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 .
Attachments
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.
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.
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.
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.
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.
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.
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
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
Hi Thomas,
Yes, IMO the Krell is better in the voice range. You should hear a tenor sax through it. Just sweet. If you are building this amp for pure listening enjoyment, you may be happier with the Krell.
We had a large group buy recently on the Leach Low TIM. I haven't really heard much feedback from the other folks who were in on it. I'd really like to hear how well they liked theirs. Like I said, I used a Hafler tranformer in mine and I don't know if that has a big effect on the sound. I was listening to some realy good jazz last night and it was very good through the Leach, but when I woud switch over to the Krell, there would be tlimes when I would have to stop and say, Oooooooo.
It would just grab me.
Can you tell I'm becoming a Krell fan?
Give me until next week and I should be able to report on the Aleph-X too. It is a more difficuly amp to build though.
Blessings, Terry
Yes, IMO the Krell is better in the voice range. You should hear a tenor sax through it. Just sweet. If you are building this amp for pure listening enjoyment, you may be happier with the Krell.
We had a large group buy recently on the Leach Low TIM. I haven't really heard much feedback from the other folks who were in on it. I'd really like to hear how well they liked theirs. Like I said, I used a Hafler tranformer in mine and I don't know if that has a big effect on the sound. I was listening to some realy good jazz last night and it was very good through the Leach, but when I woud switch over to the Krell, there would be tlimes when I would have to stop and say, Oooooooo.
It would just grab me. Can you tell I'm becoming a Krell fan?
Give me until next week and I should be able to report on the Aleph-X too. It is a more difficuly amp to build though.
Blessings, Terry
Dear Terry,
Don't forget to inform me when you finished the AX.
"Know & How"
I know :
"Live simply. Love generously. Care deeply.
Speak kindly. Leave the rest to God"
Don't know how to :
"Live simply. Love generously. Care deeply.
Speak kindly. Leave the rest to God"
You are the lucky one can do it !
Peace be with you !
Thomas
Don't forget to inform me when you finished the AX.
"Know & How"
I know :
"Live simply. Love generously. Care deeply.
Speak kindly. Leave the rest to God"
Don't know how to :
"Live simply. Love generously. Care deeply.
Speak kindly. Leave the rest to God"
You are the lucky one can do it !
Peace be with you !
Thomas
250W Leach amp
I made and have been using a pair of 250W Leach Superamp monoblock for many years already. It was absolutely a struggle to find all the needed parts in Hong Kong and making the boards. The cost of the project was four times higher than the smaller version of Leach so was the difficulty. Because of the size of capacitors (40000uF per rail) and the transformer (1200VA per channel), I had to design a two stages soft-start circuit for them to keep the tranformer inrush and capacitor charging current in check as well as a master-slave controls to allow sequential energization between two mono-blocks, basically a timing control circuit. With all kind of limitation in the place I live, the whole project took me more than half a year to complete. There was also some degree of hazard involved too as the amp operates on +/-85 to 90V rails. Fortunately, with my previous experience with the 120W Leach, I did not blow up anything.
With all the hard work, the result was the most satisfactory. I am not sure how to describe their performance, but I find them sound natural, accurate, linear, fast, good tonal balance, clean and deep bass. The trebles and mid-band are smooth, transparent, never harsh or shouting even at high volume and just as good as any single-end tube amp.
If you really want to build a Leach amp and you can read Chinese, there is a web site in Taiwan selling a very beautifully made DIY kit. I think it is the best place to start your project. You probably knew it already. If not, please check out this link. Unfortunately, I still cannot find any DIY kit for the 250W version.
My3c.net
I made and have been using a pair of 250W Leach Superamp monoblock for many years already. It was absolutely a struggle to find all the needed parts in Hong Kong and making the boards. The cost of the project was four times higher than the smaller version of Leach so was the difficulty. Because of the size of capacitors (40000uF per rail) and the transformer (1200VA per channel), I had to design a two stages soft-start circuit for them to keep the tranformer inrush and capacitor charging current in check as well as a master-slave controls to allow sequential energization between two mono-blocks, basically a timing control circuit. With all kind of limitation in the place I live, the whole project took me more than half a year to complete. There was also some degree of hazard involved too as the amp operates on +/-85 to 90V rails. Fortunately, with my previous experience with the 120W Leach, I did not blow up anything.
With all the hard work, the result was the most satisfactory. I am not sure how to describe their performance, but I find them sound natural, accurate, linear, fast, good tonal balance, clean and deep bass. The trebles and mid-band are smooth, transparent, never harsh or shouting even at high volume and just as good as any single-end tube amp.
If you really want to build a Leach amp and you can read Chinese, there is a web site in Taiwan selling a very beautifully made DIY kit. I think it is the best place to start your project. You probably knew it already. If not, please check out this link. Unfortunately, I still cannot find any DIY kit for the 250W version.
My3c.net
Hi,
I looked at the possibility of biasing Leach into classA. I think the topology will suit it well. Instead I bought Krell Klone for my treble speaker units.
For 100W ClassA I guess you will need about 6 to 8 pairs of 250W output transistors(BJTs) and probably 2 pairs of driver transistors.
The PSU and heatsinks are going to be enormous!
I looked at the possibility of biasing Leach into classA. I think the topology will suit it well. Instead I bought Krell Klone for my treble speaker units.
For 100W ClassA I guess you will need about 6 to 8 pairs of 250W output transistors(BJTs) and probably 2 pairs of driver transistors.
The PSU and heatsinks are going to be enormous!
Andrew, pushpull class A have typical efficiency 25-30 %, so dissipation will be cca 350 W. If you use 3 pair of these transistors, with summary dissipation 1.5 kW, safety coefficient will be in this case very good. Stay on earth.... Different question is, if all have sense, 'cos average output power will be a few watts... ( by maximum driving... )
Different question is, if all have sense, 'cos average output power will be a few watts... ( by maximum driving... )- Status
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