Hypex amps and DC protection

[Silly Valley]

I have been following a few threads with discussion about Hypex NC amps and DC protection. It seems the very popular NCxyzMP amps leave something to be desired in protecting speakers from DC damage, not triggering protection until at least 12v or more. This seems to be supported by a warning on Audiophonics website regarding these amps and in the Hypex data sheets.

This is also reinforced by a discussion between some of our more knowledgeable members on PMA's NC252MP in depth testing thread. From what I understand, it appears Hypex is relying on OEM's to implement amp output protection as what is provided for by Hypex seems to be very lenient. I am not aware of any of the vendors popular here making any reference to protection circuitry beyond what the modules ship with.

Even some traditional amps which do not measure very well and are seen as mediocre here, Parasound for example, go to considerable lengths to highlight their DC Servo circuits even in the affordable New Classic line. Why do I never see mention of protection with all the class D amps? Do you need to purchase from a company like NAD before you start to see these additional protections? I assume Hypex UcD is similar in regards to protection. How about Purifi?

I know Amir gets never ending requests to add to his testing regiment but is this something that could efficiently added to reviews? What is someone shopping for an amp to conclude, is this a real issue? Based on my limited knowledge of amp design, I feel like this situation is tempting Murphy. It would be very helpful if Amir and the members with relevant knowledge could bring some clarity on this topic.

 

[Extreme_Boky]

What needs to be defined first is DC protection from what/where&why.

Suppose it is a DC protection from the (catastrophic) failure of those noise generators (or amps... if you choose to call them amps...). Once the amp module fails, its protection circuit cannot operate correctly and detect DC conditions at the output. Hence, the only way to reliably provide protection is to use an external PCB that will continually monitor DC/current (as the result of either a short at the speaker output, or high DC condition as the result of catastrophic failures of amp module. There's an excellent IC that will do everything, and provide delayed turn-on, and instant turn off of the speaker relay in addition to all of the above. The protection PCB would require its own power supply, indipendent of the amp power supply.

 

[boXem]

First, if a speaker is damaged from 12V DC, that means that it cannot withstand more than 18 W @ 8R. One can wonder why choosing a 75-350W @8R amplifier for such a speaker...

Then DC protection needs to be properly defined.
- If the DC comes from the amplifications module, it can only be due to a failure causing the output stage to be stuck to one of the rails. The protection implemented works perfectly.
- If the DC comes from the outside, that means that the upstream component doesn't remove it. Is it actually the role of the power amplifier to do so? That can be discussed. I do not see anybody complaining that the myriad of DACs used as preamp are all outputting DC.

Finally (my little rambling), quality implementations are unfortunately not rewarded by the majority of sales. Most buyers look at module used and price. Period.
Most of the external buffers I see on the market not only do not remove DC but add some (looking at you bipolar opamps). That's how the market is nowadays.

 

[Silly Valley]

Thanks for your replies Extreme_Boky and boXem.

I was curious to see how one of the established brands handles DC speaker protection. I emailed NAD. Their response:

"On the C268 and C298, there is no additional DC protection on the circuits. The detection circuit is built-in in the amplifier modules.
The C268 holds an output relay that will immediately cut off output lines when the DC protection has been triggered. The C298 has a MOSFET output that also turns off when triggered."

Granted the C268 is UcD and the C298 purifi. I would assume UcD being an earlier hypex generation, it would not likely have anything that surpasses Ncore. Does anyone know if purifi brings any additional DC protection circuitry or is it similar to what hypex does?

Apparently, NAD does not consider this to be an issue.

 

[NTK]

First, if a speaker is damaged from 12V DC, that means that it cannot withstand more than 18 W @ 8R. One can wonder why choosing a 75-350W @8R amplifier for such a speaker...

Then DC protection needs to be properly defined.
- If the DC comes from the amplifications module, it can only be due to a failure causing the output stage to be stuck to one of the rails. The protection implemented works perfectly.
- If the DC comes from the outside, that means that the upstream component doesn't remove it. Is it actually the role of the power amplifier to do so? That can be discussed. I do not see anybody complaining that the myriad of DACs used as preamp are all outputting DC.

Finally (my little rambling), quality implementations are unfortunately not rewarded by the majority of sales. Most buyers look at module used and price. Period.
Most of the external buffers I see on the market not only do not remove DC but add some (looking at you bipolar opamps). That's how the market is nowadays.

That is not true. Speaker driver diaphragm travel is related to the signal frequency, because it is mechanically a spring-mass-damper system. A DC will send a constant force to the diaphragm (= DC current * BL factor), and the amount of excursion is only restrained by the spring constant (i.e. stiffness) of the spider and surround or more likely some other hard stop.

