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SPL HPm headphone amplifier

Postby ef37a » Thu Nov 02, 2017 7:38 am

I am surprised Hugh did not comment about the vulnerability of this device?

I am yet to be convinced that headphone amplifiers need a near zero output resistance but if manufacturers feel they do they should also make them bomb proof. Especially at nearly half a bag a pop!

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Re: SPL HPm headphone amplifier

Postby Hugh Robjohns » Thu Nov 02, 2017 10:56 am

ef37a wrote:I am surprised Hugh did not comment about the vulnerability of this device?

Was "The manual contains explicit warnings about not plugging in mono (TS) jack plugs, as the resulting right-channel short-circuit could damage the amplifier circuitry" insufficient in some way?

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Re: SPL HPm headphone amplifier

Postby ef37a » Thu Nov 02, 2017 11:07 am

Hugh Robjohns wrote:
ef37a wrote:I am surprised Hugh did not comment about the vulnerability of this device?

Was "The manual contains explicit warnings about not plugging in mono (TS) jack plugs, as the resulting right-channel short-circuit could damage the amplifier circuitry" insufficient in some way?

H

Sorry H but no! I would have expected you to say this is poor design. A similar warning in the manual of any decent power amplifier would immediately brand it as junk!

As we all know, *t'appen.

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Re: SPL HPm headphone amplifier

Postby Hugh Robjohns » Thu Nov 02, 2017 11:29 am

I'm not convinced it is a poor design. SPL claim the sonic benefits of their design outweigh the small risk and I would agree. Moreover, if someone is spending that kind of money on a headphone amp I'd expect them to be competent enough to use and look after it properly, heeding all relevant warnings in the manual.

Equipment of all kinds can be broken by careless or inappropriate use... That has always been the case and always will be. Should I brand all capacitor and ribbon microphones as 'a poor design' because they're likely to break if dropped on the floor? ;)

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Re: SPL HPm headphone amplifier

Postby ef37a » Thu Nov 02, 2017 1:03 pm

Hugh Robjohns wrote:I'm not convinced it is a poor design. SPL claim the sonic benefits of their design outweigh the small risk and I would agree. Moreover, if someone is spending that kind of money on a headphone amp I'd expect them to be competent enough to use and look after it properly, heeding all relevant warnings in the manual.

Equipment of all kinds can be broken by careless or inappropriate use... That has always been the case and always will be. Should I brand all capacitor and ribbon microphones as 'a poor design' because they're likely to break if dropped on the floor? ;)

H

Oooo! THAT's a bit of a stretch! Cables can get shorted and studios are full of idiots, some on substances!

You evidently don't take the same line as me Hugh. We have come a long way from fragile electronics, with only a few exceptions you can plug any audio gear into any other, even if the total noob gets it wrong there is unlikely to be smoke. I consider a puny headphone output a retrograde step.

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Re: SPL HPm headphone amplifier

Postby Hugh Robjohns » Thu Nov 02, 2017 4:14 pm

Hmmm. I do get where you're coming from, but I have little time for the nanny state approach where nothing bad can happen. I think people should know what they are doing and look after their gear... ;-)

In this case the warning in the manual says could, not would cause damage -- a point I felt important (and unusual) enough to reiterate in the review.However, I imagine the output drivers can cope with short term shorts, but don't like it for too long. It didn't go pop when I plugged headphones in and out, which would also cause momentary shorts! ;-)

SPL gear is well designed and generally pretty robust in my experience. I would hazard a guess that if they could have built-in a protection circuit that didn't compromise performance they would have done... but few protection systems have no unwanted side-effects.

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Re: SPL HPm headphone amplifier

Postby ef37a » Thu Nov 02, 2017 4:28 pm

Ok Hugh, we can agree to differ. I agree about 'nannyism' but really in terms of restrictions on headphone levels, however well intentioned.
Some valve amplifiers will die from even a few second's drive into an open circuit, others will tolerate it until a valve blows a fuse but the handbook will usually condemn the practice in both cases with equal vigour!
One question, are your beloved Brystons unconditionally short circuit proof?

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Re: SPL HPm headphone amplifier

Postby johnny h » Thu Nov 02, 2017 5:08 pm

ef37a wrote:We have come a long way from fragile electronics, with only a few exceptions you can plug any audio gear into any other, even if the total noob gets it wrong there is unlikely to be smoke. I consider a puny headphone output a retrograde step.
I agree with this. People share studios, and something as innocuous as plugging a jack lead into a jack socket should not break a headphone amplifier. A very poor design choice.
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Re: SPL HPm headphone amplifier

Postby Hugh Robjohns » Thu Nov 02, 2017 5:48 pm

ef37a wrote:Some valve amplifiers will die from even a few second's drive into an open circuit... but the handbook will usually condemn the practice ... with ...vigour!

