Distribution Amplifier

[Sizif]

Hi to everyone!

I need to split analog out from Apogee Rosetta 200 (balanced, +4 dBu/max +26 dBu, output imp. 25 Ohms) to feed monitor amp (balanced, input imp. 20 kOhm) and headphone amp (balanced, input imp. 40 kOhm) at the same time.

The best way, I suppose, should be Distribution amplifier (stereo in/ 2 stereo or more out) and I'm trying to find one the most neutral, clean and transparent to build.

Schematics, advices, opinions, links?

Many thanks in advance

Kind regards

 

[babyhead]

[quote author="Sizif"]Hi to everyone!
...
Schematics, advices, opinions, links?
[/quote]

Hello back!

http://www.cinemag.biz/line_input/CMOL-3x600T2.pdf

Put 2 of these in a box and away you go. Simple and bullet proof. If you needed more outs or separate levels then a DA(distribution amp) or another DA(convertor) would be the way to go. I don't believe there are any distro projects here in these pages... but LOTS of other stuff...

BTW:
http://www.jensen-transformers.com/as/as041.pdf
:guinness:

 

[NewYorkDave]

Splitting with a transformer is appealing, but one serious drawback is that the load on any winding is reflected to all other windings. So the system falls well short of the degree of isolation that's often required in a distribution system.

A distribution amplifier is really the way to go. The classic configuration is a low-distortion amplifier with high input impedance, low output impedance and healthy drive capability--a small power amplifier, really--feeding a resistive splitting network, which in turn feeds the various outputs. Since the impedance looking back from the splitting network into the amplifier is very low, the degree of isolation between outputs is high. And, with a suitably beefy amplifier, a number of the outputs could even be shorted with little or no effect on the remaining outputs. An engineer from Benchmark once told me that his criterion is that a DA should tolerate up to one-third of its outputs being shorted. I'm inclined to agree.

Are you determined to build something from scratch? If you are, I can dig into my files for some representative schematics. If not, a number of ready-made examples exist, many of them available at low cost on the used market.

Hmmm, come to think of it, I have a pair of never-used Dynair 1X6 DA cards which I'll probably never need. Maybe I should put 'em up in the Black Market



BUT, having said all that, in your particular case a simple resistive splitting network might do the job just fine. Try 1K resistors in series with the "hot" and "cold" going to each destination. Looking back into the 25-ohm output impedance of your converter, this will give almost 40dB of isolation between outputs, with only slight loss when feeding 20K and 40K loads. Keep the lines fairly short because the output impedance--now built up to about 2K by the series resistors--can cause high frequency rolloff with long lines.

I rarely advocate direct "multing" of destinations as some do, because it would be a shame if a disturbance on, say, a monitoring amp line caused a good take to be ruined. Or, in this case, caused someone's eardrums to be blown out from said disturbance making it into the headphone amp :shock:

 

[PRR]

This is exactly what I do for live public concert recording:

Main recording chain ends in a DBX limiter, output impedance R_out 22Ω. This feeds a buss. A bunch of outputs are fed through 470Ω resistors R2-R5: monitor amp, DAT, CDRs, cassette. Their input impedances range from 100K to 22K. I also feed a very long line to a remote point, through a 150Ω resistor R6 and a 1:1 transformer (not shown).

Assume the limiter is outputing 1.000 volts. Makes the math easier. Call that zero dB.

The monitor amp gets 0.983V or -0.16dB. The lowest-Z main recorder gets 0.967V or -0.29dB.

Actually I did line-up on the DAT. If I did it with 0.1dB precision, the DBX would be outputting +0.2dB, which it can do (I'm really asking it to make +6.2dBV to get +6.0dBV at the DAT).

Then the CDR gets -0.09dB. I call this sufficient accuracy.

The monitor amp gets +0.04dB up from the DAT, but the monitor amp knob is freely adjustable (no Dolby Certification in my shop!).

The remote location take what they get. They get two outputs with 2K resistors in series. If they short one of their two outputs, the other gets a half-dB less than my DAT (and another part-dB of transformer loss). If they don't short or low-Z their jacks, they get my DAT's level +/-0.2dB.

