Agreed. The cost of testing DAC boards might even exceed that of just designing one yourself. I look forward to how the amp performs.
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March Audio HPA1
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Agreed. The cost of testing DAC boards might even exceed that of just designing one yourself. I look forward to how the amp performs.
The attitude I see here doesn't make sense. You have a brand which is objective measured quality, with quality build and a quality company backing it up. Everything you are doing is fine. Just explain all of it. This is like the old Mac vs. PC debate. Some of us are cheapskates and nerds and will go elsewhere, but there is an audience for this.
Just be as honest as is reasonable about the design of the amp (and all of the other products). The Khadas DAC is an excellent OEM board. The matching pair will look good and sound extremely good (if your amp is as good as you say it is). Your brand should be telling everyone exactly what you are doing and why it makes their lives as audio entusiasts easier/better. Some of us will go with cheaper and more nerdy options. But there is nothing wrong with your approach (especially given the absurdity of the audiophile world). Just be transparent about everything and roll with the punches.
Just be as honest as is reasonable about the design of the amp (and all of the other products). The Khadas DAC is an excellent OEM board. The matching pair will look good and sound extremely good (if your amp is as good as you say it is). Your brand should be telling everyone exactly what you are doing and why it makes their lives as audio entusiasts easier/better. Some of us will go with cheaper and more nerdy options. But there is nothing wrong with your approach (especially given the absurdity of the audiophile world). Just be transparent about everything and roll with the punches.
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Acknowledged and taken on board. I think all of us want to move on.
Alan, it's your business, so your decision about what to reveal or not reveal. Reality is that if there's anything worth stealing in there, it will get stolen. Putting it off a few weeks doesn't seem to me to be worth it, but I'm not the one with skin in the game.
What I would ask for, and not begrudge a non-answer, is a general description of the circuit and some whys. And, of course, measured performance data. But that's because I'm a design geek, not a potential customer.
Ok
First design decision was to use a primary external smps psu. Several reasons:
-Simplicity of safety and mains related approvals. Already done for you. Phone battery dying hang on
-Didn't want internal mains transformers. At - 150dB measurements it is all too easy for mains and it's harmonics to get picked up. Real world its a non issue for a low level of pick up but still wanted to avoid it.
-Weight. A minor consideration for shipping costs.
Next, internal psu. Takes single rail DC input and uses low leakage switching supply to generate +- voltage rails. This is further carefully filtered low esr caps etc and has low noise regulation to supply amp circuit.
Volume control. No way would I put and analogue volume pot anywhere near this. Balance tracking will always be significantly compromised. They do degrade over time and become progressively noisier with often worsening balance. Simply not an option afaic. So whats the choice? Stepped resistor ladder is a big improvement but not perfect. Expensive and makes remote control more complex to implement. PGA is another option but it would degrade the amp performance. So fixed gain it is. Switchable gain is an option but could degrade amp performance and I am just not worried about catering for every single headphone out there.
I will talk about digital volume control in another post. Its a topic on its own and as it has been mentioned by several other posters.
Amp. Differential input. Xlr / trs combo sockets with shield connected directly to the case at entry. Absolute requirement as far as I'm concerned (instrumentation background). My view is that single ended interconnectivity is a dumb idea. Having the potential for loop currents and noise flowing in a signal conductor is just a crazy idea. I know single ended works adequately for most of us most of the time, but if you have a choice why do it? Contrary to some beliefs differential doesn't equal more circuitry and more distortion.
The common mode noise rejection of differential inputs is an additional benefit. This can even be of some benefit if ultimately you input a single ended signal.
The circuit itself is simple. Careful attention to local decoupling. Has input EMI filtering. Careful attention to low impedance pcb ground plane. Stupidly small SMD. Single ended output.
Headphone socket. Standard 6.3mm Trs. The amp output is single ended with common ground anyway. My measurements show that whilst a 4 pin connector does have a benefit to crosstalk, it's not that big if you have low impedance ground plane and connection to the socket. The bigger issue is the headphone wiring. It needs to have separate ground going to each driver only commoned back at the Trs jack. Beyond this you just ain't going to hear the difference between 120 and 115dB crosstalk. So keep it simple, standard trs everyone can use.
Other. Power switch is a piezo touch variety. I've had too many power switches fall apart over the years. It's a bit of a pita to implement as it's output is momentary and requires a bistable latch to make it work. But it looks good flush mounted and will last forever.
Case. Solid milled anodised aluminium. Should help with EMI. Note that is totally untested. I don't have an RF anechoic chamber.
Oh yeah, it's got a red power led.
I think that's about it. Hope that helps
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rebbiputzmaker
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The time involved in designing a board from scratch is tremendous. Board layout is complex to say the least. It doesn't make any sense to do so as the Khadas really does extract pretty much all there is to get out of the 9038q2m. I looked at a lot of boards and existing commercial implementations performance.
Edit I have been looking at thd compensation features, have to see if that leads to any benefit and as an aside a supplier has been in contact and assisted by reducing cost. This has been passed straight on so its now cheaper.
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@March Audio are you expecting better measurements than the Massdrop THX AAA 798?
Some basic measurements on the prototype.
Into 300 ohm load
Into 33 Ohm load
Into 300 ohm load
Into 33 Ohm load
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Ok digital volume. As mentioned already properly implemented digital volume that's dithered and operating at 32 or 64 bit precison and delivering 24/ 32 bit to the dac does not reduce resolution.
Also remember that there are no recordings that actually deliver more than around 18 to 20 bits. Not forgetting to mention how the recordings are mixed these days. It's predominantly not analogue, it's digital volume mixing in a DAW.
