600 Ohm inputs

[Brian Roth]

This one has me "scratching my head...."

I work with more than a few studios located here in what is called "fly over country" by the "experts" on each coast...

MORE than a few of my clients own devices with a 600 ohm input impedance.  Examples:  LA-2 or LA-3 (new or reissue)..earlier 1176 comps. 

Many/most desks use stuff like a TL072 for the driver to those units.  A 5534 is supposed to happily drive a 600 ohm load...but then, some folks decide NOT..and do things like "pull up/down" loads.

It becomes a task, making a patch bay "work every time" with the possibilities running into 600 Ohms as well as a bridge/10K load.

Shrug...to make it "bullet proof'... then EVERY output HAS  to pump out...say...  +24 dBM.

Best,

Bri

 

[pstamler]

It's unfortunate that many console makers (on the low end) choose an output amplifier which will not drive a 600 ohm load cleanly. A 5534 or 5532 will do a better job, but still produces more distortion driving a 600 ohm load than a 10k load, and nastier distortion too (higher harmonics, more IM).

The fact is that there are very few ICs which are really happy driving a 600 ohm load, and while output stage pulldown helps, the amount of output stage bias needed to drive 600 ohms in Class-A would fry the IC in short order, since to drive +24dBu into a 600 ohm load requires (mumble, mumble) about 29mA of bias, minimum, which dissipates 0.61W with +/-21V supplies.

Really, driving 600 ohms from an IC opamp requires either a buffer stage incorporated into the feedback loop (there are several of them on the market) or a discrete-transistor output stage, with heatsink in either case.  Some of the better manufacturers do the latter, but mostly on the bus output stage, not on the opamp driving the channel inserts.

A worthwhile project for this board would be a really clean buffer amp, preferably useable balanced or unbalanced, with a nice high input Z and the oomph to drive a 600 ohm load. A National LME49860 with a couple of transistors and a good bias network would be just the ticket. (And in fact there's just such a circuit on the LME49860's data sheet.)

Peace,
Paul

 

[mulletchuck]

I've been here a while on the forum, and built up a bunch of 51x gear so I have a eensy-weensy bit of DIY experience.    But i don't know what any of what you wrote means.  Can you explain it?  "input load" "drive an input" "+24dBu" etc...

I know that impedance is the combined resistance a circuit provides on an AC signal being applied to an input somewhere, which is very different than the resistance of, say, a DC signal thru a normal resistor. 

 

[Samuel Groner]

To drive +24 dBu into a 600 ohm load requires (mumble, mumble) about 29 mA of bias.

+24 dBu is 17.36 V peak, makes 28.9 mA peak. Which means a push-pull stage needs 14.5 mA quiescent current.

An AD797 happily drives 200 Ohm, with very low distortion. Several composite opamp topologies exist which produce even better results. No need for discrete...

Samuel

 

[CJ]

tell the jokers not to use the aux loop for La2a/1176.

drive those boxes with the pultec or langevin mic pre,

or put a 2:1  output trans on the end of the opamp driver, gives you twice the current, but then you have to overdrive the front end,

 

[ruffrecords]

To drive +24 dBu into a 600 ohm load requires (mumble, mumble) about 29 mA of bias.

+24 dBu is 17.36 V peak, makes 28.9 mA peak. Which means a push-pull stage needs 14.5 mA quiescent current.

An AD797 happily drives 200 Ohm, with very low distortion. Several composite opamp topologies exist which produce even better results. No need for discrete...

Samuel

So you need +- 18V rails. That means 14.5mA quiescent x36V = 522mW dissipation in the chip with no signal.

Cheers

Ian

 

[Jean Clochet]

Post removed.

 

[Samuel Groner]

That means 14.5 mA quiescent x 36 V = 522 mW dissipation in the chip with no signal.

If you check the AD811 datasheet this is not entirely unfeasible for an IC standard package.

