Line level Input and Output Impedances - Legacy and Modern Gear

I'm trying to wrap my brain around input and output impedance of studio gear. I believe I'm understanding most of it. Where I'm confused is on what the best practice is for connecting legacy and modern gear together in a signal chain. As I'm understanding it, some legacy gear expects to see a certain input impedance in order for the transformers to give the most accurate frequency response. I have also read that some gear expects to see a certain load on the output to give the sound that it was designed for. For example, a preamp connected inside a console would sound different if the next stage had an input impedance of 600 ohms, vs connecting straight to a modern interface at 10k. I don't know if this is accurate or not.

I am under the impression that if I want to increase the impedance that I should put two matched resistors in series on the positive and negative signals. If I want to decrease the impedance, I should add a resistor as a shunt between the positive and negative signals, thus having the effect of adding resistance in parallel to the original output impedance.

In order for me to understand this better, here are a few components in my signal chain, some different ways they might be connected and the specs that I could find on them. I have more 600 ohm gear, but am just using the 1176 as a placeholder for all of them. Please correct me if I'm wrong.

Interface (input: 10K ; output: 100 ohm)
EZ1290 preamp (I don't know the output impedance, but have seen that some Neve pre's are about 75 ohm output?)
Hairball 1176 (input: 600ohm ; output 600 ohm)
Passive HPF/LPF combo (requires 600ohm to be connected at input and output)


EZ1290 > Interface - I shouldn't need to change the impedance.

EZ1290 > 1176 > Interface - I should increase the EZ1290's output impedance from 75 ohms to 600 ohms by connecting two 525 ohm resistors in series.

Interface > 1176 > Interface - I should increase the interface's output impedance from 100 ohms to 600 ohms by connecting two 500 ohm resistors in series.

EZ1290 > Filter > 1176 > Interface - I should increase EZ1290's output from 75 to 600 ohms. Since the 1176's input impedance is already 600, I do not need to change it for the filter.

Am I close here?

I prefer to leave legacy gear in the museum but if you must the main interface problem will be trying to drive 600 ohm input terminations with semi-pro or prosumer gear.

The problem is not driving that 600 ohm input with 600 ohm source impedance while that might be nice, the real problem is having enough drive current to put expected voltage into that 600 ohms.

There are many modern driver ICs that can handle that, but lots of inexpensive gear won't.

JR

Transformers may or may not ring audibly with different loads.
Output stages may or may not work correctly without expected loads. 

Okay, so the way to hook up a Neve preamp directly to something like a Neve compressor and have it sound it's best is to create a modern driver IC for it? If so, I don't understand why there aren't already popular "in-between" solutions out there for people that use vintage (or clones of) rack gear.

Or is this subject all together only relevant to a small portion of gear that shows apparent problems?

Hi,
A bit of background history might help.

The first thing to appreciate is that 'legacy' equipment was (is) based on the same techniques used in telephone systems where equipment was designed to drive twisted-pair telephone lines that have a characteristic impedance of 600 ohms. 

The second point are the concepts of 'Termination' and 'Bridging'.  In order to drive the telephone line the longest distance, the telephone exchange equipment matched the telephone line's 600 ohms so as to get maximum power in to it.  This is the Termination situation and if the equipment was designed to drive a 600 ohm load, it is best to maintain this, particularly if it is a passive device such as an attenuator or frequency dependant/altering device such as passive EQ.

So, we have some kit driving a line and now our telephone engineer wants to measure the power down the line without disconnecting anything and without affecting the power (or frequency response etc.)  If he used a 600 ohm device in parallel with the Termination, this would half the signal power and screw things up, so he used a device that has a 10Kohm input impedance with his croc clips on the line since this is the minimum load that drops the signal by 1dB - this is Bridging.

Modern equipment has moved away from the telephone exchange working since we (usually) don't have to drive telephone lines and we just need to drive a signal down a cable (not a twisted pair).  So we have resorted to voltage  driving  from  a low output impedance voltage source.

The output is usually from a buffering amp  with a small output impedance so it can drive the cable some distance before the cable capacitance and output impedance starts to roll off the higher frequencies.  So the Termination and Bridging concepts are not important as long as the following input impedance is 5 to 10 times the output impedance. 

Hence if the equipment is designed to drive a termination load of 600 ohms, I suggest you Terminate it with a 600 ohm load.  If you are linking it to modern equipment that does not Terminate it, make sure it Bridges it.

Hope this helps

Mike

critterkllr said:

Okay, so the way to hook up a Neve preamp directly to something like a Neve compressor and have it sound it's best is to create a modern driver IC for it? If so, I don't understand why there aren't already popular "in-between" solutions out there for people that use vintage (or clones of) rack gear.

