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PostPosted: Sun Mar 24, 2013 12:59 pm 
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Changed the input stage of my phono preamp design to reduce noise floor of first stage, and increased gain of said stage to improve sn through eq stage. Last stage is now just a unity gain buffer to negate any loading effects on RIAA curve accuracy. Off to the workshop now to breadboard it and try it out. One channel shown only.

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PostPosted: Wed Apr 03, 2013 5:37 pm 
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Just bought another Marantz CD player for a snip from eBay as a modification hack so I dont keep having to cart mine t the shed.


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PostPosted: Wed Apr 03, 2013 5:37 pm 
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Just bought another Marantz CD player for a snip from eBay as a modification hack so I dont keep having to cart mine t the shed.


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PostPosted: Thu Apr 11, 2013 9:24 pm 
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So the Marantz is in bits now, and I've started on mods. The list is :-

1) secure DAC clock crystal to the board, ground and damp the can. Doing this to my CD67 had a quite significant effect.
2) Separate PSU for the analogue stage. This may have a bigger effect on this player as the stage is currently fed from unregulated 15v rails so noise performance is not optimum. I have built a separate regulated +/- 15v psu with its own transformer as there is space for it in the case.
3) Replace the caps in the signal path with new ones, and match the channels by replacing the carbon resistors with lower noise 1% metal film.
4) Screen the uP with RF absorbing material to prevent radiated digital hash getting into the rest of the circuitry.
5) Replace the audio Op-Amp with an LME xxxx (forget the part number - they're in the shed!)

This is the main board from the CD48

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PostPosted: Sat Apr 13, 2013 10:04 pm 
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Well, we're done!

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Mainboard again. Analogue O/P stage replaced with LME49720 op amp, a studio grade audio specific device. Decoupling caps replaced, output DC blocking caps replaced with Silmic (again, and audio grade type) of higher value (100uF instead of 47uF to extend LF further). Wurth Elektronik RF absorber added to CPU, DAC and decoder (DAC and decoder under the board). Clock crystal can grounded, bonded and damped with rubber grommet (crystals are very microphonic - this is a seriously good idea on ANY cd player!). External low noise regulated supply built for LME49720.

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Everything back together again, new supply on veroboard on right, with it's own independent transformer as the one in there doesn't supply sufficient headroom for regulated +/-17v rails. I went for +/-17v as the LME allows it, and a higher voltage swing = better signal headroom / lower distortion.

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Soak testing, spinning The Pixies 'Surfer Rosa' :D

Sounds pretty damned excellent, noticed a smooth almost 'analogue' style of presentation and some seriously good 'attack' and transient ability. Drums on 'Where is my mind' are in the room!

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Just for laughs, this the distortion plot for the LME on a 17v supply. As you can see, at standard CD output voltage level of 2V, the distortion figure is 0.00004%, which is to all intents and purposes, un-measurable, so the analogue stage in this player isn't contributing to the overall distortion performance, which is now down to the DAC. Nice...


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PostPosted: Sat Apr 13, 2013 10:30 pm 
King of the Skip Monkeys
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Location: Moomin Valley
show-off...


:mrgreen:


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PostPosted: Sat Apr 13, 2013 11:08 pm 
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Location: Malvern
:lol:

Confident enough to send me your phono stage yet? :wink:


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PostPosted: Sun Apr 14, 2013 7:24 am 
retrobike rider
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To quote the late great Ian Dury - "There ain't half been some clever b*stards"


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PostPosted: Sun Apr 14, 2013 7:55 am 
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Joined: Sun Jan 06, 2008 2:36 pm
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Location: uk
I have a battery powerd Nico sound labs head phone amp that i use with full size CD player, the twin batteries last for ages, goes loud and is very very well reviewed in say places like rockgrotto.........if you get back on the head amp idea, i'd be interested to see if you could make one as good as his stuff at the same price or better........i've had his main supply amp stuff too, very nice, but i got my portable converted to take RCA plugs and well it sounds a lot better some some stuff i've heard...........


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PostPosted: Sun Apr 14, 2013 8:42 am 
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Location: Malvern
Coincidentally, I have an idea (based on something in one of that straight taking genius, Doug Self's books) for a bit of a unique topology for a headphone amp. Will give it a shot soon and let you know how it pans out.


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