debunking the '600 ohm' inputs for tube gear

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themagicmanmdt
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debunking the '600 ohm' inputs for tube gear

Post by themagicmanmdt » Thu Apr 12, 2012 4:10 pm

so...

a transformer is, primarily, a impedance 'reflector', to put it simplified.

in relation to the turns ratio, as well as impedance termination to ground, thus reflects impedance to the other side of the transformer, as well as current and voltage gains/losses.

so....

there's quite a few times when i'm reviewing schematics for tube gear that has a transformer input (specifically mic input transformers), and I find it interesting that, say, there's a 1:10 input transformer that terminated into the first stage grid resistor of 220k... or, sometimes, 1M.

if the grid resistor was 220k; 1:10 turns ratio is a 1:100 impedance ratio: 220k/100 is a load impedance as seen by the mic as 2.2k.

a 1M grid resistor would be 10K as seen by the mic. (!)

these are all figures, of course, for transformers that behave fine without having to strap a load resistor on the primary for correcting frequency response or other issues with the transformer design or implementation.


is this thinking accurate? am i missing something? (i always defer my conclusions out for possible errors).

perhaps... lots of tube gear has always been terminating mics beyond and past the 600ohm input impedance we've been told is linked with older gear...


further, other 'line' input devices are about the same: a 1:4 line input transformer, but terminated into 220k... that's about 14k!



of course, this is a 'some, not all' post.... but may help explain certain preamps having different sounds in the tube days.



eh?
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Scodiddly
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Post by Scodiddly » Fri Apr 13, 2012 5:28 am

At this point the 600 ohm thing is somewhat of a historical footnote. "Back when the telephone company hadn't even invented the first amplifier", and they had to transfer passive signals long distances. So it was all about maximum power transfer. Ma Bell knew a *lot* about transformer design back in the day.

In the modern age (which goes back to maybe WWII for pro audio?) we've had relatively cheap amplification and so the exact impedance matching isn't as much of an issue. So mic input impedances do tend to be several thousand ohms, which makes it possible to split a source between more than one mixing console and other useful tricks. And tubes are much happier at high impedances anyway.

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Post by RodC » Sat Apr 14, 2012 10:30 am

I think I have posed this about 100 times, Shure article that helps you calculate the loss and explains this a bit more.

http://tinyurl.com/el9bh
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Post by The Scum » Mon Apr 16, 2012 11:13 pm

I'm not doublechecking your math, but it looks like you've got the concept.

One other piece of it is that the stated specs for a transformer are it's design parameters...they describe how it will be optimally applied. That doesn't mean that it stops working when loaded differently...though the frequency response and other characteristics may be less than ideal as you get away from the design parameters, and won't correlate to the mfr's docs.

600 Ohms is the characteristic impedance of small gauge wires hanging from telegraph/telephone poles.

When wires get significantly long (longer than 1/4 wavelength of the frequency in question, but at the speed of electrons in copper, not sound in air), they start to show effects similar to room acoustics. And the results are similar - nodes, modes, standing and traveling waves, etc. Matching the source, line and load minimizes those effects.

My father wrote a bunch of animation stuff to demonstrate these effects...but unless you've got Matlab, they'd just be useless source files...though searching youtube for "transmission line animation" will pull up some similar stuff.

Outside of the phone and power companies, we don't see too many transmission lines in analog audio. Thus matching is somewhat moot in most studio circumstances. Word clock and video cables are a different story.

There are some other effects to be considered. There's that stuff Paul Stamler has done by adding a load resistor to an SM57, to smooth out the presence peak. Resonant systems become much less resonant when heavily loaded. It can really alter a dynamic mic's response...though an active mic shouldn't change much, as the output impedance should be really, really low.

That Shure link sums it up nicely. I don't think I'd ever seen that before.
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themagicmanmdt
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Post by themagicmanmdt » Thu Apr 19, 2012 1:32 pm

RodC wrote:I think I have posed this about 100 times, Shure article that helps you calculate the loss and explains this a bit more.

http://tinyurl.com/el9bh
i was more of saying that most tube gear seems to already be reflecting loads greater than 600ohms.
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