SMSL SU-1 DAC Question/issue?

[FreDJ]

After good reviews and fine measurements, purchasing an SMSL SU-1 was practically a no-brainer. I'm powering it with a linear 5VDC 1A supply, using it with my decades old NAD 512 CD Player, still working well. The NAD/SMSL combination sounds OK with my Schiit Headphone Amp. BUT I found that volume control adjustment sounds scratchy. This ONLY happens when using the SMSL DAC; other sources don't exhibit this issue. So I wondered if the SMSL DAC has any DC voltage at the output jacks? Took a measurement and YES, I'm seeing 1mVDC on the right channel output and 1.5mVDC on the left channel while idling.
My question is if DC voltage at the SMSL SU-1 output jacks is normal?

Thanks in advance for any help to answer my first post here!

 

[Doodski]

After good reviews and fine measurements, purchasing an SMSL SU-1 was practically a no-brainer. I'm powering it with a linear 5VDC 1A supply, using it with my decades old NAD 512 CD Player, still working well. The NAD/SMSL combination sounds OK with my Schiit Headphone Amp. BUT I found that volume control adjustment sounds scratchy. This ONLY happens when using the SMSL DAC; other sources don't exhibit this issue. So I wondered if the SMSL DAC has any DC voltage at the output jacks? Took a measurement and YES, I'm seeing 1mVDC on the right channel output and 1.5mVDC on the left channel while idling.
My question is if DC voltage at the SMSL SU-1 output jacks is normal?

Thanks in advance for any help to answer my first post here!

Yes, a very low voltage at the output of the DAC is normal. So 1mV to 1.5mV is OK.

 

[AnalogSteph]

A DC offset of this magnitude should not be nearly enough to cause a noticeably scratchy pot. That's like what, -66 / -62 dBFS? If we were talking several hundred mV, that would be a different story. So there may be more to it. Does the scratchiness actually occur under measurement conditions, i.e. at idle?

For S&Gs, I would want to test something - does the scratchiness possibly occur even when the headphone amp's inputs are terminated with shorting plugs or equivalent? Maybe it is the presence of a DC current path or a low source impedance that triggers it.

 

[FreDJ]

I tested with another Schiit headphone amp (different circuit design) and the same happens, yes, at idle. My other source is a Wiim Ultra Streamer. It has no DC voltage at its output jacks--and no scratchiness is heard when pots are turned on either headphone amp. This only happens with the SMSL SU-1 DAC.

A potential issue is over the DAC's DC voltage impacting proper operation of the headphone amps. Currently I don't have schematic diagrams, and I haven't (yet) opened cases to look over circuitry to really get into this. The DC at the DAC's output jacks could be cured by adding coupling caps, but then there's another component in the signal path.

I believe that DC voltages should not be present at the output jacks of any well-designed line level source. Hence my question if this is normal for the SU-1 DAC, or if possibly I received a defective unit.

Thanks again

 

[Doodski]

I believe that DC voltages should not be present at the output jacks of any well-designed line level source. Hence my question if this is normal for the SU-1 DAC, or if possibly I received a defective unit.

Contact SMSL if you don't like our answers.

 

[FreDJ]

It's not a question of not liking answers. I wanted to determine if DC voltage at the outputs is normal or if I received a defective unit.

 

[Doodski]

It's not a question of not liking answers. I wanted to determine if DC voltage at the outputs is normal or if I received a defective unit.

You asked and don't like the answers so contact SMSL.

 

[FreDJ]

Have you measured the SU-1 DAC for any DC output voltage at the output jacks?

As an electrical engineer I can explain why DC output voltages in front of an amplifier, especially a DC-coupled amplifier and even more so if used in front of one capable of amplifying down to 0 Hz, can be problematic.

I first asked the question here out to determine if others observed DC at the outputs of the SU-1. Looks like I will try to contact SMSL and hope that this overseas company will be able to provide an answer.

 

[theREALdotnet]

As an electrical engineer I can explain why DC output voltages in front of an amplifier, especially a DC-coupled amplifier

Does the 1mV DC at the input of the headphone amp turn into 20-30mV (or whatever, depending on amp gain) at the output?

 

[FreDJ]

I've not yet checked for DC (gain-dependent) at the headphone output. Indeed that would be a concern, as you are aware.
I'm still wondering if 1mV-1.5mV DC out from the SU-1 is a normal characteristic of the SU-1 design or if mine is defective. When I initially posted, I hoped to learn this (normal/defective) from others before progressing further.

 

[Roland68]

I've not yet checked for DC (gain-dependent) at the headphone output. Indeed that would be a concern, as you are aware.
I'm still wondering if 1mV-1.5mV DC out from the SU-1 is a normal characteristic of the SU-1 design or if mine is defective. When I initially posted, I hoped to learn this (normal/defective) from others before progressing further.

