Review and Measurements of Topping DX7s DAC and Headphone Amp

[Mad_Economist]

Despite the measurements suggesting that it doesn't have much of an impedance response.. NwAvGuy did a writeup on this talking about despite the response that it would still have other sonic effects on the headphone since the voltages themselves would be different. I have tested with my Aeon also and its the same also plugging it into the DX7 directly (essentially the same kind of Amp as the A30) vs plugging it into the O2.
My friends Cayin is the same with its 0-ohm balanced output, when we test it in balanced the output is cleaner with the Aeon and PM-2 vs in its single ended output which has 10 and 100 ohm output impedance IIRC).

A voltage difference would be due to the voltage divider formed between the load (the headphone voice coil/circuit) and the output impedance, and would be audible when it differed by frequency (pushing up the frequencies where the impedance was higher on the headphone). I suspect you are referring to changes in electrical damping, which NwAvGuy wrote about extensively as a rationale for very-low-impedance outputs. In concept, I can get behind that, but in practice, I have never seen a change in damping factor consistently perceived without an attendant change in frequency response due to nonlinear impedance, and in the case of the planar magnetic designs, mechanical damping rather than electrical damping dominates regardless.

Is this using the OPA1622?
https://www.kadenken.com/shopdetail/000000000558/ct103/page1/order/

It appears so! Unfortunately, unless I am reading their specs wrong - which may be the case, google translate is not always kind - the output voltage seems rather limited - less than 4Vrms into high impedance loads. Not surprising for such an adorably-sized portable amp, of course, but it would leave it a bit limited for those wanting high outputs from higher-impedance headphones. Oddly, it also appears to be configured as a differential amplifier, with one OPA1622 for each channel - wouldn't have expected that for such a cheap unit, but I'm not sure why you'd want a differential portable amplifier.

If you actually look at their own lineup, and the Element. The DAC in the Element is about comparable to the USB-only OL DAC, it's not really spectacular. Slightly different, but similar enough for this comparison. If you subtract their market-price of the O2 enclosure+front/backplate(30.97), you're left with about a 70-dollar market price DAC. 350-70=280. The Amp and it's enclosure is the bulk of the product that is the Element. I'd call it 50 for the knob, and 230 for the amp itself. It's still a bit expensive, but the price of Element vs. EL Amp is reasonable relative to the rest of their product line.

A very fair point. I always forget that the Element =/= EL DAC + EL Amp, and I admit I'm somewhat spoiled by the BOM:MSRP ratios you see from some of the mass-produced Chinese stuff, which I'm sure isn't nearly as viable for something lower volume and predominately American made like the Element, particularly with the fancy housing.

 

[Don Hills]

... the output voltage seems rather limited - less than 4Vrms into high impedance loads. ... it also appears to be configured as a differential amplifier ...

A common reason for a differential (BTL) output stage is to double the available output voltage without having to include a voltage step-up. "Less than 4Vrms" implies a 5 volt supply.
Neglecting losses, this is roughly:
(5v pk-pk) / 2 = 2.5v pk
2.5 * 0.7 = 1.75v rms
1.75 * 2 = 3.5v rms
The device appears to have a single Li-ion cell (3.7v nominal) so it likely uses a standard step-up circuit to generate 5v used for the logic chips and the headphone amp.
Their 120mW into 70 ohms implies about 2.9 volts (presumably RMS). Allowing for voltage drops in the chip etc, this sounds reasonable based on the above calculation. I haven't looked at the OPA1622 datasheet, but many such chips can reach close to full rail-to-rail voltage swings on high impedance loads so "less than 4V rms into high impedance loads" also sounds reasonable.

 

[Jinjuku]

Over the years I have examined the headphone circuits of literally dozens of amplifiers dating from the 60s to the present. Of those where the headphones were driven from the PA stages, I have yet to find one that did not have series resistors in the headphone outputs, apart from one I modified myself...
The advantage of such resistors is that they evened out the level differences caused by differences in headphone impedance.

Not being a headphone buff... Have there been headphones in the past that come with their own HP amp?

 

[RayDunzl]

Have there been headphones in the past that come with their own HP amp?

Electrostats may generally come paired with an amplifier due to their bias and possibly drive voltage requirements.

http://www.stax.co.jp/Export/ExportProducts.html

My electrostatic speakers require a 2500V membrane bias and 100:1 step up voltage of the amplifier signal to drive the stators, but this is accomplished by a couple of watts from the wall driving a power supply for bias and a step-up transformer for signal inside the speaker

 

[amirm]

Not being a headphone buff... Have there been headphones in the past that come with their own HP amp?

My Stax headphones do.