Or is this subject all together only relevant to a small portion of gear that shows apparent problems?

Click to expand...

Yes market too small.

You might find some old used bump boxes that were designed to step up between  -10dBV gear and +4dBu and back down again.  The -10dBV side was typically unbalanced but for short runs should be OK.
These were popular back in the '80s/90s of last century.

JR

How about a balanced circuit that I could build to see if I hear any difference? It seems like it should be a pretty basic circuit.

Basically, I just want to be able to hook up some popular GroupDIY compressor clones (during tracking or mixing) and have them sound like they would if they were plugged into an insert on a console.

It seems kind of crazy to me that there are all of these people looking for that "classic sound", with vintage gear or clones, but there is little to no discussion as to how to properly interface the gear in a modern recording environment.

critterkllr said:

It seems kind of crazy to me that there are all of these people looking for that "classic sound", with vintage gear or clones, but there is little to no discussion as to how to properly interface the gear in a modern recording environment.

Click to expand...

Yes isn't it crazy....  8)

JR

So, does anybody know where to find a schematic of a circuit that is designed for this? Or one that can be pretty easily modified. I can create something from a schematic, but circuit design from the ground up is pretty far beyond my skill set.

JohnRoberts said:

critterkllr said:

Okay, so the way to hook up a Neve preamp directly to something like a Neve compressor and have it sound it's best is to create a modern driver IC for it? If so, I don't understand why there aren't already popular "in-between" solutions out there for people that use vintage (or clones of) rack gear.

Or is this subject all together only relevant to a small portion of gear that shows apparent problems?

Click to expand...

Yes market too small.

You might find some old used bump boxes that were designed to step up between  -10dBV gear and +4dBu and back down again.  The -10dBV side was typically unbalanced but for short runs should be OK.
These were popular back in the '80s/90s of last century.

JR

Click to expand...

I have sold thousands of those from mid-80's to late 90's. Defunct market today.

critterkllr said:

So, does anybody know where to find a schematic of a circuit that is designed for this? Or one that can be pretty easily modified. I can create something from a schematic, but circuit design from the ground up is pretty far beyond my skill set.

Click to expand...

I would suggest you piggy-back a THAT 1246 and a THAT 1646.

You are seriously overcomplicating this, you strap resistive loads, or you don't.  There is already encyclopedic discussion of the various issues to be found all over this site. 

emrr said:

You are seriously overcomplicating this, you strap resistive loads, or you don't.  There is already encyclopedic discussion of the various issues to be found all over this site.  You are just new to the problem and haven't dug deep enough to find all the answers already given, year after year since the '80's.

Click to expand...

I'm simply responding to the responses given to me. I don't understand how I am over complicating this. I'm just trusting that the senior members of this forum are giving me reliable information.

I've spent a lot of time searching for this. Everything that I've read has led me to my original post. Which I was told wasn't right. I'd really  appreciate some clear direction.

> techniques used in telephone systems where equipment was designed to drive twisted-pair telephone lines that have a characteristic impedance of 600 ohms.

Not to spoil your conclusion, but....

Twisted-pair is always around 100 Ohms.

600 Ohms was typical of Spaced-Pair, and specifically the first Transcontinental Telephone, which used house-wire (#12) on very wide spacings to reduce all losses.

As telephone traffic increased, and insulation and amplifier technology improved, twisted-pair multi-pair cable became the telephone backbone.

For short runs the convention stayed "600 Ohms", because the telephone itself is near 900 Ohms. On short cables, the cable impedance is unimportant. On not-short runs, cable series resistance (not impedance) matters most. The 900 Ohm instrument gives fairly equal performance on short or not-short lines. 600 is a fair approximation.

On long runs through repeaters... it gets complicated. Usually there are hybrids to save amplifiers. These require carefully matched "artificial lines" to cancel the echo. They may also try for a better match to the line. However the first long voice lines were "loaded" with series inductors. This raises the line impedance at higher frequencies (up to a critical freq), to well over 600 Ohms. So again 600 Ohms was a best-compromise.

In broadcast, 200 and 500 were the nominal impedance.

The VU meter committee picked 600 Ohms, perhaps to coordinate with telco practice. But also because rectifier voltage drop hurts more for lower impedances.

Of the three (+/-1) broadcast networks, two went to 600 Ohms and one went to 150 Ohms. It is not clear why. Probably each CE's personal preference and no need to co-ordinate between. (When they linked-up it was often via Ma Bell so the conversion was in the drop; when they had to connect direct it was simple to insert a transformer or just overlook the mismatch and make-up the loss elsewhere.)
____________________________________

You "match impedances" when gain is exPENsive. Used to cost $10/TU (around $500/dB today).

