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Old 28th May 2006, 04:16 PM   #1
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Default Grounding MFA Luminescence

I have been fiddling with teh grounding of my early model MFA Luminesence preamp. I think Iíve improved the original, but am interested in ideas and suggestions. Itís a unique design (to my knowledge), and Iíve never seen anything written that quite applies to it.

The design:

Phono stage and line stage. Uses octal tubes (5691, 6SN7, etc) with tube regulated power supply plus added filament regulators.

Separate boxes for power supply and preamp.

The power supply has three transformers--one for B + and two separate filament transformers, each regulated.

The original design has the preamp circuits separated by channel, bussed to a preamp chassis ground. The phono stage is closest to the chassis ground. All this is connected by an umbilical system (actually two umbilicals) that connects filament system and (regulated DC) as well as B+ and ground wires to power supply box.

Right now, the power supply box has a single star ground plus a separate mains ground both to chassis. Everything now connects to the first star, i.e., the power supply caps, the regulator ground and the ground from the preamp box. (Iím not home now, but will try attaching the preamp ground to the regulator ground.

One of the problems is that the main power transformer is very noisy, a defect of these early models. Still by experimenting, Iíve quieted the hum, though Iím not sure Iíve hit on the best solutions.

Any suggestions or general principles for such a design will be appreciated.
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Old 29th May 2006, 03:28 AM   #2
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Actually, I can simplify this. My problem is how to deal with umbilcals, in particular what to do with the ground connection.

1) Connect it to the single star ground?
2) To the ground of the power supply?
3) To its own chassis ground.

I plan to experiment, but I'm interested in any cautions, warnings, observations, etc.
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Old 12th June 2006, 08:28 PM   #3
KBK is offline KBK  Canada
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I JUST, I mean JUST bought the same three chassis Lumi off of ebay for..$106.05!!!! HOLEY SMOKES! WHAT A DEAL!!! (The preamp sells for nearly $5kUS, in ANY condition) No-one recognised it, it was so badly presented. I knew what it was, from a glimpse at the very first photo..and I nearly fell out of my chair. It sat at $29.99 for three days, no views, no bids. I still can't believe it. I'm a huge Bruce Moore fan, and this in my "Dream Preamp". I'm still pinching myself, and will be sweating until it actually arrives.

I'll be modding the crap out of mine, when it gets here.

I'll be able to contribute then.
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Old 13th June 2006, 04:52 PM   #4
kevinkr is offline kevinkr  United States
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Hi Roger,
I think you want to keep the umbilical ground on the power supply star ground. Technically the AC ground should also be connected to this point.

I've forgotten whether your lumi has a bridge or pair of rectifier diodes for the high voltage, but the rectifier negative return (or center tap) should go to the EL input filter cap first and then from there on to the star ground, this will keep high cap charging currents out of the star ground connection.
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Old 13th June 2006, 04:54 PM   #5
kevinkr is offline kevinkr  United States
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I think the 3 chassis lumi is actually later (or possibly earlier??) than the one trombone is talking about, his iirc is a two chassis affair. (I worked on it a lot in the old days, and the modified regulators in his are based on my designs.)
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Old 13th June 2006, 05:51 PM   #6
KBK is offline KBK  Canada
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It's a closed auction, so you'll have to be a member, possibly, to look at it.

The chassis' are missing all three top covers, but they are going to do a good packing job for me, I paid them extra. The open chassis gives you a chance to take a decent look.


http://cgi.ebay.com/ws/eBayISAPI.dll...MEWA%3AIT&rd=1

At that price? I take a loan out and buy at least a few dozen.

That's obviously my pre-amp deal of the decade, and I've got nothing to complain about. It originally sold for $9 or 12k, I believe, depending on year. I think the three chassis was the last one, with very few examples made. As you say, it coulda been the earlier one, as the three chassis may have been too expensive to actually sell to unbelieving folks (with their mouths hanging open-stunned to silence-at the price) in the..what..early 90's, late 80's? I can't remember. It was certainly a contender for the finest tube preamp ever made. Unique in sound,as well. But I want neutral sound, not unique, so I'll be re-vamping where nessessary. No matter what, I'd be hard pressed to find any gear like this, that is so well laid out to - start modifying.
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Old 20th June 2006, 03:17 AM   #7
KBK is offline KBK  Canada
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Default yummy preamp

OK, the beast with three heads (chassis) has arrived...anyone got a schematic?

Or will I have to draw it out by hand? I'd hate to publish it, just to see some cheap *** boards show up on fleabay from the far east. Not fair to Bruce.

I guess either my Magus C or Couterpoint 3.0 will be for sale soon. Gotta keep one of them as a spare.
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Old 21st June 2006, 04:45 AM   #8
KBK is offline KBK  Canada
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OK. One small problem. Damn, I'll come back in again.

Two small problems.

1) click/pop when switching volume level on switched ladder attenuator, but only in left channel, line or phono. Ladder attenuator only. Mostly lower settings. Not reproducable with other switches. Oops. I lied, just checked again. Reproducable with source switching, or level. Sounds a bit like RF or DC to me?

2) I noticed that the tonality is not linear, it depends on exactly what position the combination of left right gain (stepped, resistive) and stereo attenuator are in. I'll likely switch both to a shunt design to keep the loading even as possible. That should cure that weirdness.

The unit is DEAD QUIET. Zero hum, or buzz. This is interesting, as Bruce tends to put the gain control on the input of the line stage,...soo...this means the output section is so darned quiet..it sounds like silence when it's not doing anything. No mute output switch, which is alot of fun when you run fully DC capable 20 amp slo-blo fused power amps like I do. Usually the breaker pops... after the magic smoke gets out.

Gain is ridiculous. It is unusable, outside of having the -20db switch on.

I remember hearing that the Lumi would swing 100V p-p. I'm thinking that this might be true. (depending on loading, obviously)

And DAMN.. this preamp is dynamic. Whoa.

There is a set of RCA connectors on the back of the Phono section box, that says 'loading'. Presumably this is for phono loading. Is it strapped to 47k internally and then you load it down to where you want? Or does the unit need some 47k plugs to work properly?
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Old 23rd June 2006, 05:57 PM   #9
KBK is offline KBK  Canada
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Does anyone care to see extremely detailed photos of the Lumi 3 chassis preamp? It's a relatively rare beast and the chance may not come along again.

Let me know, and I'll post some.
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Old 23rd June 2006, 06:08 PM   #10
kevinkr is offline kevinkr  United States
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Sounds like an offer too good to refuse.. I'm very familiar with the two chassis version, I have never seen the three chassis version.
What is the chassis breakdown?
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