any benefit...

Well, speaking of benefits, how many dB benefits can it bring, in terms of VSWR?
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Old 5th October 2009, 07:39 PM   #4
remi207 is offline remi207  France
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Hello Nicolas

Pending make my SPDIF module, I have passed the PCB to TDR to attain a 75 ohms track on mass plan with FR4 material in the middle ( 1.31 mm).

No Zobel network is necessary if the pcb is well designed ( strip line).

Rémi
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