Using 600 Ohm vintage gear in conjunction with modern stuff

Hi all,


All the very popular vintage audio gear was designed with a 600 Ohm input and output impedance; the current thru all devices in a chain remained constant (matched impedance).


However a lot of solid state gear made nowadays has an input impedance which is at least 10 x higher than the output impedance (1 K input vs 100 Ohm output). The voltage over daisychained devices remains constant (Bridged impedance).


When I want to use a 600:600 ohm piece of gear in conjunction with my newer 1K:100 ohm gear, is there any need to use impedance converters to get an optimum result?


Please advice!


Many thanks in advance,


Rogy

Level differences aside, here are some basic rules of thumb that will keep you satisfied most of the time.

If the old gear specifies a 600-ohm load, and you want to run it into a modern "bridging" input, simply strap a 620-ohm resistor across the output of the old gear.

If the old gear has a true "terminating" 600-ohm input, make sure that whatever you plug into it is rated to drive loads as low as 600 ohms.

If the old gear is transformer-coupled with an "unloaded" 600-ohm input, you may need to build-out the output impedance of the gear you're plugging into it to 600 ohms if you want flattest frequency response. This can be done with series resistors.

As you can see, you really need to know the specs of your gear to be able to interface it effectively. There's no "universal" solution.

maybe it´s just me, but I alwyas think it´s a good thing to mod your old "true 600 ohms" input gear to something like 6k or more...

Problem is that it´s not always desirable, because sometimes you have to substitute the input transformer, cahnging the sound signature of the gear in question.

Is there really a standard input/output impedance nowadays? It doesn't really seem that way...

It seems that there isn´t a standard, but a 60-600 output and a 2k-20k input is what we usually get.

> Is there really a standard input/output impedance nowadays? It doesn't really seem that way...

There is a broadcast standard, but not slavishly observed.

In modern "Pro" audio: all inputs are way over 600Ω (usually 10KΩ to 47KΩ), most outputs are under 600Ω (usually 47Ω to 470Ω), most but not all outputs will actually drive 600Ω effectively, and you can plug one or more inputs onto one output without significant loss of level or response.

And most ins and outs have some pretense of being balanced (often quite lame).

---------------------

> All the very popular vintage audio gear was designed with a 600 Ohm input and output impedance (matched impedance).

Actually no. In the 1920s, matched impedances were necessary to minimize the number of very expensive low-gain tubes needed to get from mouth to ear. By the 1940s, tubes were affordable, feedback was fashionable. Many inputs are really "Bridging", 20KΩ. Many outputs, while intended to drive 600Ω, have actual source impedance closer to 60Ω. Voltage-transfer interfacing is very old.

And yet there are maddening differences. Many old-time limiters had a constant high output level, and a pad so you could get lower levels. The pad was generally 600Ω when padded-down, though maybe dropping to 60Ω at full-up. The LA2 might be used on the far end of a long telco line, with a passive line-EQ on front, so it uses a large step-up input transformer for cheap gain and really is "about" 600Ω input. Some transformers were originally designed for true 600Ω systems, never re-designed, and give peaking when sourced from 60Ω or loaded in 20KΩ. And if you like iron-sound, you get less of it with 60Ω sources than with 600Ω sources.

But be straight about what you are doing. If you wanted smooth predictability, you'd use high-quality modern gear. Messing with old boxes, whatever sounds good is right, even if it isn't what the designer intended.

Thanks for the clarification.

Another question is how to hook up older gear (take Teletronix as an example) in an otherwise fully balanced setup.

The LA3 for example has a +/-, a common and a chassis input terminal. Output is +/- and common. How to best hook up? What is hot, cold and Gnd?

Regarding NewYorkdave's suggestion of strapping a 620-ohm resistor across the output of the old gear, when connecting this gear to a balanced input do I need to use one 620 Ohm resistor in between +/- and common on the LA3 output side of the cable or two 620 Ohm resistors (one in between Hot and Gnd and the other in between Cold and Gnd) on the balanced input side of the cable?

Thanks again for the time.


regards,


Rogy

Rogy

put 620r accross the hot and cold (+ -) terminating the secondary of the output transformer .I built a rack mount affair with pig tails that terminates pins 2&3 with 620 ohms.This makes it easy to interface with
only one added connection.

cheers
Lance

> how to hook up older gear (take Teletronix as an example) in an otherwise fully balanced setup.

Old Pro gear IS "fully balanced".

Or to be accurate: it is "Floating", which is better than Balanced.

> The LA3 for example has a +/-, a common and a chassis input terminal. Output is +/- and common. How to best hook up? What is hot, cold and Gnd?

