Hi everyone,
the first half of my Zen-to-be is finished. More or less, that is. I've attached a photo of it.
The problem is: it doesn't work. It doesn't work at all. More precisely:
DC is at 28.5 V and there is no point in the amp where there is any other voltage but 0 or 28.5. There is no drop over the 0.33 resistor, the drains of the two MOSFETs are at 28.5, the output is at 0. Nothing gets warm either.
Is it that I use a slightly
underpowered transformer at a whopping 5VA? (Yes, I know. I did order a 225 VA one, but the estimated delivery date is somewhen in November. I then ordered another one from Schuro, but that one isn't here yet. This is only for trials.)
Any points where I should begin looking for dead solders or misplaced connections?
Thanks everyone,
Jan
the first half of my Zen-to-be is finished. More or less, that is. I've attached a photo of it.
The problem is: it doesn't work. It doesn't work at all. More precisely:
DC is at 28.5 V and there is no point in the amp where there is any other voltage but 0 or 28.5. There is no drop over the 0.33 resistor, the drains of the two MOSFETs are at 28.5, the output is at 0. Nothing gets warm either.
Is it that I use a slightly
underpowered transformer at a whopping 5VA? (Yes, I know. I did order a 225 VA one, but the estimated delivery date is somewhen in November. I then ordered another one from Schuro, but that one isn't here yet. This is only for trials.)Any points where I should begin looking for dead solders or misplaced connections?
Thanks everyone,
Jan
Attachments
1. Yeah, sure. But of course I haven't triple- and quadruple-checked.....
2. But shouldn't it get closer to working anyways? I mean, if the total voltage dropped to soemthing close to zero I'd understand that it is because of the transformer. But this can't really be due to it, can it?
Thanks for your reply!
Jan
2. But shouldn't it get closer to working anyways? I mean, if the total voltage dropped to soemthing close to zero I'd understand that it is because of the transformer. But this can't really be due to it, can it?
Thanks for your reply!
Jan
Just a couple of SWAGs since I don't have the schematic handy- are the correct Fets in the correct spots? Are they working? pull them out of the circuit and test.
since you have 28V at the drains, it seems like the current source is working - its FET is fully on (or blown/shorted). the gain transistor is not turning on. check the bias connections - is the pot the correct value? (too small and you'll never get enough bias to turn on the gain transistor.) Is the bias resistor the correct value? what is the voltage at the gate of the gain transistor? It should be 4-5 volts (may vary with output devices.)
Hope that this helps.
since you have 28V at the drains, it seems like the current source is working - its FET is fully on (or blown/shorted). the gain transistor is not turning on. check the bias connections - is the pot the correct value? (too small and you'll never get enough bias to turn on the gain transistor.) Is the bias resistor the correct value? what is the voltage at the gate of the gain transistor? It should be 4-5 volts (may vary with output devices.)
Hope that this helps.
Please, what are SWAGs?
Actually, I'd expect 28/2=14V there, at least that's what N. Pass indicates.
But now, everything has changed. I did find a fault, which apparently was the "guilty one". Now it is working much better, the gain transistor gets pretty hot.
The voltage is now at about 13 volts, which seems more reasonable, given the terrific transformer. There is a voltage drop across that resistor too, so I'm pretty happy. However, I'm puzzled that the current source transistor doesn't get warm at all.
Attaching a speaker to the amp results in pretty distorted stuff, but as long as the transformer isn't up to the task, I don't think I should care too much.
One thing however really bugs me: measing the value of the bias pot is very difficult: it is "directional", i.e. measuring from + to - I get a *very* different result as from - to +. When taken out, the pot is just fine. Is this normal behaviour?
Jan
DorcyDuck said:
Scientific Wild A** Guesses - sorry for the slang
yes - if you measure in circuit the pot is in parallel with the gain transistor Gate/Source, which SHOULD measure differently based on polarity
The current source transistor is turning on almost fully trying to get the designed bias, so there is little voltage drop ==> small power dissipation. This will change when you get a proper power transformer.
Yes, you'd expect roughly half the rail voltage at the drains when everything is working. the current source turns on trying to source its design current. if the current is low, it will drive its drain voltage towards the rail and if the current is high the drain voltage will go towards ground.
OK, I am curious, what was the mistake? was it a cold joint of miswiring? waiting a day to check my connections usally helps my prototype board troubleshooting if it isn't something immediately obvious.
Your transformer is woefully underpowerd. That is probably your
main problem right there.
When you test this amp you better have those FETS mounted on
a nice BIG heatsink cause they get HOT fast.
Be sure to have the potentiometer turned all the way down when
you first fire up the circuit.
Wait until you get all the correct parts together before you try to
start troubleshooting the circuit. Makes the process alot less
frustrating! no guessing about this part or that!
I'm sure you can get it working!! It's a fun amp.
main problem right there.
When you test this amp you better have those FETS mounted on
a nice BIG heatsink cause they get HOT fast.
Be sure to have the potentiometer turned all the way down when
you first fire up the circuit.
Wait until you get all the correct parts together before you try to
start troubleshooting the circuit. Makes the process alot less
frustrating! no guessing about this part or that!
I'm sure you can get it working!! It's a fun amp.
Again, thanks everyone for all those replies!
Fuling: I am fully aware of that, but I still have fun!
BobEllis: I simply placed the feedback resistor R8 at the wrong side of the output caps, i.e. really at the output instead before the capacitors.
moe29: No, my problem was this misplaced resistor. That it sounds awful now is just fine with me.
I'm very satisfied now about the amp. Less satisfied that Schuro also is out of the transformer I've ordered. Hrmpf!
I'll let you know about how it works when I finally get a decent transformer.
Jan
Fuling: I am fully aware of that, but I still have fun!
BobEllis: I simply placed the feedback resistor R8 at the wrong side of the output caps, i.e. really at the output instead before the capacitors.
moe29: No, my problem was this misplaced resistor. That it sounds awful now is just fine with me.
I'm very satisfied now about the amp. Less satisfied that Schuro also is out of the transformer I've ordered. Hrmpf!
I'll let you know about how it works when I finally get a decent transformer.
Jan
Just a quick update:
A proper transformer has arrived, the circuit seems to be alright. I can't do any serious measurements because again I've been smarter than I am: I attached the heatsinks with plastic screws in order to have it easy electrically insulating them. What I didn't think of is that plastic tends to melt at high temperatures.. after two screws gone, I decided to wait with further proceedings until I get metal screws. Sigh!
But still, it's fun!
Jan
A proper transformer has arrived, the circuit seems to be alright. I can't do any serious measurements because again I've been smarter than I am: I attached the heatsinks with plastic screws in order to have it easy electrically insulating them. What I didn't think of is that plastic tends to melt at high temperatures.. after two screws gone, I decided to wait with further proceedings until I get metal screws. Sigh!
But still, it's fun!
Jan
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