However it is not my point that an IC could deliver 29 mA peak class A output current. My point is that class A/discrete is not necessarily needed for clean 600 Ohm drive. I have designed headphone amps which drive far lower impedances with harmonic distortion well below -120 dB using just a few standard IC opamps, and as I noted above some opamps (e.g. AD797) have very low crossover distortion even without additional tricks.

Samuel

 

[pstamler]

But compare the output spectra on an AD797 driving 600 ohms and a Hardy 990C, or Sam's own HVA-1 discrete amplifier. For sheer simplicity of specrum there's no contest.

 

[JohnRoberts]

600 ohm input terminations are IMO archaic and unnecessary, but a fact of life in pro audio for as long as I can recall.  Only the -10dBV bedroom recording gear could safely ASSume nominal 10K inputs, while lots of cheap semi-pro gear is surely marginal in this regard. 

While ability to drive 600 Ohms is a way to separate the men from the boys, so to speak, it is an archaic, obsolete, standard, kept alive by antique gear lovers.

Note: You can still plug 600 ohm gear into non-600 ohm compliant interfaces and it will sort of work. You will mainly run out of headroom as it runs out of drive current way too early.

JR

PS: I always used 553x or equivalent in professional gear, and for semi-pro gear either used enough build out resistance so that output drivers kept their smoke inside, or used one of the cheaper GP opamps with decent drive capability widely used in value designs, while not as high slew rate or low noise as TL07x so YMMV. Yes Virginia there are some performance corners cut in low end value designs.

 

[Samuel Groner]

Brian, can you clarify why you started this thread? Are you looking for a proper line driver circuit, or is this more of a general lament against ancient 600 Ohm inputs..?

Samuel

 

[Brian Roth]

Brian, can you clarify why you started this thread? Are you looking for a proper line driver circuit, or is this more of a general lament against ancient 600 Ohm inputs..?

Samuel

It began as a lament, but I believe it also was a method to seek a relatively inexpensive solution for a very common problem in this Real World.  Indeed, I could concoct a solution myself by throwing some components against it all <g>, but I was perhaps hoping there was/is a universal "pre-existing" answer" as I trolled around with the "less than stellarly designed" desks in my world.

Best,

Bri

 

[JohnRoberts]

While this may sound flippant, IMO one should not expect to be able to mix legacy (old) recording gear, with modern value gear.

For one example that comes to mind, I recall doing some bench testing on a cheap Mackie mixer (one of my competitors at Peavey), and I discovered the insert point was not buffered, so driving it from a 600 ohm source impedance would cause a response error interaction with the unbuffered Baxandall tone control that followed the insert since it presented a different input load based on tone control settings... While this was not a huge flaw, it is a typical compromise made in value designs.

As I already answered, there were some GP opamps perhaps a step below the 553x but still usable. I am embarrassed to share how cheap we were able to buy these for (in millions) but the 4560 dual opamp could drive 600 ohms, had 5V/uSec slew rate,  1.2uV input noise etc. Another tidbit that wasn't on the data sheet was that it's turn-on/off characteristic was reasonably benign if you sequenced the PS rails properly. http://www.rlocman.ru/i/File/dat/Texas_Instruments/Operational_Amplifiers/RC4560ID.html

I didn't use this in my premium audio paths, but used truck loads for non-audio stuff, and in lower end value designs. Those opamps were price competitive with a single discrete transistor, especially when you consider glue parts and assembly cost. 

JR

PS: I don't recall the old 4560s being quite that fast, but they were significantly faster than the old cheapo 4558 and fast enough to handle most audio paths without slewing.

 

[fazer]

I always look at the console and find where it likes to work.  If there are say 24 to 36 inputs, and the bulk of the outboard gear is 10k bridging,  I fix the 600 ohm gear with a driver on the input (mounted external) and a pad if needed on the output. 

Makes no sense to me to modify all 24 outputs on prefade and postfade for 2 to 3 pieces of gear that need special treatment.  If there are many pieces of old outboard gear, than I would recommend a console change to an API , Neve, Ameck, fill in the blank.

