Output Termination for preferred response?

Whew, I've been having a lot of questions lately... Thanks for all the help...

I've been playing with terminating the output of my various mic preamps with different values (600 ohms and up), and doing frequency sweeps and listening to music. Pretty drastic differences in some cases. I've found that a lot of my "vintage" gear that likes to see a 600ohm load often benefits in the high end (often extending the hi roll off point without causing a noticeable bump) with a 1K termination driving a 10K A/D converter input. I got the thinking that 1K (or slightly higher) would be a great value to terminate 600ohm output gear, so that interfacing with both 600 and more modern high Z loads wouldn't cause too drastic of freq. response changes.

My question I suppose is, is this "acceptable" to do? Are ouputs of commercial units generally terminated for "best" compromise frequency response?

I've been wondering about 600-ohm termination a lot lately as well. Could anyone recommend a good general read about it?

[quote author="Ethan"]I've been playing with terminating the output of my various mic preamps with different values (600 ohms and up), and doing frequency sweeps and listening to music. Pretty drastic differences in some cases. I've found that a lot of my "vintage" gear that likes to see a 600ohm load often benefits in the high end (often extending the hi roll off point without causing a noticeable bump) with a 1K termination driving a 10K A/D converter input. I got the thinking that 1K (or slightly higher) would be a great value to terminate 600ohm output gear, so that interfacing with both 600 and more modern high Z loads wouldn't cause too drastic of freq. response changes.

My question I suppose is, is this "acceptable" to do? Are ouputs of commercial units generally terminated for "best" compromise frequency response?[/quote]

If you terminate an output with 1000 ohms, then when it has a 10k load it will see a total of 909 ohma termination, whereas when it has a 600 ohm load it will see a total of 375 ohms termination. So yes, you'll be seeing less change in the response between the two external loads. BUT...375 ohms will probably make the frequency response of a tranformer-coupled circuit designed for 600 ohms excessively droopy, and will probably increase the distortion, perhaps drastically.

A better solution might be a 634 ohm load resistor that you switch in wnen the external load is 10k and switch out when it's 600 ohms.

Peace,
Paul

We discussed this at some length here:

http://www.groupdiy.com/index.php?topic=2845

The two instances where you should really be concerned about providing a "real" 600-ohm load are in the cases of transformer-coupled outputs, or when the output Z has been deliberately "built out" to 600 ohms with series resistors, as was done with some early solid-state gear.

To choose the value of shunt resistor that will reflect 600 ohms to the output:

Rshunt = (Zin * 600) / (Zin - 600)
where Zin = the input impedance of the following device.

So, if Zin is 10K,
Rshunt = (10K*600)/(10K-600) = 6M/9400 = 638 ohms.

Neve used a clever scheme for their transformer-coupled outputs running into high-Z inputs. They'd strap 1.2K shunt resistors across both the output and the input. So when the output was normalled to the input, it'd see 600 ohms. But if another piece of gear with a high-Z input was patched in, the output would still be working into about 1200 ohms, which would keep the HF rise somewhat under control--in other words, better than nothin'.

Well, I was kind of thinking of a one-size-fits-most deal around 1k-1k5. That way with high Z inputs the output of the pre's would be working into the neighborhood of 1k and with a 600ohm load you wouldn't get too far below 600? So you would get kind of a halfway compromise no?

Dan Kennedy has in the MP2NV preamp manual some tips and explanations about loading. As you know the MP2NV is an updated version of the classic N*VE preamp and the ouput transformer needs to see 600 ohm loading for optimal freq response.
Read here:
http://www.greatriverelectronics.com/pdf/NVUsersGuideWeb04.pdf

chrissugar

[quote author="chrissugar"]Dan Kennedy has in the MP2NV preamp manual some tips and explanations about loading. As you know the MP2NV is an updated version of the classic N*VE preamp and the ouput transformer needs to see 600 ohm loading for optimal freq response.
Read here:
http://www.greatriverelectronics.com/pdf/NVUsersGuideWeb04.pdf

chrissugar[/quote]
Yes, indeed! He put a "LOADING" button right there on the front panel.