Leach Low Tim frontend tweak

[gmphadte]

I have assembled a Leach amp to drive my Klipsch RB 81 and am in the process of chasifying(engrish) it. But before completing it, I would like to know if anybody has modified the frontend by incorporating a ccs and can I get the schematic of the same.

Can't wait too long to listen to them through Leach amp

Thanks and regards,

Gajanan Phadte

[AndrewT]

The front end already has a CCS. Each LTP has a CCS tail.

[djk]

WM Leach felt the passive CCS (resistor from a regulated supply) sounded better than an active CCS.

MacIntosh uses a resistor pulled to a very high voltage instead of an active CCS in many of their pieces.

Just because something may measure better, doesn't mean it always sounds more like music.

[Bonsai]

Actually, I did some simulation on zener reference based LTP current generator a la Leach, and a number of the more conventional current sources for LTP use. Below a few KHz, the active current sources are better by about 8dB (in sim 156dB vs 148dB) when I t comes to rail noise rejection. But the Zener based reference rejection performance is flat out to 100's of KHz. The Zener noise issue can easily be fixed with filtering. My view: the Zener current source is more than a match for active current sources, so I would not fiddle with the Leach design in this area.

[gmphadte]

Thanks all for your valuable replies.

I will keep it as Prof. Leach designed.

Gajanan Phadte

[jcx]

you can add resistive bootstrap to the tail R for a huge increase in common mode impedance with just 2 added R (and changing the 2 tail R value)

basically connect a new R from the output (feedback take-off point) to each diff pair, calculate the ratio of the added R with the tail R to equal the feedback ratio

then the "voltage source" providing the "CCS" thru the tail R (Thevenin equivalent) is a blend of the Zener V and the output - which is set to closely match the input common mode V from the positive input/non inverting amp structure

you also have to calculate the parallel R, divided bootstrap V to get the original current setting


I am highly skeptical of people offering advice that circuit change X gives Y sound change in basically good amplifier circuits - amps are way behind source, Loudspeaker, Room, volume setting, EQ, listener's mood, ect. in effect on perceived "sound" – you have to ask how they controlled for any of those effects – ask if they ever actually had 2 complete circuits with only the one variable changed to listen to at the same time – human auditory memory is really poor too

[osscar]

I'm also think that the Leach amp does not need improvement, its sounds good (I have two Leach amps) , you speakers are quite sensitive- there is lot of power in Leach amp. Some believe that Leach sounds better with higher bias than in original article - around 150mA or even 200mA-I can not confirm that.
according to B. Cordell book protection circuit can be a problem in certain circumstances ...but this is not a problem for home listening imho.
amplifier is very quiet-there is no hiss or hum with professor boards.
in my second Leach I place output coil near resistor instead wounding around resistor.

[Bonsai]

"I am highly skeptical of people offering advice that circuit change X gives Y sound change in basically good amplifier circuits - amps are way behind source, Loudspeaker, Room, volume setting, EQ, listener's mood, ect. in effect on perceived "sound" – you have to ask how they controlled for any of those effects – ask if they ever actually had 2 complete circuits with only the one variable changed to listen to at the same time – human auditory memory is really poor too "


I am Absoloutely 100% in agreement with this JCX.

[djk]

Running a service department for a hi-fi store I could do some interesting mods. One time I modded six of the same piece of equipment with a series of mods. I had originally done several mods to one piece all at once and the unit sounded quite different. Doing them one at a time I was able to see what gave the biggest bang for the buck.

[Bonsai]

I still think the room and speakers have a bigger effect.