Below is from Speakerbuilder Pro 2.0 (reflex box, max flat alignment, SB-Acoustics SB15MFC30-4 driver) [link]. You can see the diaphragm excursion skyrockets when the excitation frequency goes below the tuning frequency of the speaker.

 

[restorer-john]

First, if a speaker is damaged from 12V DC, that means that it cannot withstand more than 18 W @ 8R. One can wonder why choosing a 75-350W @8R amplifier for such a speaker...

Then DC protection needs to be properly defined.
- If the DC comes from the amplifications module, it can only be due to a failure causing the output stage to be stuck to one of the rails. The protection implemented works perfectly.
- If the DC comes from the outside, that means that the upstream component doesn't remove it. Is it actually the role of the power amplifier to do so? That can be discussed. I do not see anybody complaining that the myriad of DACs used as preamp are all outputting DC.

Finally (my little rambling), quality implementations are unfortunately not rewarded by the majority of sales. Most buyers look at module used and price. Period.
Most of the external buffers I see on the market not only do not remove DC but add some (looking at you bipolar opamps). That's how the market is nowadays.

There's a very big difference between 12VDC and LF content of the same effective power and you know it.

Any constant DC offset will cause long term magnet deterioration, distortion and unneccesary dissipation.

Bottom line is DC protection at a 12V knee point is fine for OPT/module failure and most amplifiers with on board analog DC/overpower/OC protection trip around the same point. I have no problem with the threshold, but it is the job of a power amplifier to prevent destruction to the connected speakers in the case of failure or offset, not just protect itself.

And I totally agree, any source that is DC coupled and is outputting an offset of significance is a faulty product and should be called out for that.

 

[restorer-john]

I have been following a few threads with discussion about Hypex NC amps and DC protection. It seems the very popular NCxyzMP amps leave something to be desired in protecting speakers from DC damage, not triggering protection until at least 12v or more. This seems to be supported by a warning on Audiophonics website regarding these amps and in the Hypex data sheets.

This is also reinforced by a discussion between some of our more knowledgeable members on PMA's NC252MP in depth testing thread. From what I understand, it appears Hypex is relying on OEM's to implement amp output protection as what is provided for by Hypex seems to be very lenient. I am not aware of any of the vendors popular here making any reference to protection circuitry beyond what the modules ship with.

Even some traditional amps which do not measure very well and are seen as mediocre here, Parasound for example, go to considerable lengths to highlight their DC Servo circuits even in the affordable New Classic line. Why do I never see mention of protection with all the class D amps? Do you need to purchase from a company like NAD before you start to see these additional protections? I assume Hypex UcD is similar in regards to protection. How about Purifi?

I know Amir gets never ending requests to add to his testing regiment but is this something that could efficiently added to reviews? What is someone shopping for an amp to conclude, is this a real issue? Based on my limited knowledge of amp design, I feel like this situation is tempting Murphy. It would be very helpful if Amir and the members with relevant knowledge could bring some clarity on this topic.

You need to understand that all amplifers will output some DC (except transformer tube/SS and perhaps ancient capacitively coupled gear).

A small amount (<50mV) is not an issue.

A DC servo is not there for DC protection- they will do nothing to stop DC if there is an amplifier failure. They are generally used to allow amplifers to have a flatter response in the lower octaves without distortion caused by large value capacitors in the NFB loop.

DC protection is a whole other circuit element and the threshold and cutoff frequency are chosen by the amplifier designer to weigh up the level of protection required.

Generally, with conventional amplifiers, you will only get significant DC if the amplifier has catastrophic failure in some active devices. That usually involes a full rail latch-up to the supply voltage which can be 30-120VDC with almost infinite current. Basically a woofer destruction scenario in a second or two. So, your DC protection needs to be fast and effective or it's too late as there's a ton of energy in the PSU/Caps which has to go somewhere.

The Class Ds you are asking about can shut their supplies down in a few cycles and they don't contain a whole lot of stored energy. Also the OP devices are more likely to fail open circuit, rather than short circuit.

 

[gene_stl]

Guardian-86

The Guardian-86 is a high-end solid state speaker protection circuit intended for use in audio amplifiers. Two are used for stereo. The Guardian-86 uses MOSFETs rather than relays to protect your speakers in the event of DC on the amplifier output. This results in better protection and...

neurochrome.com

On my Be tweeter four system I used 250 milliamp fuses on the tweeters and midranges. I had 500ma at the ready but never blew the 250s.