In article , Arny
Krueger
wrote:
"Jim Lesurf" wrote in message
In article , Arny
Krueger wrote:
The big missing piece in this project is the measured performance.
There's some reason to believe that the transformer, which is rated
at -7 dBU @ 50 might be pushed to over load in some applications.
I think I said in the report that the response was flat with -1dB
points at about 20Hz, 20kHz, and that the distortion level was below
0.01 percent for 0dBFS.
0 dBFS at what frequency?
I did a variety of measurements using various frequencies. See below for
some details.
It appears that the tests were done with a 100 ohm resistor, not
headphones.
Actually I used 100 Ohms, 33 Ohms, *and* headphones as loads. Also did
measurements on the headphones at the same time. But decided not to put all
the details onto the webpage as I didn't want to make it seem overpowering
to someone who was considering just trying making something similar who
didn't have much background in electronic engineering. I also did
measurements with 'no load'. i.e. high load resistance of just the test
kit.
On the webpage I deliberately gave a minimum of test results. Just enough
to let people see how the design/test process worked. e.g. so they could
see how to estimate the impedance effects of the transformer, etc.
32 ohm headphones are very common as are 16 ohm headphones.
Yes. IIRC I mention one of the phones I used were about 18 Ohms, but the
three others I tried all came in around 32-35 Ohms.
I did measurements repeatedly at 20Hz, 35Hz, 500Hz, 1k, and 5k. Did that at
0dBFS and at a few lower levels like -3, -6 and -10dBFS. Also used other
frequencies and levels for various checks.
Overall, the measured distortions were of the order of 0.01 percent or
less. However as you would expect from the point you made the distortion
did rise at LF and high level.
For 33 Ohm resistor loads the distortion at 20Hz 0dBFS was about 1 percent.
This fell below 0.5 percent for -6dBFS 20Hz. It also fell rapidly as I
increased the frequency. So was below 0.1 percent once into 50Hz
territory. I don't have all the results listed as I just watched my
analyser at the time as I played with the test signal, etc. But values in
the 0.01 percent or less range became typical even for 0dBFS for
frequencies in the region above 100 Hz. Didn't plot curves as I wasn't
intending to have them on the webpage for the reason I explain above. :-)
Using 100 Ohm loads *increased* the LF saturation distortion effects. And
going 'open circuit' made them *much* higher!
Worth also saying having detailed the above that the LF distortion
measurements tended to be less readable as I was using an FTT with limited
resolution. I find that active nulling tends to give more accurate results
here as it helps avoid the window spreading effects.
I did also do measurements using the headphones as loads, but they gave
inconsistent values for a predictable reason. They also have nonlinearity
and the source doesn't have zero impedance. Hence I was getting extra
nonliniarity due to the headphone impedances not being linear.
I'd be quite happy to recommend people use better transformers. But I found
these worked OK, and the results sound fine to me. In practice I find I
have the volume wound down, so 0dBFS corresponds to a lower level though
the transformers. This all depends on the choice of headphones and if the
user is a head-banger, though. That said, I doubt head-bangers would be
satisfied with being excluded from their desired territory above 100dBA...
8-]
If you are interested and have other transformers to recommend I'd be
interested in seeing measurements using them, etc. I just used the ones
that were easy for inexperienced home DIYs to get. CPC are an easy source
for people in the UK. But one of my aims was that people might promptly
make 'better' versions their own way. Wasn't trying to make a product, just
to encourage people to dip their toes into the water. :-)
Slainte,
Jim
--
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DIY Headphone DAC
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