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Old 9th July 2014, 02:46 PM   #1
semola is offline semola  Italy
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Default Very good Headphone Amplifier

I would like to bring to the attention of the “forumers” experts in design and simulation, (I’m not...) this headphone amp schematic. I found it on a Swedish forum, if I'm not mistaken, and dates back to 2007. The author is “MCZ”. Thing that struck me, in addition to the low number of active components, used with skill, of course, are the real performance measured, that in my opinion nearly perfect. Never seen distortion curves, which in this case curves are not, so low and constant v.s. frequency! Note that were obtained with a 32 Ohm load, and then pushing the circuit supplying power of some importance for a headphone amp. Spectacular then, the distortion curve at 1 kHz v.s. output power that reaches, to the power of 500mW the remarkable value slightly less than 0.0003 % which translates, if not mistaken, in - 110dB.... In the latter case the analysis band is wide 30kHz, while in others it is 80kHz. This scheme is the latest variant of a version with MOSFETs as output devices which has slightly worst performance on the test bench. I must point out that, although they are indicated in diagram as smd transistors, in reality, the prototype measured, is made with “through hole” components, so that is indicated as BC856 should be a BC557C and BC846 a BC547C. Note the ordinary quality of output transistors used, which suggests that the circuit could be further optimized using the best (fast?...) performing transistors. At this point my invitation to the experts who will be left with curiosity, trying to run a simulation of this circuit to see if everything is ok or there is any stability problem with reactive loads that the measure did not detect with pure resistive load. In this regard, there is mention in the swedish forum in a 1.2 MHz oscillation detected in the first prototype, using long cables for loading and solved, with a capacitor placed in "right place" .... It would be interesting to see the outcome of the simulation bandwidth, square wave response, power supply rejection ratio and especially the slew rate. Personally, given the circumstances and the relative simplicity and low cost of implementation, I think it would be very interesting for those who have the expertise and knowledge about, to investigate thoroughly on this project, which has amazing performance, detected moreover, and it is a rather rare case, with a instrument, the Audio Precision, of great prestige.

Greetings to all
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File Type: jpg THDvsFRQ-New.jpg (73.3 KB, 134 views)
File Type: jpg THDvsPWR-New.jpg (72.2 KB, 131 views)
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Old 9th July 2014, 06:58 PM   #2
oshifis is offline oshifis  Hungary
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I am concerned about the THD vs. power curve. If it is higher at low power and falling with increasing power, it indicates crossover distortion and masking of low level audio information.
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Old 9th July 2014, 09:13 PM   #3
jcx is offline jcx  United States
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CCS Class A output won't have crossover distortion

THD+Noise - always rises to the left as Noise becomes a larger fraction of the decreasing signal level

you can reduce the noise by lengthening your analysis time, Geddes points out how to average many fft for much lower noise floor to explore low level linearity

the circuit is pretty much Self blameless Class A - Doug's book covers most of the design issues

sharing the ccs between input diff pair tail and VAS can give slew rate asymmetry

I believe the beta enhanced VAS can benefit the whole amp more with 2-pole compensation rather than simple Miller

and even at headphone levels the output may benefit from a added driver/buffer Q rather than just the single EF hanging off the VAS

Self plotted Sziklai /CFP output in his Class A amp section, his "Trimodal" amp is quite a bit more complex
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Old 9th July 2014, 10:23 PM   #4
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Originally Posted by jcx View Post

the circuit is pretty much Self blameless Class A - Doug's book covers most of the design issues
That's what I thought too.
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Old Yesterday, 12:44 AM   #5
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There is missing a "saturation" protection of Q3. In case of negative clipping or short circuit collector current may grow up to undesirable levels. If I'm not wrong it is covered in D. Self book too.
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