If any of my recorder inputs goes dead-short (it has happened, usually a panic plugs an output onto the output), the other recorders get 0.4dB less level. I know from years of working with step-attenuators that you do not hear a 0.4dB change in the middle of music.

A rule of thumb is that, on a bad day, a third of your outputs may get shorted. I really have a dozen outputs but not that many recorders: 3 shorts would give a 1.2dB loss of level which is not going to ruin the recording, even if all three shorted at once in the middle of the concert.

Things are a little more risky on the remote feed. The 2K in the far end protects the line from the expected idiots, but if the wire to the remote gets shorted then my recorders get -1.2dB of calibrated level. Even this would almost never be audible; though I would like to add a $2 buffer and get that risk out of the system. Not so much for level-drop, but because if they back-feed a signal into their jacks, it lands on my buss at -39dB, which would be audible after the fact though maybe not noticed in recording (I like to nap until the applause).

So I say just split your outputs with series resistors. It works and adds zero distortion. The part-dB loss can be adjusted out, and is negligible for monitoring with a volume knob in hand.

Since your loads are 20K and higher, the maximum series resistance for 1dB drop is 2K. Since your source is rated to drive 600 ohms, and more than one short in a 2-load system puts you out of business, that's your minimum series resistance.

Since your system is balanced, you want two resistors per load. Max 2*1K, min 2*300 ohms. Take 330 ohms as a standard value. Each load will see 682 ohms which is so close to the traditional 600 ohms that they can't complain. You'll need eight resistors total.

Though myself, I'd just Y them together. It appears this is just monitoring. If things go wrong, you can finish the take and capture the moment without hearing it. And there are just two loads to go wrong, not like my pile of assorted recorders plus mystery guest loads.

 

[Kev]

really great post
simple practical real world hook-up

and shows there can be good things when mixing 600ohm outputs with bridging inputs
might get a little tricker if one of the inputs were unbalanced with a funny ground
but a little experimentation in the work shop before the gig and most of them can be sorted with the soldering iron

[quote author="PRR"]The monitor amp gets +0.04dB up from the DAT, but the monitor amp knob is freely adjustable (no Dolby Certification in my shop!). [/quote]that made me laugh
:grin:

 

[Sizif]

Guys, you are great. Thanks so much for the input. :thumb: :sam: :guinness:

I was thinking about passive solution first, but after contact with Apogee support, they said:

The impedance rating of the Rosetta 200 is 100k on the input and 25 ohm on the output. It may be best to acquire active splitter for your setup. This would insure a good level from the analog o/p of the Rosetta 200 to your monitor and headphone amps considering your i/p impedance is low on your amps.

After that, I compared prices of few stock models including Henry Engineering and ATI but here in Europe it's pretty hard to find the distributors and the prices are significantly hotter than US.

Friend of mine mentioned DIY, because making distribution amplifier should not be a rocket science. He said: ?Just find the schematics of world top model, we will try to find the best parts (not always included in stock models) money can buy and I could make one for you? He is experienced electrician, more on yours level than I am. I could only split a hair and be anal about my recordings and mixes. :green:

I'm trying to keep all my cables short as possible. I moved PC out of my control room to the next room (finally it's quiet enough) therefore I have to move my rack closer to the right wall (PC is behind that wall) to keep firewire connection (Rosetta/PC) reasonably short. Now, there is one more problem, could be connected with the choice of distribution amp type. I could move my monitor amp from the rack, closer to the speakers (short speaker cables - 5 feet each, but longer interconnection ? 18 feet) or do opposite (longer speaker cable - 21 feet, short interconnection ? 2 feet).

What do you think?

If I go with passive one, like dr. PRR mentioned above, am I at risk of messing the frequency spectrum?

Many thanks for your time and effort

Best regards

 

[Freq Band]

Try to get hold of one of these, used if need be (cheaper$). They work great. Transformer outputs are an option on these units.

http://www.oxmoor.com/files/cut%20sheets/MDA%20cut%20sheet.pdf

I noticed one on ebay today.


=FB=

 

[Sizif]

After all, I decided to try passive first.

PRR, what kind of resistors for your schematic? Tantalum by AudioNote or metal film by Caddock or Vishay?

