600 Ohm inputs

[Matt Syson]

Many 'audio' op amps can't really drive and worse many of the output capacitors that are used are far too small to drive into a 600 Ohm load (at LF). You should investigate WHY you think the supposed 10K input of a Motu 'acts' like 600 Ohms. It would be highly unusual and raises suspicions you are not measuring things correctly.

 

[abbey road d enfer]

Almost any audio op amp can drive 600 ohms

Many opamps are spec'd for 2kohms load. They may be capable of driving 600 ohm loads, but at the cost of reduced headroom and increased distortion.

 

[JohnRoberts]

Many 'audio' op amps can't really drive and worse many of the output capacitors that are used are far too small to drive into a 600 Ohm load (at LF). You should investigate WHY you think the supposed 10K input of a Motu 'acts' like 600 Ohms. It would be highly unusual and raises suspicions you are not measuring things correctly.

True 600 ohm I/O is mostly found on legacy (old) equipment. Most modern gear should interface with whatever is patched into it, some legacy gear, may encounter issues.

Many opamps are spec'd for 2kohms load. They may be capable of driving 600 ohm loads, but at the cost of reduced headroom and increased distortion.

Just to clarify, op amps specified to drive 2k ohm loads to full scale, will current limit (distort) before driving 600 ohms full scale.

JR

 

[Cranehazard]

2k is close enough to 600 ohms for me for me to consider them the same

 

[Cranehazard]

In any case my advice, build a preamp that can drive your interface without losing signal.

 

[Cranehazard]

True 600 ohm I/O is mostly found on legacy (old) equipment. Most modern gear should interface with whatever is patched into it, some legacy gear, may encounter issues.

Just to clarify, op amps specified to drive 2k ohm loads to full scale, will current limit (distort) before driving 600 ohms full scale.

JR

I actually think there lying and it's the same 1k input as the mic. But now we're talking conspiracy.

 

[Audio1Man]

OK Guys, how many miles of audio cable interconnect cable does it take for the VSWR to get >1.000 (causing ringing) to create a problem? Termination of short audio cables driven from a low Z source typically is not a problem, however TUBE EQUIPMENT is typically higher Z and the reflected Z must be controlled.

 

[user 37518]

OK Guys, how many miles of audio cable interconnect cable does it take for the VSWR to get >1.000 (causing ringing) to create a problem? Termination of short audio cables driven from a low Z source typically is not a problem, however TUBE EQUIPMENT is typically higher Z and the reflected Z must be controlled.

Playing it safe, If we take lambda/10 as the cable length at which one should start considering an audio cable as a transmission line, at a frequency of 20 KHz, and considering, say, a propagation speed of 0.8Xc0, I would say that it requires a 1.2 km cable, or 0.75 mi for those of you who insist on being impractical (for the record, just kidding). So Duke is right.

BTW I believe Dave Rat did some sort of experiments with this on YouTube, he is not what I would consider scientific, but he does a good job.

 

[Brian Roth]

This thread started 11 years ago (!) and perhaps my following comments have already been covered....

There is still an elephant in the room! While some posters seem to think 600 Ohm input impedances apply to antique/vintage gear, that is NOT a fact. More than a few new "replica" outboards still have 600 Ohm input impedances. I just verified that these PRICEY reissues fall into that category.

-Universal Audio 1176 and LA-2
-Purple Audio MC77 (1176 clone)
-Pulse Techniques Pultec equalizers

Probably others. I also believe that some of the new DIY kits also have 600 Ohm input impedances.

Just sayin.....

Bri

 

[user 37518]

This thread started 11 years ago (!) and perhaps my following comments have already been covered....

There is still an elephant in the room! While some posters seem to think 600 Ohm input impedances apply to antique/vintage gear, that is NOT a fact. More than a few new "replica" outboards still have 600 Ohm input impedances. I just verified that these PRICEY reissues fall into that category.

