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Frank Stearns
 
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Default Doing the Splits

I need to cobble together a single-channel two-way mic splitter -- one
output "flow through" and the other transformer isolated.

I've got an ancient Triad line-to-line audio transformer: each side has
600 ohms with taps for 250 and 50 ohms. (Model number is rubbed off, but
it's a metal can, 3/8" stud mount, roughly the size of a shot glass with
leads coming out through the stud,)

This old beast actually looks pretty good on the 'scope; respectable
square wave performance once I put a 220 ohm resistor across the output
(otherwise rang like a church bell).

The flow-through is for recording; the iso-out is for PA.

A few questions to The Wisdom:

1. I figured on using the 250 ohm taps on both sides. For the transformer
output side, should I continue with that 220 ohm resister across the
output? (I assume "yes" given most modern preamps.)

2. On the input side to the transformer, should I "bridge" the
flow-through mic line with some resistive value, or just make a hard-wired
"Y"? (The flow-through line will be carrying phantom power.)

3. I'm assuming I should float the 50 ohm legs on both sides of the
transformer... or should I terminate them in some way?

Thanks in advance,

Frank
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Paul Stamler
 
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"Frank Stearns" wrote in message
...
I need to cobble together a single-channel two-way mic splitter -- one
output "flow through" and the other transformer isolated.

I've got an ancient Triad line-to-line audio transformer: each side has
600 ohms with taps for 250 and 50 ohms. (Model number is rubbed off, but
it's a metal can, 3/8" stud mount, roughly the size of a shot glass with
leads coming out through the stud,)

This old beast actually looks pretty good on the 'scope; respectable
square wave performance once I put a 220 ohm resistor across the output
(otherwise rang like a church bell).


That could be a problem, as many modern mics prefer to see a higher
impedance, like 1.5k. Still, try it and see.

The flow-through is for recording; the iso-out is for PA.

A few questions to The Wisdom:

1. I figured on using the 250 ohm taps on both sides. For the transformer
output side, should I continue with that 220 ohm resister across the
output? (I assume "yes" given most modern preamps.)


I'd try the 600 ohm taps on both sides. You may be able to use a higher
termination resistor, which at least some of your mics may like. Oh, and
don't forget the recording preamp will be in parallel with the loading
resistor, so it should be higher (parallel resistances add using the formula
1/total = 1/R1 + 1/R2).

2. On the input side to the transformer, should I "bridge" the
flow-through mic line with some resistive value, or just make a hard-wired
"Y"? (The flow-through line will be carrying phantom power.)


Try just a Y.

3. I'm assuming I should float the 50 ohm legs on both sides of the
transformer... or should I terminate them in some way?


Float.

Peace,
Paul


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Frank Stearns
 
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Hi Paul -

Thanks for your suggestions... A follow up (I did some snipping; hope
not too much context has been lost.)

I'd try the 600 ohm taps on both sides [of the transformer]. You may be
able to use a higher termination resistor, which at least some of your
mics may like. Oh, and don't forget the recording preamp will be in
parallel with the loading resistor, so it should be higher (parallel
resistances add using the formula 1/total = 1/R1 + 1/R2).


Let me clarify -- I hadn't planned on any termination resistors on the
*input* to the transformer ("Y" split point from the flow-through), only
on the output of the transformer. The one resistor on the output cured the
ringing; I had nothing on the input. (But I *should* get all the
pieces on the bench -- actual mic and pre with the xformer hanging of
the lines -- and see what happens.)

So we're talking a hard Y from the flow through, directly into the
transformer, with no other resistor(s) present unless we think that's a
good idea...

Thanks again,

Frank



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Scott Dorsey
 
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Default

Frank Stearns wrote:
I need to cobble together a single-channel two-way mic splitter -- one
output "flow through" and the other transformer isolated.

I've got an ancient Triad line-to-line audio transformer: each side has
600 ohms with taps for 250 and 50 ohms. (Model number is rubbed off, but
it's a metal can, 3/8" stud mount, roughly the size of a shot glass with
leads coming out through the stud,)


This is not the right sort of transformer for a bridging splitter. For
a bridging splitter you want a high-Z-to-low-Z transformer, so that the
bridging load does not much affect the load that the microphone sees.

