Neurochrome HP-LOAD Review (Headphone Dummy Load)

[JohnYang1997]

@amirm , I don't fully understand the purpose of the "dummy load" after reading the review and couldn't envisage the connections between the different gear & how you use it & for what purpose? Is it to calibrate/verify your measurement devices to ensure they're performing correctly?

A dummy load is essentially a replacement for headphones as a load in the testings.

Headphones come in with various impedances hence different value resistors in the dummy load.

It's what's used when measuring distortion vs power and the one shown above (distortion vs voltage with different loads).

A high quality dummy load will give consistent, accurate results when performing measurements.

 

[jtwrace]

Great job as always @tomchr!

Sure would be super awesome if many others would use it to compare.

Willing to share (with approval) who has or might be investing in one?

 

[Robbo99999]

A dummy load is essentially a replacement for headphones as a load in the testings.

Headphones come in with various impedances hence different value resistors in the dummy load.

It's what's used when measuring distortion vs power and the one shown above (distortion vs voltage with different loads).

A high quality dummy load will give consistent, accurate results when performing measurements.

Oh, ok, I'm with you, Amir had one of these already in order to actually produce the reviews he's been doing of headphone amplifiers & speaker amplifiers......just this "dummy load" is proven to be more accurate?

EDIT: as well as one box rather than three that Amir was using before, from his review: "I actually used three different ones which I used for different circumstances."

 

[P_M]

Does this dummy load present a true L+C+R type of load simulating an actual speaker/headphone driver ? or is it only resistive plus capacitance added ? Asked another way what would you see if you measured impedance of this load ? Would you see a low frequency resonance peak followed by rising impedance and falling phase ? If not then its not a true dummy load.

Also why does a dummy load need power input ? ground loop while testing dummy load is quite absurd.

In any case $1500 for a low power dummy load is obscene.

 

[PeteL]

Does this dummy load present a true L+C+R type of load simulating an actual speaker/headphone driver ? or is it only resistive plus capacitance added ? Asked another way what would you see if you measured impedance of this load ? Would you see a low frequency resonance peak followed by rising impedance and falling phase ? If not then its not a true dummy load.

Also why does a dummy load need power input ? ground loop while testing dummy load is quite absurd.

In any case $1500 for a low power dummy load is obscene.

It's resistive, and the power is for display and relay, that's all. You wouldn't be able to caracterize an amp under load with L+C+R, the goal is not to simulate a speaker, they are all different, what would be the reference? You want a measurement at a load X to not become load Y if you change frequency. We asses that when measuring speakers, headphones, measuring the load itself.
edit: ah I see you can add capcitance, so yes I guess there could be some use to measure at different power factors. But it's not in the common suite of measurments.

 

[mac]

@tomchr also makes excellent amplifiers that are used by many LXMini & LX521 users (including himself).

 

[peniku8]

Speakers?

I also have a lot of high power resistors. About 30 years ago, I was servicing high power pro-sound amps and my dummy load was heating up. So I measured the resistance of my toaster and found it worked in a pinch. Also, two 3500 watt electric water heater elements parelleled makes about 8-10 ohm load.

That's what I've done

More about it:
https://data-bass.ipbhost.com/topic/917-robs-amp-tests/?tab=comments#comment-21517

 

[tomchr]

How much power can this load measure/handle?

It depends on which load resistance is selected. Minimum is 8 W, but some settings (12, 16, and 32 Ω) can handle higher power.
The power dissipation on the 300 Ω and 600 Ω settings is limited by the maximum voltage that can safely be applied to the HP-LOAD. The max voltage is 63 V RMS.

Thank you very much for the review @amirm. I appreciate it.

Tom

 

[amirm]

Shouldn't that say "If you are doing any high performance headphone amp measurements, you need a high quality dummy load."?

Thanks. Fixed.

 

[tomchr]

I also have a lot of high power resistors. About 30 years ago, I was servicing high power pro-sound amps and my dummy load was heating up. So I measured the resistance of my toaster and found it worked in a pinch. Also, two 3500 watt electric water heater elements parelleled makes about 8-10 ohm load.

Have you ever measured the distortion of the amplifier when presented with that load and compared with a low-inductance, low-TCR resistor? Heating elements are not exactly linear resistors. I would be shocked if they didn't have a very steep temperature coefficient (TCR). This means that the load changes as function of the signal swing, which can result in distortion. That distortion would be induced by the load and result in a falsely high THD measurement.

For plain torture testing of amps, the heater element/toaster approach certainly works.

For amp testing I have a large fan-cooled heat sink that came from a cellphone base station fitted with an array of non-inductive power resistors.

The HP-LOAD is a different animal, though. The HP-LOAD is a headphone dummy load intended for testing headphone amplifiers. Characterization and testing of headphone amps is rather different than torture testing PA amps. It is important that the dummy load does not impact the measurement (aside from providing a load that is). As Amir demonstrated in Post #1, that is the case for the HP-LOAD. Given that many headphone amps operate at distortion levels below -120 dBc with some reaching near -140 dBc that's a pretty tall order. That's why I went with the Caddock resistors, WIMA polypropylene caps, and small-signal relays with AgPd switch points.

Like Amir I was getting tired of my point-to-point resistor ladder for headphone amp testing. I don't know how many times I forgot to move the clip leads to select a different resistance during production testing of the HPA-1 headphone amp. I've also lost track of how many issues related to bad connections on those clip leads I had to resolve.
I used the HP-LOAD for production testing the HPA-10 headphone amps. With the APx control interface I was able to test them faster than I could build them. The test script takes 7-8 minutes to run and tests many variables, including THD+N, gain, frequency response, noise floor, IMD, etc.