-Universal Audio 1176 and LA-2
-Purple Audio MC77 (1176 clone)
-Pulse Techniques Pultec equalizers

Probably others. I also believe that some of the new DIY kits also have 600 Ohm input impedances.

Just sayin.....

Bri

That is correct. However, it should be noted that the main reason to terminate those pieces of equipment is not due to reflections in the cable/maximum power transfer as required in RF or even video equipment, but rather due to level matching and frequency response. If you don't happen to terminate them with 600 ohms, it is not as critical as, say, an RF power amplifier, which can melt down.

 

[JohnRoberts]

This thread started 11 years ago (!) and perhaps my following comments have already been covered....

There is still an elephant in the room! While some posters seem to think 600 Ohm input impedances apply to antique/vintage gear, that is NOT a fact. More than a few new "replica" outboards still have 600 Ohm input impedances. I just verified that these PRICEY reissues fall into that category.

-Universal Audio 1176 and LA-2
-Purple Audio MC77 (1176 clone)
-Pulse Techniques Pultec equalizers

Probably others. I also believe that some of the new DIY kits also have 600 Ohm input impedances.

Just sayin.....

Bri

Whats worse than old archaic gear, is brand new gear masquerading as old...

Do whatever floats your boat...

Back when I was still designing gear, I made sure that serious audio outputs could drive 600 ohm loads, but I did not attempt to mimic 600 ohm source impedance.

JR

 

[user 37518]

Whats worse than old archaic gear, is brand new gear masquerading as old...

Do whatever floats your boat...

Back when I was still designing gear, I made sure that serious audio outputs could drive 600 ohm loads, but I did not attempt to mimic 600 ohm source impedance.

JR

You weren't a good designer then, good designers are those who use a lot of tubes, 600 inputs/outputs and warm musical transformers that give depth and completeness to the sound. [\satire]

 

[abbey road d enfer]

2k is close enough to 600 ohms for me for me to consider them the same

Are you serious or joking? Let's say you have a 6kW contract with the electricity company. You plug a 9kW range/oven. What happens?

 

[abbey road d enfer]

In any case my advice, build a preamp that can drive your interface without losing signal.

Could you have some kind of scientific data, like something with figures, impedance, level, distortion...
It's like saying build a car that can drive any road anywhere.

 

[user 37518]

It's like saying build a car that can drive any road anywhere.

That would be a Jeep, of course....

 

[abbey road d enfer]

I actually think there lying

Who's lying?????

same 1k input as the mic

????

 

[abbey road d enfer]

That would be a Jeep, of course....

It would need to be amphibious, equipped with arctic equipment.
It's feasible, but not a reasonable proposal.
There is room for some specialization.

 

[Brian Roth]

That is correct. However, it should be noted that the main reason to terminate those pieces of equipment is not due to reflections in the cable/maximum power transfer as required in RF or even video equipment, but rather due to level matching and frequency response. If you don't happen to terminate them with 600 ohms, it is not as critical as, say, an RF power amplifier, which can melt down.

Forget all the reflectiving and terminations, etc.
I am merely stating two facts.....

1. More than a FEW brand-new, expensive ($2000+) gizmos are spec'ed with a 600 Ohm input impedance. Talking Universal Audio, etc. Refer to my earlier post <g>.

2. Mackies, Behringers, yada yada will choke and and barf driving that low Z.

If it's all antique designs, then complain to the makers of the CURRENT manufacturers....not me...LOL!

Bri

 

[JohnRoberts]

I actually think there lying and it's the same 1k input as the mic. But now we're talking conspiracy.

1.5k-2k is a typical mic input (bridging) termination.

Lots of semi-pro gear has been built using 2k output drive capable op amps but with series resistance large enough to prevent current limiting even if loaded down with 600 ohm loads. Of course it will lose level, but not distort when heavily loaded.

JR

 

[user 37518]

Forget all the reflectiving and terminations, etc.
I am merely stating two facts.....

