Please help me understand audio interface impedance

[abechap024]

I too was once very lost when it came to impedance and matching and all that. But it is pretty simple once you figure it out (like anything) I will try my best to help. Just remember these simple rules:


Low Impedance (about 150ohms) = Is like Superman...Less susceptible to noise and HF loss from long cable runs.

High Impedance (anything >50k about) = Very susceptible to noise and HF loss from long cable runs

Low Impedance is commonly found with microphones, and High impedance you will usually find with guitars and basses.

Now the LINE level interfacing lies somewhere in between those two at around 10k.

If your doing a long cable run and have a DI it would probably benefit from being converted to a lower ohm impedance.


Also another thing since its brought up....you hear in the old days about old equipment having more balls etc....because back then they wanted to match impedance (at 600ohms) instead of bridge them like we do today. Good thing about bridging in a lower impedance output can drive many high impedance inputs.

Sooo low impedance output is happy when the input impedance of the unit its connected to is higher.

Common output impedance for microphones is around 150ohm, and the common input impedance for microphone preamp is usually above 1k.

Also common output impedance for line level gear should be as low as possible <100ohms but sometimes it can be much higher than that. and ususally the input impedance is 10k. And a line level output can easily drive 10 pieces of gear. Kinda sweet right?

Guitars and guitaramps/equipment is high impedance and thats why they are so noisey and seem to suck the noise out of the environment. They have impedance anywhere from 50k to 1M and more

So hopefully I haven't confused you more....but impedance is important for best noise, distortion and frequency response from your equipment.

[Steck]

Originally Posted by abechap024

Low Impedance (about 150ohms) = Is like Superman...Less susceptible to noise and HF loss from long cable runs.

High Impedance (anything >50k about) = Very susceptible to noise and HF loss from long cable runs


Low Impedance is commonly found with microphones, and High impedance you will usually find with guitars and basses.

There's another factor, which is whether the particular signal is balanced. Typical studio microphones are balanced. This is in my opinion what provides noise immunity, not the fact that the impedance is low in the circuit.

Also, when quoting specific impedance values, it's important to identify whether we're talking about output or input impedance.

Originally Posted by abechap024

Now the LINE level interfacing lies somewhere in between those two at around 10k.

Line level has low output impedance (anywhere from almost nothing to 600 ohms) and high input impedance. Some line-level devices are balanced, some aren't. As you noted, most designers try to have input impedances in the order of 10x the output impedance so the input doesn't load down the output. It also provides the ability to 'bridge" multiple inputs onto a given output without much degradation.

Originally Posted by abechap024

Also another thing since its brought up....you hear in the old days about old equipment having more balls etc....because back then they wanted to match impedance (at 600ohms) instead of bridge them like we do today. Good thing about bridging in a lower impedance output can drive many high impedance inputs.

The critical thing about the older equipment was that the output was usually a transformer, which works correctly when it is properly loaded.

This is one of the reasons why older gear can sound different depending on the chain it is connected in - if the gear was designed to expect a 600 ohm load, and you're not matching the impedance properly with the input of the following gear, the frequency response of the transformer will change (not to mention the gain).

For example: Cinemag makes high quality transfomers. Here's a data sheet for one of their output transformers:

http://www.cinemag.biz/output/CMOQ-2.pdf

Notice that in all measurement graphs, it is noted that "RL=600", meaning that to get the performance specified, there needs to be a 600 ohm load.

Here's another PDF from an authoritative source that will shed light on transformers and impedances:

http://www.jensen-transformers.com/an/an002.pdf

Originally Posted by abechap024

Guitars and guitaramps/equipment is high impedance and thats why they are so noisey and seem to suck the noise out of the environment. They have impedance anywhere from 50k to 1M and more

I would also point out that most guitar amps are very high gain, which serves to amplify noise very effectively. Proper shielding techniques and "hum-bucking" pickups also matter although nothing sounds quite like a single-coil pickup... *shrug*

back to the original question:

Originally Posted by rmedek

• Do the outputs to the PA need to be run through a DI?
• Do the outputs to the amp need to be run through a reamp box?

Since the PA mixer is typically 100 +/- out into the audience, it's a good idea to use a DI box (or boxes, for stereo) to take advantage of the noise immunity that a transformer balanced run can provide.

Assuming your amp onstage is a typical guitar or keyboard amp....

