I stated that balanced microphones are designed to work into a mixer at 600 Ohms

This simply isn't true. A common setup is 150 Ω balanced mic output into 1.5 kΩ balanced preamp input.

 

@bogosort,

Lets agree to disagree,

have good day

 

This simply isn't true. A common setup is 150 Ω balanced mic output into 1.5 kΩ balanced preamp input.

I agree with you, I’ve never seen a 600Ω mic input unless it’s got an input transformer (generally driving a triode valve).
I’ve seen 10k more often than 1.5k, but the exact value makes little difference, the noise generated by a higher value of input resistor is shunted by the resistance of the microphone.

 

Yes the 600 ohm "standard impedance" was concerned with audio ("low frequency") transmission lines, but also with the line transformers used to connect to those lines, the balanced "pads" (resistor networks) used to maintain precise levels on those lines, and the interactions between all of the above, and also with preserving the ability to interconnect networks of jacks of outputs (sources) and loads (inputs) with "patch cords" and to quickly change the interconnections while a program was on the air and not have to worry about changes in gain or program material quality. And to be perfectly accurate, the goal of maintaining precise impedance matching was actually to allow the use of the LEAST expensive transformer that was adequate for the purpose! (Anyone who has used an old Western Electric REP 111C "repeater coil" knows that if cost is almost no object and you're designing for a nominal 40 year service life, it's more than possible to design a transformer that will sustain a +30dbm level from a few hertz to over 100 kilohertz, but they take up a lot of space and weign several pounds EACH.) And you have to understand that these standards evolved in an era where MOST amplifiers used vacuum tubes which took up a lot of space and weight and power and weren't exactly free of distortion, so the fewer of those that were in the signal chain the better, which helps explain why so much efffort was put into precise "pads" to soak up excess gain. And an output that "saw" a 600 ohm resistor was a "terminated" output, and all outputs were designed to be terminated in ONE and EXACTLY ONE 600 ohm load, but that didn't preclude the possible use of a large number of "bridging" loads as well. And one of the BEST reasons for the use of all these transformers is if all your circuits are "balanced floating" (at least on one end) then you NEVER have any common ground circuits, and your odds of developing hum-inducing ground loops is exactly zero. (The organizations I was associated with were of the opinion that microphone loads were supposed to be designed to be bridging, for what it's worth.) So in review, unless you are using equipment containing audio transducers or transformers or you're counting on "gain pads" to produce precise attenuation results, there is usually no good reason to worry about installing non-functional load termination resistors anywhere in the circuit, but I would worry A LOT about all that hum and ground loops from NOT using balancing transformers where they are needed!!

 

Inputs and outputs are electronically balanced these days.

 

Yes but at what common-mode rejection ratio (CMRR)? A decent transformer input can easily exceed 90 db CMRR, most "balanced inputs" don't exceed 40 db CMRR and many are 26 db or less. Tie both inputs together and connect a -10 dbV source from ground and measure the input level and reference to that, then you'll understand why transformers are still frequently necessary. (Especially if you're running a microphone "snake" a hundred feet across a stage filled with a couple hundred kilowatts of triac-controlled, EMI-generating stage lighting fixtures!)

 

Measurements done by Self* indicate that a standard balance input is generally rather better than the 40dB that a calculation based on 1% tolerance resistors would predict.

Microphone snakes are a thing of the past now that we have DANTE.

*Self. D. Small Signal Audio Design (Elsivir 2010) pp.349-359

 

No matter how many digits a calculator spits out, that still is a +/-5% tolerance value.

Better to use one 604 ohm resistor at an input, and two 301 ohm resistors at a balanced output.

ak

Why 5%? Two 1.2k 1% resistors and you are done. Cost circa 2p.

 

Why 5%? Two 1.2k 1% resistors and you are done.

No, you are not.

1.2 K is not a standard E-96 (1%) resistor value. It is a standard value in the E-12 (10%) and E-24 (5%) value sets.

When combining resistors in series or parallel, the resulting value's error tolerance never can be better (less) than the worst of the resistors being used.

ak

 

Why 5%? Two 1.2k 1% resistors and you are done. Cost circa 2p.

But as I said back in post #11, a 600Ω on a balanced line is 300Ω in each side, and 300Ω IS a E24 value.

1.2 K is not a standard E-96, 1% resistor value.

True, the E96 is 1.21k, but you can buy 1% tolerance resistors and 0.1% tolerance resistors in E24 values

 

you can buy 1% tolerance resistors and 0.1% tolerance resistors in E24 values

I've seen that, but that is a relatively recent change. Given how many decades the E-xx values systems have been in place, I think using an exception needs to be called out explicitly.

ak

 

But I suppose that the E24 series should really be every fourth value from the E96 series. So the E24 value should really be 1.21k.
E24 is really the 5% series, so presumably both 1.2k and 1.21k are correct, the extra decimal place being superfluous for a 5% tolerance component. It would only be wrong if it were labelled "1.20k"

 

Hi Wendy.
Yes, even though I am a retired old geezer, all my functions still work like new.

Now your just bragging.