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I suspect the gate bias network is not adequately decoupled - i.e. C1 is not adequate. You could try adding another electrolytic capacitor, connected from the junction of D1/R2/R1 to the power supply ground.
Are you trying to drive a speaker or headphones ? With only 20V dc supply and a 128:8 output transformer you don't have very much voltage headroom to get a decent power level out of this circuit. Let's imagine that you get 2 x the power rail as a voltage swing since it is choke loaded - so you have +/- 20V which then gets stepped down 128:8 and that results in +/- 20 x 128 / 8 = +/- 1.25V on the load. If my math is right, that's 100mW rms into your speaker. Maybe OK for a small desktop radio.
As a single ended circuit that output transformer should be designed to be happy with 100mA of standing current - i.e. a gapped core. If not, you'll be risking saturating the magnetic core and the output will be distorted.
Looks like a fun project - maybe you can post some photo's of it.
Are you trying to drive a speaker or headphones ? With only 20V dc supply and a 128:8 output transformer you don't have very much voltage headroom to get a decent power level out of this circuit. Let's imagine that you get 2 x the power rail as a voltage swing since it is choke loaded - so you have +/- 20V which then gets stepped down 128:8 and that results in +/- 20 x 128 / 8 = +/- 1.25V on the load. If my math is right, that's 100mW rms into your speaker. Maybe OK for a small desktop radio.
As a single ended circuit that output transformer should be designed to be happy with 100mA of standing current - i.e. a gapped core. If not, you'll be risking saturating the magnetic core and the output will be distorted.
Looks like a fun project - maybe you can post some photo's of it.
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I just finished it and it hasn't been used yet.
It should feel good to use it as a headphone amplifier.
Thank you very much for your explanation. I'll try it!
It should feel good to use it as a headphone amplifier.
Thank you very much for your explanation. I'll try it!
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Well, I'll change the layout. Thank you.
I'll try this method. Thank you.
The circuit is an amplifier. So any small ripple that gets on the gate gets amplified. It can easily become larger at the output (drain) than it is on the supply.
Try feeding the gate bias from a battery, that will give you an idea whether the drain ripple comes from the supply through the bias circuit.
Jan
Hi,
this topology would especially benefit from a more elaborate power supply (employing a smoothing choke).
I would also:
• increase the value of R4 (2.2 kohm), R5 (10 ohm), and C3 while inserting a parallel resistor and a capacitor (10 kohm / 100-220 pF KS).
• plainly implement an RC snubber network across the primary winding (say 1kohm / 470nF PP), instead.
• refer the secondary winding to ground.
Is it a headphone amp?
this topology would especially benefit from a more elaborate power supply (employing a smoothing choke).
I would also:
• increase the value of R4 (2.2 kohm), R5 (10 ohm), and C3 while inserting a parallel resistor and a capacitor (10 kohm / 100-220 pF KS).
• plainly implement an RC snubber network across the primary winding (say 1kohm / 470nF PP), instead.
• refer the secondary winding to ground.
Is it a headphone amp?
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Joined 2009
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The rectifier causes large current spikes in the secondary of the transformer at twice the mains frequency.
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