600 Ohm inputs

[Jean Clochet]

Post removed.

 

[Brian Roth]

Perhaps I came of as a bit of a whiner in my other posts <g>.  But, the problem I mentioned just came up again with a current project.  A friend/client recently bought a Tangent 3216 desk, which I'm working on.  For those unfamiliar with that brand, the company was based in Phoenix, AZ and sold quite a bit of product from the late 1970's until the mid 1980's.  Their consoles were positioned above, say, a Tascam and below, say, an MCI.  IIRC, a new Tangent desk sold in the $10K range.

While this desk has P&G faders (!), it is full of nothing but TL071 opamps, so "here we go again".  My friend/client does have a few pieces of outboard with 600 Ohm inputs, and I need to deal with the likely problems which will arise.  Over the years, I've kept my eyes open for an off-the-shelf solution, either manufactured or more recently, as a DIY item.

A box like an Aphex 124a (designed as a bi-directional -10 dBV to +4 dBu interface) comes close, but is pricey for just 2 channels, and also provides (unwanted) signal gain.

The "ideal" product could be something like a THAT InGenius balanced front end paired with a THAT OutSmarts output chip.  I think the balanced input is a Good Idea to make it completely universal.  Why?  Years ago, I discovered the (relatively pricey) DigiDesign 192HD analog outputs had a "brain dead" design when dealing with unbalanced loads, such as an API 550 EQ.  In addition, the 192HD doesn't seem really happy driving a 600 Ohm load.

Soooooooooo...I may need to quit whining <g> and lay out a PCB to deal with the various system possibilities, and then fuss with powering and packaging the PCBs.  I HOPED someone else had already done some of the legwork!  <G!>

While John Roberts makes a very valid point about 600 Ohm input stages being archaic, they are still everywhere.  Just think how many Mnats 1176 Rev D limiters are now in outboard racks; another client just finished assembling his fourth unit.

Best,

Bri

 

[CJ]

for the LA2a you can swap the input with a 10K:10K , since it has too much gain anyway, this is a well known mod, jensen has a paper on it,

maybe the same thing would work for other gear, maybe not.

pultec with a 10K:10K, input might have problems with the tone circuit dragging it down?

1176, i do not know that circuit yet, but maybe you could also give it the same treatment.

 

[gyraf]

pultec with a 10K:10K, input might have problems with the tone circuit dragging it down?

Yep - worst-case load impedance from the filter is down at 75 Ohms (when full boost hi, narrow Q). This makes hi-z input transformers unusable because of too much winding resistance.

On the other hand - has anyone wondered why the 600 Ohms input units are coming increasingly popular at the same time as not much left is able to drive it properly..

.....could it be that we actually LIKE the sound of inadequately-driven-input-transformer...?

Jakob E.

 

[jensenmann]

While fiddling with my Amek M2500 i ran into a similar problem. I bought a bunch of german Rundfunktechnik distribution amplifiers (Telefunken V382) which I use for buffering and multiplying outputs to the patchbay. They are dirtcheap, class A discrete, transformer in and 6x out (!) each card and are speced to drive 300Ohm at every output. Perfect, even for Pultecs.

 

[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

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.

I know, which is why I was careful to qualify my answer by saying just that. I think the important point to establish is just what 'modern' gear is designed to drive. Is it 10K bridging or 600 ohms?


Cheers

Ian

 

[JohnRoberts]

And as also previously discussed there are two standards... "pro" or wannbe pro, have driven 600 ohm terminations as long as I can recall... While many value products, don't anticipate encountering Low input Z gear.

There is a huge difference between being 600 ohm capable at the output, and expecting the rest of the world to be 600 ohm capable to drive your input.

FWIW the Tangent board that Brian is ranting about, is perhaps a design flaw. I was designing studio gear around the same time, and my gear often used discrete transistor buffers on the major output lines so I could drive parallel 600 ohm loads.  Unbalanced TRS inserts are another story. There was little expectation that unbalanced inserts would drive long lines or 600 ohms, while there were lots of bump boxes around back then to interface all the bedroom recording gear's -10dBV level up to normal studio level.  These bump boxes have pretty much gone away with the low end recording gear, but some are still available. http://www.peavey.com/assets/literature/manuals/ia10_4.pdf  Note: from looking at the picture, this has been repackaged since the original version I designed back in the '80s, so I have no idea what the internal circuitry looks like. The data sheet does not say that it drives 600 ohms (my old one did) so caveat emptor.

I suspect a premium version if that old stereo bump box, using THAT chips could satisfy this niche market need.

JR

 

[PRR]

Tascam used to market a 1U rack box with 4 RCA-to-XLR and 4 XLR-to-RCA, specifically to interface Tascam -10dbV 10K stuff in and out of "pro" systems. Possibly to update radio production rooms to 4-track with the 3340 deck instead of a staggering invoice from Ampeg.

As I recall the performance was flawed; not so the early bedroom studios (or AM jingle-ad producers) would care but I didn't like to 'scope it at high level.

The case and connectors and PS were very fine and the circuit was not flawed, it just wanted something better than 4558 and some attention to buffer dynamics. Could keep the existing PCB. Or rip and re-do.

 

[ibvee]

Very interesting topic!

If I may suggest to make things a bit more practical, I'd really like to know which THAT chip John is talking about, and what some of the DIY solutions have been for other people so far.

Would a INA134 and a DRV134 for most applications?

