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Author Topic: Speaker level pad for measurement.  (Read 570 times)

Helge A Bentsen

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Speaker level pad for measurement.
« on: May 05, 2023, 04:06:32 AM »

I wish to measure output of amplifiers with Smaart, done it in the past with a pair of Contryman type85 in the signal chain, but looking for a more elegant solution. Preferably speakon in and XLR/speakon out.
Is there a ready-made pad available for this?
Or could anybody point me in the direction of a schematic with resistor values so I can DIY my own?


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Mac Kerr

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Re: Speaker level pad for measurement.
« Reply #1 on: May 05, 2023, 10:17:39 AM »

I wish to measure output of amplifiers with Smaart, done it in the past with a pair of Contryman type85 in the signal chain, but looking for a more elegant solution. Preferably speakon in and XLR/speakon out.
Is there a ready-made pad available for this?
Or could anybody point me in the direction of a schematic with resistor values so I can DIY my own?

If you want to measure them at high power, into a typical speaker load impedance you will need a very high power resistor to provide the load.  Bink built one out of a couple of hot water heater coils in a bucket of water to cool them.

Mac
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John Schalk

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Re: Speaker level pad for measurement.
« Reply #2 on: May 05, 2023, 10:36:59 AM »

My method is to use a DI and I connect it up with a speaker cable that has a Neutrik on one end and a 1/4" tip-sleeve on the other end.  I plug the 1/4" end into the input of the DI and take the XLR output of the DI into the measurement channel of my USB pre-amp.  I wasn't trying to do any kind of load testing though.  I was looking at the transfer function of the amp after applying some recommended DSP values.
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Helge A Bentsen

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Re: Speaker level pad for measurement.
« Reply #3 on: May 05, 2023, 10:40:46 AM »

If you want to measure them at high power, into a typical speaker load impedance you will need a very high power resistor to provide the load.  Bink built one out of a couple of hot water heater coils in a bucket of water to cool them.

Mac

I forgot to specify that this isnít for load testing, itís for verifying DSP settings with a speaker connected :)
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Steve-White

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Re: Speaker level pad for measurement.
« Reply #4 on: May 05, 2023, 11:12:48 AM »

I use these and make my own adapter cables and resistive 10:1 dividing networks as needed for higher power measurements.

ART DTI 2-channel Hum Eliminator

https://www.sweetwater.com/store/detail/DTI--art-dti-2-channel-hum-eliminator
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David Sturzenbecher

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Re: Speaker level pad for measurement.
« Reply #5 on: May 05, 2023, 12:25:34 PM »

I forgot to specify that this isnít for load testing, itís for verifying DSP settings with a speaker connected :)

I have used a VIBox from LinearX in the past for this.   Here is a snippet of my response to this exact question on the signal to noise discord a few days ago.

Sadly the owner of LinearX passed away a number of years ago, so the VIbox is no longer available.  Physical-lab appears to make a similar unit (IMP Box), but I have no first hand experience.   The VIBox was nothing more then a voltage divider circuit with precision matched components (actually 2 voltage dividers as it had a 0dB "low" side and -40dB "high" side).    I smoked the low voltage side of my VIBox, and the one resistor was something like $15 to replace... with a minimum purchase quantity of 400.   You can easily make your own with slightly less precise components for way cheaper.    You can also use a passive DI but depending on the quality, you may see some phase deviations due to the internal transformer.   This can of course be calibrated out, but something to be aware of.


Another posted posted a link to this.
http://www.atelier-der-tonkunst.de/produkte/pol-ddi

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David Sturzenbecher

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Re: Speaker level pad for measurement.
« Reply #6 on: May 05, 2023, 12:30:37 PM »

I also built my own VIBox into a 1RU panel.



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David Sturzenbecher

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« Last Edit: May 05, 2023, 01:05:49 PM by David Sturzenbecher »
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Ivan Beaver

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Re: Speaker level pad for measurement.
« Reply #8 on: May 12, 2023, 06:46:46 PM »

One thing that is very important with a lot of moderm amplifiers who do not have a speaker terminal that is NOT ground (ie floating) is to have an isolation transformer between the amplifier pad and the input to the test system.

Depending on how you wire it up, you could damage the amp or the test system if you don't physically isolate the two systems.

What I prefer is a normal pad to drop the voltage, and then go into a line level transformer (not the mic level ones used in DI boxes).

I had a bunch of old Altec plug in transformers that are quite good and could take a lot of level.  Of course there are others, Triad, Jensen etc.

You could also use a "hum eliminator" type transformer as most of those are line level.

The pad I use is typically 100:1, so that I can run amps at full output and still stay around line level.

A quick description:  From the speaker outputs, 1 leg goes through a series resistor (say 100K) and then to a parallel resistor (1K).  One side of the transformer is hooked across the 1K resistor, and the other side of the transformer goes to pins 2 and 3 of the measurement system

NOTE: There will be a little bit of a low freq rolloff (depending on the transformer used and freq), but you can measure that before you measure the DSP in the amp so you can compensate for that.
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Ivan Beaver
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Re: Speaker level pad for measurement.
¬ę Reply #8 on: May 12, 2023, 06:46:46 PM ¬Ľ


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