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Old 27th February 2010, 02:18 AM   #11
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Cool Yay!

it works!

i changed my resistor divider from 30k/100k to 30k/30k and the dc offset went away, since the opamp could provide enough voltage for the new bias point. currently i'm listening to the amp in mono form, and i'm using the other half of the AD823 to buffer the "ground" point between the dividing resistors, meaning no output coupling cap on a single supply. the sound is quite good for something i threw together from junkbox components.

i'll have a schematic of what i've hacked together up soon.

~ brad.
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Old 27th February 2010, 07:20 PM   #12
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Post here's what is working...

ok, attached is the circuit which i've breadboarded. it's working with no dc offset between the resistor divider and the source. i have listened two ways:
  1. 470uF output electrolytic: panasonic FC, regular ground is used
  2. other half of ad823 buffers resistor divider groundpoint, no output cap
so far, my impressions are mixed. on the one hand there is a clear difference in bass response without the output cap, even though buffering the groundpoint through the opamp is sub-optimal. on the other hand, using such a low B+ voltage means low headroom, so i can't really make a final judgement at the moment. one thing is for sure, the input 22uF ceramic cap is making the highs too bright. :P

i'm very interested in testing this further. i'll post my results here. please give me your suggestions or comments, if you have any.

~ brad.
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Old 28th February 2010, 03:18 PM   #13
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it seems after working further into the servoed szekeres, i have to say i still prefer the sound of my first build. this is probably more for religious reasons, as i've realized that adding an opamp into the audio chain is leaving the innate simplicity of the szekeres. for that amount of complexity, it'd probably be more worth my while to think about an M^3. and honestly, if any mod to the stock szekeres is going to raise performance, it's the replacement of the source resistor with a fet-ccs, not the removal of the output cap. (IMHO, correct me if i've made a grievous error here.)

so i've decided to stick with my original design, more or less. i'll post the new schematic here soon.

~ brad.
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Old 1st March 2010, 08:56 PM   #14
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listening both my first amp and the servoed design with my sennheisers, it seems to me that the first has a less reserved sound, possibly due to the act of buffering the ground through an opamp in the servoed version. just my attempt at an explanation.

another interesting tidbit, not related to the comparison between topologies, is the fact that the szekeres reveals sonic differences in the source it's driven by. for example, my iriver (wolfson wm8731l) seems to have a bit less detailed sound than my pc line out (???). for instance, beat frequencies between notes played on a synthesizer were only audible using the szekeres with my pc.

i've attached what i feel to be a fairly complete description of my next headphone "amp" project... i've decided to spring for salas shunt regulators as an upgrade to the previously used lm350k series reg, and i've separated the power rails and grounds for each channel to minimize crosstalk without venturing into true dual mono territory.

~ brad.
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Old 8th March 2010, 11:23 PM   #15
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Lightbulb then again, ...

ok, working with the simulations i found discomforting hf instability with my newest design. the frequency response would peak at 1.8mhz, phase response was no better, and since i didn't like the input low-pass filter to begin with, i decided to fix it.

taking note of the base resistor in the ccs of the zen-like (in topology, not simplicity) headphone amplifier, i upped mine to 1k and moved the ccs capacitor to a safer and more useful location. since this fixed the issue at hf, i was able to remove the input lpf.

i've attached the newest revision to this post.

~ brad.
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Old 8th March 2010, 11:49 PM   #16
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ok, here is the new PDF of the design, as well as a first draft BOM, for those who are interested... a part number prefixed with 'd:' is from digikey, with 'm:' is from mouser.

~ brad.
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File Type: txt bom.txt (1.4 KB, 19 views)
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Old 9th March 2010, 05:29 PM   #17
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Keep in mind that if you're using the 100uF cap to fix your HF oscillation issues, that in reality a 100uF cap, which is likely to be an electrolytic will act as an inductor at anywhere near 1.8MHz. You'll need a parallel ceramic or film cap (100nF should do) just to get those high frequencies.

It's been interesting watching your design evolve. Keep it up.


Mike.
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Old 9th March 2010, 08:01 PM   #18
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Quote:
Originally Posted by miket6000 View Post
Keep in mind that if you're using the 100uF cap to fix your HF oscillation issues, that in reality a 100uF cap, which is likely to be an electrolytic will act as an inductor at anywhere near 1.8MHz. You'll need a parallel ceramic or film cap (100nF should do) just to get those high frequencies.
actually the modification of the base resistors in the ccs fixed the hf issue, but bypassing the 100uF cap with a film cap can't hurt. thanks.

Quote:
Originally Posted by miket6000 View Post
It's been interesting watching your design evolve. Keep it up.
thanks. i'm very glad my postings are of use.

~ brad.
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Old 9th March 2010, 10:56 PM   #19
Gordy is offline Gordy  United Kingdom
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Usually a lot more people interested than posting. Such is life with forum projects.
Good luck with it.
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Old 10th March 2010, 02:01 AM   #20
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thanks.

well, i'm looking at heatsinks currently... my last design had the FET and CCS mounted via mica spacers to the case, which was barely large enough to act as a decent heat sink. it was a bit more artful than functional...

so my plan here is to use natural convection through the case to cool the FETs, which will be mounted on actual commercially available sinks. it seems that the wakefield 657-15abp should do the job, with R_theta=6.3 K/W under natual airflow. they seem to be capable of handling the 6.6W of dissipation that M101 and M201 will produce, right?

~ brad.
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