600 ohm output transformer termination?

So... with no termination, if you go to a transformer based compressor or eq (1176,LA-2A, Neve or API EQ) after the preamp and before the converter there should be no issue ?

Are you sure? On old Urei stuff and other types of preamps that were built a long time ago you would just build your term resistor into the pigtails and move on, but modern day manufacturers normally fix the impedance of their gear properly to avoid ringing. 

After enquiring with Heritage Audio whether transformer termination resistors were fitted to my Series 80 modules and/or 'Frame 8' rack here is the email reply I received...
I don't know whether or not 500 series modules are also unterminated but I will enquire about those as well.

If you are finding that in some circumstances the upper frequencies seem somewhat harsh or too sibilant especially when using condenser mics CHECK THE OUTPUT TRANSFORMER LOAD IMPEDANCE!

I suspect many pieces of gear using output transformers are incorrectly terminated.
Back in 'the old' days pretty much every bit of gear in most studio were 600ohms.

Today we have a mixture of new and old (or re-issued vintage gear) so the loading is likely to change as soon as you patch in a different piece of gear.

If your are already feeding into a 600 ohm input impedance then you are okay.
If you are feeding into a typical high impedance input on most modern, non vintage gear then you are not.

This is a good case for fitting switchable load terminating resistors to the transformer outputs of any gear that doesn't already have them fitted.

Without sufficient transformer loading your acoustic guitar tracks may sound a bit more sparkly even with no EQ but your vocal tracks may end up too sibilant and possibly even little harsh.

This is the result of output transformer ringing which appears as waveform overshoot on an oscilloscope.

I wish it were really this easy, but it ain't so. Meant for a 600 ohm system does not mean everything is necessarily actual 600 ohms, or anywhere near it, it frequently means SHOULD BE fed by 600 ohm sources, which can be difficult to determine.  It really takes test gear measurements to determine optimal loading, and you can't just look at most spec sheets and determine input or output impedance, nor can you really even with a good understanding of schematic basics.  If it's transformer coupled, test gear is going to clue you to the transformer specs that aren't printed either, such as does it give target response with a matching or bridging condition. YES...the transformer might require either matching or bridging condition source impedance AND the equipment it is a part of might be adhering to that condition, or not, and the piece after might be....or not. Good news is it's seldom sonic make or break, and common minor effects are frequently up at or past our accepted hearing range. Too heavily loaded sometimes roll off highs, sometimes it extends response. Depends on the transformer!  Too low a source Z can be a problem on some rare occasions, and then you need series resistance to RAISE source impedance.   There are some good Altec papers on the various issues with power amps and EQ's online.  

My Neve experience has only been within the context of a complete console, and I'm assuming correct loads attached.

If you really want to cover all bases, a switchable load using a rheostat with build out resistor to set minimum resistance load value would let you dial to taste, or you could use a rotary switch to set steps that align with particular interface needs. In practice I haven't found too many things that need variable options after tweaking in the right loading, which tends to be passive EQ's here. Build outs to raise source Z are sometimes needed to get passive filter sets flat on average. Closest example to the Neve situation I can think of is RCA SS preamps, which are described as 600 ohm system pieces, yet deliver a source impedance well under 100 ohms, and oscillate with anything higher than around 1K5. They don't show difference in response comparing 600 with 1K5, so 1K5 is a safe load resistance in that case to prevent need for an 'off' switch when connecting to a true 600 input, 1K5/600 not really impacting response either where 600/600 will have effects.

Indeed. Here's another. Langevin AM-16 and Altec 9470A. Both designed by John Hall, very similar designs (compared to anything else on the planet) done within a few years of one another. AM-16 needs a load or there's a large high treble boost, 9470A does not. Combination of circuit specifics and transformer winding differences. AM-16 loses more than 3.5dB comparing 10K to 600R load, 9470A loses less than 1dB, this tells us something about actual output Z.

Are you sure you don't mean the 300R/1K2 switchable input impedance?

Just get a 600R 3dB or 6dB pad and put it on the output. You should notice a difference in the frequency response if it's not terminated normally.

However, a lot of clones have snubber/zobel terminations on the output transformers to keep them from ringing. These serve as an overall termination as well.

Bumping up this topic that I wanted to research.

I have a few pieces that might need to be terminated: 1176 clone, Heritage TT73 (edit: it's factory terminated), Tree Audio The Branch II.

I don’t have an oscilloscope - is this the moment to get one and learn how to work it?

Is there something that I could patch on my patchbay (TRS) to add the 600 ohm termination?

Is there something I could listen for, or is the oscilloscope required?