Review and Measurements of Topping DX7s DAC and Headphone Amp

[Dana reed]

Question for any DX7s owners out there...

In DAC mode, is it OK to connect both the single ended and the balanced outputs on the back of the unit at the same time? They would both be feeding the same amp that has an input selector so only one of the DX7s outputs would actually be used by the amp at a time, but if it is a bad idea to connect to both DX7s outputs at the same time I would rather not risk it!

Using the input select on my amp I could easily check out the sound difference between SE and balanced inputs...

I haven’t tried connecting both line outs at the same time, but I have tried connecting both headphone outs. I found if one load has lower impedance than the other, the output level on the higher impedance headphones is dropped.
I’ve seen a similar thing when splitting a DAC’s output to two preamps. If the input impedance of the amps are different, a similar thing happens where the higher impedance device level drops when the other is connected

 

[jsmiller58]

I haven’t tried connecting both line outs at the same time, but I have tried connecting both headphone outs. I found if one load has lower impedance than the other, the output level on the higher impedance headphones is dropped.
I’ve seen a similar thing when splitting a DAC’s output to two preamps. If the input impedance of the amps are different, a similar thing happens where the higher impedance device level drops when the other is connected

Thank you Dana! I have always treated hooking up two headphone outputs on the same amp the same way the Ghostbusters of old treated crossing proton streams

"There's something very important I forgot to tell you! Don't cross the streams… It would be bad… Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light.
— Egon Spengler (Harold Ramis) on crossing proton streams"

 

[Ayreonaut]

New here, just discovered the site. Is the review of the DX7s amplifier section still in the works?

Based on the specs, it look s like an A30 + D50 might best the DX7s. Is that right?

 

[Grave]

New here, just discovered the site. Is the review of the DX7s amplifier section still in the works?

Based on the specs, it look s like an A30 + D50 might best the DX7s. Is that right?

I was told the amp in the Topping DX7s is the same as the Topping A30. I thought about buying a Topping D50, but it would not stack with my Topping A30.

 

[jsmiller58]

I was told the amp in the Topping DX7s is the same as the Topping A30. I thought about buying a Topping D50, but it would not stack with my Topping A30.

I am not sure how to go about determining if the amps are the same or not...

The Topping website says the DX7s single ended jack’s output impedance is <10 ohms... from the basic rule of cans should have at least 8-10x the impedance of the amp that means the cans driven by this should have min impedance of around 80-100 ohms.

The A30 page does not seem to list output impedance, but says that it is suitable for driving cans with a non of 16 ohms, up to 600 ohms...

This leads me to question that the amps (or more precisely amp section as opposed to specific amp chips) are the same, but my analysis is all sorts of weak in this regard, including whether Topping recognizes the 8-10x rule for min can to amp impedance in their marketing...

 

[amirm]

The A30 page does not seem to list output impedance, but says that it is suitable for driving cans with a non of 16 ohms, up to 600 ohms...

I measured it in my review and it was the same 10 ohm: https://audiosciencereview.com/foru...g-a30-and-schiit-magni-3-headphone-amps.4024/

 

[jsmiller58]

I measured it in my review and it was the same 10 ohm: https://audiosciencereview.com/foru...g-a30-and-schiit-magni-3-headphone-amps.4024/

Sorry, I missed that!! I guess I am surprised by their website saying that it was suitable for headphones with impedance values as low as 16 ohms then. But maybe planars that have flatter response curves?

 

[derp1n]

Sorry, I missed that!! I guess I am surprised by their website saying that it was suitable for headphones with impedance values as low as 16 ohms then. But maybe planars that have flatter response curves?

Suitable as in it works, not that it's necessarily ideal. Some people might enjoy the FR change it causes, too... preferences are subjective.

Yes, the 8:1 impedance rule generally doesn't apply to planars.

 

[Ayreonaut]

The specs are different:

A30 "maximum output power" is 291 mW into 300 ohm through the 1/4" jack and 258 mW through the 1/8" jack. (Why are these different?)

DX7s "output power" is 146 mW into 300 ohm unbalanced and 546 mW balanced. (146 seems disproportionately low?)

I use Senn HD650. Not sure whether I'd be better off with the A30 + D30 or going balanced out of the DX7s. Never tried balanced before. I guess the 20 ohm output impedance should be no problem for the HD650?

 

[Veri]

The impedance is no problem for the HD650, no.

 

[Pavano]

Would the impedance be a problem for a Focal Clear?

 

[derp1n]

Would the impedance be a problem for a Focal Clear?

The Clear are 55 ohm, so using the conservative 8:1 rule you'd want an OI less than 7. But do note that it's already conservative at 8:1 (people who use 10:1 are belts, suspenders, and superglue).

From InnerFidelity's measurements:

So you'd expect a high impedance output to cause the frequency response to increase (i.e. the headphone is more sensitive) around that 55 Hz spike in the graph. With a really high impedance output amp like a Bottlehead Crack (120 ohm OI), it's likely the Clear would sound boomy or muddy. With the small mismatch between the DX7s and the Clear, the change around that 55 Hz spike is likely to be too small to be perceptible.

