The headphones look inductive, that's what he means I think. You could try putting a smallish cap across the HPs, say 2n2, that might also get rid of those annoying 'spikes'.
The decoupling is related to the power supply pins of the DAC, not pin7.
Thank you Richard, as usual. A small cap in parallel to the headphones definitely eliminates the spikes. Higher values around 47n seem to remove them more effectively without compromising much high frequencies in my case, but I still see a bit of the sampling ladder and the sound doesn't convince me yet. High frequencies are far from sine and synths tend to sound harsh. I guess this mode needs much more time and research.
Decoupling the supply is not making much of a difference to me.
Decoupling the supply is not making much of a difference to me.
That's encouraging. I tried to ground lift using PNPs, using what I had at hand and avoid using an opamp which was noisy on a previous trial. Connected the negative of the headphones to the ground lift (BD139's emitter), which effectively measured 1.6V from GND. The positive of the HPs to a CCS sinking roughly half current (11mA). I probably did something wrong, the scope shows now only half wave.
One problem with your 'ground lift' circuit is it can't source any current, it can only sink it. Therefore positive half cycles are fine, but on the negative half cycles there's nothing to support the 1.6V reference and it'll collapse. Perhaps this explains the truncation of those negative half cycles in the scope pic. You could fix the BD139's inability to source current with (yet another) current source from VCC to its emitter.
Do you mean something like this? If this goes in the right direction, I don't know if the CCS biasing resistor R4 should be grounded to BD139's emitter, or to GND as drawn in the diagram.
I'll be happy to listen and measure this I-out DC free, hoping to get a working circuit.
But I suspect now that the aggressiveness I hear might come from digital signal integrity. It is quite noticeable as artifacts when digital volume is low, even with an I/V resistor and passive RLC filter... Might be the lack of an USB isolator or buffered I2S streams... trying to find it out.
I'll be happy to listen and measure this I-out DC free, hoping to get a working circuit.
But I suspect now that the aggressiveness I hear might come from digital signal integrity. It is quite noticeable as artifacts when digital volume is low, even with an I/V resistor and passive RLC filter... Might be the lack of an USB isolator or buffered I2S streams... trying to find it out.
When I lower the digital volume I get indeed a nasty sound. It remains to volume changes like a dim replica of the track, distorted like in overdrive. This is very noticeable with 4x oversampling.
The strange thing is I tried with other stacks and I perceive the same thing just dimmer, but anyways I measured the voltage at vref of each chip in the 24x stack. They all show around 0.83V, only one is lower at 0.816V. I grounded each vref pin with a 50ohm resistor, which seemed to deactivate them one by one, but the nastiness did not disappear.
A good thing along the way is that I found out that a 2200uF at power supply brings a lot of life, so I left it installed. A small 1uF in parallel to it helps also a bit with the referred noise.
The strange thing is I tried with other stacks and I perceive the same thing just dimmer, but anyways I measured the voltage at vref of each chip in the 24x stack. They all show around 0.83V, only one is lower at 0.816V. I grounded each vref pin with a 50ohm resistor, which seemed to deactivate them one by one, but the nastiness did not disappear.
A good thing along the way is that I found out that a 2200uF at power supply brings a lot of life, so I left it installed. A small 1uF in parallel to it helps also a bit with the referred noise.
Good news here!
The 'DC sink' circuit works fine on breadboard. With the values on the previous diagram DC went down to 200mV. Lowering CCS resistors to 47ohm it went down to 27mV, which seemed to be optimal because further down it would start increasing again. Also shrinking the CCS saturation helps: increasing the 'CCS bias resistor' from 6.8k to 13k brought finally DC down to -3mV, even though sometimes it comes back to 30mV or so.
The case is there is almost no more constant high DC I suppose this will increase dynamics on the HP membranes and time will show me the actual benefits of it.
I also got quite rid of the annoying noise. Yes... it was 'ringing'. A 100n at supply helped. Then had to solder another 4 along the tower, a 5th didn't make any audible difference for me. I know this makes in total 500n, probably I should get some smaller 10n ceramics and solder 10x along, as @Zoran suggested. Thanks for that advice!
How (un)linear should this expected to be? Does the droop grow when so many chips are stacked? At 0dBFS I'm getting around -6dB@40Hz, while only -14dB@15kHz. That's much more than the normal -3dB@20kHz droop. My scope is quite basic and I'm getting familiar to it, but those are currently my measurements.
Any suggestions to my primitive 'ground lift' @abraxalito ? I'm not sure if I should use diodes instead at the BD136's base.
Have a nice weekend you all.