0.001 or 0.0015 volts DC offset are really no problem and not abnormal. I have measured more with similar devices. My SMSL PO100 AK (almost the same device with XU316 & AK4493, only smaller) has 0.0019 volts on one channel.

What kind of DC offset value do you have with a music signal?

Even 20 millivolts/0.020 volts should not be a problem and should not cause the potentiometer to scratch.
That is really very unusual and I have never experienced it with several hundred devices.

When all devices are connected, please measure the DC and AC voltage (3 digits before the dot) from the ground of the RCA plug of the SU-1 to the protective earth of the socket.
Then please do the same again with the SU-1 and the headphone amplifier (also ground RCA) without any further cables, just with the power supply. The headphone amplifier also has a plug-in power supply without protective earth?

 

[antcollinet]

When I initially posted, I hoped to learn this (normal/defective) from others before progressing further.

Yes

Yes, a very low voltage at the output of the DAC is normal. So 1mV to 1.5mV is OK.

A DC offset of this magnitude should not be nearly enough to cause a noticeably scratchy pot.

And you got confirmation that it is normal/should not cause issues.

You've since had further confirmation.

 

[FreDJ]

0.001 or 0.0015 volts DC offset are really no problem and not abnormal. I have measured more with similar devices. My SMSL PO100 AK (almost the same device with XU316 & AK4493, only smaller) has 0.0019 volts on one channel.

What kind of DC offset value do you have with a music signal?

Even 20 millivolts/0.020 volts should not be a problem and should not cause the potentiometer to scratch.
That is really very unusual and I have never experienced it with several hundred devices.

When all devices are connected, please measure the DC and AC voltage (3 digits before the dot) from the ground of the RCA plug of the SU-1 to the protective earth of the socket.
Then please do the same again with the SU-1 and the headphone amplifier (also ground RCA) without any further cables, just with the power supply. The headphone amplifier also has a plug-in power supply without protective earth?

Thanks for the additional advice. Just earlier I went back to explore/measure further. (I had discontinued using the NAD/SU-1 combination, substituting another CD Player with internal DAC. No 'scratchy pot' issue was observed, 0.000 VDC was measured at the input to the headphone amp; all was well.) Both the headphone amp and the SU-1 utilize 2-prong external linear 'wall wart' power supplies; all audio equipment has 2-prong plugs plugged into a surge suppressor power strip at this location for my headphone listening set-up.

Here's what I uncovered thus far. No DC output voltage was found with a totally unconnected SU-1 powered on. Connected to a powered-on headphone amp, voltage readings still remained at 0.000 VDC. Next the NAD CD Player was connected and powered on, digital coax out to SU-1. Well, now the 0.001 / 0.0015 appears. Can hear a little 'scratchy pot' when headphone volume control is rotated, however after some time it disappears. DCV reading with a music signal varies; I should measure with my oscilloscope instead of digital VOM.

Yes, it is kind of a strange problem. I'm wondering if initial turn-on voltages (CD Player/SU-1 combination) may need to stabilize over time. I can leave everything turned on and measure after a longer period of time. However at this point I do feel better that the SU-1 isn't defective, and that others here observed similar DC voltages on line level outputs.

Thanks for a memorable first post!

 

[Roland68]

Thanks for the additional advice. Just earlier I went back to explore/measure further. (I had discontinued using the NAD/SU-1 combination, substituting another CD Player with internal DAC. No 'scratchy pot' issue was observed, 0.000 VDC was measured at the input to the headphone amp; all was well.) Both the headphone amp and the SU-1 utilize 2-prong external linear 'wall wart' power supplies; all audio equipment has 2-prong plugs plugged into a surge suppressor power strip at this location for my headphone listening set-up.

Here's what I uncovered thus far. No DC output voltage was found with a totally unconnected SU-1 powered on. Connected to a powered-on headphone amp, voltage readings still remained at 0.000 VDC. Next the NAD CD Player was connected and powered on, digital coax out to SU-1. Well, now the 0.001 / 0.0015 appears. Can hear a little 'scratchy pot' when headphone volume control is rotated, however after some time it disappears. DCV reading with a music signal varies; I should measure with my oscilloscope instead of digital VOM.

Yes, it is kind of a strange problem. I'm wondering if initial turn-on voltages (CD Player/SU-1 combination) may need to stabilize over time. I can leave everything turned on and measure after a longer period of time. However at this point I do feel better that the SU-1 isn't defective, and that others here observed similar DC voltages on line level outputs.

Thanks for a memorable first post!

I also briefly connected the PO100 AK to the Topping L30 II and there was absolutely no noise when turning the potentiometer.
Then I connected an old preamp in between, which I know has a 25-30mV DC offset, and it was also dead silent when turning the potentiometer, L30 II at high gain.
Anything else would have surprised me.

