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Old 6th January 2007, 02:02 AM   #1
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Default MFA Lumi - possible to "un-darken" the sound signature?

I wonder if there are any mods to "un-darken" the sound of the Lumi? I am getting very tired of it's charachteristic dark sound, want to open it up, and add brightness. Thx, Mark
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Old 6th January 2007, 08:18 AM   #2
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Why am i not surprised about the darkness? This monstrosity has just too many tubes, stages, coupling caps, wire. No idea how good are the regulators either. A simple first step would be to cut down the tube count by a factor of three

A more realistic approach is to look at the types of coupling and PS decoupling caps and improve on those. This will very possibly bring in some improvement but at a cost.

It's a nice chasis - very tempting to reuse. And yes, i know it's the greatest preamp known to mankind.
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Old 6th January 2007, 02:17 PM   #3
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Default Lumi

Contrary to most postings I have read here and other places, I do not think it is the best preamp known to mankind. However, I find a lot of things that it does very hard to beat. If I could afford AtmaSphere, I wouldn't be using it!

Thanks for your reply. I have tried coupling and interstage caps upgrades, they improve resolution but sonic signature remains exactly the same. Later Lumi C that I used to have (but sold because was offered ridiculously high amount of money) was much more open and brighter with same tube complement but different power supply. I don't want to mod mine to that extent. If it is what it is, I'd rather sell it.

Rgds, M
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Old 6th January 2007, 10:06 PM   #4
kevinkr is offline kevinkr  United States
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I'd recommend selling it and applying the funds to something you like better. Lots of people like the Lumi, and you should be able to get something more suited to your taste.

I'm well acquainted with the sound of same and agree it is very dark sounding, and actually not totally my cup of tea either. For the time though it was one of the best pre-amps available that would not put you in the poor house..

Perhaps consider building an Aikido if you don't need the phono stage.
Or build both... There's lots of good designs here.
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Old 7th January 2007, 03:21 PM   #5
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Default Lumi

Quote:
Originally posted by kevinkr
I'd recommend selling it and applying the funds to something you like better. Lots of people like the Lumi, and you should be able to get something more suited to your taste.

I'm well acquainted with the sound of same and agree it is very dark sounding, and actually not totally my cup of tea either. For the time though it was one of the best pre-amps available that would not put you in the poor house..

Perhaps consider building an Aikido if you don't need the phono stage.
Or build both... There's lots of good designs here.
Thanks Kevinkr

I do need a phono stage, and an MC at that. I was just wondering why it is so dark. May be there is a do-able solution. I know I can sell it in a bit, however I have a very hard time finding affordable preamp which would be in the same league but open sounding. I tried a lot (CAT, PAC, VTL, Counterpoint) but no sigar. The only pre I liked a lot was a solid state Metaxas Opulense, but that thing nearly burned my speakers down (a famous Metaxas fringe benefit!). I'll check out designs available on site. M
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Old 7th January 2007, 03:51 PM   #6
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Default Re: Lumi

Quote:
Originally posted by marknoir
I was just wondering why it is so dark. May be there is a do-able solution.
I don't have a schematic in front of me, but as I recall the Lumi used octal tubes. The grid-plate capacitance in octals is typically about two or three times as high as it is in "comparable" 9-pin tubes. If the grid is driven by a fairly high source resistance, such as by a volume pot or a fairly high preceding plate resistance, then bandwidth can suffer due to the Miller effect. In a balanced design there are neutralization tricks that can be played. However, in a single-ended design the only recourse is to drive each octal grid with a fairly low preceding resistance (leaving feedback out of it for now). This isnít usually possible without changing the entire topology. I donít know for sure if this explains the dark sound of the Lumi, but it might. I agree with Kevin; itís probably better to sell any valuable vintage amplifier that you arenít really happy with rather than to tweak it, ruining its value. Use the funds to build your own. BTW, I think octals are a marvelous choice for new designs, with the above design constraint kept in mind.
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Old 7th January 2007, 04:03 PM   #7
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Good point Brian. I think the input tubes are also run in parallel which only makes things worse.

I recall excessive darkness in my ARC SP-10 clone as well. Ended up getting rid of all those parallel sections.
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Old 7th January 2007, 10:03 PM   #8
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It's always a trade off, paralleling sections reduces tube produced noise that is essentially gaussian in nature in this case by as much as 6dB, however this is at the expense of 4X the miller capacitance of a single triode.

Moving coil cartridges generally can drive quite high capacitive loads without much difficulty, but if you are using a step up transformer the reflected capacitance can be a bit of a problem at higher step up ratios.

I am not sure what kind of tubes you are running in the LUMI, however a lot of people running these report better performance in the phono stage with 5691's in place of 6SL7 and 5692 in place of the 6SN7 in the line section. This may help lighten up the sound a tad.

Depending on version carefully selecting 6DN7/6EM7 may help the sound. Note that you do want to avoid the later coin base types if possible, and IIRC the Sylvania label tubes might have sounded slightly better.

Operating currents are high in most stages and source impedances quite low. I have never heard anyone complain about bandwidth in one of these pre-amps.

Try to contact Trombone or I will direct his attention to this thread as I did a whole lot of mods for his pre-amp and Scott Frankland has also passed along a lot of tips that substantially improve the sonics of this design.

I've worked on a number of Lumis, but unfortunately ceased all commercial activity nearly 7 yrs ago, and my memory on the details of these mods is sketchy. I seem to recall redesigning all of the power supply filament regulators, and the HV supply as well.

Both the line stage and phono stage use global feedback, and in the line stage in particular the margin is rather high. IIRC there is a mod to sub a 6SN7 in place of the 6EM7 which results in about a 10dB reduction in feedback. This should open up the sound a bit I would think.

The Lumi is not that rare (relatively speaking) and doesn't command particularly stellar prices on the used market so given your previous comments I would be inclined to reverse my recommendation and suggest you follow the same path Trombone did. (I was going to recommend one of the more recent versions of the CAT SL1, so we won't go there.. )
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Old 8th January 2007, 01:37 AM   #9
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Smile MFA DARK SOUNDING

HELLO, YOU MIGHT TRY USING ALL PURE SILVER WIRING LIKE KIMBER AG . THAT MAY HELP A BIT? ALSO , HOW MUCH TUBE ROLLING HAVE YOU DONE. MANY 6SN7 TYPE HAVE DIFFERING SONIC SIGNATURES. GOOD LUCK
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Old 9th January 2007, 12:50 AM   #10
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Default MFA

Thanks to all.

I corresponded quite intensively with Scott on this subject and there were no mods specifically to address this problem, except for a complete upgrade to a later "C" schematic with a different power supply and other changes from the early one. I will look for another pre with signature that will be closer to my taste. However, as I said before, there are some things that Lumi does great. As to Lumi not being that rare, beg to differ: every one offered for sale flies out of the window for a ridiculous price (mostly to Asia). So I'll probably stick mine into closet for now. As to building mine from scratch - interesting project, but perhaps over my technical knowledge... And then, what if I don't like it? :-)))
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