Mini mono Zen - pic and Q

[DorcyDuck]

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

[Fuling]

1: Have you checked and doublechecked every single connection?
Wiring errors are not exactly uncommon when building veroboard prototype stuff.

2: 5VA is *slightly* underrated, yes Maybe if you turn the bias down to 100mA or so...

[DorcyDuck]

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

[Fuling]

At the risk of sounding like a moron, but...
Have you tried adjusting the bias trimmer?

[DorcyDuck]

No, you don't sound like a moron. Mind you, this is my first power amp at all!
But yes, I've tried playing around with the bias pot. It turns nice and smoothly. Oh, whether anything else changes? Not at all.

Men tack för idén

Jan

[DorcyDuck]

In case anyone is interested: further pictures are here.

[BobEllis]

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.

[DorcyDuck]

Just a couple of SWAGs

Please, what are SWAGs?

since you have 28V at the drains, it seems like the current source is working

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

[Fuling]

I don´t want to spoil your fun in any way, but you cannot expect anything as long as you power it from a 5VA transformer.

[BobEllis]

Originally posted by DorcyDuck


Please, what are SWAGs?

Scientific Wild A** Guesses - sorry for the slang

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?

[/B]

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.