A typical reamp box takes "the studio signal" (in this case, your Avid's output) and simply attenuates it appropriately for use with guitar and keyboard amps (which expect a low-level signal). This same level is about the same as what you'd need for a DI box. So simply turning the Avid down to a level that's good for the DI box(es) should result in a good level for your amp.

[rmedek]

Thanks for all the good info. Let me see if I'm getting the gist of things…

For the PA path, impedance might not be so much of an issue, because it's a low impedance balanced output (100 ohms), into an input expecting a low impedance output. But a DI is helpful because it's a transformer balanced run to the PA.

A straight path to the amp isn't ideal (because the amp expects a high impedance input) but not terrible because it's low impedance output to high impedance input. (?)

This is where I get a little confused…

A typical reamp box takes "the studio signal" (in this case, your Avid's output) and simply attenuates it appropriately for use with guitar and keyboard amps (which expect a low-level signal). This same level is about the same as what you'd need for a DI box. So simply turning the Avid down to a level that's good for the DI box(es) should result in a good level for your amp.

I've powered amps straight out of the interface before, so I know things will work, but I always thought I was setting myself up for long-term damage to the amp because of the impedance mismatch. I thought a reamp box was essentially a transformer that changed the impedance from low to high, not just an attenuator?

[Steck]

Originally Posted by rmedek

Thanks for all the good info. Let me see if I'm getting the gist of things…

For the PA path, impedance might not be so much of an issue, because it's a low impedance balanced output (100 ohms), into an input expecting a low impedance output. But a DI is helpful because it's a transformer balanced run to the PA.

If you use your Avid's output without a DI box and the guys at the far end are expecting a mic-level signal, then you could easily overload their inputs with anything approaching a line-level signal. Prior planning by letting them know you're going to send them a balanced line-level signal will avoid this (although they may frown and want you to use a DI box anyway, since that's what they expect [that everything coming from the stage is a mic-level balanced signal, from a mic or a DI box]).

The DI box doesn't fix the level difference, it's you turning down the level that will fix it. (Some DI boxes have 20db pads that effectively do the same thing as "turning the volume down"). A transformer DI box usually can provide ground isolation, possibly handy in a live situation depending on how the power is distributed.

Originally Posted by rmedek

A straight path to the amp isn't ideal (because the amp expects a high impedance input) but not terrible because it's low impedance output to high impedance input. (?)

It's not because of the impedances, it's because the amp (assuming it's a guitar amp) is expecting a low-level unbalanced signal - your Mbox can very easily send a signal that will be clipped by the first active element it goes through.

Originally Posted by rmedek

This is where I get a little confused…



I've powered amps straight out of the interface before, so I know things will work, but I always thought I was setting myself up for long-term damage to the amp because of the impedance mismatch. I thought a reamp box was essentially a transformer that changed the impedance from low to high, not just an attenuator?

Any kind of impedance problem is more likely to damage output circuitry than input circuitry. But, many active low-impedance outputs won't fry themselves when connected into a dead short (worst case scenario) - however, you really don't want to be the one to find out your Mbox is the exception. Connecting it to a guitar amp is not that kind of situation, so no worries.

To answer your last question, in its simplest form, a re-amp box consists of two parts.

One is basically a DI box (sometimes with an active input instead of a transformer so the input impedance is really really high and therefore doesn't load a guitar or guitar-pedal-effects or tube-amp effects-loop patch points at all, resulting in better/different tone). If it's active electronics, the DI box might have enough gain to produce a line-level output; if not, you'd run it into a mic pre channel like any other DI box would typically go.

The other is a level-pad that reduces line level down to guitar amp level (and potentially converts from balanced to unbalanced with active electronics). The level-pad permits running a console signal into a guitar amp [E.G. your Mbox could be such a source] such that a guitar signal originally recorded 'direct' with no effects could be run through your pedal chain/amp of choice.

clear as mud?

[rmedek]

Sorry for the late follow-up, I've been traveling and hadn't had a chance to read this yet.

But it is actually clear, not just as mud. Thanks for the detailed explanation!

[Steck]

Cool - we will want to know how things worked out based on the advice, okay?

[rmedek]

Finally had the first show with the new setup (minus one buggy Mbox and plus one MOTU Traveler). Since the setup I've been using was the one recommended above — i.e. straight from the interface to the amp — there were no surprises except for now I didn't worry about the impedance mismatch so much. I miced the amp versus using a DI (my personal preference) so no DI issues.

But…based on your advice I attenuated the line out from the Traveler before it hit the amp, which I think helped the amp project a much louder and clearer signal.