Benchmark DAC1 now available with USB

[audioengr]

Quote:

Originally Posted by EliasGwinn /img/forum/go_quote.gif I'd be interested in hearing the dual-active configuration, but I can't see how it would make a difference aside from having a signal with twice the magnitude. Plus, they would have to be driven from a source with very low (near zero) output impedance. The problem with using the DAC1 for such an application is the output drivers are designed to drive a line input device, hence the 60-ohm output impedance. With headphones, this output impedance will cause the damping factor to drop significantly, causing ringing, overshoot, and all sorts of distortion based on driver resonances. Let me elaborate on this further... Damping factor is a ratio of [load impedance vs. source impedance]. In the case of the HPA2, the output (source) impedance is less then 0.11 ohms. Therefore, the damping factor with 300-ohm headphones will be near 3000. The damping factor with a 30-ohm output impedance and 300-ohm headphones will be 10. The damping factor with 60-ohm outputs and 300-ohm headphones will be near 5. As you can see, we lose the tight control over the drivers which is essential for the reactive load which a speaker presents. Another problem with a significant (>1-ohm) source impedance: the headphones have frequency-dependant resistance. Therefore, the voltage divider that is created between the source impedance and the load impedance is now a frequency-dependent voltage divider. This means the frequency response is going to be anything but flat. I would be hesitant to use this type of configuration. In fact, I don't recommend driving headphones with any source with an output impedance >1 ohm to drive headphones at all. With that being said, it will not hurt the DAC1 to do this, so feel free to experiment with this setup without worrying about causing any damage. My philosophy is...if it sounds good to you...go for it! However, don't evaluate the quality of the DAC1 based on driving headphones with the XLR outputs. Thanks, Elias

Elias - actually there are several advantages to balanced output drivers for headphones. Aside from the standard balanced advantage of common-mode noise rejection, the power supply di/dt delivery to the drivers will be balanced out and therefore power decoupling of these drivers will be easier. For instance, providing di/dt current for the BUF634 is particularly difficult and the bass usually suffers because of the lack of direct ground-return path for the output current. There are some modding tricks that can improve this a lot, but it would be much better to make it balanced IMO. the BUF634 is not a bad part, just difficult to feed.

Steve N.

Electrical Engineer/chief scientist
Empirical Audio

 

[schaqfu]

I alternate between HD 650s and Shure E500 IEMs with my DAC1-USB, but the E500s are way too sensitive to be much use. Even with the -10Db jumpers on the headphone output enabled, it's way too loud past the 4th detent. I am wondering what is the preferred way to further attenuate volume output from the headphone output? Should I use an in-line hardware attenuator on the wire itself, or should I reduce the wave output volume in Windows? Any ideas? I know Elias has said if we're going to use any software volume reduction it should be the Windows volume slider because it's the best implemented without degrading the quality. But will I be degrading the quality more with that or with an in-line attenuator?

Thanks for any ideas.

 

[schaqfu]

One more question... I note that there are two jumper pins that control the 10Db headphone output gain reduction, J8 and J9. I'm curious, does each one control a different headphone jack? And if so, is it possible to leave only one with the pin on, and one with the pin off, and have different gain settings on each of the headphone jacks? That would be really cool because it would allow me to plug in different resistance headphones into different jacks and get closer to the same volume with each.

 

[JimP]

X

 

[lowmagnet]

Quote:

Originally Posted by schaqfu /img/forum/go_quote.gif I'm curious, does each one control a different headphone jack?

Nope, I pulled just one last week. Got one -10dB channel and one -0dB channel.

Having tried a pair of 16Ω ER-6i IEMs I feel your pain here. (literally, unfortunately)

 

[lowmagnet]

Quote:

Originally Posted by JimP /img/forum/go_quote.gif I think these are old tape transfers so some hiss in background,

Yeah, they do sound nice and dynamic recordings, but the hiss was a big distraction. Also, changing system-wide sample rate for one or two source recordings is kinda a big pain in the butt.

I wonder if I can write an Applescript to change settings quickly. Maybe someone already wrote something.

