Vpinchoff combined with Idss will confirm whether the jFET is high gm or low gm.
Idss alone does not confirm the correct TYPE of jFET.
Idss alone does not confirm the correct TYPE of jFET.
Try connecting one channel at a time.
Connect left channel RCA by itself. Does it buzz?
Connect right channel RCA by itself. Does it buzz?
I'm not even close to that point yet. New jfets arrive tomorrow.
And, in the unlikely event that my mosfets have any problems, I have 4 extra of each channel.
Jfets came a day early and are soldered in. I've got the mosfets up to 0.7V right now. I'm confused about the DC offset measurement. Measuring across R12 gets you nothing with that big cap on one side. Measuring on the fet-side of R13 and ground gives you the offset of the voltage at the drain of each mosfet. Isn't that what you want? Measuring across R12 just gives me zero.
measure "inside" output cap , so where two drains meet
point is in symmetry , not output offset literally
but , if you write output node offset , then it's sorta clearer
point is in symmetry , not output offset literally
but , if you write output node offset , then it's sorta clearer
I am measuring inside of C3. The voltage drop across R12 is zero and never changes.
EDIT: I found my answer. Post #45 by 6L6 in the preamp build guide. I was right. Measuring across R12 doesn't give you anything. You just measure from the top of the output cap to ground.
EDIT: I found my answer. Post #45 by 6L6 in the preamp build guide. I was right. Measuring across R12 doesn't give you anything. You just measure from the top of the output cap to ground.
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Let it cook awhile, reset bias and output. Once stable, hook up the outputs! Adjust offset and let it rip!
The single ended is a little funny. The bias boards on the outputs only have one 500r pot. I have two meters monitoring FET voltage and two meters on the speaker outputs. I'm assuming bias is permanently set on the SE amps, and all I adjust for is zero volts at the binding posts?
bias is predetermined with Q113 biasing mechanismus , while P101 is for offset setting
all that ref. to The official BA #1 single-ended output stage , on p2 of BA3 article
all that ref. to The official BA #1 single-ended output stage , on p2 of BA3 article
She's up and away. Letting it cook for a while and monitoring voltages.
Here's a shot of the front. Made the volume knob too small. Just need to polish the new one and put it on.
Here's a shot of the front. Made the volume knob too small. Just need to polish the new one and put it on.
So I get the usual large voltage spike upon powering up the amp. If I put a cap across the binding posts, what value should I use?
She's up and away. Letting it cook for a while and monitoring voltages.
Here's a shot of the front. Made the volume knob too small. Just need to polish the new one and put it on.
Congrats!
what's voltage spike if you're having dummy load across binding posts ?
cap across binding posts is no no - shortie
in series with speaker , maybe ........... but unnecessary , if everything is ok
Mine powers up/down dead silent....it is the complimentary version, maybe different? You used the Diy Store slow start, right?
Russellc
Russellc
The soft start, yes. I have the speaker protection board, but didn't use it. With no load, a meter across the binding posts spikes to 4 or 5 volts as soon as I turn the power on. It calms down in a second or two.
I never measured mine. dc offset was good, so I fired it up on beater speakers. All was fine, so real speakers. If you have some old beater woofers, try it with them, look for cone movement as well as listen for sound....I am assuming this is DC you are measuring? Sorry, I dont know how much dc for how long is detrimental to voice coils of woofer. When I get home I can measure mine at turn on. I get no sound or movement upon turn on.
I had a large tube amp I built that had a good little crack on turn on. I used a wirewound radio shack 10 watt 8 ohm resistor for turn on and off. A Mcintosh repair person sussed out the problem and I no longer needed the resistor. If you do this, it must be removed once turn on is over or no sound of course. I considered installing a switch, but my repairman friend solved problem for me.
Seems like folks talk about the Aleph J having a little turn on thump, is this something single ended amps do? I know dc isnt a good thing on the amps outputs as a general rule...
Russellc
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A single ended amp usually biases the output node to around half supply voltage.
This mode of operation demands that the uncharged output capacitor be charged from zero Vdc to ½Vcc during start up.
That charging current has to flow around a circuit.
Make the speaker part of that charging route and you get a Thump.
It's endemic to single ended.
Much the same at shut down.
This mode of operation demands that the uncharged output capacitor be charged from zero Vdc to ½Vcc during start up.
That charging current has to flow around a circuit.
Make the speaker part of that charging route and you get a Thump.
It's endemic to single ended.
Much the same at shut down.