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Help me interpret the unusual HP amp specs of my audio interface

Barncore

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Heya!

I own an interface called the Discrete 4 Synergy Core (non-Pro version) by Antelope Audio. I recently upgraded my headphones (to Hifiman Ananda Stealth V3) and I'm trying to figure out whether these headphones are getting used to their full potential or whether they're not getting amped properly.

I don't have any HP amps to compare against, and I've never bought a HP amp before so I don't know how to know for sure.

The headphones don't sound bad by any means but I'm wondering if there's more juice to be squeezed out of it with a dedicated HP amp. I'd like to optimise my planar magnetic headphones purchase.

By all accounts the DA conversion in these interfaces is great, that's the reason I bought it in 2021 for $1,300 AUD (I'm an audio engineer), so I'm assuming the HP DAC is good enough? (Don't know how to know that either though)


These are the HP amp specs that are publicly available:
Output Level ( unloaded ) 20dBu max
Output impedance 120 ohms
Dynamic Range: 121dB
THD + N: – 100dB
Recommended HP Impedance 16Ω to 600Ω

(It should also be noted that the interface has 4 HP 1/4" outputs, that i'm fairly sure are able to play at the same time.)

I asked Antelope support for more info and this is what they said:

Thank you for reaching out. Victor here with Antelope's tech support.
I'm copy-pasting the answer from our hardware engineer: 7.5Vrms / (120ohm resistor in series + hp impedance)
Wattage: 7.5 * 7.5 / (120 + hp impedance)

I have no idea what to do with that information.

Is it enough to drive these Hifiman headphones? (16Ω and 93db sensitivity) How do i calculate it?

How do I use the data they provided compare the interface HP amp against other HP amps?

I'm looking at maybe buying either the Topping L70 amp or the Topping DX5 Lite DAC-amp, but I'm not certain whether that would be a lateral move or not.

Thanks for any help
 

AnalogSteph

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Do you get more than enough volume out of the setup as-is in any situation? If so, it's basically sufficient. Planars are just about entirely ohmic loads so their frequency response will be unbothered by output impedance, and maximum output will be the dominant concern.
Obviously a 120 ohm output impedance is going to cost you a lot of level with a 16 ohm load, you'll only get about 0.9 Vrms out maximum. Not to mention those 120 ohm resistors might get a bit toasty...
I'm looking at maybe buying either the Topping L70 amp or the Topping DX5 Lite DAC-amp, but I'm not certain whether that would be a lateral move or not.
Both seem fine but if all you really need is an "afterburner" for the headphone out, I would suggest going with a much less expensive L30 II instead, which can muster the same output power into 16 ohms. (Get a cable 1/4" stereo TRS --> 2x RCA for that.) Even the minimum (-14 dB) gain setting normally intended for IEMs would go slightly louder than what you have now, and by medium (unity) gain it would be a whopping 18 dB louder, simply because the amplifier's output impedance is only a fraction of an ohm instead of 120.

A possible argument for the L70 might be its hardware mains power switch. The external transformer supplying the L30 II / L50 (think the kind that comes with inexpensive portable mixers) uses 2-3 W doing absolutely nothing.

If you want MOAR POWAH, there's still the A30 Pro which can muster 2x 6 W into 16 ohms. That would need to take up one of your regular balanced outputs though, using unbalanced input is not recommended under these circumstances (it doesn't have a ground lift feature like the L70).
 
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Barncore

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Do you get more than enough volume out of the setup as-is in any situation? If so, it's basically sufficient. Planars are just about entirely ohmic loads so their frequency response will be unbothered by output impedance, and maximum output will be the dominant concern.
Obviously a 120 ohm output impedance is going to cost you a lot of level with a 16 ohm load, you'll only get about 0.9 Vrms out maximum. Not to mention those 120 ohm resistors might get a bit toasty...
I do get enough volume, yeah. I was under the impression that a higher powered amp can offer more kick, body, dynamic, soundstage, liveliness, stuff like that?
But if volume is the only thing a headphone amp does, then I guess I don't need one. The headphones are loud enough.