Here's @solderdude's comparison of the Clear with a 0.2 ohm OI and a 120 ohm OI:

So around a 7 dB boost in the low bass region. I'd expect 10 ohm OI graph to be very close to the 0.2 ohm OI graph, less than 1 dB difference probably (guessing)... so you're most likely fine with the Clear and the DX7s.

 

[jsmiller58]

The Clear are 55 ohm, so using the conservative 8:1 rule you'd want an OI less than 7. But do note that it's already conservative at 8:1 (people who use 10:1 are belts, suspenders, and superglue).

From InnerFidelity's measurements:

So you'd expect a high impedance output to cause the frequency response to increase (i.e. the headphone is more sensitive) around that 55 Hz spike in the graph. With a really high impedance output amp like a Bottlehead Crack (120 ohm OI), it's likely the Clear would sound boomy or muddy. With the small mismatch between the DX7s and the Clear, the change around that 55 Hz spike is likely to be too small to be perceptible.

Here's @solderdude's comparison of the Clear with a 0.2 ohm OI and a 120 ohm OI:

So around a 7 dB boost in the low bass region. I'd expect 10 ohm OI graph to be very close to the 0.2 ohm OI graph, less than 1 dB difference probably (guessing)... so you're most likely fine with the Clear and the DX7s.

This is awesome. Have been avoiding trying out the headphone outputs on my DX7s but I suppose I should be more adventurous and give it a shot when I get back to my gear tomorrow... these curves are encouraging, though the 20 ohm impedance on the balanced output may be interesting...

 

[Ayreonaut]

The Clear are 55 ohm, so using the conservative 8:1 rule you'd want an OI less than 7. But do note that it's already conservative at 8:1 (people who use 10:1 are belts, suspenders, and superglue).

From InnerFidelity's measurements:

So you'd expect a high impedance output to cause the frequency response to increase (i.e. the headphone is more sensitive) around that 55 Hz spike in the graph. With a really high impedance output amp like a Bottlehead Crack (120 ohm OI), it's likely the Clear would sound boomy or muddy. With the small mismatch between the DX7s and the Clear, the change around that 55 Hz spike is likely to be too small to be perceptible.

Here's @solderdude's comparison of the Clear with a 0.2 ohm OI and a 120 ohm OI:

So around a 7 dB boost in the low bass region. I'd expect 10 ohm OI graph to be very close to the 0.2 ohm OI graph, less than 1 dB difference probably (guessing)... so you're most likely fine with the Clear and the DX7s.

I did the calcs using: max 325, min 70, and output 20.

The effect would be +1.66 dB in the high impedance region.

 

[Grave]

I wonder if the 20 ohm balanced output would affect the 150 ohm HD 660 S? How do you do this calculation?

I am still tempted to buy a DX7s at some point.

 

[pwjazz]

I wonder if the 20 ohm balanced output would affect the 150 ohm HD 660 S? How do you do this calculation?

I am still tempted to buy a DX7s at some point.

The general rule of thumb is that you want output impedance to be 1/8 of the transducer's impedance, so 20 ohm is in the right ballpark. For some reason, the DIY Audio Heaven review recommends an output impedance of 10 or less. I'm not sure if Solderdude actually tested anything other than 0.2 and 120 ohm, but he's very friendly and you can contact him on http://diyah.boards.net to ask (or maybe even start by searching the boards).

 

[Veri]

The general rule of thumb is that you want output impedance to be 1/8 of the transducer's impedance, so 20 ohm is in the right ballpark. For some reason, the DIY Audio Heaven review recommends an output impedance of 10 or less. I'm not sure if Solderdude actually tested anything other than 0.2 and 120 ohm, but he's very friendly and you can contact him on http://diyah.boards.net to ask (or maybe even start by searching the boards).

Yep @solderdude

 

[amirm]

For output impedance of an amp mattering, the frequency response needs to have variations. Those variations need to be broad before they are audible. Otherwise, they may just hit one note and hence, not be audible across much of the music out there. So it is important to look at the shape of the frequency response of the headphone to make judgements.

 

[pwjazz]

For output impedance of an amp mattering, the frequency response needs to have variations. Those variations need to be broad before they are audible. Otherwise, they may just hit one note and hence, not be audible across much of the music out there. So it is important to look at the shape of the frequency response of the headphone to make judgements.

@solderdude's measurements show a 2.5 dB mid-bass boost at a 120 ohm output impedance, so the HD660S is definitely sensitive to output impedance. The thing that's not clear to me is that he then recommends staying below 10 ohm, and I just don't know if that's an educated guess or if he also measured these with a 10 ohm output impedance device.

 

[Edelweiss]

@solderdude's measurements show a 2.5 dB mid-bass boost at a 120 ohm output impedance, so the HD660S is definitely sensitive to output impedance. The thing that's not clear to me is that he then recommends staying below 10 ohm, and I just don't know if that's an educated guess or if he also measured these with a 10 ohm output impedance device.

The 1/8 Rule exists to keep the tonal impact below 1 dB, which is pretty much inaudible. Solderdude most likely just wanted to say "Stay away from tube amps." A tasteful increase in mid bass is perceived as pleasant by most people anyways.