Stupid Mic Pre-amp Output impedance question.

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Michael.E
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Stupid Mic Pre-amp Output impedance question.

Post by Michael.E » Sun Sep 28, 2008 12:29 pm

Ok.. here's my question! If I have a choice of either wiring my Mic pre-amp's
output at either: (A) 150 Ohm or (B) 600 Ohm resistance, which should I be using
Note: it's a TRS balanced lead connection going to a line input of my DIGI 002R.

Which one should I be using? I'm feeling stupid at the moment :)

thanks everyone

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digitaldrummer
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Post by digitaldrummer » Sun Sep 28, 2008 7:12 pm

going to the 002R it probably won't make a diference but since you have a choice I'd go with the 600 ohm option.

Mike
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Michael.E
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Post by Michael.E » Sun Sep 28, 2008 8:52 pm

digitaldrummer wrote:going to the 002R it probably won't make a diference but since you have a choice I'd go with the 600 ohm option.

Mike
why 600? I'm unsure a bit, because I always assumed
it was mostly better to pass the audio signal through the
the least possible resistance ?

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Post by The Scum » Sun Sep 28, 2008 10:05 pm

I take it this is an API-style pre? The 150/600 setting selects whether the output transformer secondaries are in series or parallel. In parallel, you get twice the available current, for a given voltage. In series, you get the input current, but at twice the voltage...though you probably don't need either capability in this situation.

If you were interfacing vintage 600-ohm input impedance gear, you'd want the 600 ohm setting.

But the line inputs of the digi are probably 10,000 ohms or greater (the spec sheet should say). It's a high enough impedance that it's not going to load the output...so it's largely moot. Try both, and see if you hear any difference.

Does the pre have an option to select whether it loads the output transformer?

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Post by Michael.E » Mon Sep 29, 2008 4:36 am

I'm unsure about this.. but here's what I'm trying to do basically.
I want to make a short 2 ft TRS Lead for an Altec 1592B Preamp.
I got an old one from a friend's basement, he basically said i can have it.
So far, I wired up a 2 conductor mogami mic cable to a Neutik TRS plug and
on the corresponding ends I wired up 3 spade connectors, by first stripping
(+, -, shield) and then dripping a bit of solder into the spade connector lugs
and then crimping a bit (w/ some shrink tubing on it).. but I'm a bit confused
on where I should be connecting these 3 rings on the 1592b output barrier strip.

Any ideas? (Click on thumbnails below for more info)

Image Image Image

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Post by The Scum » Mon Sep 29, 2008 10:40 pm

Strap it for 600 ohms, as shown on page 4, fig 6. Add a snip of wire between terminals 2 and 3.

Than add a 604 ohm resistor, and hang it between the hot and cold outputs(pins 1 & 4) to load the output transformer properly.

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Post by Michael.E » Tue Sep 30, 2008 8:59 am

thanks again!

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Post by Michael.E » Thu Oct 02, 2008 9:27 pm

The Scum wrote:Strap it for 600 ohms, as shown on page 4, fig 6. Add a snip of wire between terminals 2 and 3.

Than add a 604 ohm resistor, and hang it between the hot and cold outputs(pins 1 & 4) to load the output transformer properly.

quick question.. just to clarify.

I just screw in a 'normal' 604 ohm carbon resistor into the
barrier strip? Like I put one end of the resistor on pin 1
and other end of resistor wire onto pin 4 (underneath the
spade lug)? Is this the typical way to do this? Thanks again!

Note: I pretty much understand the gist of what your saying, but
I'm still kinda new to this so, I'd appreciate it if you can just confirm
that the above is the 'correct' way to do this.

(Resistor type ---> such as below?)
Image

:oops: obviously using the multimeter to test the resistance is correct first!

BTW: How did you come up with the 604 ohm resistor part of this?
I mean i didn't see anything mentioned in the manual about this!?
(you've been really helpful, and will take your advice on this though!)
thanks again! seriously! :hearts:

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Post by The Scum » Thu Oct 02, 2008 11:31 pm

Long ago, gear interfaced using impedance matching - 600 ohm outputs driving 600 ohm inputs. It allows for a maximum power transfer from device to device...but it makes things like multing very hard - because two inputs in parallel would be 300ohms.

Somewhere along the way (sorta marking the end of the tube era, and ushering in the opamp era), the power matching fell away. Input impedances climbed (10K, 22K and 47K are all common), and output impedances dropped (50 ohms is common). This allows for maximum Voltage transfer, because the inputs don't appreciably load the output.

This preamp of yours dates from the 600 Ohm era...and you're trying to interface it with modern gear. That doesn't change the fact that it was designed to interface to a 600 ohm input...the circuit will behave optimally if it is (often, the unterminated transformer will ring, adding an HF boost. That story of Geoff Emerick hearing ultrasonic frequencies has it's genesis in an unterminated transformer, IIRC).

So to hook it to a modern input, you fake the 600 ohm input using a resistor. This isn't mentioned in the manual, because 10K line inputs didn't exist when it was written!

604 is about as close as a modestly affordable resistor will get to 600...any value in that range will work - 640 or 560 might be easier to find. I'd use a metal film 1% resistor, because they're quieter and not really any more expensive than carbon.

You can indeed just hang it between the screw terminals, just make sure it doesn't accidentally contact the chassis or intermediate terminals...maybe with some heat shrink or spaghetti tube. Alternatively, you can hide the resistor in the connector on the other end of the cable - inside the barrel, between the pins of an XLR, or a speaker-sized TRS.

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Post by Michael.E » Fri Oct 03, 2008 10:09 am

WOW! amazing information!
thanks so much.. this is really great
info. extremely interesting as well..

can't thank you enough!

-mike :^:

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Post by arkestra » Mon Oct 06, 2008 9:04 pm

im dealing with the same thing now, and again, to clarify this is like putting a resistor in parallel across pin 2 and 3 if the output were an xlr?

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Post by The Scum » Mon Oct 06, 2008 10:19 pm

im dealing with the same thing now, and again, to clarify this is like putting a resistor in parallel across pin 2 and 3 if the output were an xlr?
Correct.

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