 

[Jean Clochet]

Post removed.

 

[ruffrecords]

While ability to drive 600 Ohms is a way to separate the men from the boys, so to speak, it is an archaic, obsolete, standard, kept alive by antique gear lovers. s.

Not entirely obsolete. If you need to drive significant cable lengths, as many studios, OB units and PA rigs do, then you need to ensure your driving source impedance is low enough. If your kit has for example a 1500ohm output impedance it will drive a 10K bridging input with no problems. However, use it to drive 30m of cable which has 100pF/m cable capacitance and you will be 3dB at 15KHz at the other end. Reduce that output impedance by tenfold to 150 ohms and the -3dB point moves out to 150KHz but the phase response starts to alter at 15HKz. For negligible phase shift at 20KHz under these conditions you really need an output driving impedance of no more than 50ohms. Now I know a low output impedance is not the same as drive capability into 600 ohms, but any amplifier with a 50 ohms or better output impedance will drive a 600 ohm load to a reasonable level.

Cheers

Ian

 

[JohnRoberts]

While ability to drive 600 Ohms is a way to separate the men from the boys, so to speak, it is an archaic, obsolete, standard, kept alive by antique gear lovers. s.

Not entirely obsolete. If you need to drive significant cable lengths, as many studios, OB units and PA rigs do, then you need to ensure your driving source impedance is low enough. If your kit has for example a 1500ohm output impedance it will drive a 10K bridging input with no problems. However, use it to drive 30m of cable which has 100pF/m cable capacitance and you will be 3dB at 15KHz at the other end. Reduce that output impedance by tenfold to 150 ohms and the -3dB point moves out to 150KHz but the phase response starts to alter at 15HKz. For negligible phase shift at 20KHz under these conditions you really need an output driving impedance of no more than 50ohms. Now I know a low output impedance is not the same as drive capability into 600 ohms, but any amplifier with a 50 ohms or better output impedance will drive a 600 ohm load to a reasonable level.

Cheers

Ian

Huh... apples and oranges,,, drive capability and source impedance are two different things.  An audio power amplifier may drive 2 ohms but have a source impedance of tens of milliOhms.

But getting back you your supposition, I made a piece of budget test equipment (LOFTECH TS-1) with a 51 ohm build out resistor for the sine wave output, but driven by a TL074 so maybe 2k drive capability... apples and oranges... FWIW I could drive a speaker with the TL074 just loud enough to measure the impedance of the speaker (looking at dB drop across the 51 ohm vs no load). 

Premium gear makers have to design expecting the exceptional.. but they have the budget to pay for it.

JR

 

[Jean Clochet]

Post removed.

 

[Biasrocks]

I think we should take a deep breath.  8)

Perhaps even do a search on Brian's contributions to this board, you'll no doubt see they are significant.

To say these issues don't exist and to launch an attack on Brian as not in the know is short sighted at best.

With that said, I've got a bunch of 600 ohm input gear here that I use regularly and don't see myself not using it in the near or distant future.

My Apogee D/A can put out +26dbu into 600 ohms without breaking a sweat and I'm sure just about all my outboard can do close to the same. It's when you start mixing the 'prosumer' gear in that you run into issues.

The last time I checked the studio biz is changing, so I don't doubt Brian runs into these situations more often than not.

A rack space of patchable 'buffer/driver amps' seem like a good solution to me.

Regards,
Mark

 

[lassoharp]

If it's just to foot the few odd 600r inputs that venture in, perhaps an outboard box using bridging transformers?  Good ones will be somewhat pricey but 3 to 5 channels worth should cover some ground.  1176/La2A & all the big tube limiters can stand ~14db loss (they're often padded down to begin with).  The Pultec EQPxx is the odd one out with no provisions for makeup before hitting the input.

Brian has probably been through this 10 times over but worth mentioning that the insert point in most consoles is a risky zone for the older vintage '600r' comp/limiters and the working chain is often a group buss out and return to line input.