I hooked up the scope. 1k square waves look ok. 20k Square waves show a little ringing on thh leading edge. Another weird thing is that using a light bulb tester, the amp starts normally with not load attached but with a load attached the light bulb goes bright and will not go out until I remove the load. I'm too afraid to try it without the light bulb.
Ha ha ,i ended up with 3 burned FET due to a small piece of solder between the driver(soldering,desoldering for input ballance using a desolder pump)).
It isn't a good thing that this amplifier won't start normally using lower voltage.
I have used 5R6 instead of fuses and i see 15v across the resistors. One fet failed!
Second try without resistors,two fet failed!
Now mine is fully functional.
I will report again with the results.
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Terry, does the scope show something strange when the bulb goes bright?
Is it some static offset or some oscillation?
Not really - just VHex showed very high stability in different conditions. It can be added at the output terminal as an option.
Valery i ask about zobel,why you haven't use zobel in this amplifier?
Question has been answered.🙂
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I know the way i will update the topology for the next version - fully push-pull VAS, utilizing both sides of the input LTP - this way the offset will not fluctuate that much.
R17, R18 = 100R is good, by the way.
C8 may need to be slightly increased for taming that ringing.
R17, R18 = 100R is good, by the way.
C8 may need to be slightly increased for taming that ringing.
I have use a 200R trimmer for idle current adjustment and R21=680R.
The current easily adjusted for 22mV on 0.22R =100mA BUT now i see that this must be 11mV /0.22R or 22mv/ pair.
The current easily adjusted for 22mV on 0.22R =100mA BUT now i see that this must be 11mV /0.22R or 22mv/ pair.
The line goes completely off the screen. I'm guessing very high offset. I'm afraid something may blow is the light bulb is not in place. It starts fine if there is no load on the output.
Are you saying we should be setting it for 50mA? 100mA is 22mV across one 0R22 emitter resistor. I can't get it that low with the 200R trimmer and 620R resistor. Maybe with 680R like Thimios used.
We need to figure out why it won't start with a load attached.
Sons of Vhex
A first test.
1)Offset isn't stable but vary continuously between 0-15 mV.when input is shorted.
2)This amplifier isn't quiet,i see 1mv A.C output when input is shorted.
3)It is stable,i see no ring no oscillation up to 50 khz, the square wave is good.
4)Idle current is rock stable.
A first test.
1)Offset isn't stable but vary continuously between 0-15 mV.when input is shorted.
2)This amplifier isn't quiet,i see 1mv A.C output when input is shorted.
3)It is stable,i see no ring no oscillation up to 50 khz, the square wave is good.
4)Idle current is rock stable.
Attachments
Does yours start up ok with a load on the output? Maybe I have something still wrong. I have definite ringing at 20khz.
Hi Terry,when you start this with a load at the out some leds starts latter but i don't think that is a problem.
Why you don't start this up to the vas first?
How will that help? There is clearly no load on it in that case. The amp starts fine if the output is open. When I attach a load it causes the light bulb to burn brightly and stay that way.
Hi Terry,
Can you power your front end up off a different supply?
It sounds like you have a ground problem. Did you return your speaker load to the power supply directly so that the speaker (or load) current doesn't share any conductor with the amplifier?
Best, Chris
Can you power your front end up off a different supply?
It sounds like you have a ground problem. Did you return your speaker load to the power supply directly so that the speaker (or load) current doesn't share any conductor with the amplifier?
Best, Chris