"Can hear a little 'scratchy pot' when headphone volume control is rotated, however after some time it disappears." To me that indicates some kind of load, a ground or ground loop problem.

 

[FreDJ]

I also briefly connected the PO100 AK to the Topping L30 II and there was absolutely no noise when turning the potentiometer.
Then I connected an old preamp in between, which I know has a 25-30mV DC offset, and it was also dead silent when turning the potentiometer, L30 II at high gain.
Anything else would have surprised me.

"Can hear a little 'scratchy pot' when headphone volume control is rotated, however after some time it disappears." To me that indicates some kind of load, a ground or ground loop problem.

Would be great to have schematics for these units. That the scratchy pot issue seems to disappear over time makes me think of a capacitor discharging, i.e., initial turn-on bump stored in cap, then it slowly discharges thru the pot, etc.
I could search further for a ground loop, yet I'd expect hum or additional noise to be associated with that issue. In the past I have observed ground loops originating with wall-warts. AC hum in the audio resulted; had to track where/how it originated. Murphy's Law is a thing.
Speaking of schematics, do you know what op amps are utilized in the SU-1? I haven't opened its case.

 

[Roland68]

Would be great to have schematics for these units. That the scratchy pot issue seems to disappear over time makes me think of a capacitor discharging, i.e., initial turn-on bump stored in cap, then it slowly discharges thru the pot, etc.
I could search further for a ground loop, yet I'd expect hum or additional noise to be associated with that issue. In the past I have observed ground loops originating with wall-warts. AC hum in the audio resulted; had to track where/how it originated. Murphy's Law is a thing.
Speaking of schematics, do you know what op amps are utilized in the SU-1? I haven't opened its case.

You can find information about the OPAmp in the SU-1 thread, but also in other places, just use the Google image search and look for opened devices.

The measurements I suggested could quickly provide clarity and only take a few minutes. It is best to do it once when the devices have been running for a while and then again shortly after switching them on. I have already measured very high voltages to protective earth on the housing and on the ground.

 

[FreDJ]

"When all devices are connected, please measure the DC and AC voltage (3 digits before the dot) from the ground of the RCA plug of the SU-1 to the protective earth of the socket." (AC power strip ground socket)
Approx: 41 VAC and 0.01VDC All units on
Approx: 26 VAC and 0.01VDC CD Player off

"Then please do the same again with the SU-1 and the headphone amplifier (also RCA ground side) without any further cables, just with the power supply." (Both power supplies connected; RCA ground side to protective earth).
Approx: 16VAC and 0.01VDC similar for both SU-1 and headphone amp independent/not connected to each other
Both connected together (w/o CD Player) Approx. 26 VAC

Additional measurements:
VAC line to neutral: 122 VAC
VAC between protective earth and 120 VAC neutral: approx. 0019 VAC, reasonably consistent.
VAC between SU-1 5VDC external power supply to protective earth: 15VAC (neg/barrel side)
VAC between Headphone external 12VDC power supply to protective earth: 13 VAC (neg/barrel side)

I note 'Approx' because over time voltages measurements drifted with units running for awhile.

 

[Bwaslo]

Sorry I haven't read through this longish thread, but a thought -- might the problem be input current of the stage just AFTER the pot? If you dc short the input jack rather than connect the dac, does the pot then make scratchy sounds? Maybe series blocking caps are in order?
Again sorry if this has been covered (or may not be correct)

 

[FreDJ]

I'm glad that we can all share with and learn from each other here. Yes, series blocking caps may be a solution to try. Many older audio components made abundant use of coupling caps. As circuity advanced, DC-coupled audio circuits increased; eliminating coupling caps potentially eliminated a source of sonic coloration assuming as appropriate for a given design. However another potential issue is the very small case size and circuit/internal layout of the SU-1 which may not easily allow coupling caps to be added.

Your suggestion to test by shorting is logical. My issue is that I observed the 'scratchy pot' sound only when using with the SU-1 DAC, but not with any other audio component nor when headphone amp's inputs were left open (no source connected). I began to wonder if possibly I received a defective SU-1. Excessive DC seemed like it could be a possible issue, so I started this post to ask if others measured DC at the outputs of the SU-1. BTW, hoping to improve my decades-old NAD CD Player, this is my first adventure with an external DAC.

Others have stated that the DC values I measured are normal, so I believe my SU-1 is not defective. Thankfully this is not a big deal. Yet we're still discussing what may be causes of this rather strange, inconsistent issue. I guess it's just what we audio-types do!

 

[Bwaslo]

What I was suggesting is that dc effects might not be from device outputs but perhaps current into their inputs (with bipolar devices) might also affect scratchiness? Not sure that actually makes any sense, but shorting the preamp input would be a quick test -- a short would look a lot like the output Z of an opamp with only a mV of offset