Don't buy those, they are counterfeit! There is no such thing as high Idss 2SJ74BLs, they stop at 12.5ma. I purchased a quantity of those and returned them after I tested them, very low Gm and very high Vp.
The total front end current is around 8.5ma so each jfet in your pair should be able to pass that amount. You want the Idss to be around 10ma for each one.
Best, Bill
2SJ74-GR or 2SJ74-V or 2SJ74-BL
Hi,
Do you know anything about The 2SJ74-GR or the 2SJ74-V, or should I just forget about these and Buy the 2SJ74-BL, I have two sets of 4 matching, and dont want to break up a matched four 2SJ74-BL for the CSS on the front end, I want to use these on the next amp I build for 5.1 surround sound movies.
Thank You
Ian
Hi,
Do you know anything about The 2SJ74-GR or the 2SJ74-V, or should I just forget about these and Buy the 2SJ74-BL, I have two sets of 4 matching, and dont want to break up a matched four 2SJ74-BL for the CSS on the front end, I want to use these on the next amp I build for 5.1 surround sound movies.
Thank You
Ian
Look up the 2sj74 datasheet. GR, BL, and V are clearly defined groups. Is the Idss of the j74s you have high enough to use in the front end?
You can use BJTs or mosfets for CCSs, you don't need jfets for that. Sounds like you're in way over your head. There's plenty of help available on the forum, but no one is going to hold your hand all the way through a fairly difficult project such as this. You still have a lot of homework to do on the Passdiy website. No disrespect intended, but you should build a standard Aleph before you attempt an X amp.
Best, Bill
You can use BJTs or mosfets for CCSs, you don't need jfets for that. Sounds like you're in way over your head. There's plenty of help available on the forum, but no one is going to hold your hand all the way through a fairly difficult project such as this. You still have a lot of homework to do on the Passdiy website. No disrespect intended, but you should build a standard Aleph before you attempt an X amp.
Best, Bill
Q2 or J74
Hi, I believe that I have only one more thing to ask about, for which I already have the answer in another thread but I need to verify everything I read b4 I move ahead on the project, do I just use either the Q2 or the J74 on the front end circuit board but not both, yes.
Thanks for your concern about me building this project, I have built many amplifiers without anyone to communicate with and had to resolve everything by myself, I have talked alote about things with just need to hear other peoples thoughts are the same as mine, or different from mine where I am then required to reevaluate my origional conclusions, in other words I ask allot of stupid questions, someone told me once there is no such a thing as a stupid question, and of those my glass is always full.
Thank You For Everything
Ian
(scatterbrain001)
Hi, I believe that I have only one more thing to ask about, for which I already have the answer in another thread but I need to verify everything I read b4 I move ahead on the project, do I just use either the Q2 or the J74 on the front end circuit board but not both, yes.
Thanks for your concern about me building this project, I have built many amplifiers without anyone to communicate with and had to resolve everything by myself, I have talked alote about things with just need to hear other peoples thoughts are the same as mine, or different from mine where I am then required to reevaluate my origional conclusions, in other words I ask allot of stupid questions, someone told me once there is no such a thing as a stupid question, and of those my glass is always full.
Thank You For Everything
Ian
(scatterbrain001)
I am inclined to agree with this statement because I am befuddled by the behavior of my plain Aleph J. It is still under construction but giving me headaches. Built with all new components (all of which are matched and measure to spec) on PD boards, I am getting way too much heat. I'm only idling them to test heat-sinking, not even trying to make music. Papa lists 3 reference measurements:
R8 listed @ 8.3v - I have 9.96
R7 listed @ 4.3v - I have 8.1
R10/R17 listed @ 4.3v - I have 5.55
Power supply - mono 18V 400VA torroids, idling they read 21.x volts.
I am a little suspicious of the 2SJ74BL transistors and have replacements from a different source on the way...
Meanwhile, any and all suggestions greatly appreciated!!!
Frank
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There are no mistakes in the resistors. Again, I also measured to be sure that values were 'matched' on the boards. The real clunker is R7 - it and R8 are CMF501K 1% 1/4 W 25PPM from Digikey. Were could 8.1v be coming from?
Frank
That means your input pair is running at 10mA instead of 8.3mA. First of all, check that the zener diode Z1 is indeed a 9.1V part by measuring voltage drop across R5 - it should read rail minus 9.1V.
