Leach Amp Modification with Mosfet

[thomasfw]

Is it possible to use the Power Mosfet in Leach Amp ?

Thomas

[rajeev luthra]

You will have to change the output stage from current driven to voltage driven , you can also give higher voltage to the output mosfets than the rest of the circuit , I am attaching a diagram and I think this should work , only see that the drive voltage from the vas is at least 7volts .

[Leolabs]

But does anyone try this before???

[BobEllis]

Leach mentioned on his site that he tried it and did not like the sound.

If you want to try it, change the resistors in the Vbe multiplier to allow a bias adjustment range of 3-5 volts if you use vertical MOSFETs like the IRFP devices mentioned. You may be able to use lateral output devices without changing anything.

You'll lose some voltage swing due to the Vgs of the output devices. So, unless you run the front end and VAS at higher rails than the output stage you will lose some power compared to the original bipolar design on the same output rails.

[thomasfw]

Thanks you for your information !
How about adjust it into Class A Amp
with the orginal design ?

Thomas

[BobEllis]

Ensure adequate heatsinking and turn the bias pot clockwise. you may want to add output device pairs.

Adequate can be quite large, depending on how high you want to go with class A bias. See the KSA50 thread for some ideas - although if you keep the rails around 60V, you'll want more than the KSA50 guys use. My modified A75s run 60 V output rails and have 40 fin 14"x10"x1.5" sinks per channel. This allows me to bias up to about 45 watts class A, with 200W dissipation and about 30C rise above ambient. You probably wouldn't be able to get that high on these sinks with only two pairs of output devices - watch junction temperatures.

[thomasfw]

Dear BobEllis & All,

Actually I am stand at the cross road to chose making Leach Amp with high bais, Krell 50 or Alpeh 2 !

Are any one has experience ?

Thomas

[BobEllis]

I know at least one person has built a Leach, Krell and AX, perhaps Terry AKA Still4given can comment. His Leach amps are standard bias, though.

I have 4 channels of Leach (standard bias) and 4 of Pass/Thagard A75 (reduced bias to ~10W class A, but 60V rails). To my ears, the A75 is a little sweeter and more detailed sounding, with better bass control and power. The difference is not great (barely audible), and I am perfectly satisfied with my Leach amps in my bedroom system.

[thomasfw]

Would please intoduce Terry !
Please forwards my doubt to him !

Thanks !

Thomas

[still4given]

Hi Friend,

I have not quite finished my Aleph-X. As I was working on the case for it last night I got a chance to listen to quite a bit of music through the Leach and the Krell. The Leach is very detailed. It has really good bass and clear highs but the midrange may be a little too revealing. the Krell is just a joy to listen to. I'm hoping that the AX will be as good or better. The only draw back the Krell is the heat and fan noise. I don't have the Leach biased high. I used a Hafler Transformer in it so I probably can't go a lot higher. I may try bumping it up a bit more just to see if it will smooth out a little. I'd love to hear what someone thinks of a Class A biased Leach. If it works well I might consider replacing the trans in mine.

Blessings, Terry