I read that planars are hungry for current more than wattage. How do I calculate the current of my interface's HP amp?


Both seem fine but if all you really need is an "afterburner" for the headphone out, I would suggest going with a much less expensive L30 II instead, which can muster the same output power into 16 ohms. (Get a cable 1/4" stereo TRS --> 2x RCA for that.) Even the minimum (-14 dB) gain setting normally intended for IEMs would go slightly louder than what you have now, and by medium (unity) gain it would be a whopping 18 dB louder, simply because the amplifier's output impedance is only a fraction of an ohm instead of 120.

A possible argument for the L70 might be its hardware mains power switch. The external transformer supplying the L30 II / L50 (think the kind that comes with inexpensive portable mixers) uses 2-3 W doing absolutely nothing.

If you want MOAR POWAH, there's still the A30 Pro which can muster 2x 6 W into 16 ohms. That would need to take up one of your regular balanced outputs though, using unbalanced input is not recommended under these circumstances (it doesn't have a ground lift feature like the L70
Well the thing is I found great used deals for the L70 and DX5 online. $350 AUD / $232 USD for the L70, and $450 AUD / $300 USD for the DX5.

There's a certain appeal to getting DAC-amp instead of just an amp, cos then I could set a systemwide EQ on the HP DAC source and not have to worry about forgetting to turn it off when switching to my speakers, lol.
But that's neither here nor there for this conversation I guess. Just wanted to explain the reasoning behind L70 and DX5.

Also from what I can glean from reviews, the L70 and DX5 are closer to the sound I'll like, moreso than L30 II and A30
 

Vincent Kars

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Output impedance 120 ohms
I'm baffled by this value.
As a rule of the thumb a damping ratio of 8 is recommended as a bare minimum.
So you need a 960 Ohm headphone to obtain this.
Reminds me of the days of old when amps didn't had a headphone amp but a series of resistors to turn the power of the speaker output into a much lower value to drive the headphone.
 

Sokel

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Zen has 27 Ohm output impedance,it should be close to that if not lower.
 
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Barncore

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I'm baffled by this value.
As a rule of the thumb a damping ratio of 8 is recommended as a bare minimum.
So you need a 960 Ohm headphone to obtain this.
Reminds me of the days of old when amps didn't had a headphone amp but a series of resistors to turn the power of the speaker output into a much lower value to drive the headphone.
Well if you're baffled, then i haven't got a chance :D
 

AnalogSteph

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I do get enough volume, yeah. I was under the impression that a higher powered amp can offer more kick, body, dynamic, soundstage, liveliness, stuff like that?
Most of these are simply a function of output and frequency response, which we have already discussed. Low-impedance headphones can be taxing on channel separation due to finite ground return resistance, but when adding that much output impedance in series this effect is greatly diminshed.
I read that planars are hungry for current more than wattage. How do I calculate the current of my interface's HP amp?
The current limit, you mean? It'll be at most (+20 dBu)/(120 +16 ohms) or about 57 mA rms, which isn't so outlandish that one 4556A or a pair of 4580 opamps couldn't deliver it (rule of thumb: ~80 mA), so I don't think it'll be any less either. It doesn't take a super powerful circuit to achieve this.
Well the thing is I found great used deals for the L70 and DX5 online. $350 AUD / $232 USD for the L70, and $450 AUD / $300 USD for the DX5.

There's a certain appeal to getting DAC-amp instead of just an amp, cos then I could set a systemwide EQ on the HP DAC source and not have to worry about forgetting to turn it off when switching to my speakers, lol.
Fair enough.
Also from what I can glean from reviews, the L70 and DX5 are closer to the sound I'll like, moreso than L30 II and A30
Don't believe everything you read on the internet.

Operated within their respective limits, Topping amps pretty much qualify as the proverbial "wire with gain". As such, they shouldn't have a "sound" - they amplify (or attenuate) and that's about it. As do a number of other similar amps.
 