By the way, the alephs run hottest during idle as they're ClassA-single ended. Playing music in fact reduces the heat in the amp.
Hannes
Hi Frank,
That strange; see 9.5V across Z1 means voltage drop of 9.5-0.6=8.9 V across R8 and consquently 8.3/1000=8.3 mA for the differential pair. If you still measure nearly 10V across R8 that could mean that there is for some reason too much current in the differential pair which is then out of balance and therefore too much voltage drop across R7.
However, if Z1 is fine I run out of information for debugging
It could be that there is DC-offset pulling down the gate of Q1B which would also cause too much current through R7.
Any case, you will certainly find the problem! Hannes
That strange; see 9.5V across Z1 means voltage drop of 9.5-0.6=8.9 V across R8 and consquently 8.3/1000=8.3 mA for the differential pair. If you still measure nearly 10V across R8 that could mean that there is for some reason too much current in the differential pair which is then out of balance and therefore too much voltage drop across R7.
However, if Z1 is fine I run out of information for debugging
It could be that there is DC-offset pulling down the gate of Q1B which would also cause too much current through R7.Any case, you will certainly find the problem! Hannes
Thanks Hannes. There is no signal entering the amp yet so no DC offset.
I just re-measured on the cold boards and got R7 of 8.45v and R8 of 9.81v. Next step will be to replace the JFETs as soon as the new ones arrive - I don't know what suppliers to trust...
I take this as a learning project and certainly we all learn by making mistakes. So I don't rule out discovering some stupid assumption or misunderstanding in the future.
Cheers,
Frank
PS: One mistake I did not make was to limit the power supply. ...so if all else fails, I can switch to higher rail voltages if required for a different aleph design.
Hopefully unnecessary!
Hi Frank,
Sure that can happen as there is a feedback signal coming in from the output (R4). Don't forget to short the inputs to ground for stable measurements.
8.45V across R7 definitely means that all current is running through Q1A instead of only half of it. There is something wrong around Q1B.
I looked back a few pages and found your initial post; I can't see how wrongly valued resistors for R2 and R4 can cause overheating the jfets and neither why the resistors themselves should run so hot to fail. Definitely a problem somewhere!
I think all you can do is post a schematic with the measured voltages at all points.
Hannes
Sure that can happen as there is a feedback signal coming in from the output (R4). Don't forget to short the inputs to ground for stable measurements.
8.45V across R7 definitely means that all current is running through Q1A instead of only half of it. There is something wrong around Q1B.
I looked back a few pages and found your initial post; I can't see how wrongly valued resistors for R2 and R4 can cause overheating the jfets and neither why the resistors themselves should run so hot to fail. Definitely a problem somewhere!
I think all you can do is post a schematic with the measured voltages at all points.
Hannes
XLR ballanced input
Hi,
Dose anyone know what the order is for the positive, negative, ground, on the XLR male, or female inputs ? I have never had an amplifier with this quality, all of my equipment is RCA jacks for input and output, my next project will be to build a DAC with ballanced outputs.
Thank You
Ian
Hi,
Dose anyone know what the order is for the positive, negative, ground, on the XLR male, or female inputs ? I have never had an amplifier with this quality, all of my equipment is RCA jacks for input and output, my next project will be to build a DAC with ballanced outputs.
Thank You
Ian
thermal runaway
Hi Everyone,
I know there is a component that is attached to the output transistors heat sink and when it reachs a specific tempiture it cuts off the AC, or DC ?. What is this part called and what specifications should it have to be used in my Aleph J-X mono amps ?
I need to add this part because I relying on fans for cooling my amps and I dont have a way of determining if a fan malfunctions, which would result in thermal runaway ?
Thank You For Your Help
Ian
Hi Everyone,
I know there is a component that is attached to the output transistors heat sink and when it reachs a specific tempiture it cuts off the AC, or DC ?. What is this part called and what specifications should it have to be used in my Aleph J-X mono amps ?
I need to add this part because I relying on fans for cooling my amps and I dont have a way of determining if a fan malfunctions, which would result in thermal runaway ?
Thank You For Your Help
Ian
The part is called a thermostat, and is usually fitted to primary side of the transformer, the specification should be able to handle the mains voltage and enough current for the transformer - limit the inrush with CL60 thermistors, the other thing to keep in mind is you probably should use a latching type, not auto reset type.