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CMoy based preamp for home stereo?

post #1 of 21
Thread Starter 
Hi

This is my first post here, so bear with me. I recently built my first cmoy headphone amp from a kit I got off ebay. I use it daily to drive my DT770 pro 32 Ohm headphones, which works great. It has 2*470uF 25V power caps, 18V power (2*9V batteries on a homemade adaptor) and AD8620 opamp. Source is rockboxed 5.5g ipod video full of flac files, interconnect is a LOD I got off ebay. Very happy with the results

Last night when going to sleep I got a crazy idea. What if I use the cmoy directly on the power amp part of my NAD C340 and C352 amps? Came home from work today and hooked it up. A bit of a bang came out of the speakers when I turned the cmoy on, but nothing loud enough to worry me. Turned on the ipod and started playing back one of my "reference" tracks; Bruce Springsteen - Vigilante Man.flac and was just blown away. The NAD preamps are jokes compared to the cmoy "preamp". The very subtle details are there, the soundstage is super wide. My B&W 602s have never sounded this good before. My Beovox CX 50 speakers never sounded good, but the cmoy somehow made them sound quite allright for what they are.

So I have decided to build me a "super cmoy" preamp for both the livingroom and office. When I say "super", I mean an optimized circuit with the best components I can get, ultraquiet PSU and maybe an attenuator in stead of the alps pot with power switch that is in my portable one.

Has anyone done something like this? Anything special I need to keep in mind when building it? How do I make the simplest possible input selector while avoiding clicks and bangs when switching sources?

Suggestions on design principles and practical implementation would me most welcome.

And btw, thanks for this great forum, it is truly an amazing source of information and inspiration
post #2 of 21
Of course you can use Cmoy as preamp, with good RCA input/outputs, good volume pot. Maybe you need change gain for better operation. Put opamp on good socket.

Which Ebay kit you use, Chinese? I ask, because in one Chinese Cmoy kit is dangerous thing - no virtual ground schematic, ground is simply split of 2 batterys.
This work good, until both batterys are similar voltage, if for some reason one battery discharge little faster, or you use different brand batterys, you can get large, dangerous DC offset on output.
If you have this type kite, I suggest you add TLE2426 virtual ground, or at least 2 resistors virtual ground.
post #3 of 21
Thread Starter 
The kit I used was sold by a guy in the UK:

Cmoy Zippo Headphone Amp ---FULL KIT OF PARTS --ChuMoy - eBay (item 280370372088 end time Jul-17-09 10:36:20 PDT)

It uses the TLE2426 for virtual ground, decoupling caps and a seemingly good opamp socket. I tested the AD823 and AD8620 chips in it, both work fine with 9 an 18V battery power. I am planning to use this design along with a "better than decent" AC->DC power supply.


I did buy the chinese kit with the 2 battery and no real virtual ground, mostly to study it. It did work quite well with a resistor virtual ground psu which I built on a breadboard, but only as an educational novelty.

Any suggestion on a good and click/pop free input selector? I'd need 4-5 different inputs for CD, DVD, video games and iPod
post #4 of 21
you can use a video switch chip (I'm serious) as your analog a/b/c switch. but its a bit of effort and smd soldering.

you could use a cmos analog switch chip. maybe its good enough. those can be clickless.

you can switch at the spdif level before your dac (for the digital sources, at least).

finally, you can use relays and then just momentarily mute as you make an input transition.
post #5 of 21
Thread Starter 
Any ideas about how to design the muting thing? I want to make it in a way that there are none or very few non intrusive components in the signal path. The whole point of the excercise would be to make it as quiet and simple as possible
post #6 of 21
these days, I'm all about controller chips so I'd so an arduino controller that would take input via a front panel or IR and then have it send a binary address out to a relay board and also have control over a mute relay. switch the mute relay on, do the a/b switching, unmute; for all input switch events.

with a controller, its easy. so I'd go that route. I could help a bit more if you are interested in doing it that way.
post #7 of 21
Thread Starter 
As long as I don't have to deal with surface mount chips, I guess that I could be ok with something like that. My soldering hand isn't stable enough for those small things. Not even after a few beers,

How did they do it before everything was easily solved with some digital gadget?
post #8 of 21
post #9 of 21
Thread Starter 
Quote:
Originally Posted by Zigis View Post
Seems that elfa doesn't appreciate direct linking. What's the part number?
post #10 of 21
Quote:
Originally Posted by Mannevond View Post
How did they do it before everything was easily solved with some digital gadget?
Vacuum tubes. There's a large amount of plans for inexpensive tube-based preamps, too. Yes, I'm a confirmed tubehead.
post #11 of 21
35-490-78
or
35-490-60
post #12 of 21
Thread Starter 
Zigis, excellent! I hafe a few scavenged parts from old equipment, but I'd rather get something that hasn't already collected dust for 30 years. Seems that elfa are going to get an order from me soon
post #13 of 21
Many of today's integrated amps do not really have preamps in them- they just have a volume control and a selector switch- aka a passive preamp. I don't know if the NAD is done that way. Passives can sound very good, but an active preamp stage can do a better job. This is probably what you are experiencing. There may be better preamps out there than a Cmoy, but none for what even a hot-rodded Cmoy will cost.

Happy listening!
post #14 of 21
Quote:
Originally Posted by Mannevond View Post
Has anyone done something like this? Anything special I need to keep in mind when building it? How do I make the simplest possible input selector while avoiding clicks and bangs when switching sources?

Suggestions on design principles and practical implementation would me most welcome.

And btw, thanks for this great forum, it is truly an amazing source of information and inspiration
Use separate op-amps instead of a dual. Pay attention to temperature coefficents when selecting components, esp from different manufacturers. Use bandwidth limiting caps in the feedback loop. Use good decoupling caps (more importantly, have them). If you haven't, consider the use of trimpots / caps to get the best possible tracking between the channels. Don't overdo the PSU.

There is a fair bit of difference between CMOY's, yours seems to be a fairly basic one.
post #15 of 21
Thread Starter 
Quote:
Originally Posted by fzman View Post
Many of today's integrated amps do not really have preamps in them- they just have a volume control and a selector switch- aka a passive preamp. I don't know if the NAD is done that way. Passives can sound very good, but an active preamp stage can do a better job. This is probably what you are experiencing. There may be better preamps out there than a Cmoy, but none for what even a hot-rodded Cmoy will cost.

Happy listening!
I don't understand much of the details in these amps, but it seems fairly obvious to me that there is an active preamp in both of them, in fact quite similar. These amps are both between 8 and 10 years old, the designs could be many years older.

I think the components in these cheap amps are quite much lower grade than those in my cmoy. The opamps I identified are jrc chips, probably of the cheaper kind. Couldn't readily see the exact model upon quick inspection. The same seems to be the case with caps and resistors, which in many places are 5% tolerance.
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