The 'DC sink' circuit works fine on breadboard. With the values on the previous diagram DC went down to 200mV. Lowering CCS resistors to 47ohm it went down to 27mV, which seemed to be optimal because further down it would start increasing again. Also shrinking the CCS saturation helps: increasing the 'CCS bias resistor' from 6.8k to 13k brought finally DC down to -3mV, even though sometimes it comes back to 30mV or so.
The case is there is almost no more constant high DC
I suppose this will increase dynamics on the HP membranes and time will show me the actual benefits of it.I also got quite rid of the annoying noise. Yes... it was 'ringing'. A 100n at supply helped. Then had to solder another 4 along the tower, a 5th didn't make any audible difference for me. I know this makes in total 500n, probably I should get some smaller 10n ceramics and solder 10x along, as @Zoran suggested. Thanks for that advice!
How (un)linear should this expected to be? Does the droop grow when so many chips are stacked? At 0dBFS I'm getting around -6dB@40Hz, while only -14dB@15kHz. That's much more than the normal -3dB@20kHz droop. My scope is quite basic and I'm getting familiar to it, but those are currently my measurements.
Any suggestions to my primitive 'ground lift' @abraxalito ? I'm not sure if I should use diodes instead at the BD136's base.
Have a nice weekend you all.
Attachments
As regards your ground lifting circuit I can only suggest maybe use an IR LED as a voltage reference (fed from a resistor or CCS with ~5mA) and delete the two BC559s. The anode terminal of the IR LED goes to BD139's base and its cathode goes to GND. The IR LEDs I use are very low noise and have a forward voltage ~1.2V. I think they're 940nm.
Your droop numbers are very suspect, -14dB@15k isn't droop, its a canyon!
Your droop numbers are very suspect, -14dB@15k isn't droop, its a canyon!
Thanks for the suggestion, I'll dig into the IR LED alternative. I suppose that the amount of current injected into the BD136's emitter, aka. ground lift, should be doubled when having the two channels of the HP mounted on it? Also, R3 should be somewhere around 30-22ohm?
I really need to do those frequency response measurements again. It is very strange.
I really need to do those frequency response measurements again. It is very strange.
The saga continues...
The canyon I was having (-11dB @ 15kHz) was mostly due to the CIC digital filter I was using. Unfortunately I don't have proper knowledge to modify it and, without any filters, the 24x I-out design appears very flat to me but a bit of an harmonics festival within the audible range.
White noise on unfiltered NOS 24x (10dB/div)
20kHz on unfiltered NOS 24x / with 80% attenuation (20dB/div)
The way things are, one can clearly hear a 20kHz sine wave–which is probably impossible for me. On the scope it can be seen that loud harmonics (-20dB quieter the 20kHz signal) form at 18kHz and 14kHz. Also, a -40dB peak is visible at 8.2kHz and milder ones down to 4kHz and 2kHz. Probably it's those peaks, rather than the effect of ultrasonic images, what I'm listening to. Or maybe both, as one might be the cause of the other. I also don't know if this is technically called THD or IMD.
When listening to music instead of a sine-wave this effect seems to render into an immersive experience, or a sense of space–maybe similar to a reverb? I'd dare now to say. Infinite harmonics interact with each other, I assume, quite the way our perception works in real life. The drawback however being the loss of sharpness or punch quite needed sometimes, particularly to feel the contour of synthesis or filters. It was maybe my misconception so far that an I-out design would be more transparent in this sense. Unless I'm doing something wrong, I'm left to the idea at the moment that a filter is quite inescapable to mild down 'images' at least a little bit.
The canyon I was having (-11dB @ 15kHz) was mostly due to the CIC digital filter I was using. Unfortunately I don't have proper knowledge to modify it and, without any filters, the 24x I-out design appears very flat to me but a bit of an harmonics festival within the audible range.
White noise on unfiltered NOS 24x (10dB/div)
20kHz on unfiltered NOS 24x / with 80% attenuation (20dB/div)
The way things are, one can clearly hear a 20kHz sine wave–which is probably impossible for me. On the scope it can be seen that loud harmonics (-20dB quieter the 20kHz signal) form at 18kHz and 14kHz. Also, a -40dB peak is visible at 8.2kHz and milder ones down to 4kHz and 2kHz. Probably it's those peaks, rather than the effect of ultrasonic images, what I'm listening to. Or maybe both, as one might be the cause of the other. I also don't know if this is technically called THD or IMD.
When listening to music instead of a sine-wave this effect seems to render into an immersive experience, or a sense of space–maybe similar to a reverb? I'd dare now to say. Infinite harmonics interact with each other, I assume, quite the way our perception works in real life. The drawback however being the loss of sharpness or punch quite needed sometimes, particularly to feel the contour of synthesis or filters. It was maybe my misconception so far that an I-out design would be more transparent in this sense. Unless I'm doing something wrong, I'm left to the idea at the moment that a filter is quite inescapable to mild down 'images' at least a little bit.