 

[audioengr]

Quote:

Originally Posted by schaqfu /img/forum/go_quote.gif I alternate between HD 650s and Shure E500 IEMs with my DAC1-USB, but the E500s are way too sensitive to be much use. Even with the -10Db jumpers on the headphone output enabled, it's way too loud past the 4th detent. I am wondering what is the preferred way to further attenuate volume output from the headphone output? Should I use an in-line hardware attenuator on the wire itself, or should I reduce the wave output volume in Windows? Any ideas? I know Elias has said if we're going to use any software volume reduction it should be the Windows volume slider because it's the best implemented without degrading the quality. But will I be degrading the quality more with that or with an in-line attenuator? Thanks for any ideas.

I tried digital volume control on the USB DAC-1 that I just modded. Not bad results at all. Other than this, the feedback resistors on the predriver op-amps must be changed to reduce gain.

 

[schaqfu]

Quote:

Originally Posted by lowmagnet /img/forum/go_quote.gif Yeah, they do sound nice and dynamic recordings, but the hiss was a big distraction. Also, changing system-wide sample rate for one or two source recordings is kinda a big pain in the butt. I wonder if I can write an Applescript to change settings quickly. Maybe someone already wrote something.

Why do you have to change system-wide sample rate in order to play back different sample rate tracks? Doesn't the DAC1-USB take care of that automagically?

 

[lowmagnet]

Quote:

Originally Posted by schaqfu /img/forum/go_quote.gif Why do you have to change system-wide sample rate in order to play back different sample rate tracks? Doesn't the DAC1-USB take care of that automagically?

It's my output sample rate I'm changing, generally kept at 44.1kHz, for obvious reasons. Switching isn't so much a pain as 99% unnecessary. I could set it at 96kHz but Elias mentioned issues with bit-perfect and Mac OS X when doing that. Leopard may fix this.

 

[EliasGwinn]

Quote:

Originally Posted by kool bubba ice /img/forum/go_quote.gif With all due respect, driving my headphones out of the DAC1 XLR output is a much more enjoyable listening experience then using the single ended HPA2. You should try it for yourself. I'm sure you will be surprised by the results. If going balanced with the DAC1 was a bad experience I doubt headfiers would be using the DAC1 as a balanced set up.. I'm sure amps that are made for balanced headphones will sound better cause they are tailor made for that purpose.. But the DAC1 does more then a decent job IMO..

Kool bubba,

I would like very much to hear it. I am very curious. And if you enjoy it, by all means, continue to do so!!

Thanks,
Elias

 

[EliasGwinn]

Quote:

Originally Posted by audioengr /img/forum/go_quote.gif Elias - actually there are several advantages to balanced output drivers for headphones. Aside from the standard balanced advantage of common-mode noise rejection, the power supply di/dt delivery to the drivers will be balanced out and therefore power decoupling of these drivers will be easier. For instance, providing di/dt current for the BUF634 is particularly difficult and the bass usually suffers because of the lack of direct ground-return path for the output current. There are some modding tricks that can improve this a lot, but it would be much better to make it balanced IMO. the BUF634 is not a bad part, just difficult to feed. Steve N. Electrical Engineer/chief scientist Empirical Audio

Steve,

I've read things like this on some websites. However, I can't say I agree with it. First of all, common-mode interference is not a problem with headphone drivers because there isn't a seperate ground path. In other words, if their was a seperate ground path from the headphones, then their may be noise issues due to the new reference. However, without this reference, it will not be manifested in the driver, mechanically or acoustically.

As for the current rating and slew rate (di/dt) of the BUF634, we are not even approaching any limits that would require any additional driving devices.

Thanks,
Elias

 

[EliasGwinn]

Quote:

Originally Posted by schaqfu /img/forum/go_quote.gif I alternate between HD 650s and Shure E500 IEMs with my DAC1-USB, but the E500s are way too sensitive to be much use. Even with the -10Db jumpers on the headphone output enabled, it's way too loud past the 4th detent. I am wondering what is the preferred way to further attenuate volume output from the headphone output? Should I use an in-line hardware attenuator on the wire itself, or should I reduce the wave output volume in Windows? Any ideas? I know Elias has said if we're going to use any software volume reduction it should be the Windows volume slider because it's the best implemented without degrading the quality. But will I be degrading the quality more with that or with an in-line attenuator? Thanks for any ideas.

Schaqfu,

It is much, MUCH better to use the Windows Volume Mixer.

The reason I stress this so much is because an in-line attenuator will raise the source impedance and ruin the frequency response of the headphones. It is very important to maintain a extremely low (<1 ohm) source impedance for the headphone driver.

