24ch. tone generator

[earthsled]

Hi all,
I need some help in designing a passive 24 channel splitter or "mult" to send tone from my tone generator to the 24 inputs of a tape machine. My tone generator has a 600Ω unbalanced output and the tape machine inputs are 10kΩ balanced. I've already tried the "y-cable" approach to split the signal (simply joining together the hot, cold, and grounds of 24 lines), but this seems to be giving me some issues at the tape machine.

Is there are way to add resistance between the lines for isolation? Could the unbalanced to balanced connection be problematic?

Thanks for your help!

 

[PRR]

> giving me some issues at the tape machine.

What "issues"? Low level? Level changing? Buzz on top of tone?

24 10K loads is near 417 Ohms. A "600 Ohm" output would normally drive this OK, at lower level. "0dBv" un-loaded should be -7.75dBv at each input. But many "600 Ohm" sources are not true 600 Ohms. My Heath was very-very close on the lower ranges, but my H-P was more like 200 Ohms.

 

[pucho812]

If you have a daw and a converter would be easy to generate a balanced line level +4dBu sine wave. At any frequency you want.  Then mult it out.

 

[earthsled]

What "issues"? Low level? Level changing? Buzz on top of tone?

Well, I thought I was having issues on a couple channels with input bleeding into playback while monitoring the record head (modulating the tone from the MRL tape). I tried to recreate the scenario today, but now it looks okay. It seems this issue was probably tape machine and not the harness.

The tone generator I'm using is a B&K 3010, and I'm sending 1.228 VAC to the machine.

 

[earthsled]

If you have a daw and a converter would be easy to generate a balanced line level +4dBu sine wave. At any frequency you want.  Then mult it out.

Ideally I would love to have a DAW with 24 i/o to test, but unfortunately I don't. My 24 ch "y-cable" is essentially a mult, but I don't have any isolation between channels.

From what PRR was saying, it doesn't seem like isolation is necessary as long as the tone generator can drive the load. (Please correct me if I'm wrong).

 

[PRR]

> can drive the load.

I got a lot more than 24 AC devices running from one source. All my lights and PCs, TV, a clock, etc.

"Can drive the load" is up to the situation. My power is 125V no-load and can sag below 115V with lots of loads. None of my loads really cares. I have computed that if my power sags much below 90V, the street-wire will get hot and the main fuse will blow. In this case, before that, the lights would be mighty dim and the TV would be re-booting itself.

Your twenty-four 10K loads seems to be 416.6 Ohms. I do not trust this, as the "10K" may be very nominal. Also maybe deviates at 20Hz or 20KHz. But pencil 417 Ohms.

Are there specs on the signal generator? Yes. Better: is there a circuit so as we know exactly what is in there? Yes.

From R71 and Q13 we suspect the output at point {12} would be under 27 Ohms naked. NFB through Q10 Q11 promise much lower, probably under 1 Ohm. So the true output impedance is dominated by R74+R75, 600 Ohms on the dot (+/-5%).

600 driving 417 is a substantial voltage divider. We need near 3Vrms at internal point {12}, 8.5V p-p. We see the amplifier has +/-15V rails and can likely swing +/-10V or 20V p-p. It should swing it.

3V into 1017 Ohms is around 10mW output power. Dynamic power in Q12 Q13 may be 5mW. As these are 300mW devices, it should not smoke.

The only tricky spot is you have to find your "1.228V" with your total load connected. And you should watch that as frequency varies because even small capacitance (shunt OR series) applied twenty-four times will make it deviate at frequency extremes.

 

[earthsled]

The only tricky spot is you have to find your "1.228V" with your total load connected. And you should watch that as frequency varies because even small capacitance (shunt OR series) applied twenty-four times will make it deviate at frequency extremes.

Thank you for your help, PRR. I appreciate your comprehensive answer!

 

[earthsled]

And you should watch that as frequency varies because even small capacitance (shunt OR series) applied twenty-four times will make it deviate at frequency extremes.