The issue that surrounds digital volume is this. With say a dac output you have an inherent noise level present, plus the signal (music). So when you feed this into a subsequent amplifier via an analogue volume pot as the volume is turned down both the noise component and the the signal component get reduced. This maintains the available dynamic range \ signal to noise ratio. However it only does this until such point that the dac noise falls below the noise floor of the subsequent amplifier. From that point the amp noise is dominant and you start losing snr / DR.
With a digital volume only the signal (music) gets attenuated as you turn the volume down, the noise level remains the same. So as the volume goes down the dynamic range / snr goes down.
Now here's the thing. That's a theoretical disadvantage. No 2 ways about it. However let's enter the real world.
A typical power amp might have about 26dB of gain. Dac noise is now so quiet that even with that gain its virtually impossible to hear the dac noise. An example. testing with my B&W c5 speakers with a 88dBm/watt sensitivity, 26dB hypex power amps with a DAC1 plugged directly in. If I place my ear flat against the tweeter I *think* I can hear the faintest of hiss. I am not sure if this is the noise of the amp or dac. This is totally inaudible in the quietest of listening rooms and mine is acoustically treated and very quiet. In many set ups it's the power amp noise that will dominate. So basically the dac noise is a non issue. I will note that in some setups that unusually high sensitivity speakers (100dB/w +) you may start to have some audible noise, but these really are the outliers.
Now let's consider this with the HPA1 headphone amp. It's gain is only 3dB, might even go to unity 0dB in the production version. The dac noise is very close to the noise floor of the amp. From the plots above you can see how low it is. It is completely inaudible under any circumstances, I would suggest with even the most sensitive of headphones. So in this circumstance you lose nothing at all with digital volume.
You gain perfect channel balance and app based remote control and you still have all the real world useable dynamic range/snr/resolution.
Next thing, fear of being blasted. This is a set up issue. I have left analogue volume controls and my digital AV amp volume set high and blasted myself BTW.
The dac doesn't have to be set as a Windows audio device. Turn it off. You might argue that some may want to use it for multiple duties which is fair comment, but I am aiming the market that wants hifi, not trying to suit everyone. Decent media players, foobar, Roon, J River just don't have issues with random full blast noises. Myself and many others I know just use digital volume and have done so without issue for years.
The efficacy of Windows own volume control I will leave for @amirm to comment on as I think he was direrectly/indirectly involved with its design.
Hope that helps
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Let's just discuss the issue and explain the positives and negatives.
There are negatives but they need to be put in context which people may not have thought about so I'm happy to discuss.
There are negatives but they need to be put in context which people may not have thought about so I'm happy to discuss.
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You are too nice. It hasn't worked so far.
I wouldn't say so. There's been crash and burn, and some interest. Such is life. I'm hoping this turns out interesting. It's unique, that's for sure. It'll be the first headamp I've ever heard of skipping a pot.
Had to register to applaud for doing a low gain potless amp, atleast balances out all that DAC fiasco. I don't really get why there aren't any commercial headphone "power amps" around. If you don't like digital volume, then use a pre or whatever dac with internal volume. No different from speaker world, I optimized my previous 99dB/W speaker setup with a low 14dB gain Modulus86 amp, no probs using digital volume.
I don't really get why there aren't any commercial headphone "power amps" around. If you don't like digital volume, then use a pre or whatever dac with internal volume. No different from speaker world, I optimized my previous 99dB/W speaker setup with a low 14dB gain Modulus86 amp, no probs using digital volume.
I've been listening now for months Topping D10 connected to my amps while volume is controlled by Volumio via XMOS chip in D10 and haven't encountered any issues at all.
I think that's about it. Hope that helps
It does, thanks. A few questions (which again, I will not begrudge non-answers):
The internal switching supply, is this generally done on the same lines as Jan Didden's Silent Switcher?
Are the output stages chip-based or discrete?
Is the input bipolar or FET?
Measurements look great, fine work there!
No, not like Jans, its simpler. I wasnt aware of that product, its interesting - I will have to oem it....................................too soon?
I hope you dont mind but Im going to keep you guessing about the amp circuit
The measurements tell the important story We have performance beyond the capabilities of any current DAC output.
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Thanks, Alan. Hope you're having fun today!
https://stoneracoustics.com/udxa
This headphone has no volume control. It's been out for a while. Think it's digitally controlled
This headphone has no volume control. It's been out for a while. Think it's digitally controlled
I think some of us think that if, for unknown reasons (Windows updates perhaps), DAC's volume will get to the max., then our ears might blow away. It's a valid hypothetical scenario after all, but that's why jRiver has "Startup Volume" setting for, to setup manually our startup desired volume.
However, perhaps in the near future an ALPS pot might fit HPA1's case...who knows.
L.E.: HPA1 is like a power amplifier, but for headphones, so we can always use a preamp between DAC1 and HPA1. Why not give a $49 credit to Schiit for http://www.schiit.com/products/sys and see if it works well in the above scenario. At $49 I'm not sure I could even build the aluminium case, not speaking about the PCB, plugs and potentiometer inside.
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The volume gets adjusted from A & B buttons, so it has a digital volume inside.
However, from my understanding, this combo gets its power from the USB port or from the included 5V, 3A power brick. No matter what boost-converters are inside, there's no way to have a good punch for planars; there's just no way, after boost regulators, to have enough current and voltage to output more than 4-5V RMS/channel (definitely lower than Objective2, for example), but I could be wrong here. However, can't be compared with HPA1 for sure, it plays in a different league for sure.
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