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Output Impedance in Ohms and its meaning

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Output Impedance in Ohms and its meaning

Postby siderealxxx » Mon Oct 10, 2016 11:46 am

Aloha,

I'm trying to ascertain if I can use the Output 9-10 on a Fireface 800 (the headphone output) to feed a click track to an IEM transmitter for a drummer.

According to the manual the 1-8 line outs are:

• Output level at 0 dBFS @ Hi Gain: +19 dBu
• Output impedance: 75 Ohm

And the phones 9/10 are:

• Maximum output level at 0 dBFS: +17 dBu
• Output impedance: 30 Ohm

So the output levels are roughly comparable but the impedance is not. I don't however understand ohms etc and what this means in real terms. I assume the headphone output is no longer line level and is perhaps insufficient for the IEM transmitter?

I could use output 3-4 instead, I simply don't have long enough cables to get to the transmitter for that! Will happily buy if necessary.

Thanks for any thoughts.
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Re: Output Impedance in Ohms and it's meaning

Postby Hugh Robjohns » Mon Oct 10, 2016 12:10 pm

siderealxxx wrote:So the output levels are roughly comparable but the impedance is not. I don't however understand ohms etc and what this means in real terms. I assume the headphone output is no longer line level and is perhaps insufficient for the IEM transmitter?

Both are nominal line level, albeit with slightly different digital/analogue alignments. Line level is defined either as 0dBu or +4dBu, depending on where you live and what you're doing.

However, audio doesn't have a fixed level, it varies above and below the nominal level, typically with a peak level of 8dB above the nominal level for 'controlled' signals. The signal space above the nominal level is called headroom and in your case one output provides 15dB of headroom and the other 13dB.

This is all fine, but how does it relate to impedance?

The general rule for modern analogue signal interfacing is that we employ a 'voltage-matched' configuration.

A lot of the confusion with impedances relates to the concept of 'impedance-matched' interfaces which dominated pro-audio until the early 70s because we borrowed the core technology from the telephone industry where impedances make a big difference due to the length of telephone cables involved between cities!

However, while this concept is still valid in telephony, RF, video, and for digital audio interfacing (where very high frequency signals are involved and where, in relation to the signal wavelength, extremely long cables are being used), it is completely obsolete and irrelevant in the analogue audio studio interfacing world.

So... in a voltage-matched interface environment the requirement is to ensure that the signal voltage generated by the source is passed in its entirety (or as close as we can get) to the destination. (This is definitely not the case in an impedance-matched system!)

To achieve this goal, we need the destination's input impedance to be very much higher than the source's output impedance. The reason is very obvious if you consider Ohm's law. The source's output impedance forms a simple potential divider with the destination's input impedance:

Image

In this diagram, Vin is the source's output signal voltage and Ra is its output impedance. Rb is the input impedance of the destination and Vout is the signal voltage it receives.

So, to maximise the received signal voltage, Ra should be as low as possible and Rb as high as practical... and the typical implementation is to make Rb about 10 times greater than Ra.

For example, if you consider a typical microphone and mic preamp, the standard mic output impedance is between 150 and 200 Ohms, while the standard preamp input impedance is between 1.5 and 2.5k Ohms -- ie around 10 times higher!

For line level sources, it is usual to make the output impedance around 100 Ohms or so as well, but the line input impedance is typically at least 10K Ohms and is often as high as 47K Ohms. Clearly, this is much more than ten-times the output impedance... but there's a reason for that.

Whereas a mic is (normally) only ever connected to one preamp input, it is not unusual to need to distribute a line level signal to multiple destinations (eg, several tape recorders, and a monitoring system). For this reason, line inputs are sometimes referred to as 'bridging inputs', and the idea is to enable the passive splitting and distributing of one line-level source output to feed several destinations simultaneously. Multiple destinations can then 'bridge' the input signal without 'loading it down'.

If you do that, wiring a bunch of destinations across the same output, there's still only one output impedance to consider (Ra in the diag above), but all of the separate destination input impedances (Rb in the diag above) are effectively placed in parallel wuth each other.

Parallel resistances act to reduce the overall resistance, and so two identical destinations will result in a halving of the total combined input impedance, four will quarter it... and so on. This means that slightly more of the signal voltage will be developed across Ra and less across Rb, thus reducing the received signal voltage.