Thanks!
Elilas

 

[EliasGwinn]

Quote:

Originally Posted by schaqfu /img/forum/go_quote.gif One more question... I note that there are two jumper pins that control the 10Db headphone output gain reduction, J8 and J9. I'm curious, does each one control a different headphone jack? And if so, is it possible to leave only one with the pin on, and one with the pin off, and have different gain settings on each of the headphone jacks? That would be really cool because it would allow me to plug in different resistance headphones into different jacks and get closer to the same volume with each.

Schaqfu,

The jumpers attenuate respective channels (right and left). The attenuation affects both headphone jacks.

Thanks,
Elias

 

[EliasGwinn]

Quote:

Originally Posted by schaqfu /img/forum/go_quote.gif Why do you have to change system-wide sample rate in order to play back different sample rate tracks? Doesn't the DAC1-USB take care of that automagically?

Schaqfu,

Mac's audio software does not dynamically change the sample-rate ever. This is not hardware or software specific (this even applies to Mac's built-in optical ports!!). It is very unfortunate, however, because many times, the user will not know that the OS is performing sample-rate conversion, and the audio suffers horribly because of it.

It is very important to make sure the output sample-rate matches that of the audio when using Mac for audio. This is done in the "Audio MIDI Setup" page.

Thanks,
Elias

 

[jsiau]

Quote:

Originally Posted by audioengr /img/forum/go_quote.gif Aside from the standard balanced advantage of common-mode noise rejection, the power supply di/dt delivery to the drivers will be balanced out and therefore power decoupling of these drivers will be easier. For instance, providing di/dt current for the BUF634 is particularly difficult and the bass usually suffers because of the lack of direct ground-return path for the output current. There are some modding tricks that can improve this a lot, but it would be much better to make it balanced IMO. the BUF634 is not a bad part, just difficult to feed.

Both of the above statements are incorrect. First common-mode rejection is not an issue. The headphone wiring usually provides a balanced connection to the 1/4" TRS plug. The TRS jack on the DAC1 is wired such that the common sleeve connection is wired to the analog ground reference at the opamp that drives the BUF634. There are no additional loads on this ground trace that would induce an interfering signal. In other words, the TRS jack is back-referenced to the analog ground at the BUF634 drivers.

Common-mode rejection is not an issue with either wiring scheme. Headphones have near perfect common-mode rejection as there is no return path for any common-mode signal. The drivers are isolated from the headphone frame (by many MegOhms). A voltage balanced drive offers no advantage, and the drivers have now way of "knowing" that they are being driven from a balanced drive. Headphones do not respond to common-mode voltage.

There are two BUF634s in the DAC1. These are not difficult parts to drive when they are inside the feedback loop of an opamp. In this configuration, the BUF634/Opamp combination has excellent an PSRR (power supply rejection ratio) and is completely insensitive to audio-band noise on the +/- 18 volt power supply rails. In our lab tests we are able to inject a 3 volt AC waveform on top of either voltage rail without measuring or hearing a change in SNR. The BUF634's in the DAC1 have more than sufficient bypassing and do not suffer from any loss of low-frequency response even when driving 30-Ohm loads at the maximum rated output level of the HPA2.

By the way, the BUF634 should never be used in an open-loop configuration. We have seen headphone amplifiers that use BUF634 buffers open-loop and they perform very poorly due to high distortion and poor PSRR.

I can assure you that THD is higher when driving headphones from the XLR outputs of the DAC1. We have done this measurement. The difference is dramatic. Also, you will encounter some change in frequency response due to the fact that no headphone has perfectly constant input impedance over the audio band. There is also no guarantee that the left and right drivers will match each other. Low impedance drive solves these problems.

Driving headphones from the DAC1 XLR outputs may create a warmer sound with some frequency-response and phase-response contouring. These changes may or may not be pleasing, but will definitely be less true to the original. At the end of the day, the increased THD may also add listening fatigue.

Also, please remember that the coils in headphone drivers cannot tell if they are being driven from a single-ended amp or a balanced amp unless there is an additional leakage path back to the source. The voltage and current delivered to the drivers is identical in both cases, and the diaphragm movements will also be identical. Draw the circuit and trace the current paths if you have any doubts.