We've been trying to use the hard-wired 24-ch output harness for about a month now. I'm re-posting about it because what PRR had mentioned regarding "frequency extremes" seems to be evident when we attempt to calibrate a 24-track tape machine. When using the harness, the signal generator's output drops considerably. This is not so much of an issue, because we can adjust the amplitude back up to 1.228 VAC to compensate. The problem seems to be with frequency response. The generator's 10kHz and 20kHz seems to be getting rolled off with the use of the 24-ch harness connected to the tape machine.

I'd really like to know more about how to properly isolate or buffer the outputs to avoid the frequency issues. Is there anything that can be done with passive components that would be effective to help with the frequency response of the harness?

I imagine that the simplest solution might be an analog mixer that has 24 outputs and can distribute the signal across them. I wonder if we could re-purpose some VU meter buffers to do the job of distributing the signal. (We happen to have a bunch lying around for spares).

Thanks for your help!

 

[JohnRoberts]

We typically build oscillators into console master section with a feed to every bus.  The line up tones travel through the console sub master output sections so are already feeding the tape machine inputs.  Easy peasy...

Just trick whatever you are driving your tape machine with now to supply the line up tones.

JR

 

[gar381]

Use 24 busing resistors the same value as the busing resistors in your desk.
Feed each of the 24 buses from these resistors pre summing amp. 

Just think of your generator as just another channel.

GARY

 

[earthsled]

Just trick whatever you are driving your tape machine with now to supply the line up tones.

Use 24 busing resistors the same value as the busing resistors in your desk.

The problem is, I don't have a console to use in the shop. I'm trying to build something inexpensive that would do the job of distribution. Not sure if that's possible.

Is there a standard value resistor that would work for distributing 24 channels at line level? Something that would basically have the opposite function of a passive summing bus. If so, how would a schematic look for this?

Thanks!

 

[mjrippe]

Is there a reason you can't calibrate one channel at a time?

 

[JohnRoberts]

24x10k loads in parallel is around 400 ohms so marginal for your 600 ohm source...

5k resistors in series with each 10k input would bring the parallel load up above 600 ohm.

5k in parallel with 10k would drop less than 4 dB and result in a roughly 3k source impedance...

JR

 

[earthsled]

Is there a reason you can't calibrate one channel at a time?

Good question. We do calibrate one channel at a time. The 24-ch harness is so that we don't have to plug and unplug our signal generator for each channel.

5k resistors in series with each 10k input would bring the parallel load up above 600 ohm.

Great! Is it best to add the 5k series resistance at pin 2 of the XLRs, or is it best to do 2.5k in series with both pins 2 and 3 for balanced?

 

[PRR]

OK, pull that old Sansui hi-fi stereo amp out of the closet. Feed your tone to that. The Speaker output can drive 8 Ohms, so a thousand "10K" loads, or hundreds of loads that fall-off from 10K at the frequency extremes. Go in at Tape. The Vol knob will have to be way down. Do monitor the Speaker output to watch for the "hi-fi" amp's fall-offs.

 

[JohnRoberts]

Good question. We do calibrate one channel at a time. The 24-ch harness is so that we don't have to plug and unplug our signal generator for each channel.
Great! Is it best to add the 5k series resistance at pin 2 of the XLRs, or is it best to do 2.5k in series with both pins 2 and 3 for balanced?

5k in series with pin 2, 5k in series with pin 3... done.

JR

 

[moamps]

I will suggest here one different method. Just open your generator and mod it to "50 ohm" (or less) source by soldering 56 ohms resistor  across R74 and 75. The output stage of the generator is capable to deliver +10dBu into 400 ohms including increased capacitive load at high frequencies, so it should work, IMO.

 

[sodderboy]

Ive never had good experiences with mults above 12 channels or so.  There is always additional artifact no matter what the math and specs shows.

If you are doing this in a tech shop and will be doing it more in the future I would recommend making a 12 channel balanced distribution amplifier and multing two channels 12 times.  You can do it with 13 dual opamps and some resistors from the parts drawers.  Make it to two dsubs so that you can use it on other equipment.

Back in the ol' days dupe racks with more than 5 or 6 devices used DA's.  SSL 4K "Main Outputs and Dist." was just a mult under the patchbay and you had to be test a new install for any heavy loading there.

Mike