So . ...
I have built a little stereo pcl82 amp simple open ended.
It's running off a 240ac to 24dc converter with a buck up converter to obtain 170dc smoothed by a 20w 380ohm resistor and two 100uf capacitors.
I have added a 910M resistor and 1.8uf capacitor in high pass config in place of the volume pot to avoid amplifying anything below 100hz
I am hearing a hum.
Using my phone and an app it seems to be 183hz
Is there anything obvious I should look for?
I have built a little stereo pcl82 amp simple open ended.
It's running off a 240ac to 24dc converter with a buck up converter to obtain 170dc smoothed by a 20w 380ohm resistor and two 100uf capacitors.
I have added a 910M resistor and 1.8uf capacitor in high pass config in place of the volume pot to avoid amplifying anything below 100hz
I am hearing a hum.
Using my phone and an app it seems to be 183hz
Is there anything obvious I should look for?
How is that connected? 910Meg to ground, top via 1.8uF cap to grid?
I don't see a grid stopper. Can you show a scribble?
BTW an up converter is called a boost converter. Buck is down.
Jan
I don't see a grid stopper. Can you show a scribble?
BTW an up converter is called a boost converter. Buck is down.
Jan
Yes, add one for stability.
R2 is much too high, usually the grid leak is between 100k and 470k. This very high resistor you have here makes the grid pick up anything floating around even from power supply wiring (180Hz is found in the power supply circuitry with 60Hz mains).
Is that R really 910Megohm? And the cap is it 1.8pF or 1.8uF?
Don't put me on the wrong foot! ;-)
Jan
R2 is much too high, usually the grid leak is between 100k and 470k. This very high resistor you have here makes the grid pick up anything floating around even from power supply wiring (180Hz is found in the power supply circuitry with 60Hz mains).
Is that R really 910Megohm? And the cap is it 1.8pF or 1.8uF?
Don't put me on the wrong foot! ;-)
Jan
Hi absolutely the input filter is avoid below 100hz being amplified as my subwoofer fills in that area. Cheers Ian
Unless the input is sort of floating and can pick up anything radiated.
Not saying it is, but a distinct possibility.
Get the low hanging fruit first.
Did you see that input network and the values ... ?
Jan
Not saying it is, but a distinct possibility.
Get the low hanging fruit first.
Did you see that input network and the values ... ?
Jan
Hi absolutely the input filter is avoid below 100hz being amplified as my subwoofer fills in that area. Cheers Ian
910k and 1.8 nF gives a cut-off of 97hz. 1.8 pF gives a cutoff of 97 kHz which I am sure is not what you want!
You're absolutely right, I am incapable of typing the correct values.... It is 910k and 1.8nf cut off 97hz... 8 was aiming for roughly 100hz
Apologies for being so utterly useless in this respect 🙂
Cheers Ian
First thing to do: Ground the input grid. If the hum goes away, it's on the input. If it's still there, it's in the plate circuit.
Since I've played with using SMPS and DC-DC converters with my stuff, what you describe sounds like the SMPS is a noise bomb and it's radiating into the circuitry. Try putting the SMPS on the floor and run the amp with a 6 foot wire. If the SMPS is the source, the noise should mostly go away.
Also, what kind of DC DC board are you using? I've found some of the adjustable ones will oscillate and the effect is a ~200-400Hz howl on the power. The fixed voltage ones (with no feedback loop) don't do that.
Since I've played with using SMPS and DC-DC converters with my stuff, what you describe sounds like the SMPS is a noise bomb and it's radiating into the circuitry. Try putting the SMPS on the floor and run the amp with a 6 foot wire. If the SMPS is the source, the noise should mostly go away.
Also, what kind of DC DC board are you using? I've found some of the adjustable ones will oscillate and the effect is a ~200-400Hz howl on the power. The fixed voltage ones (with no feedback loop) don't do that.
Been doing some testing
1. 2k7 stopper resistor added. No issues left in circuit.
2. Changed r2 to 300k. Hum reduced. Got very odd distortion in sound. Removed.
3. Grounded the input. Hum present as before.
The boost board is adjustable
I will look at testing the smps on a long wire next.
Thanks for helping me
Cheers Ian
1. 2k7 stopper resistor added. No issues left in circuit.
2. Changed r2 to 300k. Hum reduced. Got very odd distortion in sound. Removed.
3. Grounded the input. Hum present as before.
The boost board is adjustable
I will look at testing the smps on a long wire next.
Thanks for helping me
Cheers Ian
Okay it's not the smps.
So I am an guessing that the issue is the boost converter. I think what I will do is get a transformer to supply the HT and remove the boost from the circuit. I'll order one and get back to you all once it's fitted and tested. Cheers Ian
So I am an guessing that the issue is the boost converter. I think what I will do is get a transformer to supply the HT and remove the boost from the circuit. I'll order one and get back to you all once it's fitted and tested. Cheers Ian
Which boost converter?
See this post: https://www.diyaudio.com/community/...ns-for-tube-preamp.315604/page-2#post-7054686
See this post: https://www.diyaudio.com/community/...ns-for-tube-preamp.315604/page-2#post-7054686
Ya... I've found those to be noisy trash in my experience (I still have a couple that I'd give away). Still better than the one I had imagined though https://www.ebay.ca/itm/255028122842
Good for charging a capacitor or for a tube tester, though.
If you change the voltage while it's powered up, the frequency of the hum will probably change.
The one I linked from Ali is isolating, the two others are not.
Good for charging a capacitor or for a tube tester, though.
If you change the voltage while it's powered up, the frequency of the hum will probably change.
The one I linked from Ali is isolating, the two others are not.
This device regulates by on/off. Try a resistor at the output ( 100 ohm to start with ) and see how your hum reduces.