1. More than a FEW brand-new, expensive ($2000+) gizmos are spec'ed with a 600 Ohm input impedance. Talking Universal Audio, etc. Refer to my earlier post <g>.

2. Mackies, Behringers, yada yada will choke and and barf driving that low Z.

If it's all antique designs, then complain to the makers of the CURRENT manufacturers....not me...LOL!

Bri

Agreed, they could probably re-issue the LA2 and so on without the low input Z, but they would have to do some modifications and people would probably claim "it is not the real deal". If it is not a re-issue of some old gear, I don't know why would someone would want to have 600 ohm impedances.

 

[user 37518]

1.5k-2k is a typical mic input (bridging) termination.

Lots of semi-pro gear has been built using 2k output drive capable op amps but with series resistance large enough to prevent current limiting even if loaded down with 600 ohm loads. Of course it will lose level, but not distort when heavily loaded.

JR

Don't you miss the good old days when they used to add a discrete push-pull stage to the outputs of a TL070 for it to be able to handle 600 ohm loads?

 

[Matt Syson]

The world has gone downhill since Beethoven* died. At least at one of his gigs you had to actually be there and nobody was rustling sweet wrappers behind you.
* Also other long deceased 'popular musicians' from before recording (fakery) was possible.
For entertainment I read old magazines produces in the early 1940's which show that many electronic princi[ples were understood even if the technology did not exist to use them.

 

[abbey road d enfer]

If it's all antique designs, then complain to the makers of the CURRENT manufacturers....not me...LOL!

I don't think anybody's complaining at you, but rather at those who don't fully understand the reasons for this state of affairs.
You're a victim here, but it's also an opportunity for you to provide practical solutions to this issue.
Look on the bright side.
As I mentioned earlier, I sold a lot of helpers, different times, the context was adapting consumer equipment to pro, Sony 501/701, Tascam dbx coder/decoders, cassette recorders, CD players.
Today, it's adapting bridged connections to matching connections. So many recreations of vintage gear, I think there's a market.

 

[JohnRoberts]

Don't you miss the good old days when they used to add a discrete push-pull stage to the outputs of a TL070 for it to be able to handle 600 ohm loads?

I've used truckloads of 553x to drive 600 ohm lines.

I made one headphone amp with a discrete push-pull stage hanging off a TL07x. It could comfortably drive loudspeakers until the 1A wall wart opens it's thermal fuse. I ended up adding a R in series with the charging rectifier in the PS so the rails would sag under heavy use and protect the wall wart.

JR

 

[ruffrecords]

Don't you miss the good old days when they used to add a discrete push-pull stage to the outputs of a TL070 for it to be able to handle 600 ohm loads?

Probably the best thing about the 'good old days' was the much greater reliance on actual musical talent rather than electronics that can turn a sow's ear into a silk purse.

Cheers

Ian

 

[SmokeyJoe]

What happens if you replace r109 & R110 with 10 ohms?
Your G-Pultec has an input xfmr, right? What is it?

Thanks for the suggestion. Excellent results with 10 ohm - doesn't seem to be any roll off at all. Although I've not performed any empirical tests yet other than listening, and looking at the white noise response on a graph.

(I may also have been a complete goon and forgotten to turn on the power to the opamp circuit when I was testing the 300 ohm output resistors... with the power on the rolloff is back in more reasonable territory (not 5k!), but still too high for my liking).

As for next steps - is there any benefit to finding a trade off between increasing the output resistor value and acceptable low end roll off? Or ok just to leave them as 10 ohm? I believe increasing my supply to +/-15V would be worthwhile to increase headroom.

Perhaps there is a more suitable transformer to use on the input (further research required...)

Edit: Preliminary tests are showing quite a bit of THD with the driver and Pultec in the loop (~50dB below signal level). The driver alone is very clean, and the Pultec alone is even worse. I'll try and conduct some proper measurements.