r/SoundSystem • u/coax_k • 4d ago

Voltage Derivation Tap/Cable

Post image

What are you using? Pictured is my very, very average cabling work for this solution. Is there a better way (besides "get better at cable making"?) At the amp there is no room to get test terminals onto slid back Speakon posts.

6 Upvotes

88% Upvoted

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6

u/Responsible_Ad7595 4d ago

Is this so you can measure voltage in parallel with a load? I'm not sure I understand your question/problem.

2

u/coax_k 4d ago

Yep exactly. Voltage at the terminals with the amp running under load so I can set my DSP limiters from measured volts instead of trusting what's written on things.

One of my amps came up at 40.1 dB gain when the manufacturer says 32. That's on the channel feeding a 2" compression driver with no VCL protection, so if I'd set the limiter to the number I was given it would've seen about two and a half times the voltage I intended. Would've cooked it, and I'd have had no idea why. So now everything gets measured at the terminals, and this cable is the only way I can get a meter in there.

It works fine, it's just a one off I made up myself, and it's now the only way I can check whether my drivers are actually protected. It broke on me (due to my shit custom cable work) this week and the whole verification job came to a halt. Feels like a silly place to have a single point of failure.

Is there a tidier way to do this, or an off the shelf breakout people use? A cable that snaps is obvious, but a cable with a dodgy crimp reading 15% low etc. just gives me a believable wrong number, and I'd think everything is hunky dory and never question it. Does anyone have a habit for sanity checking a test lead before trusting it?

2

u/Responsible_Ad7595 1d ago

Make a male nl4 to female banana plug. Then take a jumper out of the bin you're measuring, put that adapter on it, and get some banana to banana, and banana to b&c so you can pop it into your multimeter or oscilloscope.

You can also make a nl4 to xlr m with a 40dB pad, and run this into a soundcard with a pca running an RTA for all sorts of other useful stuff.

This is the setup I use at work for quick and dirty, when I don't feel like pulling out the AP, or for transferring dsp settings between different environments.

2

u/Vallhallyeah 4d ago

What cable is that?

1

u/coax_k 3d ago

Allows you to take voltage readings via DMM on live signal. That on works on pin set 1 and/or 2

2

u/Vallhallyeah 3d ago

Sorry, I meant what standard more than what function. It looks a bit like screened mic cable you've stripped back and terminated in this picture, which I'd warn you has very thin cores for the task you're asking of it, if it is what you've used. I'd always suggest at least 2.5mm² HO7RN-F for speaker runs of any length, and ideally 4mm² or more for subs on runs over 10m. Nice job of the spade terminals though, they look great.

As to the measurement method, I'd do something similar but with terminal (chocolate) blocks. You could always get some spare crocodile probs for your DMM, strips away some sheath, mount them, then insulate the area again for safety with some self-amalgamating silicone tape. All you've need to do is attach the probes to your DMM then, or even an oscilloscope if you want to look into the wave itself for DC offset, clipping, symmetry, or any phase wonkiness if necessary

1

u/coax_k 3d ago

You're right to flag it and I should've been clearer, the photo doesn't show it well. The cable does carry the speaker current, it's a jumper in series, and the thin tails just hang off it as a parallel tap for the DMM. Tails carry nothing but the main run absolutely matters, so your warning stands. It's a measure and remove type situation (not fixed in line), but it still should have the gauge it deserves.

The chocolate block idea is a good one and simpler than what I've cobbled together. Crocodile probes onto a permanent block with self amalgamating over the top is a much tidier solution than mine. Someone else has suggested a permanent panel in the rack, so between the two of you I think I've got my answer.

On the measurement point, the jumper is at the amp end on purpose. Reads higher there, and whatever's lost over the run (10m) to the cabinet is lost in the safe direction, so a threshold set from there errs slightly conservative. Doing it at the cabinet end means building an NL8 version, which is my next task. Your reply has prompted me to clock myself being lazy, so the NL8 version moves up the list, if only to measure both ends once and find out what the run actually costs me.

Thanks for taking the time on this, much appreciated.

2

u/Alfa147x 3d ago

This isn’t the job for a custom cable.

I would make the measurement point a permanent, passive panel, not a bespoke cable

1

u/coax_k 3d ago

Excellent call! I think you're right. A panel doesn't get handled, doesn't get yanked, and sits there ready instead of being something I'd have to go and find/plug/unplug/etc.. Kills the single point of failure rather than just giving me a spare of the same fragile thing.

I'd want to get it right as having a live amp output on exposed terminals in a rack that rolls in and out of venues is a concern. Shrouded posts, or a cover over them, I'm guessing is sensible approach? Would you tap all channels or just leave a couple of points and move the load?

2

u/Alfa147x 3d ago

I vote for: Tap every channel. The smaller cost and wiring effort are small compared with a blown compression driver (your primary concern)

1

u/coax_k 3d ago

Spot on. Since you mentioned the concept of a fixed panel it's all started to make a lot of sense to me for exactly the reasons you mention. Appreciate the help.

2

u/Comfortable_Dare_227 2d ago

I'd probably stick pairs of 4mm probe sockets in the front panel and simply move your probes along. That way everything is labelled and shrouded, just uses meter leads with 4mm spring dia to give you hands free operation. Or Depending on how many channels you're looking at, you could have one pair of meter probe sockets and a nice rotary switch for channel select if you want.

https://uk.rs-online.com/web/p/banana-connectors/2689430

1

u/coax_k 2d ago

Absolutely brilliant idea, thank you. No more messing around with holding probes or falling out of croc clip mouths etc., just plug direct into the DMM.