Since you don't have one, the best you can do is probably to strap it as
a 600:600 transformer and bridge the 600 ohm input across the first console's
input. This will mean the mike will see somewhat lower impedance than it
is used to.

If it has dual primaries and secondaries instead of just taps, you could
probably strap the primary for 600 ohms and then use the two secondary
windings for two isolated outputs. This gives you a full isolation splitter
rather than a bridging splitter.

1. I figured on using the 250 ohm taps on both sides. For the transformer
output side, should I continue with that 220 ohm resister across the
output? (I assume "yes" given most modern preamps.)


This is way, way too low impedance. Leave the load resistors off and let
the preamp input do the loading.

2. On the input side to the transformer, should I "bridge" the
flow-through mic line with some resistive value, or just make a hard-wired
"Y"? (The flow-through line will be carrying phantom power.)


No, make a hard-wired Y.

3. I'm assuming I should float the 50 ohm legs on both sides of the
transformer... or should I terminate them in some way?


Anything that isn't used on the transformer should be floating.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Scott Dorsey
 
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In article ,
Frank Stearns wrote:
(Scott Dorsey) writes:

Frank Stearns wrote:
I need to cobble together a single-channel two-way mic splitter -- one
output "flow through" and the other transformer isolated.

I've got an ancient Triad line-to-line audio transformer: each side has
600 ohms with taps for 250 and 50 ohms. (Model number is rubbed off, but
it's a metal can, 3/8" stud mount, roughly the size of a shot glass with
leads coming out through the stud,)


This is not the right sort of transformer for a bridging splitter. For
a bridging splitter you want a high-Z-to-low-Z transformer, so that the
bridging load does not much affect the load that the microphone sees.


Well, heck. I rumaged around and found a UTC 0-1: 50K on one side and
500/200/50 on the other and also another Triad with 85K on one side and
600/250/50 on the other.

I figured 50K+ through a transformer might be too high to drive the low
side with any respectable amount of level. I'll try it.

Thanks, Scott.


A 50K to 600 is going to be just fine from the standpoint of the mike...
your only worry is that this is such a high ratio that you may wind up
with poor S/N on the split output because you're knocking the voltage down
so much. Try it and see!

Note that the $12 Tamura 600:600 microtran types from digi-key are actually
not bad if you use them for dual isolated splits. They can't handle very
high levels before the low end drops off, but for stage mikes they're really
not bad. Their higher ratio transformers are pretty awful, though.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Paul Stamler
 
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"Frank Stearns" wrote in message
...

I'd try the 600 ohm taps on both sides [of the transformer]. You may be
able to use a higher termination resistor, which at least some of your
mics may like. Oh, and don't forget the recording preamp will be in
parallel with the loading resistor, so it should be higher (parallel
resistances add using the formula 1/total = 1/R1 + 1/R2).


Let me clarify -- I hadn't planned on any termination resistors on the
*input* to the transformer ("Y" split point from the flow-through), only
on the output of the transformer. The one resistor on the output cured the
ringing; I had nothing on the input. (But I *should* get all the
pieces on the bench -- actual mic and pre with the xformer hanging of
the lines -- and see what happens.)


With a 1:1 transformer, the resistance loading the secondary appears as
though it is present at the primary.

Peace,
Paul


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Paul Stamler
 
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"Frank Stearns" wrote in message
...

Well, heck. I rumaged around and found a UTC 0-1: 50K on one side and
500/200/50 on the other and also another Triad with 85K on one side and
600/250/50 on the other.

I figured 50K+ through a transformer might be too high to drive the low
side with any respectable amount of level. I'll try it.


The first transformer, set up to use the 500 ohm secondary (50k:500 ohms)
will load the microphone very little, which is good. It'll also step down
the level 20dB. If that's not a problem, I'd go with it.

Peace,
Paul


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