Quite -- and that's exactly what SPL has done in this case. I note that they include very similar warnings in all of their headphone amplifier designs.

One question, are your beloved Brystons unconditionally short circuit proof?

I believe they go into a protective shut down under that condition. Never had reason to test it! ;-)

Personally, I wouldn't be concerned about this potential risk of failure -- I think it highly unlikely to be a problem in normal applications with intelligent users. But in cases where a studio is shared with others of dubious or unknown technical competence, alternative headphone amplifiers are available and may afford greater piece of mind.

I wouldn't lend my ribbon mics to just anyone either...

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Re: SPL HPm headphone amplifier

Postby ef37a » Fri Nov 03, 2017 9:05 am

johnny h wrote:
ef37a wrote:We have come a long way from fragile electronics, with only a few exceptions you can plug any audio gear into any other, even if the total noob gets it wrong there is unlikely to be smoke. I consider a puny headphone output a retrograde step.
I agree with this. People share studios, and something as innocuous as plugging a jack lead into a jack socket should not break a headphone amplifier. A very poor design choice.

Thank you Johnny.

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Re: SPL HPm headphone amplifier

Postby Mixedup » Fri Nov 03, 2017 5:20 pm

But who plugs TS jacks into a headphone amp? I mean, no headphones have that. If the shorting caused by sliding in a TRS headphone jack (and at a stretch these days, I suppose, a TRRS headphone+input jack) plug doesn't cause damage, surely that's sufficient? (Unlike the countless guitar pedals where there's no protection when plugging in the wrong polarity of power cable...)
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Re: SPL HPm headphone amplifier

Postby Hugh Robjohns » Fri Nov 03, 2017 5:35 pm

Mixedup wrote:But who plugs TS jacks into a headphone amp? I mean, no headphones have that.

Quite -- but I think SPL are covering their backs in case someone thinks, 'wouldn't it be clever to use the headphone output as a drive signal for an active speaker' (...or IEM system etc), and use an unbalanced/mono TS instrument cable for the purpose. That situation would short the right channel to ground for protracted period and (I'm presuming) possibly cause that channel's output driver to overheat and fail.

Dave and Johnny think the product should be idiot-proofed against such abuse, and that's certainly a valid point of view. However, the designers at SPL are not idiots and I'm sure they designed it the way they did because it sounded better than the alternatives. So I'm of the opinion that someone investing £500 in a product like this should know how to use it, or at least read the manual to know how not to use it!

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Re: SPL HPm headphone amplifier

Postby ef37a » Fri Nov 03, 2017 9:51 pm

I have in my hands a TRS jack and plug, The entry sequence is..

Tip hits common, tip and ring hit R and common then all is fully home. At no point is anything shorted to anything else. No I am not ga-ga (yet) I just wanted to clear THAT matter up first because I have read of 'shorting jacks' before but did not believe it but this is the first time I have actually dug one out and checked.

So, SPL reckon the amp sounds better with a very low source impedance? They are entitled to that opinion, cannot see it myself and have never seen a shred of objective evidence. Headphones after all are of about the same DC resistance as nominal impedance so the 'damping factor' is one at best. Having a bit, 20-50 Ohms in the circuit in any case tends to compensate for the wild range of headphone impedances (any idea why that is Hugh?)

But, there are other high end HP amps with a similarly low Z and high drive capability but AFAIK they do not come with such a warning? Then, we have had S/C proof power amplifiers of impeccably quality for decades. If The Men at SPL are that clever why can't they do the same for a relatively puny headphone amp?

Of course Hugh, you and I and all the treasured SOS followers would be very careful with such a piece of kit but busy studios are full of twits! Twits moreover with TS guitar cables!

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Re: SPL HPm headphone amplifier

Postby Hugh Robjohns » Fri Nov 03, 2017 10:52 pm

ef37a wrote:So, SPL reckon the amp sounds better with a very low source impedance?

I'm not sure they've actually said that. I don't think the output impedance is specified and I didn't mention it in my review. But I imagine it is a very low output Z.

But, there are other high end HP amps with a similarly low Z and high drive capability but AFAIK they do not come with such a warning?

You're really fixated on this aren't you! as I said earlier, I expect it is a back-covering exercise as much as anything.

If The Men at SPL are that clever why can't they do the same for a relatively puny headphone amp?

Perhaps you should enquire of them if it concerns you so much.