 

[JohnRoberts]

I don't have any specific THAT PNs in mind they have a few different input receivers and output drivers,,, I always rolled my own from opamps and resistors but they have invested some engineering effort into the project so precision matched IC level resistors are nice.

A universal High input Z, low output Z, low drive Z black box could be a nice fixer circuit.

JR

PS: Back int the 80s/90s I also made a 8 channel bump box (2U 19" eia), but it was not a very strong seller and faded almost as fast as the small format tape machines...

PPS: PRR, it seems you might want to buy the bump box from a company working from the +4 side of the equation... While Peavey is not a high end company, I designed the bump boxes so boys and girls could plug up to my +4 consoles. So they worked OK. 

 

[Brian Roth]

In the circumstances I'm familiar with, the issue isn't typically level matching (requiring what John reminded me was called a "bump box"), but rather a "value desk" (thanks again, John! <g>) which has wimpy chips which feed into the "outside world", and yet operate at a nominal +4 dBu level.

The Tangent desk I'm working on is certainly not the first.  I've seen the same thing on some Soundcrafts, etc.  Interestingly, in some designs, the manufacturer would toss in a 553x chip in a few circuits which had to drive "the outside world", but would leave something like an insert send with a TL07x as an output driver.  For example, a channel insert patch point is where something like an 1176 or LA2/3 would be commonly patched.

I am definitely leaning towards "buffer the inputs of the 600 Ohm input impedance outboard gear", which would be a LOT less extensive than "fixing" all the wimpy outputs on a "value" desk.  Once a circuit design has been worked out/tested, then it becomes an interesting project in terms of packaging/powering the buffer circuits, then distributing the audio lines around in the outboard rack.

One question becomes: how many channels of buffering are required for any given studio system.  Two?  Four?  Eight?  etc

Depends on how many Mnats 1176 REV D compressors (or, various other devices) are expected.  <G!>

In addition, for many years, I've included "guest equipment" XLR patch panels into new studio installations.  Each of THOSE will also require buffering with the "value" desks to ensure the ability to connect anything which may wander into the doors of the studio.

I figure you can fit a total of 8 channels of Neutrik XLR I/O into 1 RU.  But, then you have the expense of all those XLR's (both on the box, and on the cables).

Too bad the required circuitry won't fit inside of an XLR connector <G!>

Best,

Bri




Based on that ASSumption (thanks again, John...hehe), that might help define some sort of "universal fixer-upper".

 

[emrr]

Aphex 124a's are $25-50 on ebay all the time.  Pretty hard to beat. 

 

[Brian Roth]

OK, then hack them to be essentially unity gain....

Bri

 

[pstamler]

Too bad the required circuitry won't fit inside of an XLR connector <G!>

But it could easily fit onto a circuit board in the back of a patch-bay chassis, combining the patch-bay and the buffering for 600-ohm loads.

Peace,
Paul

 

[AP carrier]

Hey Brian,

This 8 pack circuit board looks like it can be configured balanced input to balanced output with a 5532 or 2604 and trimmed to unity. Could be an option if you don't mind a bit more soldering?

http://www.jlmaudio.com/forum/viewtopic.php?f=2&t=164&sid=bd485d221e09cf01360667d56e86b9b1

Andrew

 

[JohnRoberts]

The most efficient fix, is to build a THAT receiver chip or the small JLM Regurgitator  (not sure I care for that name imagery) onto a small PCB and located inside the low input Z unit, so it can use that power supply.  It is certainly possible to squeeze a SMD version of this into an XLR but you still need to provide external power.

Of course now we face, modifying a perhaps classic, surely antique, product, that may be worth more to other audiophools in stock  unadulterated condition. Even though we all know the product is eminently more useful with the higher input impedance.

I would consider building a multi channel custom bump box that you can rack into the same rack with however many classic products there are that need buffering, with a single power supply and chassis. This becomes more cost effective the more units you have to deal with, while an off the shelf stereo bump  box (surely there are several people still selling these) would allow you to recommend a fix for your customers that you aren't married to service for the rest of your life... OTOH if you want lifetime customers, a custom solution may accomplish that too.

JR

 

[emrr]

The JA Electronics mid-side matrix pcb is also useful as four THAT receivers and four THAT line drivers, see the note on the sidebar.

http://www.ka-electronics.com/KAelectronics/MS_Matrix/MS_Matrix.htm

That JLM looks like a useful device too. 

 

[gemini86]

I have pcbs for the ssl9k aux send section made up. Im also going to do some that balancer boards soon... would a 4 channel board work or would single units be more ideal?

 

[Brian Roth]

Hey Brian,

This 8 pack circuit board looks like it can be configured balanced input to balanced output with a 5532 or 2604 and trimmed to unity. Could be an option if you don't mind a bit more soldering?

http://www.jlmaudio.com/forum/viewtopic.php?f=2&t=164&sid=bd485d221e09cf01360667d56e86b9b1

Andrew

Hmmmm!  I had missed that one.  But, without seeing a schemo, I am slightly worried the "balanced output" might have a "brain dead" circuit design, which makes it worthless as a Universal circuit....

Bri

 

[JohnRoberts]

You can probably satisfy the most common interface problems from active balanced drivers with a simple dip switch, so the end user worst case may have to try a couple options until they get a good result. The circuitry can be made reasonably bulletproof so it will tolerate shorts or opens where they shouldn't be, without failure.

JR