Gain has got a LOT cheaper since the Transcontinental.

MOST gear since 1955 will tolerate either 600r or "infinite" loading on the output. An exception is passive tone-controls which depend on a load to suck against. Such things ought to have 620r attached permanently.

Even into the early 1960s, "some" gear had true 600r inputs. Particularly HIGH-gain LOW-level inputs. A hefty-ratio input transformer is not only a handy place to find gain, but gives a better hiss-ratio into tubes and some transistors. Even so there was an awareness that Matching was usually NOT best for low hiss.

There is an infinity of situations to think about, but nobody gets his ya-yas-out thinking about impedances. (Well, maybe me.) The real work is The Music. "Mismatched" impedances do not stop the music. It may cause some small issues. IMHO, possibly less than the color of the studio walls.

Generally you plug it together and try it. Any gain-"loss" due to "mis-match" is easily compensated elsewhere in a modern studio (turn-up a knob).

There will be cases where a transformer has a 17KHz ringiness when driven with a too-low *or* too-high impedance. Usually the music is not ruined. In modern work it may be the "vintage tone" you seek. Any 1/4-decent DAW can mostly-correct such errors if they are unwanted.

There's all sorts of discussion, basically most of it saying slap a 620-680 ohm resistor on it, or even better with a switch so you can or you can't as needed.  But that is highly simplified. 

http://groupdiy.com/index.php?topic=2845.msg681575#msg681575
http://groupdiy.com/index.php?topic=29509.0
http://groupdiy.com/index.php?topic=41032.msg508997#msg508997
http://groupdiy.com/index.php?topic=25786.msg307094#msg307094
http://groupdiy.com/index.php?topic=24376.msg290858#msg290858
http://groupdiy.com/index.php?topic=27406.msg329258#msg329258
http://groupdiy.com/index.php?topic=52435.msg668501#msg668501

critterkllr said:

Okay, so the way to hook up a Neve preamp directly to something like a Neve compressor and have it sound it's best is to create a modern driver IC for it? If so, I don't understand why there aren't already popular "in-between" solutions out there for people that use vintage (or clones of) rack gear.

Or is this subject all together only relevant to a small portion of gear that shows apparent problems?

Click to expand...

I would suggest that Neve is not legacy gear.

To my mind, legacy gear is the type that has a 600 ohm output impedance and/or presents a 600 line input impedance. It may or may not also require these to get the right 'sound'.

Modern gear has a low output impedance ( a few tens of ohms) and and presents a 10K bridging line input impedance.

By those definitions, Neve is definitely not legacy gear (and indeed nearly all the semiconductor gear from the late 60s onwards - Cadac, calrec, Audix, API, SSL etc) and therefroe does not need any special loading for it to sound  or work 'right'.

Cheers

Ian

ruffrecords said:

By those definitions, Neve is definitely not legacy gear (and indeed nearly all the semiconductor gear from the late 60s onwards - Cadac, calrec, Audix, API, SSL etc) and therefroe does not need any special loading for it to sound  or work 'right'.

Cheers

Ian

Click to expand...

Still, there are some, such as UREI, where the 1970's specs mentioned the importance of proper loading.
My first encounter with UREI gear was at a vinyl mastering/cutting facility. The consequences of unproperly loading them were not trivial. Remember, the SX74 cutting head was designed to record up to 60kHz for the doomed quad format.
You are generally right; however it's the exceptions that generate caution.

Altec 9470's don't need loading.  Langevin AM-16's do.  At a glance they look the same, and both designed by John Hall.  You have to measure things to really know, as I said in one of my links above.  Does it matter?  Not terribly, if the ear isn't offended. 

I read  years ago somewhere by a Neve expert ( Geoff Tanner maybe) that the older Neve consoles were terminated with 1200 ohms on the line outs so that if they were driving  600 ohm inputs or 10k inputs the results were both OK.

  I'd say I prefer my  all original parts 1290 with the 600 ohm termination engaged if plugged into the higher impedance  inputs.  But as EMRR said some may  like the sound of it unterminated.

Getting back to that transmission line business. We should remember that whatever the line's impedance is, it only holds for a narrow band of audio frequencies.

This Jim Brown paper has the often overlooked details:

Transmission Lines at Audio Frequencies, and a Bit of History
The behavior of cables at audio frequencies is widely misunderstood.

http://www.audiosystemsgroup.com/TransLines-LowFreq.pdf