I'm currently building another two of the HP-LOAD boxes as I sold the one I had built for myself. I'll update the inventory on the HP-LOAD product page (https://neurochrome.com/products/headphone-dummy-load) once I get the last parts in from Mouser and can complete the build. That should happen around the end of August.

Tom

 

[tomchr]

Willing to share (with approval) who has or might be investing in one?

That would be against my privacy policy. You could ask the manufacturers, though.

A company in the UK picked one up last week, which is why I'm building more. I'm building them by hand. If they suddenly start flying off the shelf, I'll have them built by machine and make the boards Neurochrome Blue instead of purple.

Tom

 

[jtwrace]

That would be against my privacy policy.

I get it.
I wonder if you could cross market with AP...that would be ideal to market directly to the people that own an AP.

 

[tomchr]

I wonder if you could cross market with AP...that would be ideal to market directly to the people that own an AP.

Ya know... I should let my AP rep know about it. Given that I just dropped an ungodly sum on an APx555B (to arrive 4-5 weeks from now) he could at least help me spread the word. Thanks for the prod.

Tom

 

[jtwrace]

Ya know... I should let my AP rep know about it. Given that I just dropped an ungodly sum on an APx555B (to arrive 4-5 weeks from now) he could at least help me spread the word. Thanks for the prod.

Tom

Congrats!

And YES!
The answer is always "no" if you don't ask.

 

[3125b]

All these nice dummy loads ... mine is just an old lunch box with 5W resistors

But a dummy load is really handy to have.

 

[H-713]

$1500 for a well-designed headphone dummy load that interfaces with the AP box? That's fantastic. This is one of those things that's an annoying PITA to build, but very necessary for those who are designing headphone amps.

I use water heater elements for testing big power amps, and they're better behaved than I would have expected. I'm still able to measure distortion down in the 0.001% range with them, which is the limit of my current analyzer. For precise distortion measurements I have some better load resistors, but they're limited to 300 watts.

For super big power, I use a 4" diameter tube of water with an electrode at each end and an appropriate amount of salt added. What's nice is that this type of load can take enormous pulses without dying.

 

[tomchr]

Congrats!

Thank you. I've been trying for years to improve on my measurement capability with the APx525. The 525 is an awesome instrument, but can't measure below about -112 dBc THD without "helper circuits". I've through the years made various circuits that would get me a spot measurement down near -140 dBc, but they only worked at 1.000 kHz and really only at line level ... if they felt like it. But the same circuit wouldn't work for power amplifiers - or it would, but "only" to -130 dBc not the -140 dBc I was after. I just got tired of it. It was time to stop pussyfooting around and just buy the right tool.

My business has seen solid growth the past few years so I'm now finally at the point where I can start looking up and out a bit. It was time to invest more in Neurochrome and less in silly things like food and shelter.

Ya know. Buy a new car ... buy an audio analyzer. Meh. I can walk. It's healthier. Downpayment on a house ... buy an audio analyzer. Ya know... Camping is really quite nice this time of year.

The answer is always "no" if you don't ask.

Yep. They can't tell me no unless I ask.

Now, if any of you would like to buy my APx525, I'm happy to sell ... in 4-5 weeks. I figure I'd get ~$10k on the used market.

Tom

 

[jtwrace]

Thank you. I've been trying for years to improve on my measurement capability with the APx525. The 525 is an awesome instrument, but can't measure below about -112 dBc THD without "helper circuits". I've through the years made various circuits that would get me a spot measurement down near -140 dBc, but they only worked at 1.000 kHz and really only at line level ... if they felt like it. But the same circuit wouldn't work for power amplifiers - or it would, but "only" to -130 dBc not the -140 dBc I was after. I just got tired of it. It was time to stop pussyfooting around and just buy the right tool.

My business has seen solid growth the past few years so I'm now finally at the point where I can start looking up and out a bit. It was time to invest more in Neurochrome and less in silly things like food and shelter.

Ya know. Buy a new car ... buy an audio analyzer. Meh. I can walk. It's healthier. Downpayment on a house ... buy an audio analyzer. Ya know... Camping is really quite nice this time of year.


Yep. They can't tell me no unless I ask.

Now, if any of you would like to buy my APx525, I'm happy to sell ... in 4-5 weeks. I figure I'd get ~$10k on the used market.

Tom

I love your outlook! Further, I really look forward to see future offerings using this instrument. Should make it easier as a designer and also give customers even more performance. Win-Win (car, house coming)

 

[P_M]

It's resistive, and the power is for display and relay, that's all. You wouldn't be able to caracterize an amp under load with L+C+R, the goal is not to simulate a speaker, they are all different, what would be the reference? You want a measurement at a load X to not become load Y if you change frequency. We asses that when measuring speakers, headphones, measuring the load itself.
edit: ah I see you can add capcitance, so yes I guess there could be some use to measure at different power factors. But it's not in the common suite of measurments.

But isn't the purpose of dummy load to present a real world load to the amp ? And a real headphone will have varying impedance and phase across the whole frequency band. It is this type of load (and not just resistive) that is most difficult (and the real test) for the amp.

If this dummy load is resistive then why go through this trouble, why not just wire up a resistor and be done with it ? You could have 2-3 headphone plugs with different value resistors (30 ohm, 300 ohm..) wired in each.

 

[amirm]

But isn't the purpose of dummy load to present a real world load to the amp ?

Which real load? Every headphone is different in that regard. So you can't just pick one. Or two. Or even three. It is not line one or two devices dominate 90% of the market.

For a while I tested with real headphones. They generated totally bogus data due to back EMF. They can even act as microphones and inject noise into your measurements! At the end, it was wasted effort producing garbage and misleading measurements.