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PostPosted: Mon May 13, 2013 10:43 am 
Old School Grand Master
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Joined: Sun Jan 06, 2008 2:36 pm
Posts: 4007
Location: uk
Sounds very interesting..........i like headphones amp or should i say good ones, portable ones seem very popular on ebay and static /homes one have quite a strong following, with sales to back that up.........there is some 'pretty' crap out there, i know a lot of enthousiasts like valve stuff, but lots on a budget go for the cheap chinese stuff that seems very unreliable build wize..............what headphoes you using with it ?


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PostPosted: Mon May 13, 2013 11:37 am 
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Joined: Thu Oct 13, 2011 9:30 am
Posts: 958
Location: Malvern
I have a pair of Bose... errr... somethings that I bought donkeys ago when I was working in the states. They were supposed to be $300 but I got 50% off on a veterans day discount because I was working for MoD at the time. They are far far better that many 'fashionable' headphones, and an also very old pair of Sennheisers, which may or may not be 480's?

A valve headphone amp eh? So people aren't happy with saving poor linearity, poor efficiency and high 2nd order harmonic distortion for driving speakers? ;)

I can see I'll have to seal the cases of my stuff or I'll fall foul of subjectivist cries when they find I use unfashionable parts such as op-amps and (dare I say it...) the occasional capacitor! :D


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PostPosted: Mon May 13, 2013 9:09 pm 
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Location: Malvern
Dissipation sorted - if in doubt, parallel more devices! Changed the first stage to a gain stage, prior to the volume control - gives a small theoretical improvement in noise floor, it's already lower than I can measure. I need to make the second stage a voltage follower as with the gain there there is now a bit too much 'oomph' (even into the 30 ohm Bose!) and it would be easy to cause ears to bleed!

It was easier to move the CD player to the shed than move my shed to the listening room! Sounding good - very strong fast sound. Measures well, so it should do.Maybe I should give it some bling power supplies for street cred :D

Image


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PostPosted: Tue May 14, 2013 3:50 pm 
Gold Trader / MacRetro rider
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Joined: Wed Aug 22, 2007 12:23 am
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apache wrote:
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

Image

I'm not sure I really understand any of the stuff above, however as the new owner of this machine because I had fond memories of my old marantz cd BITD, I thought I would do a wee review from my living room.

First off I have been away from anything remotely hi-fi in a good number of years and my silver discs had all been transferred to my ipod via itunes (f**k bugger arse apple) so I was quite excited to get my crappy cd collection out and see if the mods above would really deliver an upgrade on the sound I remember and have the musicality that Apache mentions when he compared the marantz to his rega.

I have to say that I am very impressed with the thing- my old B&W speakers and Denon amp fairly sing now- I have on at the moment "dark side of the moon" and the sound-stage is really impressive- sounds very smooth indeed and the music just flows along with a real live sound. Overall I need to tweek the speaker positions and stands are probably a must (however the wife says no!) but without a few changes, straight out of the box it's really amazing and just need to get the house to myself more so that I can crank it up to 11, thanks Apache, I'm over the "Moon".


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PostPosted: Tue May 14, 2013 9:16 pm 
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Joined: Thu Oct 13, 2011 9:30 am
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Location: Malvern
Cool. Enjoy rediscoving your CDs!

Just finished tweaking my headphone amp, so I think the core design is done now. I reduced the input gain a bit, and changed the op amps to a low input offset voltage fet type, which allows me to DC couple the whole thing. The output stages are still bipolar types as they have better current drive. Sounds excellent, and I spent an hour listening to it when I should've been designing the power supplies for it!

Haven't done the PSU yet, but will split the regulators between the output stage and the input / buffer stages so the higher current bit can't modulate the power supplies of the voltage amplifier.

<edit>

Getting quite excited now (sad I know) as I've measured no increase in distortion on +/-5v rails (as opposed to +/-15) which means I can power the fet input stage via rail bootstrap, which means you drive the supply rails up and down with the music signal, which in turn reduces common mode distortion (thanks again Doug!). The output stages will be on the raw regulated +/-15v rails. Starting to look like a lot of effort for a headphone amp, but hopefully will be worth it! :)

Hey... +/-5v probably means it's possible to 'portable-ise' it... :o


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PostPosted: Fri May 17, 2013 8:35 am 
Old School Grand Master
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Joined: Sun Jan 06, 2008 2:36 pm
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Location: uk
Hows the tone control coming along ?


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PostPosted: Fri May 17, 2013 8:55 am 
King of the Skip Monkeys
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Posts: 26170
Location: Moomin Valley
My friend has the original Alchemsit Kraken http://www.alchemisthifi.info/ranges/kr ... plifier.ht

It had to be repaired over at The Audiofile in Cambridge after his little daughter shorted the speakers. The repaired side is quite a lot cooler than the original side. I said I would ask on his behalf to see if it was worth re-doing so it was back to its original 72c heat.

Plus, as well as that phono box to send you (when I find the damn thing) , I have a Gale subwoofer amp to send you as it went 'pop' quite unexpectedly. Cheapo circuit board and a big scorch mark...


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PostPosted: Fri May 17, 2013 10:52 am 
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Joined: Thu Oct 13, 2011 9:30 am
Posts: 958
Location: Malvern
Tone control - last of parts arrived (including OPA2134 op amps which may mean I can DC couple it)

Gale sub - maybe just send PCB?

Kraken - probably quiescent current adjustment required, though makes surprisingly little difference as long as there is 'some'. Will research that amp and see what it should be, and possible reasons why its not!


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PostPosted: Mon May 20, 2013 7:23 pm 
Anglian Deputy AEC
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Joined: Mon Oct 09, 2006 6:13 pm
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Location: Livin' in a dust bowl
I have a spare Gale subwoofer you can have for nought.


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PostPosted: Tue May 21, 2013 8:25 pm 
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Location: Malvern
We're nearly there!

Just need to wire up connectors and switches, plug it in and check all OK.

Image

Image


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