I will admit, I'm only comparing it to other headphone amps that can be used as preamps, but still...
I have the DDB HPA, an ACP+ HPA, and a Whammy HPA. I find that the music I play through the DDB sounds more dynamic and has more punch. Especially when playing classic and hard rock which are two of my favorite categories.
I have the DDB HPA, an ACP+ HPA, and a Whammy HPA. I find that the music I play through the DDB sounds more dynamic and has more punch. Especially when playing classic and hard rock which are two of my favorite categories.
Hi X,
first let me thank you for the DDB module, it literally saves my otherwise failing project. I will come back when finished with my implementation and measurements.
But I need your help too. While the module powers up and works fine it still does not bias as expected to ~40 mA. I barely get 20 mA, even less actually as it warms up. Both channels are equal. I measured all resistor values on the board, they are ok and as per schematic. The culprit appears to be the reference LEDs. They only develop 1.87 V on them while obviously there had to be somewhere around 2.7 V as follows from your simulation schematic in the PDB thread using LED NSCW100 type. 2.7 V would indeed provide enough reference for output biasing to 40 mA or so. Power rails are ±15 V.
Can you advise please how do I best go about it to bring up the bias. Replace LEDs? Reduce resistor values in CSs or emitter resistors of output BJTs?
Thanks in beforehand!
first let me thank you for the DDB module, it literally saves my otherwise failing project. I will come back when finished with my implementation and measurements.
But I need your help too. While the module powers up and works fine it still does not bias as expected to ~40 mA. I barely get 20 mA, even less actually as it warms up. Both channels are equal. I measured all resistor values on the board, they are ok and as per schematic. The culprit appears to be the reference LEDs. They only develop 1.87 V on them while obviously there had to be somewhere around 2.7 V as follows from your simulation schematic in the PDB thread using LED NSCW100 type. 2.7 V would indeed provide enough reference for output biasing to 40 mA or so. Power rails are ±15 V.
Can you advise please how do I best go about it to bring up the bias. Replace LEDs? Reduce resistor values in CSs or emitter resistors of output BJTs?
Thanks in beforehand!
I am using the Lite On LED with specs showing voltage drop as circa 2v.
https://optoelectronics.liteon.com/upload/download/DS22-2000-228/LTST-C191KGKT.PDF
It might be that your transistors HFe’s are different.
you can cut the value of the large emitter resistors R212 and 213 to half the value (2.2R) to double the current. Or change the resistors on the front end to increase the bias current (preferred way).
Perhaps @jhofland can chime in as to how to fix this.
It might be that R204, 205, 206, 211 could be cut in half to double the current in the front end?
https://optoelectronics.liteon.com/upload/download/DS22-2000-228/LTST-C191KGKT.PDF
It might be that your transistors HFe’s are different.
you can cut the value of the large emitter resistors R212 and 213 to half the value (2.2R) to double the current. Or change the resistors on the front end to increase the bias current (preferred way).
Perhaps @jhofland can chime in as to how to fix this.
It might be that R204, 205, 206, 211 could be cut in half to double the current in the front end?
Thank you @xrk971 for the quick response.
Ok, I see, the LED datasheet says that Vf = 2V is measured at 20mA. Since the LED in the DDB is fed by only ~1.2mA, my measurement of Vf = 1.87V makes sense. I can of course increase the forward current to maybe 5 mA but that would not change much the output bias current.
What did you mean exactly by "change the resistors on the front end"? The 100 Ω resistors R201 and R202? The simulator prompts that these resistors would need to be increased to at least 3 kΩ to get the output bias closer to 40 mA. Would that be right?
Ok, I see, the LED datasheet says that Vf = 2V is measured at 20mA. Since the LED in the DDB is fed by only ~1.2mA, my measurement of Vf = 1.87V makes sense. I can of course increase the forward current to maybe 5 mA but that would not change much the output bias current.
What did you mean exactly by "change the resistors on the front end"? The 100 Ω resistors R201 and R202? The simulator prompts that these resistors would need to be increased to at least 3 kΩ to get the output bias closer to 40 mA. Would that be right?
Hi X,
another happy user of DDB here!
I have found the reason for the low bias (had to change two resistors) and once at it I also took the liberty to adjust few other components to my liking. Those SMDs are so small for hand soldering but the learning curve alone was worth the effort.
The DDB board found its place inside an inferior project from a kit I picked up at Ali/Bay. I was about to take out all reusable components and ditch the rest when I came across the DDB thread. The DDB board dimensions were fitting the space for the amplifier section so perfectly that it looked like being designed just for this case. Miracle! I kept the PSU (modified to CRCRC), the protection, DAC and self-designed input switcher in their places and added the DDB board instead of the former amplifier section. Works now perfectly both as headphone amp and pre-amp.
The photo also shows some odd point-to-point additions on top of the DDB board. Those were necessary for removal of DC offset on the output. Since my DDB board came pre-populated with NE5532, the volume pot could not be connected to the board in the same way as in case of the FET input OPA1642. Rewiring the pot wiper connection to before the input DC blocking capacitor expectedly resulted in higher DC offset. I had negative ca 60 mV, which was in agreement with what simulator had told me. The added positive voltage injection network now keeps the offset within 2 mV.
I also had to retain zobel on the output even though simulator indicated no need for it. I tried the board without the zobel but got small scale local HF oscillation (~80 mVpp) somewhere in the circuit with nothing connected to the output. I did not dig further to find out.
