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Author Topic: Musings on Future Measurements  (Read 2201 times)

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Armaegis

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Re: Musings on Future Measurements
« Reply #40 on: July 31, 2015, 04:00:26 AM »

I just googled it so I'm an expert now... http://physics.aps.org/story/v27/st17

Now I just need a spinning toroidal chamber filled with molten sodium. Done. I've just created a newfangled power supply conditioner regenerator. Easy peasy. Now to print up a note telling people about the very long warmup times for optimal performance...
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Do you think there may be an acoustic leak from the jack hole? ~Tyll Hertsens

Not sure if I like stuffing one hole or both holes. Tending toward one hole since both holes seems kinda ghey ~Purrin

Solderdude

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Re: Musings on Future Measurements
« Reply #41 on: July 31, 2015, 06:58:16 AM »

We will need a way to level match the output versus input.

I can imagine listening real-time and listening for the one with the least artifacts. For those artifacts which do exist, I wonder if those which sound least bad correlate with the better sounding amplifier....

Yes, that's the idea.
You can record the dif + source file using a soundcard and analyse afterwards as well.

If it needs to be done all tube you will need to find tubes + low distortion caps that buffer the inputs and an attenuator that can be set extremely accurately.
It all hangs on how balanced you can get it (probably around 1kHz using a sinewave).
I used a 20 rotation potmeter but I suggest to use 2 potmeters instead.
One for 'course' tuning and a second, small value one, in series with the input for fine adjust.

Then comes the difficult part.

The output signal represents the difference between the 2 inputs so when you have a difference signal it could be that what you hear is something that has been added but also something that may have been 'removed' from the original signal.

That will be an anlysing thing.
A difference in FR will show in an amplitude difference, so will a phase difference.

My questionmarks with traffos vs opamps lie in the fact that opamps can go from DC to MHz, transformers well...
With transformers  a question will be how well both inputs balance.
You will have to get a baseline by connecting the same signal and see how that nulls.
Still a nice idea and as Donald mentioned you could aim for the least amount of diff (if you are looking for hifi) or you can listen out for the most pleasant sounding diff file.


Another test that may be interesting to see is the dynamics test I proposed.
Chances are the -70dB signal may not look like the original.

You may have to clip the 0dB signal (diodes in antiparallel) to avoid problems with some scope inputs.


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OJneg

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Re: Musings on Future Measurements
« Reply #42 on: August 01, 2015, 01:10:04 AM »

So that no one can say that nothing useful came out of this thread:



Check against Tyll's datasheet for reference. For actual testing, component values wouldn't have to be tight to get the gist of it

http://www.innerfidelity.com/images/SennheiserHD800.pdf
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Re: Musings on Future Measurements
« Reply #43 on: August 01, 2015, 07:40:40 AM »

That's a good place to start.
For a passive emulation that is.

However, finding a 1.2H inductor may be a tall order.
the 2.2uF cap is no problem of course.
BUT... does a real HD800 really have an equivalent 1.2H and 2.2uF cap 'hidden' inside the voicecoil ?
It does have something like that of course .... the parts that make up the membrane + voicecoil + suspencion.

Here's the thing... the rising impedance at the top is determined by the inductance of the voicecoil (mainly)
The 100Hz hump is NOT a passive something.
It's caused by the back EMF of the driver.
The resonance frequency of the driver itself is the cause for the wide band.
The wide 'band' of course is better than a narrow one for obvious reasons.

This needs to be emulated by creating back EMF.
As Marv rightfully remarked is not done by a passive load but IS present using real headphones.

This would mean apply noise with a determined SPL TO the driver and measure the open voltage on the driver.
Next load the driver with a known resistance (say 330 Ohm in this case) and determine the source resistance and calculate the source impedance and EMF.
Relate that to the applied SPL and what voltage was needed to create the same SPL by the driver.
A much simpler but potentially less accurate way would be to make an assumption.
This could be that the internal resistance of the 'microphone' portion is around the DC resistance of the driver.
Given the fact that at 100Hz the peak is about twice the resistance (so half the current) I would suggest a simpler yet more complicated load to emulate high impedance headphones with back EMF.

I suggest the 2.5mH inductor + 560 Ohm to ground and connect a small amplifier via a 680 Ohm to the inductor as well to emulate the voicecoil.
Then use the test signal applied to the amp to be tested, split that into a small amp (use your obsolete O2 or a simple opamp for this) and bandfilter the input to 'approximate' the 100Hz hump.

Missing the cable capacitance of course, which will be around 150pF, and could well matter for some designs.
This means connect that load via a 1.5 (or other length) HP cable.

Emulating a low impedance HP could be done simliarly but with other resistor values to keep things simple so you just have to switch little components..