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Barncore

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Most of these are simply a function of output and frequency response, which we have already discussed. Low-impedance headphones can be taxing on channel separation due to finite ground return resistance, but when adding that much output impedance in series this effect is greatly diminshed.
Ok so just to confirm, the L30 II would give me the same result/sound for these Hifimans that an L70 would? Or even something like an A70?

The only advantage to the L70 or A70 would be things like inputs/outputs and features?
 

AnalogSteph

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Ok so just to confirm, the L30 II would give me the same result/sound for these Hifimans that an L70 would? Or even something like an A70?
As you can see, it pretty much goes from distortion levels at least a magnitude below all concerns of audibility straight into clipping at 7+ Vrms into 16 ohms, or 3+ W, and the frequency response would make a ruler proud.
That's about as close to a wire with gain as it gets. It's not the very most powerful amp they make but it's more than respectable and a massive upgrade from what you have now (a good 50 mW, and you're not even using all of that). It's a very good performer, let alone for what it costs.

The L50 is pretty much the same circuit with balanced I/O added.
The only advantage to the L70 or A70 would be things like inputs/outputs and features?
For you, pretty much that. And a built-in power supply.
 
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Barncore

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Both seem fine but if all you really need is an "afterburner" for the headphone out, I would suggest going with a much less expensive L30 II instead, which can muster the same output power into 16 ohms. (Get a cable 1/4" stereo TRS --> 2x RCA for that.) Even the minimum (-14 dB) gain setting normally intended for IEMs would go slightly louder than what you have now, and by medium (unity) gain it would be a whopping 18 dB louder, simply because the amplifier's output impedance is only a fraction of an ohm instead of 120.
Think i'm gonna go for the L30 II.
Quick question. My interface has 2 Monitor outs (L+R), that i'll continue to use for my monitors obviously, but it also has 4 Line outputs. I could use the LineOut for the headphone amp right?
My question is, is the signal coming out of the LineOut a mono signal or a stereo signal? I'm fairly sure it's stereo summed to mono right?
So would i use a 1xTRS-->2xRCA cable? And it will end up as stereo in the amp?
Or would i use a 2xTS-->2xRCA cable, and plug the 2xTS into LineOut 1+2?

Or do i plug into the interface's headphone output?? Surely not
 

AnalogSteph

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Or do i plug into the interface's headphone output?? Surely not
Oh yes. This is your only unbalanced kind of output on this interface (as it is on many others), and with a whopping 4 of them, it's not like you couldn't spare one. It might not look all that elegant but from a technical perspective I can't see anything wrong with this.

You can't easily hook up an unbalanced amp to your line-outs. You'd have to go for at least an L50 instead.
 
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Barncore

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Oh yes. This is your only unbalanced kind of output on this interface (as it is on many others), and with a whopping 4 of them, it's not like you couldn't spare one. It might not look all that elegant but from a technical perspective I can't see anything wrong with this.

You can't easily hook up an unbalanced amp to your line-outs. You'd have to go for at least an L50 instead.
Are you sure? Advice i'm getting on another forum is telling me to go out the line outs. I actually already ordered the amp and TS-RCA cables. I emailed support for the interface and they confirmed the line outs. And something i read somewhere else said that going through the headphone output sounds worse. What's wrong with going through the line outs?
 

AnalogSteph

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I emailed support for the interface and they confirmed the line outs.
Well, if they say plugging in a TS for unbalanced output is fine, I'd take their word for it. There are several different circuit topologies for balanced outputs that will react in different ways, and a very common variant is potentially quite unhappy if you just short one half to ground, hence my reservations. I guess the fact that both the line-outs and headphone outs go to +20 dBu could be taken as a clue that things may be impedance-balanced only on the line-out side.
 

doalt

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I'm baffled by this value.
As a rule of the thumb a damping ratio of 8 is recommended as a bare minimum.
So you need a 960 Ohm headphone to obtain this.
Reminds me of the days of old when amps didn't had a headphone amp but a series of resistors to turn the power of the speaker output into a much lower value to drive the headphone.

