Matched Impedance, Voltage Matching and possible damage

[lopsided]

Hey,

we have taken a diy approach for recording with my band, which means we have a lot of stuff borrowed and I try to build the rest. This brings a new responsibility to me, because I really don't want to damage the borrowed equipment with my diy builds.
We use an eight input sound-card, which has only one working headphone output, so we are in need of a headphone preamp/distributor for three very different headphones.
I used this circuit as a building block + added decoupling caps, because I use a single supply and Vref instead of ground. From the quadruple opamps I had at home the two tl084 did the best job.
.

Now for my concern about the impedance. I have re-read this article http://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp
I understand that some device configuration operate with matched impedance (source Z ≈ load Z) and some (like our stompboxes) are voltaged matched (source Z <<< load Z).
I also understand that having a wrong input/output Z connected can damage the sound (like a stompbox with too low input Z) and in some cases damage the equipment (tube amps).

Now how is it with headphone outputs in the soundcards? They are designed with really low output impedance to drive the also quite low Z headphones. On the other hand the input impedance of the opamps is very high.
My obvious question is, is it save to connect to a low impedance headphone output a high input impedance opamps? Or is it necessary to lower it (e.g 1k resitor to ground on the input)?
In more general, are tube amps one special case, where high load Z causes trouble, or are there more instances where I cannot follow the rule that high input impedance is a good thing?

[R.G.]

Now how is it with headphone outputs in the soundcards? They are designed with really low output impedance to drive the also quite low Z headphones. On the other hand the input impedance of the opamps is very high.
My obvious question is, is it save to connect to a low impedance headphone output a high input impedance opamps? Or is it necessary to lower it (e.g 1k resitor to ground on the input)?

Soundcard output, safe for driving headphones, will not damage high impedance opamp inputs, nor be damaged by them. It is not necessary to load down the input for an opamp.

In more general, are tube amps one special case, where high load Z causes trouble, or are there more instances where I cannot follow the rule that high input impedance is a good thing?

Tube amps with high Z load are a special case. There are probably others, but they are not common. High Z load impedance on tube POWER amplifiers can cause oscillation and high voltage failure because of the oddities of output transformers and tubes.

[lopsided]

Thanks a lot, R.G.!

[PRR]

> some device configuration operate with matched impedance

Very-very-very few.

"Matching" is not easy, was mostly approximate, and went out of style over 50 years ago.

That article covers the "matched" case because some of the 1930-1960 professional gear was designed for a matched world and is still sought-after for fine sound or interesting faults.

> how is it with headphone outputs in the soundcards?

If you don't plug in headphones, what do they see? Infinite ohms. Is the soundcard damaged? No.

In general line-outputs are happy with infinity, and will drive low impedances with decresed level and increased distortion but no harm. Also nearly any transistor power amp (at least since the mid-1980s) can face infinity fine and low or shorted loads with distortion or shut-down.

As R.G. says, tube power amps are the only thing you will find that MUST have a near-right load. When driven hard with no load, voltage spikes over 2,000V puncture the transformer insulation. The high-strung fix-bias tube amps could be harmed with a shorted load; however any cathode-bias power amp can run into a short all day long. (Since you may not know what is really in there, it is best to keep a tube power-amp's load within 2:1 of what it is supposed to be.)

[lopsided]

Thanks Paul!
We have tested it at todays session and so far it works just fine, a little noisy maybe, will have to take look into the filtering, but just fine for our needs.
Thank you both for clearing the impedance issues for me!