Thanks

 

[babyhead]

Another option might be the

http://www.presonus.com/centralstation.html#tech

It's a good piece of gear. It solves a lot of problems elegantly. Not DIY though.

:guinness:

 

[Sizif]

Still need opinion about resistors

please

 

[AudioJohn]

Not come across Tantalum resistors before - so had to do a Web search. Sure enough - found some information:

These exotic resistors from Japan, sound less dry than the metal films and they do not emphasize any particular frequency range. They sound more natural, more musical.

Prices were mentioned - £2.34 - per 100?, or even 10? - no each.......!

My preferred resistors are Welwyn MFR4 series (0.6 watt 1% metal film) - have used hundreds of thousands of these - with never a problem.

No need to pay more than a few pence (or cents) - each for resistors surely?

Anyway for your proposed application - try it first with inexpensive resistors to check it works properly - then move on to the exotic components if thought necessary.

 

[laitue5]

[quote author="babyhead"]
Hello back!

http://www.cinemag.biz/line_input/CMOL-3x600T2.pdf

Put 2 of these in a box and away you go. Simple and bullet proof. If you needed more outs or separate levels then a DA(distribution amp) or another DA(convertor) would be the way to go. I don't believe there are any distro projects here in these pages... but LOTS of other stuff...

BTW:
http://www.jensen-transformers.com/as/as041.pdf
:guinness:[/quote]

Sorry to hijack this thread, but hey, this type of transformer could be a nice way to go for a console monitor: the second secondary can feed a vu-meter. Am I right or there is a better way?

 

[PRR]

> opinion about resistors

The resistors will have nearly no effect on the sound; they would have to be really awful to be "heard".

I just got a multi-pack of carbon-film resistors.

> No need to pay more than a few pence

That's the way I feel. There are places for ultra-virgin/exotic parts, and places where they have no advantage.

I suspect, in this application, you could not tell carbon from metal in blind A-B testing, but eight few-pence metal-film is not a lot of money, and might be better.

That tantalum stuff is for resistors that dominate the audio. Plate resistors. Feedback resistors. But here the resistor value is picked to have very-very-little impact on the audio, used mostly for short-protection.

 

[NewYorkDave]

For anyone who cares, here's a schematic and info sheet for a nice, solid broadcast audio DA.

1.8Meg PDF

I used to work in a plant that had a few trays of this model, in continuous service for 15 years or more. There was never a failure, to the best of my knowledge, and for all I know they may still be cranking away today.

The circuit is fairly typical for an audio DA as found in a broadcast plant.

 

[Sizif]

Thanks for the all the knowledge and experience you share. :grin:

Now it's time for soldering ...

 

[tmuikku]

Hi all.

I'm building headphone amp box with four independent headphone amps, each with their own INA143 chips at the inputs. I want to use switching TRS sockets so that succeeding amps get the signal previous amp got unless another signal is plugged in. Eg. when plugging in audio signal (A) to amp 1 inputs, the 2,3 and 4 get the same signal. If I plug another audio signal (B) into amp 3 inputs, amps 1 and 2 get the signal A and amps 3 and 4 get the signal B etc. Audio signals are balanced.

Now, is it possible to connect the TRS sockets with wire only or should I put resistors between, say 0k47 or 1k as suggested examples by NYD and PRR or do I need something else?

Another option would be to add a common audio input and put switch to each amp to select between the common input or correspondent amp input but this will cost more.

Lets assume the device providing the audio signal can drive all four headphone amp inputs easily and is within few meters, in same rack perhaps.

Thanks!

 

[scott2000]

For anyone who cares, here's a schematic and info sheet for a nice, solid broadcast audio DA.

1.8Meg PDF

I used to work in a plant that had a few trays of this model, in continuous service for 15 years or more. There was never a failure, to the best of my knowledge, and for all I know they may still be cranking away today.

The circuit is fairly typical for an audio DA as found in a broadcast plant.

Anyone ever get and still this schematic ?

 

[mjrippe]

Apparently he was talking about the Utah Scientific ADA-8, but no luck finding the schematic.

 

[scott2000]

Thanks Mike... I didn't catch that...