I thought the 1/8 rule applied more to dynamic headphones and multi-driver IEMs with variable impedance, because adding a constant output impedance to a variable headphone impedance would change the frequency response. But planar magnetic headphones normally have a constant impedance, so the amp's output impedance shouldn't affect their sound that way.

However upon further reading, I think you are right about the damping factor:
The bass performance of the headphones, as designed by the manufacture, may be audibly compromised if there’s insufficient damping. The bass might become more “boomy” and less controlled. The transient response becomes worse and the deep bass performance is compromised (the headphones will roll off sooner at low frequencies). A few, such as those who like a very warm “tube like” sound, might enjoy this sort of under damped bass. But it’s almost always less accurate compared to using a low impedance source.

My quick search on the internet and in this forum found some discussions about it but they were more targeted at speakers and the "worst case" damping factor was still above 1. I am not sure how much a damping factor of 0.13 (16/120) would affect the sound, or if the 1/8 rule also makes sense here. If you have more links for analysis or measurements regarding this, I am keen to learn.
 
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Barncore

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Well i received my new amp (Topping L30 II), and can confirm that the sound is improved, which means that not getting enough power on my previous amp was indeed limiting the sound.

I compared the new amp with the old amp side by side. I volume-matched them as best i could. (note that i'm using the same DAC in the Antelope interface for both amps, and using the same headphones)

The new Topping amp has given the sound more weight and energy. The bass vibrates my ears a bit more, and it sounds both tighter/leaner and more "alive", i.e. less woofy but also more energy somehow. The transients sound much clearer and cleaner. Feels like there's a bit more "attack" in the transients, it's a bit sharper (or should i say, it was a bit dulled on the Antelope amp). When changing from the Antelope amp to the Topping amp it's as if a veil gets lifted off the sound. The Antelope amp sounds more "grainy" in the high mids, whereas things sound slightly more high-res through the Topping amp.

One thing is for sure is that the Topping amp sounds noticeably better. It's not exactly subtle either. It's immediately noticeable. There's no doubt in my mind that my headphones weren't getting used to their full potential with my previous amp

Man, i've been reading people on forums say that lack of power through an amp doesn't change the sound profile of hifiman planar, it just has less volume. Well i can safely say from direct experience that's definitely wrong. Glad i took a gamble and tested it myself
 

doalt

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Well i received my new amp (Topping L30 II), and can confirm that the sound is improved, which means that not getting enough power on my previous amp was indeed limiting the sound.
What you experienced sounds like the effect of higher damping factor due to the lower output impedance of the new amp.

I was just discussing this right above your post. When the amp impedance is too high, the transient response becomes worse and the deep bass performance is compromised. But I was not sure how audible this is, or whether it affects planar magnetic headphones like yours as much as dynamic drivers.

Thanks for your update. I'm glad the amp worked out for you, and I learned that low output impedance could meaningfully improve the sound of planars too.
 
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Barncore

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What you experienced sounds like the effect of higher damping factor due to the lower output impedance of the new amp.

I was just discussing this right above your post. When the amp impedance is too high, the transient response becomes worse and the deep bass performance is compromised. But I was not sure how audible this is, or whether it affects planar magnetic headphones like yours as much as dynamic drivers.

Thanks for your update. I'm glad the amp worked out for you, and I learned that low output impedance could meaningfully improve the sound of planars too.
Interesting.

It's also worth mentioning... It occured to me that part of the improved sound could be the fact that the headphones are now going through the "Line Outs" part of my interface (into the new Topping amp) instead of the headphones outputs part of the interface like before. It's possible that the Line Outs are using a better quality DAC than what the headphones outputs are using. There's 4 headphone outputs in my interface, so whatever DAC it's using would have to be shared between the 4 HP outputs.

I'm only speculating of course, I have no idea, but I thought it'd be worth mentioning
 
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