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1176´s 600 ohm input impedance
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gainreduction
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25th September 2008
Old 25th September 2008
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1176´s 600 ohm input impedance

Wondering about the 600 ohm input impedance of the 1176 and MC76/77...

Is there a sonic reason to why it´s that low or is it simply beacuse it´s an old design ? (been reading about Abbey Road going as low as 200ohm in the 60´s)

And on the same token, can anyone give a simple explanation to what that "constant 600 ohm" input impedance does for the sonics ?

I have two MC76´s and, well, I´m just wondering and like to know more.

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25th September 2008
Old 25th September 2008
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Hi,

it's due to old design. Once 600 ohm termination was a standard, now it's not.

In the 1176 the constant impedance input pad is before the input transformer. It has 600 ohm impedance on the input and output side, regardless of input pot position. The thing with transformers is that they like to see a certain impedance (it has an effect especially on the low frequencies). The input transformer in 1176 is designed to be driven by 600 ohm.

There's a lot of contemporary gear that don't like to see such a low impedance, not only cheap gear, but some really expensive tube devices as well.

Hope that helps...
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25th September 2008
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Yes, that helps. thumbsup

I´ve never run into the problem of gear having difficulties interfacing with the MC76. What are the symptoms and workarounds in that case ?

Thanks for the informative reply.
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25th September 2008
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The problems are mostly headroom related. Some cheaper desks have weak opamps on sends. For instance TL07x family op-amp loaded with 600 ohms distorts very early, at +12dBu perhaps. And that sounds very ugly.
Low value output capacitors cut lows. A 22uF output cap with 600 load has -3dB cut at 12Hz, but phase shift starts above 100Hz.
Some output transformers with high leakage inductance cut highs.
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7th March 2009
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Bump. stike More questions on the subject.

What kind of problems can arise when inserting a 600ohm in/out impedance compressor as a hardware insert in PT using a digi192 ? (I can´t find the specifications about impedance for the 192).

I´m asking because I have two different compressor types that are 600ohm I/O, transformer balanced and they have the same kind of thinning out the lows and low-mids going on. Wondering if it´s purely coincidental and characteristic of their respective designs or a symptom of bad loading ? (they are MC76 and ADL1000)

I have read somewhere that "strapping a resistor across a 600ohm output transformer helps with the sonics when interfacing with other gear. Is this some kind of termination ? When is it needed and why ?
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7th March 2009
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My tech basically tells me to do it on any older gear with big transformers unless it's already terminated inside... but LA3As, 1176s, pultecs, LA-2As, etc. all need that resistor. The quick fix is going to radio shack and getting a 680 ohm resistor (close enough) and throwing it accross the output barrier strip. I use these pieces as hardware inserts and yes it helps to keep them from "thinning out".
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7th March 2009
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Don't know if this is of any value, but a 2-1176 is switchable 15k ohms or 600ohms...

I'll run some material through both if you wanna hear the difference...

M
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8th March 2009
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Quote:
Originally Posted by yeloocproducer View Post
My tech basically tells me to do it on any older gear with big transformers unless it's already terminated inside... but LA3As, 1176s, pultecs, LA-2As, etc. all need that resistor. The quick fix is going to radio shack and getting a 680 ohm resistor (close enough) and throwing it accross the output barrier strip. I use these pieces as hardware inserts and yes it helps to keep them from "thinning out".
How is the resistor to be connected ?
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Quote:
Originally Posted by gainreduction View Post
How is the resistor to be connected ?
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Across the output, pin 2 & 3.
Newer versions may have it internal, or someone may have added it,if so DON"T add another one...
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8th March 2009
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Quote:
Originally Posted by nosebleedaudio View Post
Across the output, pin 2 & 3.
Newer versions may have it internal, or someone may have added it,if so DON"T add another one...
Thanks ! If there is one already, it would be located somewhere between the OP transformer and XLR connector, right ?

Is the resistor value to be as close to 600ohm as possible ?

I think I´ll start by making a short cable with the resistor inside the connector to simply plug in and see what gives.
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And here´s a little tech-talk-for-dummies on the subject: (scroll down to "power transfer")

Power Calculations
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Like mentioned 680 Ohms is a good choice.
Some 600 ohm transformers do NOT require termination, Jensen for example..
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30th January 2010
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SO I will benefit from the throwing a a 600 ohm resistor on the output barrier strip of my vintage 1176??
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30th January 2010
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Quote:
Originally Posted by superburtm View Post
SO I will benefit from the throwing a a 600 ohm resistor on the output barrier strip of my vintage 1176??
The problem is that it depends on what your next device in the chain is. If your 1176's driving a piece of gear designed in the sixties or earlier (like Pultecs, Fairchilds, UREIs, Gates' etc. and their clones) then it's better to omit the resistor, otherwise 600 ohm input impedance and the 600 ohm resistor will create 300 ohm load for your 1176, probably destroying its tone.

Hope that helps...
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30th January 2010
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thanks..its going into an apogee rosetta 800
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30th January 2010
Old 30th January 2010
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Quote:
Originally Posted by superburtm View Post
thanks..its going into an apogee rosetta 800
Then there should be no problem with the resistor. 680ohm value is OK as mentioned before.
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