As for the zero ohm concept, Benchmark, who pioneered the idea AFAIK say this:

Benchmark wrote:Most headphone amplifiers use series resistors to maintain stability and protect against short-circuit conditions. These resistors are usually at least 30 Ohms, and have a negative impact on performance. A headphone amplifier with series resistors may measure very well on a test bench when driving resistive loads. However, the same amplifier will measure very poorly when driving a headphone load. Unfortunately, most manufacturers do not measure or specify headphone amplifier performance when loaded with real headphones. The measurements use ideal resistive loads. Our measurements show that headphones do not behave like resistive loads.

In our tests we have measured substantial distortion across resistors that are wired in series with headphones. We have conducted measurements with a variety of headphones. In general, distortion increases as headphone impedance decreases. This distortion can be eliminated with a properly designed 0-Ohm headphone amplifier.

The performance of the HPA2TM does not change when headphones are driven. THD+N measurements for no-load, 30-Ohm resistive loads, 30-Ohm headphone loads, and 600- Ohm headphone loads are virtually identical. The HPA2TM will substantially improve the sound of 30 and 60-Ohm headphones. It will make very noticeable improvements with 600-Ohm headphones.

... but busy studios are full of twits!

I'd be surprised if a studio staffed by twits was busy for long! :-)
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Re: SPL HPm headphone amplifier

Postby ef37a » Sat Nov 04, 2017 6:29 am

Thank you Hugh for jogging my memory about that other zero impedance headphone amplifier. I remember the review and thinking fleetingly at the time that..."In our tests we have measured substantial distortion across resistors that are wired in series with headphones." is an odd way to express the distortion of an amplifier?

Why not measure the distortion across the headphones? Is the amplifier distorting at its output? If neither is the case, where is the distortion coming from? The only conclusion I can come to is that headphones themselves are non-linear but surely messrs AKG, Sennheiser et al would have worked that one out long ago? After all, headphones were driven from a 600 Ohm source for many years, a far higher series R than any HP amplifier I know of.

So, I am at once "fixated" but also have a valid point! I really just wanted to say that IMHO the possible vulnerability of the device should have been flagged as a more serious design flaw than it was, not sufficient, (again IMHO) to merely bring a warning in the handbook to our attention. These days I expect professional equipment to be proof against short, open, RF and reasonable overdrive (say rail-rail).

I did not mean STAFFED by twits! I meant the customers, but I am sure you knew that really.

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Re: SPL HPm headphone amplifier

Postby Alex_Nino_ » Fri Feb 08, 2019 11:15 pm

Hi, I'm a new member here and created an account just to ask this - I just bought one of these SPL HPms and would love it if someone knows if you can buy a wooden on metal stand to fit it? I don't own any other 500 series stuff so buying a big old rack thing is not a great idea. I don't mind just using it as a desktop device, I already have an SPL Crimson elsewhere in my setup, I'm just hoping there's a manufacturer making some sort of stand or box it'll fit into snugly and look the part. Ebay and Google searches have not been fruitful so far. Thanks in advance! Alex
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Re: SPL HPm headphone amplifier

Postby ef37a » Sat Feb 09, 2019 8:21 am

I am confused Alex? You have bought a device that need the power supplies found in a 500 series "rack box" and yet do not have the box? How do you intend to power it?

Assuming you have that sorted I would think you could find a skilled cabinet maker* who could make you a wooden sleeve? You might like to search for people that make custom guitar amp head sleeves.

*Were dad, master cabinet maker still around he would have made a FORTUNE! Had a tea chest full of exotic woods collected over 70 years. Rosewood, Teaks, Sycamore is nice...

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Re: SPL HPm headphone amplifier

Postby Wonks » Sat Feb 09, 2019 11:31 am

The minimum vertical slot count racks I've found have 3 slots.

As the HPm is a dual-width unit, so will need at least 2 slots, I'd suggest considering a 3-unit rack with a blanking plate for the empty slot.

Though as you've started down the 500 series route, can you see yourself getting some more units in the future? If so, getting a bigger slot-count now would be cost-saving.
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Re: SPL HPm headphone amplifier

Postby Mixedup » Sat Feb 09, 2019 2:47 pm

Wonks wrote:The minimum vertical slot count racks I've found have 3 slots.

Sound Skulptor do a 1-slot one... Doesn't really solve this particular issue, though!
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Re: SPL HPm headphone amplifier

Postby Wonks » Sat Feb 09, 2019 3:26 pm

I've seen a 2-slot one, but horizontal mounting, so 2 x 1U slots, which doesn't help either.
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