Finally a resistive load for testing 'orthos' could be an idea.

There you have 3 loads that could become a standard SHOULD that be warranted.

Another question about the Ortho load...
Does an ortho generate any back EMF ?
It should ... how low will that be ?
Could be determined with the above described test.

As you mentioned earlier, most amplifiers won't have problems with an inductive load of 2.5mH + 300 Ohm at all.
I fully agree with that BUT the capacitance of the HP cable + rising impedance of the voicecoil could potentially spell trouble with some designs (mostly those with overall feedback from the output) .
Is it worth going through all the trouble ?
Is it cheaper to just buy an HD800, or (used) HD580 or similar ?
Is it cheaper to buy a cheap driver instead that is close ?

Yes, it probably is + you have a 'monitor'

Damn ... back to where we started.
Simply use dummy headphone instead (for 300 Ohm)
Oh wait ... if we want to standardise everyone should have the same driver .... hmmm

For 32 Ohm this may also be a problem given how much power is generated by some amps and during testing something may go wrong and burn out a driver.

The horrors of testing.

But could it still be wiser to build the 'emulator' with standard parts anyway ?
What could be the potential benefits over a real driver (aside from the obious power rating) ?

Well for one you could 'switch off' the back EMF to see how this would affect the output signal.
You could also insert the 'back EMF' only into the amps output and see how it fares ?
You could alter the frequency response of the back EMF.
You could even insert a testtone to see how feedback in an amp handles stuff into its output.

Just some musings though ... and I want to add more than just a simulation could come from this thread.
Perhaps some novel ideas or test signals that aren't applied.
Perhaps someone dares to actually make the fransformer based or (heaven forbid) a simple opamp differential amp and starts educationg themselves further in ways they are unfamiliar with till now.
Get people out of comfort zones ?

Maybe someone will be brave enough to 'record' the output signal of a high end tube amp and play that back on the Rag (or other decent SS amp) to see if the tubeness is really gone ?
« Last Edit: August 01, 2015, 08:07:24 AM by Solderdude »
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OJneg

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Re: Musings on Future Measurements
« Reply #44 on: August 01, 2015, 04:10:05 PM »

Given the fact that at 100Hz the peak is about twice the resistance (so half the current) I would suggest a simpler yet more complicated load to emulate high impedance headphones with back EMF.

No comment.

If anyone wants to push .5Vrms through their $1500 headphones they're welcome to do so. Not doing that with mine though
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Donald North

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Re: Musings on Future Measurements
« Reply #45 on: August 01, 2015, 05:06:32 PM »

0.5Vrms should not damage the HD800. Their sensitivity is rated at 102dB with 1V input.
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firev1

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Re: Musings on Future Measurements
« Reply #46 on: August 01, 2015, 05:11:18 PM »

For low ohm testing everyone could just use a standard Knowles/Sonion integrated crossover drivers(TWFK or Acupass). If you are crazy enough you could still buy the HD800 drivers for around $500 for the stereo pair.
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OJneg

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Re: Musings on Future Measurements
« Reply #47 on: August 01, 2015, 05:24:49 PM »

0.5Vrms should not damage the HD800. Their sensitivity is rated at 102dB with 1V input.

Yeah but I'd rather not push things hard especially if we're going to put constant tones through it. Paranoid perhaps
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Donald North

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Re: Musings on Future Measurements
« Reply #48 on: August 01, 2015, 05:48:29 PM »

It's good to be cautious, but unless you smell the voice coil heating up, I wouldn't worry. Plus they're replaceable. I would be more concerned about tests on a R10 or K1000. I have done a lot of high power sweep and Klippel tests over the years at Aura and now with ADX. A memorable one was when we were developing the NS15-992-4A 15" woofer at Aurasound, I did a 600W power test in a 2 cubic foot sealed box. At the end the frame was too hot to touch. Once I was able to lift it from the box, this plume of hot, smelly vapor came rushing out, caused by the hot MDF wood box.
« Last Edit: August 01, 2015, 07:40:06 PM by Donald North »
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Solderdude

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Re: Musings on Future Measurements
« Reply #49 on: August 01, 2015, 07:34:22 PM »

If anyone wants to push .5Vrms through their $1500 headphones they're welcome to do so. Not doing that with mine though

0.5V in a HD800 = 0.8mW
The HD800 is rated at 500mW (12V)
Ratings are usually continous unless it specifically mentions otherwise.

Even a Sansa clip can reach 0.5V ... you are a very cautious man indeed.


With the simpler, yet more complicated load I mean 2 resistors + 1 inductor (which is simpler) + a small amplifier that generates the back EMF (making it more complicated)

As you don't like to use a HD800 as a load (I wouldn't either so you are not alone) the suggested 'replacement' with back EMF should be interesting.

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