Hi,
I am trying to set the bias up on my AlephJ build and it keeps popping fuses. These are 1.25A slow-blow fuses for a 240v supply specified in the schematic.
The power supply is working fine and is stable giving +/- 25v. However with the board in question I did manage to have it powered up with the 3 CSS BJP transistors in backwards. I corrected the error but I wonder if I have might have damaged the transistors.
So I can either order new transistors or I can try with a bigger fuse. Not sure what the best thing is here.
Thanks,
Graham
I am trying to set the bias up on my AlephJ build and it keeps popping fuses. These are 1.25A slow-blow fuses for a 240v supply specified in the schematic.
The power supply is working fine and is stable giving +/- 25v. However with the board in question I did manage to have it powered up with the 3 CSS BJP transistors in backwards. I corrected the error but I wonder if I have might have damaged the transistors.
So I can either order new transistors or I can try with a bigger fuse. Not sure what the best thing is here.
Thanks,
Graham
What size VA transformer are you using? If larger than the one used in the factory version your inrush current may be blowing the fuses and you may need to upsize your fuses. I had this problem when building mine with a 400VA transformer.
Pics please
And, turn your bias pots all the way down, disconnect both amp boards from the psu, and confirm it turns on ok. Then connect the first amp board, and try again turning it on. Then we’ll know more to troubleshoot accordingly.
Please also describe when the fuses pop. At turn-on, or as you increase Iq/bias?
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Thanks for the quick response
I tried the other board (the left board), which has never been powered up with the transistors in the wrong positions and therefore should be good. The behaviour was identical.
I believe that the fuses blow just a few seconds after power on and I have not been able to take a bias reading.
The rating for the input transformers is 2x250VA. I have now run out of fuses so I will order an assortment of slow blow which will come after Christmas.
I will wind the bias down as suggested. I am assuming that the lowering the value of R27 raises the potential at the source of Q5 and lowers the bias.
Pics attached.
I tried the other board (the left board), which has never been powered up with the transistors in the wrong positions and therefore should be good. The behaviour was identical.
I believe that the fuses blow just a few seconds after power on and I have not been able to take a bias reading.
The rating for the input transformers is 2x250VA. I have now run out of fuses so I will order an assortment of slow blow which will come after Christmas.
I will wind the bias down as suggested. I am assuming that the lowering the value of R27 raises the potential at the source of Q5 and lowers the bias.
Pics attached.
Attachments
If both transformers are being fed through the same fuse then 2x250VA = 500W draw and your 240V x1.25A is only rated for 300W. You should order fuses larger than (500W/240V = 2.083A). Perhaps 2.25 or 2.5A 250V fuses.
I would think it is clever to use two fuse holders, since he in essence only needs the big fuses to start it up successfully. My J uses 2 x 1,6A @ 230 volts. It can take smaller ones, I just didn’t bother to order themIf both transformers are being fed through the same fuse then 2x250VA = 500W draw and your 240V x1.25A is only rated for 300W. You should order fuses larger than (500W/240V = 2.083A). Perhaps 2.25 or 2.5A 250V fuses.
Greenhorns are not allowed to give orders, only advice. Thanks for amplifying the advice
decorated the Christmas tree yet?
I see it clearly before me: OB’s with Christmas wallpaperwith ZM every day is Xmass
Fuses and NTCs all good now. Thank you.
Next issue 🙄
I am struggling to get my AlephJ set up and I think I may have made a build error but I am not sure where. Both channels are behaving the same.
Symptom 1. DC offset is extremely unstable and I have not been really able to zero it. It seems very temperature dependent. It will eventually reach a sort of equilibrium after 20 minutes but it is drifting around +/- 1 volt of so. Any tweak on the pot send the offset flying off and takes a while to restabilize.
Symptom 2. Music plays but there is more hum than music.
I have modelled the circuit in LTSpice in order to diagnose it and the model is well behaved. Measuring around the model in the quiescent state indicates a large discrepancy between my build and the model: expected voltage at Q2 emitter is 270Mv but I am reading 4.27V. (This is with +/- 23V rails wrt GND). I had Q2 in backwards initially but I corrected it so there is a possibility it is damaged. (I was unable to quickly obtain a replacement for that piece).
I have R8 set at 1K on the pot.
Any guidance really appreciated as I am a bit stuck and I am fairly new to DIY and itching to get this amp going, its been a lot of effort. (I know the BJPs are put in horribly, I had to put them in from the top as I could not get the board off the heatsink easily)
Next issue 🙄
I am struggling to get my AlephJ set up and I think I may have made a build error but I am not sure where. Both channels are behaving the same.
Symptom 1. DC offset is extremely unstable and I have not been really able to zero it. It seems very temperature dependent. It will eventually reach a sort of equilibrium after 20 minutes but it is drifting around +/- 1 volt of so. Any tweak on the pot send the offset flying off and takes a while to restabilize.
Symptom 2. Music plays but there is more hum than music.
I have modelled the circuit in LTSpice in order to diagnose it and the model is well behaved. Measuring around the model in the quiescent state indicates a large discrepancy between my build and the model: expected voltage at Q2 emitter is 270Mv but I am reading 4.27V. (This is with +/- 23V rails wrt GND). I had Q2 in backwards initially but I corrected it so there is a possibility it is damaged. (I was unable to quickly obtain a replacement for that piece).
I have R8 set at 1K on the pot.
Any guidance really appreciated as I am a bit stuck and I am fairly new to DIY and itching to get this amp going, its been a lot of effort. (I know the BJPs are put in horribly, I had to put them in from the top as I could not get the board off the heatsink easily)
Please post more photos.
Jumping offset is usually a poor solder joint somewhere.
Yes, the bipolar could be damaged.
Are you using XLR inouts or RCA? If RCA, is -in jumpered to GND?
What’s going on here with the input wiring?
Jumping offset is usually a poor solder joint somewhere.
Yes, the bipolar could be damaged.
Are you using XLR inouts or RCA? If RCA, is -in jumpered to GND?
What’s going on here with the input wiring?
I temporarily unhooked the IN- to GND. I intend to put it back. (I was confused with the IN+nto GND via R2)
Some more pics....
Some more pics....
Just for clarification, are you using BC550 for Q3 and Q4 and ZTX550 for Q2? I read that you've since corrected Q2 (installed backward in these pictures) and it's now installed correctly or is the current state what we see in post #14?
Sorry, I should have clarified - yes the BC550/560 have 'opposite' pinouts compared to ZTX450/550 if you line them up with flat side facing you but if you line them up with the printing facing you, they are the same. Are you sure Q2 is a BC560? I would expect to see writing on the flat side if it were.