 

[SpeedyRodent]

The original Sennheiser Orpheus HE90 which I believe was released in 1991. Aside from Stax, this is one of the older headphones that came with their own amp. The design and look of this headphone reminds me a lot of the HD580/600/650.

https://www.innerfidelity.com/content/sennheiser-orpheus-he-90-headphones
http://www.trustedreviews.com/reviews/sennheiser-orpheus-he-90

 

[Wombat]

My Beyers look like a fly's eyes too.

 

[March Audio]

Engineers with too much time on their hands me thinks

 

[Mad_Economist]

My Stax headphones do.

The original Sennheiser Orpheus HE90 which I believe was released in 1991. Aside from Stax, this is one of the older headphones that came with their own amp. The design and look of this headphone reminds me a lot of the HD580/600/650.

Pretty much all electrostatic headphones have "companion" amps that they're sold in bundles with - perhaps if there were enough manufacturers to agree on a unified DC standard you'd see more standalone ones, but as stands it's pretty much mandatory.

Out of curiosity Amir, what Stax do you have?

 

[Frank Dernie]

Not being a headphone buff... Have there been headphones in the past that come with their own HP amp?

I have Stax electrostatics and Kingsound electrostatics. Both come with a dedicated power supply/amp. I bought a pair of Koss ESP7 headphones in 1970 when I went to the USA on a student exchange. They had a dedicated box to connect to the speaker connection. They were an expensive disappointment, the polarising voltage came from transforming some of the power output of the amp, so on quiet bits of music they faded to silence. They were really uncomfortable too. I didn't use them much.

 

[amirm]

Out of curiosity Amir, what Stax do you have?

I have three of them. I have all three of their headphone types (gray, brown and reference round ones). And three amps: transistor, single-ended tube and fully differential tube. Model numbers escape me as I bought them more than 15 years ago.

 

[DonH56]

I went to the Stax website; is there only the USA one dealer/distributor now? One of the stores I worked for way back then carried them, and a local company was a distributor for Stax (one of several).

 

[Blumlein 88]

I went to the Stax website; is there only the USA one dealer/distributor now? One of the stores I worked for way back then carried them, and a local company was a distributor for Stax (one of several).

If there is one, is it gray market? I thought Stax withdrew from the US market a decade or more ago.

I had some of the early Lambda's with the SRD7 interface. Had a Sumo Model Nine just for the headphones at one time. Later used a VTL 75/75. Used a Mark Levinson ML9 briefly, but that was not a good match. Can you say ETCHED....................I knew you could.

When I purchased that Stax earphone second hand, I never had heard of the brand. I was buying a McIntosh amp and the guy basically refused to sell it without the phones. I listened to them and decided for the modest amount he was asking they were a deal. Headphones needing a full power amp. Tickled my sense of the bizarre at the time.

 

[DonH56]

Well, the dealer is listed on the Stax JP web site, so I think it's legit. I know they disappeared for a while and I have not followed them in ages. The ones I borrowed from the store I worked were great but way out of my price range then, and now I rarely use headphones and what I have are adequate (Sony MDR-7506 and Sennheiser 424 -- new foam and cables on the latter).

 

[amirm]

Stax has been coming to US audio shows so I imagine they are selling them here.

 

[Mad_Economist]

I have three of them. I have all three of their headphone types (gray, brown and reference round ones). And three amps: transistor, single-ended tube and fully differential tube. Model numbers escape me as I bought them more than 15 years ago.

Ahh, that will be two Lambda series and the original Omega, I believe (or possibly the SR007, depending on how much more than 15 years it was). Extremely high performance designs in either case, and very highly in demand to this day!

 

[Jimster480]

Stax has been coming to US audio shows so I imagine they are selling them here.

Have you listened to the mrspeakers Aeon? If so I wonder your personal impressions vs your Stax.

 

[TheInquiring]

• Is the DX7s designed to directly drive power amplifiers or powered speakers?
• Or, for that matter, are there any budget (let’s say, below 1K mark) DACs with such capability?
• What characteristics should match to provide the proper interface between the components under consideration?

 

[RayDunzl]

Is the DX7s designed to directly drive power amplifiers or powered speakers?

Looks like it is.

That's what "line out" is in the specs.

http://www.tpdz.net/en/products/dx7s/index.htm

What characteristics should match to provide the proper interface between the components under consideration?

Amplifiers tend to be an easy drive (high input impedance 10k to 100k ohm - you're sending the amplifier signal voltage, not much power).

You need a volume control in the path - sometimes an amplifer has that (integrated amp), more commonly, not. DX7s has it.

 

[amirm]

Is the DX7s designed to directly drive power amplifiers or powered speakers?

I have driven my JBL LSR305 Mark II and they work superbly in that application. Assuming you get a remote with it, it makes for a great entertainment system and not just desktop use.

Indeed my idea of an excellent budget system is now the Topping DX7s and a pair of powered speakers.

Of course you can also drive amps with them just the same.