You'll need to purchase an 8 Ohm L-pad to go with your test rig, or make up a reasonably high impedance 10:1 probe to give yourself some headroom.troplin said:
Harmonic distortion measurements
- Thread starter troplin
- Start date
Ok, there we go.
I've made an L-pad with a 10Ω/4W pot and performed the measurements you suggested. I raised the volume until I got visible clipping on the oscilloscope.
Input voltage (after volume pot): 0.958VAC
Output voltage: 5.36VAC
Measurements are attached.
Is this a reasonable point of reference?
One question that was not answered yet and that makes me a bit nervous:
Is is normal that I can hear the sweep sound of the measurement from the amplifier itself?
I've made an L-pad with a 10Ω/4W pot and performed the measurements you suggested. I raised the volume until I got visible clipping on the oscilloscope.
Input voltage (after volume pot): 0.958VAC
Output voltage: 5.36VAC
Measurements are attached.
Is this a reasonable point of reference?
One question that was not answered yet and that makes me a bit nervous:
Is is normal that I can hear the sweep sound of the measurement from the amplifier itself?
Attachments
Yes, that is starting to look like it should. You can see that the troughs of the waves are squashing a bit on the scope trace which implies you are at about 10%THD, and that correlates to your graph. 2nd harmonic dominates, followed by third, which also makes sense. And yes, your transformer will sing.
Ok, next measurement, 1/10 of the volume:
Input voltage: 0.095VAC
Output voltage: 0.58VAC
And two more measurements with input voltage 0.030VAC and 0.010VAC.
Those are starting to look like my initial measurements, i.e. odd harmonics are starting to dominate.
Is that still expected?
Input voltage: 0.095VAC
Output voltage: 0.58VAC
And two more measurements with input voltage 0.030VAC and 0.010VAC.
Those are starting to look like my initial measurements, i.e. odd harmonics are starting to dominate.
Is that still expected?
Attachments
The higher harmonic stuff is looking the same no matter what the volume setting is in these lower output level sweeps. They may not necessarily be generated in the amp, but could be inherent in the test setup. You would need to start playing with the setup to see if you can isolate where the distortion is coming from.
Doc B. said:
I doubt that it's something inherent in the test setup because if I take just the amplifier out of the loop, the picture looks completely different, i.e. almost no distortion (see my earlier post http://bottlehead.com/smf/index.php?topic=10046.msg93097#msg93097).
I don't know what else I could change to find out the source of the distortion.
As I've written earlier, if I use my old iPhone earbuds (TRRS with mic), some tracks sound awfully distorted. I'm not calling myself audiophile, but it's so obvious that even I can hear it clearly. Sure, the phones are bad, but there's no such distortion if I plug them directly into the DAC (which has an internal headphone amp).
Curiously, the effect is not as noticeable with a different pair of (even older) earbuds.
I'd be thankful for any pointer where to search next.
My first measurements (without load) were about the same on both channels. The latest measurements with 10Ohm load are only on the right channel, but since the picture is similar to the first measurement I'd expect that they are also the same on both channels.
But I can repeat them on the left side if you think that this will help.
The good news ist, that I've "solved" the massive distortion with the iPhone earbuds. It's not a problem with the amplifier but with the jack. TRRS jack does not seem to match well with the 6.3/3.5mm adapter. If I pull out the jack a little bit and find the right spot, it sounds ok. I can only guess, but I think that there's no proper ground connection and it connects to the mic instead.
But that also means that I'm back to just numbers and measurements. They still seem to be quite odd (haha the pun) to me, but I've no more actual audible distortions to back this up. I just haven't enough experience to hear such things.
What do you think, would a measurement between the amp stages be meaningful?
OTOH you say that those numbers are normal and to be expected I might also just give up searching and accept it how it is.
But I can repeat them on the left side if you think that this will help.
The good news ist, that I've "solved" the massive distortion with the iPhone earbuds. It's not a problem with the amplifier but with the jack. TRRS jack does not seem to match well with the 6.3/3.5mm adapter. If I pull out the jack a little bit and find the right spot, it sounds ok. I can only guess, but I think that there's no proper ground connection and it connects to the mic instead.
But that also means that I'm back to just numbers and measurements. They still seem to be quite odd (haha the pun) to me, but I've no more actual audible distortions to back this up. I just haven't enough experience to hear such things.
What do you think, would a measurement between the amp stages be meaningful?
OTOH you say that those numbers are normal and to be expected I might also just give up searching and accept it how it is.
