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Chip Amps Amplifiers based on integrated circuits

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Old 2nd October 2012, 01:11 AM   #1
DMolony is offline DMolony  United Kingdom
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Join Date: Oct 2012
Location: Cambridge UK
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Hi to the DIY audio Forum,

We (me and my housemate - I still feel like a student) decided to build a chipamp, to replace a 1983 and gradually dieing Technics SU-V6 I inherited off my dad (he replaced it with some new thing).

I've recently gained a lot of respect for op amps, having been working on a lot of op amp instrumentation in my day job, so it sounded like a good way to go....

We thought we would give our own minimalistic design a go - we haven't copied any other design from the net, simply made it up...

PSU is a 300VA 2*25Vrms toroid. 2*10,000uF caps, and a single 20A bridge rec. We put 10uF polyprops on each chipamp right next to it, to kill PSU noise, and provide a very nearby energy source for transients. With 105dB PSSR and the multiple caps, I suspect that PSU noise will not be an issue.

We did not use the DC blocking Ci, since:
1) It causes phase shifts in the lower frequencies, and
2) We did not want electrolytics in the signal path. Calculated cutoff for the one we tried was 30Hz, but the phase shift was 90 degrees by that frequency, which is hardly going to do the sound quality any favours, so we shorted it out.

Tests so far show we have:
10Hz to 20kHz (all tests) +/- 0.1dB, THD<1% (can't really measure any lower than that on the 'scope) phase distortion <<1% (0.57deg) DC offset into 8ohm of 9mV one channel, 15mV the other. Noise floor is down at 2mV, so barely audible (if atall).

Listening impressions are excellent. Simply can't fault it to be honest. We have played a range of music; Jack johnson, Lana del Ray,Pink Floyd, Dayna Kurtz, Nick Cave, DeadMaus, Aviici, ... and a whole bunch of test tones. Bass is solid, tight, highs are clear, well defined... the thing that strikes you is that there really is no colouration atall. It's not warm, not cold or harsh, nothing resonates... you could say it has no character, but I really like it. I don't really want my hifi to have character. Probably worth mentioning that so far it's only been driven to 4V peak (1Wrms/channel), since by that point it was rather loud, and we couldn't really hear each other over it... and there is still another ~18dB to go?!? Klipsch RB81s are efficient...

Admittedly it's currently being driven by a laptop sound card, but it is a very good laptop sound card; noticeably better than any other laptop/Iphone/other phone/mp3 player I've heard. Maybe I ought to get a better quality source, but I struggle to convince myself there is much point given the general standard of music recording/processing is way below its capabilities.

I'll get more pics posted up when we have stuck it in a box and tidies it up a bit... until then, any comments welcome!

Hope that's OK for a first post.

Dave
2734 - phase distortion at single frequency - should be a straight line
2736 - phase distortion in music...
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Last edited by DMolony; 2nd October 2012 at 01:22 AM. Reason: Removed a photo that added nothing except a good look at my jeans and living room...
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Old 2nd October 2012, 02:29 AM   #2
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Join Date: Jan 2012
Location: Canada
Interesting, but pardon my ignorance as I might have missed something... Which chip are you using for this? The LM3875, LM3886, or the LM1875? And do you have a schematic handy as I am a novice to this and I can't really follow your detailed description (it sounds like a nice side project for me later on if it is easy enough).

Cheers!
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Old 2nd October 2012, 08:13 AM   #3
DMolony is offline DMolony  United Kingdom
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Join Date: Oct 2012
Location: Cambridge UK
lm3886. sorry...
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