Step response is perfect in my view and the waveform remained undisturbed with any capacitive loads (I tested up to 220 nF). Note however that I am also using 2.7 Ω output resistor which does improve load immunity as well.
The THD measurement was in full alignment with your results - the 2nd harmonic dominates, there is a little bit of the 3rd but the rest stays in "grass". Below is 1 VRMS into 33 Ω:
Since my gear cannot reliably measure HD for higher frequencies, I resort to CCIF IMD. Same 1 VRMS into 33 Ω. Looks good to me:
Overall, very happy result! Lots of new learning and experience. DDB created the best thing possible out of otherwise miserable project.
Thank you very much for this board!
another happy user of DDB here!
I have found the reason for the low bias (had to change two resistors) and once at it I also took the liberty to adjust few other components to my liking. Those SMDs are so small for hand soldering but the learning curve alone was worth the effort.
The DDB board found its place inside an inferior project from a kit I picked up at Ali/Bay. I was about to take out all reusable components and ditch the rest when I came across the DDB thread. The DDB board dimensions were fitting the space for the amplifier section so perfectly that it looked like being designed just for this case. Miracle! I kept the PSU (modified to CRCRC), the protection, DAC and self-designed input switcher in their places and added the DDB board instead of the former amplifier section. Works now perfectly both as headphone amp and pre-amp.
The photo also shows some odd point-to-point additions on top of the DDB board. Those were necessary for removal of DC offset on the output. Since my DDB board came pre-populated with NE5532, the volume pot could not be connected to the board in the same way as in case of the FET input OPA1642. Rewiring the pot wiper connection to before the input DC blocking capacitor expectedly resulted in higher DC offset. I had negative ca 60 mV, which was in agreement with what simulator had told me. The added positive voltage injection network now keeps the offset within 2 mV.
I also had to retain zobel on the output even though simulator indicated no need for it. I tried the board without the zobel but got small scale local HF oscillation (~80 mVpp) somewhere in the circuit with nothing connected to the output. I did not dig further to find out.
Step response is perfect in my view and the waveform remained undisturbed with any capacitive loads (I tested up to 220 nF). Note however that I am also using 2.7 Ω output resistor which does improve load immunity as well.
The THD measurement was in full alignment with your results - the 2nd harmonic dominates, there is a little bit of the 3rd but the rest stays in "grass". Below is 1 VRMS into 33 Ω:
Since my gear cannot reliably measure HD for higher frequencies, I resort to CCIF IMD. Same 1 VRMS into 33 Ω. Looks good to me:
Overall, very happy result! Lots of new learning and experience. DDB created the best thing possible out of otherwise miserable project.
Thank you very much for this board!
Dumb question time from a newbie.
Aren’t there better performing BJT than those older Toshiba ones in the BOM?
Isn’t their transition freq lower than let’s say another older BJT like the BD140?
Or their SMD equivalent? ( I think they are called BCP56?)
Thanks !
If you can find some new old stock of 2SA1837 and 2SC4795, they are better. But the TTA004 and TTC004 are modern BJTs that will perform better than you need. As can be seen in the recent measurements by nonills, it doesn’t get much better.
thanks so much for the replies!
Appreciate it.
One more question.
When increasing the Bias voltage
thanks so much for the replies!
Appreciate it.
One more question.
When increasing the Bias current to 80mA, will this increase the THD a bit, but raise the H2 and lower the higher order harmonics ?
Appreciate it.
One more question.
When increasing the Bias voltage
thanks so much for the replies!
Appreciate it.
One more question.
When increasing the Bias current to 80mA, will this increase the THD a bit, but raise the H2 and lower the higher order harmonics ?
So it will only bring down the higher order harmonics and the H2 will be more dominant.
Not raise the H2 and lower the higher order.
Is that correct?
And would that change be audible in any qualitative way ?
Thanks so much!
Not raise the H2 and lower the higher order.
Is that correct?
And would that change be audible in any qualitative way ?
Thanks so much!
Pardon but another newbie question.
Yeah, that’s a great performance. And nice succession of harmonics.
The THD doesn’t matter too much to me, the above is perfect, but could adding a supercapacitor bank lower the noise floor more and get rid of the small ‘ humps’ below 1khz in the power section ?
Or just adding more capacitance after the voltage regulators would do that ?
I know I know it’s not a DAC to need such a low noise floor. But I do believe in a super low noise floor.
Thanks and forgive me if my questions are tiring.
It’s a combination of things to get noise floor down. Adding caps only helps so much. Physical arrangement of components and where wires are routed make a big difference. Look up “Loop area” for EMI pickup. Basically an amp with wire paths that make a large loop will pickup more noise than ones with smaller loops or loops that are balanced with a bifilar wire of opposite phase.
Using cap multipliers is a big help. About -50dB to be gained there.
Good EMI filters with CMC chokes.
Using cap multipliers is a big help. About -50dB to be gained there.
Good EMI filters with CMC chokes.
Thanks so much for all the answers.
Ok hopefully this isn’t tiring you guys.
But do you hear a difference in the input Caps C111 and C112, if using let’s say wima or a higher end Film Foil like the Audyn or let’s say Mundorf?
Or is it such a small diff you wouldn’t tell in a blind test
Ok hopefully this isn’t tiring you guys.
But do you hear a difference in the input Caps C111 and C112, if using let’s say wima or a higher end Film Foil like the Audyn or let’s say Mundorf?
Or is it such a small diff you wouldn’t tell in a blind test