Consequently, it makes sense to have as high an input impedance as is practical so as to minimise this reduction in signal level which inevitably occurs when additional units are wired across the same source!

To put some real-world numbers on this, if the source's output impedance is 100 Ohms, and the destination equipment all has a 10K input impedances, then splitting the signal to feed two devices results in a signal loss at each input of just 0.17dB, and you could share it between six devices and still only lose 0.5dB overall.

If the source output impedance was lowered to 75 Ohms instead, the signal loss reduces to 0.12dB with two devices, and 8 devices could now share the signal before the signal loss reached 0.5dB. Alternatively, sticking with 100 Ohms and going up to 47k input impedances gives just 0.04dB signal loss for two destinations, and allows almost 30 devices to be strapped across the output before the loss reaches 0.5dB!

... and that's why bridging inputs with high impedances are so useful! But I'm heading off on a tangent here, so... relating this back to your original query (at last, you cry!), the output impedances of the two options are both below 100 Ohms, which is typical and fine and of no concern at all.

However, the spec you haven't checked -- but actually the more important one -- is the input impedance of the IEM system. As you can see from the above, as long as that is more than 10-times the 30-75 Ohm output impedances of your interface ports all will be well, so anything more than 1K Ohms is great and you won't have a problem... In reality, I suspect the IEM transmitter will have an input impedance more like 10-50K Ohms anyway.

In other words, there shouldn't be a problem at all... ;) Just be careful when setting the levels...

H
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Re: Output Impedance in Ohms and it's meaning

Postby ef37a » Mon Oct 10, 2016 12:39 pm

If I may?
The other area where "impedance" causes endless confusion is speaker "matching" and especially with valve output stages.

Almost everyone says you must "match" the speaker impedance to that set on the amplifier.
This causes folks to think that the output impedance of the amplifier is the same as the speaker? Not so.

For a hi fidelity valve amp the source impedance will be very low, usually under one Ohm. However for a valved GUITAR amplifier it will be much higher. A 15 to 30 watt amp without feedback could well have an output impedance of 60 Ohms though most are around 20-30.
Even a big 50-100W amp with feedback will still be 10 Ohms or more. This is not as confusing as it sounds, just connect the right speaker to the right impedance on the amp! (this is not nearly so critical as many amp makers tell you but generally, do as you are told!) . So the "matching" really just means, match the numbers, electrically it is nonsense.

Transistor amps have an almost zero output impedance and can run any speaker impedance down to that given in the manual.

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Re: Output Impedance in Ohms and it's meaning

Postby siderealxxx » Mon Oct 10, 2016 12:55 pm

Hugh Robjohns wrote:
In other words, there shouldn't be a problem at all... ;) Just be careful when setting the levels...

H

If there were some sort of award for most detailed forum answers on the internet, I'm pretty sure you'd be in the running Hugh!

That certainly answers my question, and then some. Might have to digest some of it later... right now I have scores and backing tracks and guitar parts and vans etc to contend with!

As long as we have enough gain to the IEM transmitter then that's one less thing to worry about! Unforuntately it's not mine so will try and find out the input impedance anyway.

Thanks
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Re: Output Impedance in Ohms and it's meaning

Postby Hugh Robjohns » Mon Oct 10, 2016 1:01 pm

siderealxxx wrote: That certainly answers my question, and then some. Might have to digest some of it later...

Tiz all about the eddukayshun, innit...

H
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Re: Output Impedance in Ohms and it's meaning

Postby siderealxxx » Mon Oct 10, 2016 2:33 pm

Hugh Robjohns wrote:
siderealxxx wrote:
Tiz all about the eddukayshun, innit...

H

You mean Pink Floyd lied to us?!
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Re: Output Impedance in Ohms and it's meaning

Postby Hugh Robjohns » Mon Oct 10, 2016 2:43 pm

:bouncy:
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Re: Output Impedance in Ohms and it's meaning

Postby blinddrew » Mon Oct 10, 2016 10:19 pm

siderealxxx wrote:You mean Pink Floyd lied to us?!
To be fair, the double negative should have given it away...
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