Luxman L-405 : how to split pre-from out stage

Got myself recently a solid vintage pre-amp rather cheap (another time on that) which I am going to refurbish totally. Now, as I am looking for companion power amp, I was considering meanwhile maybe to modify a bit my trusty L-405 so it could be used as the power amp as well. Looking at the schematics
I am not sure where would the split of the stages be made... basically after the differential amp I see there the current limiter before the driver stage and the power amp. If I would like to out the output from an external pre-amp, would the connection be made before or after the current limiter?
'would appreciate advice from someone more experienced.
 

rayma

Member
2011-04-29 8:37 pm
I am not sure where would the split of the stages be made.

This integrated amplifier lacks a line stage, so look at the volume control board.
If you don't need a volume control on the power amp, disconnect the A7 cable
and use that point for the power amplifier inputs. Add a 100k resistor to ground
at each input on the amplifier board. Don't attempt to input a signal after this point.

If you do need a volume control on the amp, disconnect cable A5 instead,
and connect the inputs to the volume control board. No extra resistors needed.
 
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This amplifier is designed to operate with what we wrong call a passive preamp ....Actually this is just a selector switch and a potentiometer
There is nothing active to amplify the signal till the amp actually the signal gets a bit attenuated from the source till the input of the amp
The only active stage in the pre area is the phono stage and its work is to amplify the very low phono signal to a line level ( like a tape recorder )

To compensate for the losses of this "passive" net work, the amplifier has a bit of gain more than typical other amplifiers ....also has a quite high input impedance to also compensate with the sources of 100-200mv at the time ....

More gain in the amplifier means more noise ....more ...but no more than an active stage of preamplifier ....

So this amplifier as made is ultra clean has perfect dynamics in low listening levels extremely warm sound with a sweet sonic signature with a very warm tone eventhough outdated as a procedure ....

So advice is to restore it and keep it as was as a second but very dear set ....

there is 2 drawbacks:
one that there is way too much gain in the system as is, and ports behind are designed for a line level of about 150-200mv which means that a modern source like a Cd or a DAC with 2 volt output will make volume knob almost useless and expect 50% of the power to be there in position 2-3 of the volume knob ....
One may attenuate one input for CD /DAC use to compensate with other sources like a tuner which normally is far lower But my opinion is that there is going to be some loss with that in dynamics in low or very low listening level .

Two, the tone control is made very 70's style and is also passive, producing a very dear and warm tone control also extremely low noise ( also nothing active in this area ) but since the tone circuit is nested in the feedback area is actually not very accurate and has high value pots with probably have bad tracking between channels , involving also the trouble that a bad wiring or speaker load may actually modulate through the feedback the all amplifier and produce problems oscillation and bad sound after all

Luxman of that style love easy sensitive speakers and low power listening level At the time that was the target for the specific amplifier At this area the amplifier is extremely friendly up to addictive i 'd say ...

To close the story recap it make the best out of it , there is a few repair tips for this amplifier if you like i can post them here and keep it as was ...

A statistic will say that we repair between 20-40 per year of that line L200 and up For some strange reason 99% of them come here from the original first Owner Even though now days the same owner has a far newer and modern main amplifier, still likes to keep the Lux up and running probably for a second system for the reasons i described above ...

Kindest regards
Sakis
 
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