The use of the switchable 600 ohm output termination .

[Harry_HH]

I purchased an used GAP Pre-73 DLX to increase my mic preamp capacity.

The manual is very minimal and doesn’t give any instruction for the use of the switchable 600 ohm output termination .

I tried to find the answer in the net forum, but couldn’t find any straight answer.

The question is, should this 600 ohm output termination be activated or not - and the reasoning for that/what does this do in practice.

The use case here is mic pre before the KT-2A, mainy vocal. Mic is Neumann, condenser.

[BertKoor]

What does the manual exactly say? I can't find it on the manufacturers website. Does it state the exact output impedance? What about the KT-2A, does its manual state an input impedance?

I think the use case is that if the next thing in the chain has a rather high input impedance, then this box puts a 600Ω load on its outputs by itself.

For line level devices it is common to have an output impedance of 600Ω which connects to a ca 10kΩ input impedance. With an input impedance being in the ballpark of a factor 10 higher than the output impedance, it is like you plug something into the AC Mains power: there is always enough juice and the power delivery doesn't have to really work.
An alternative design is the input impedance being a close match to the output impedance. That's what you want with speakers: all delivered power is consumed. That design is not used much in audio over line level, but there are probably scenarios where it's needed.

It makes me think of ethernet over coax: there needs to be a 75Ω termination resistor at the end for absorption, otherwise the signal will reflect back and makes the whole network unstable. This phenomena happens only in the megahertzes region. But impedance mismatches (output impedance too high for the input impedance) are known for causing loss of high frequencies and inducing noise. I should say I've hardly encountered them in practice.

That's what I basically know about impedance matching, so you'll have to do with these 2cts.

[Harry_HH]

Thank you for commenting.
As said, the manual is very minimal:

http://www.velvetsound.net/userfiles/im ... e73dlx.pdf

[BertKoor]

Aha.

The output transformer used in the PRE-73 DLX is made for having an ideal load of about 600 ohm. The input impedance of most modern units is 10 kohm or more. The PRE-73 DLX has a 600 ohm output termination switch located at the back panel. Engaging the switch terminates the output transformer with a resistor to bring down the load to about 600 ohms. This switch should normally be engaged if you are connecting the PRE-73 DLX to a modern unit but you can also choose not to engage the switch. This will lift the high end frequency response slightly which can sometimes be a good thing.

To me this is crystal clear. You only need to look up *) the input impedance of whatever is connected to this devices output. If it is like 10kΩ or more, then switch that termination on, and if it is like 600Ω, then switch it off.

*) which I did, and it says:

Audio Input
Impedance 600 Ω, balanced and unbalanced.

In other words: keep that switch off.

[Harry_HH]

Great!