The "+" and the "-" are the signal terminals. They are the two ends of a transformer winding that has NO other electrical connection. "+" and "-" are "the same" except for phase: they are equal-but-opposite. If phase were completely unimportant, they would not need labels; they are both "hot". But phase is important: absolute phase is sometime audible, and relative phase is very important with multiple sources. So they put a "+" on the pin that will go Positive when you push on the microphone diaphragm. Just connect + to +, - to -, all the way through the system, and you are fine.

If you ever need to reverse phase, just connect + to -, - to +.

With true Floating inputs and outputs, signal is never referenced to Ground. Ground exists only for shielding. In many situations, you can disconnect ground lines between boxes. Floating is especially good for very long (miles) unshielded lines (telephone systems). Since there is no electrical connection to ground, you can run from studio at low ground potential to a transmitter with a very "hot" ground (from radio and power fields) and still get a clean signal without stray ground current.

Floating is better than Balanced when you are forced to change to an unbalanced condition. With Floating, you can ground either + or - pin, and take an unbalanced signal from the other pin. (Normally you ground - and then + is "hot".) A truly Balanced interface that forces the + and - pins to swing symmetrically both sides of Ground will resent having one output grounded (however you can take one output as "hot" to Ground at 6-dB level).

Thanks PRR, that is something I have always wanted to understand more clearly.

:thumb: :thumb:

Hi PRR,


Thanks for the refresh about absolute phase and balancing principles; this wasn't exactly the kind of answer I was hoping for since i already understand this bit and use it for the last fifteen years in my daily job. Probably my question was not well formulated.

The thing is that the LA3 has one pin which is labelled "+/-", another which is called C and a third which is named Chassis. So not one pin + and one pin -.

Now, the "+/-" is connected to one side of the primary, and "C" to the other side of the primary.

"Chassis" is connected to the core of the input trafo and is also connected to earth.

So am I correct when I assume that the "+/-" pin will be the hot, "C" will be the cold and "Chassis" will be the Gnd in a setup where all devices are connected balanced?

When connected unbalanced, "+/-" will take the unbalanced signal and "C" will connect to the Gnd of the previous device. In this situation "C" should be tied to "Chassis", correct?


thanks for the time.


Rogy

One of the reasons they used the old centertap grounded input transformer setup was to lessen capacitance to ground in the transformer.

> the LA3 has one pin which is labelled "+/-", another which is called C and a third which is named Chassis. So not one pin + and one pin -.

Oh. Well that's just wacky.

Since you've traced back to the transformer, you know what it really does. Wondering how they came up with the markings on some old stuff is a waste of time.

:grin:
Just connected a double pair of old EQs with the above 600? impedance after reading this really informative thread, works like a charm. It just needed an amp on it's output. (got it connected to my vlz mackie, as the eq really needed a clean and unflattering amp)

Does anybody know anything about this eq? I just picked it up, and can't find much info on the www.
It's called Eckmiller 86 and 86a, made in Berlin and it's a passive equaliser.
Black with silver sliding (86) and golden roller (86a) controls.
It sounds a bit like a pultec, but has fixed (60 Hz and 10 KHz) frequencies.
thanks

Guys
Is this method apply to Urei 1178 & LA4 ACompressor (1983)? I can find only 650 Om resister in my area.
Thaks
K.

didn't we used to have a 0DB operation level as pro standard as well? as for the +/- on one c and chassis maybe it's unbalanced input or output? Delt with an orban stereo not to long ago that had balanced on the way in but unblanced on the way out which means sometimes old shit is just plain wacky.

DOH. I ment orban stereo compessor unit.

reptil "it's a passive equaliser."
My understanding of passive circuits is that the output voltage/level will never be greater then the input level/voltage(i.e. can boost level) and tends to either be equal or less then input voltage/level so it would make sense that it needed an amp on output.
As for the 600OHM discussion I remember it being the standard set long ago by the phone companies and since the audio industry had no real clue how to amplify a singnal we looked to them. same reason we got patch bays and other things and as I mentioned b4 the old operational standard of 0 DBM. DBM is referencing to 600 ohm line. Now days we got a set operational standard of +4DBM well actually 4+DBU but the oldskool guys will call it DBM just out of habbit. When you loot at the 2 it's basicly a difference of 4 DB so you can plug your old stuff with your new stuff and a 4 DB difference in volume which would give you a lil more noise if my thinking cap is on straight.

The file-naming system on the webpage is kind of confusing. Was that done to confound search engines? :wink:

@kubi
thanks, really informative
any history known to you? :grin: