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Old 28th March 2006, 02:39 PM   #1
kvholio is offline kvholio  Netherlands
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Default le monstre questions

Hello all,

i have a le monstre-clone on pcb like this:
Click the image to open in full size.
Click the image to open in full size.

i made a crude powersupply with ordinary bridge-rectifier and 4x 10.000 uf total.
Both channels are fed by this supply in parallel.
Now of course i've got some hummm...not noticeable at my listening-position ,but i'm pretty sure if i used this amp to drive 100 db/w horns it would drive me nuts.
ps-setup right now is kinda messy, but i think i got the basics right: one star-ground point where all ground-connections lead to, equal wire-lengths.

Basically my questions are these:
Why is this amplifier sensitive about its ps ? poor ripple-rejection, grounding-issues ?
How can i overcome this problem in an elegant way ?
Adding enormous amounts of capacity and/or batteries is not an option to me- i like my amp to be moveable

I'd appreciate some input on this, cause i've searched the existing le monstre-threads and found the info there to be somewhat inconclusive/limited.
Questions or remarks about setup are allso welcome of course

ps: sound in this setup is ''promising''. To my ears, midrange-transparancy is already above average, but i know from previous experience it can be a lot better.
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Old 28th March 2006, 02:51 PM   #2
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Default Grounding, grounding, grounding

The Le Monstre is very sensitive to power supply because there is no buffering between the input and other stages. A single supply.
Get a voltmeter that reads 0.0 mv AC. Then start moving your grounds around to exact lengths. A star ground may not be optimal. With proper grounding you can get the measured hum down to 0.1 - 0.2 mv.
Mine is way over the top, 350,000 ufd. Also the input stage will pick up power cord radiated noise, so pay attention to ac power wire layout.
The sound of this circuit can be very good. But it is more fiddly than most. I started with 2 -3 mv of hum, and after months of screwing around got it down to 0.1 mv. My speakers are 100 dB.


George
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Old 28th March 2006, 02:55 PM   #3
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Cool Pi-filter

AFAIK the original le Monstre had a pi-filter which is far more effective than upping capacity.
Personally I would not use a resistor between the powersupply caps but an inductor.
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Old 28th March 2006, 02:58 PM   #4
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A capacitance multiplier (just google for it; also discussed in the ZenV3 article on www.passdiy.com ) is a nice way to remove AC-ripple voltage from the supply: no big cans, not too many parts.
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Old 28th March 2006, 03:12 PM   #5
kvholio is offline kvholio  Netherlands
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Thanks for all the info sofar.

Just some info:without inputs and outputs attached i measure 6.7 mv left, 4.6 mv right.(inputs not shorted)
Should i measure with inputs shorted or open ?

Also im thinking of using 7812/7912 for stabilised ps.
Would this be an option ?



Klaas
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Old 28th March 2006, 03:16 PM   #6
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Default Did not work for me

Quote:
Originally posted by Rodeodave
A capacitance multiplier (just google for it; also discussed in the ZenV3 article on www.passdiy.com ) is a nice way to remove AC-ripple voltage from the supply: no big cans, not too many parts.

I tried a cap multiplier with worse results. The prob lem was ground path length, but adding a cap multiplier made it sound worse and have more hum. Used a nice cicuit with Darlington devices.
A choke would really help here. May make up for your relative low amount of capacitance. Mine is C-L-C-L-C.

George
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Old 28th March 2006, 03:38 PM   #7
kvholio is offline kvholio  Netherlands
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George, what range of L should i be thinking about using ?
I've got quite a range of big aircored inductors for ls-filters, are these useable ?

btw: i CAN go up on capacitance, just dont like the idea of going bezerk in values like > 100.000 uf... (no pun intended)



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Old 28th March 2006, 05:44 PM   #8
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Default Maybe ferrite cored

I always use bobbin cored chokes. Big industrial hunks of iron. The air core ones are much wider bandwidth, but they are used here to pass dc, so bandwidth is your last concern.
A couple milli henries, not micro (u) henry values. And since the LeMonstre runs about an amp, rated for 3 to 4 amperes. Try to use four, two per channel. That is one per rail. Try a pair of your 10k CAPS, then go stereo with four chokes, and four 10K caps on the output. That should be real quiet.
If you can find some nice ones, the higher the value, 6 -8 mH, the more filtering, more energy storage, cleaner dc. Chokes store energy just like a capacitor, so a C-L-C filter has more reserves than just a C-C or C-R-C.
The air core will work fine also. Just usually are much larger and more expensive. For speaker crossovers they are the ticket. For PS filters not worth it.

George
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Old 28th March 2006, 05:58 PM   #9
kvholio is offline kvholio  Netherlands
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thanks again George for you input.
i also have a range of mundorf ferrocores lying around (3.9 to 15 mh) so i guess its okay to experiment with these.
I'll keep you posted on the results

Klaas
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Old 28th March 2006, 06:25 PM   #10
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Quote:
Originally posted by kvholio
thanks again George for you input.
i also have a range of mundorf ferrocores lying around (3.9 to 15 mh) so i guess its okay to experiment with these.
I'll keep you posted on the results

Klaas

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