[Steven Everett]

I’ve bread boarded my Szekeres headphone amp and it sounds good except for a slight hum. My power supply concise of a car battery eliminator (12volt at 3 amp) with 8000mf caps across the output. The power supply says it is filtered. I did some test on the power supply to find out how a load would pull it down. I have some very large resistors (about a foot long) that I hooked across the output and took a voltage reading. Here are the results:
No Load...................16.91 Vdc
416 ohm...................16.75 Vdc
210 ohm...................16.67 Vdc
79.4 ohm..................16.24 Vdc
20 ohm.....................15 Vdc
15.3 ohm..................14.64 Vdc
6 ohm.......................12.78 Vdc

I did not use a regulator and hook up this power supply directly to my amp. I then took some voltage readings:
Power supply reading with amp running…………….15.27Vdc
Voltage across the 20 ohm power resistor…………….6.30Vdc
Voltage across the Mosfet (source to drain)…………..8.90Vdc

I do want to fix the hum but I also included all this information to check with you guys on how I’m doing? This is the first non-kit I’ve built and before I solder it together I wanted to check and see if, other that listening to it, the amp looks okay? Any other test I can do? I also notice that if I put my fingers on the end of the output caps, the two 470mf electrolytic, just right on the can, the hum’s volume goes down a lot.
severett

 

[Apheared]

Hey Steven, see you found the place...

re: supplies, they're rated at load. Notice your 2 amp load is close to the 12V rating. You're fine; this is expected... the question is, where did you end up with a selection of low-ohm footlong power resistors?!

re: your amp measurements; right on.

re: the hum, too many things can do it... ground loop (does it make this buzz with no source connected?) radiated transformer noise (what type of Xformr and how close to the circuit?) etc etc hum infinium.

And yea not regulating could be it. Szekeres needs a pretty stable supply but 8000uF filtering cap should be ok. Do you have any regulators handy? I mean for a buck and a few passives it's really worth it. (rickster will get you to add a 2nd bridge and regulator and run the channels quasi dual-mono, for like $5 or so.)

 

[Steven Everett]

Apheared,
Thanks for showing my where you guys moved to.
I'm glad to see my power supply is up to the task and my voltages are correct.
You ask where I got the long power resistors?
Well, I confess, I am a dumpster diver. I can't help myself. I work for the railroad and everyday, on my break, I walk past the Electric Shop and check their dumpster. Any boxes I find I tear apart and take out whatever I think I might need. Heat sinks, capacitors, aluminum boxes, transformers and power tools have all been found this way. One day there were about 50 Milwaukee resistors (from 3 to 2000ohm) laying there waiting for me to collect. Many were brand new still in the GE boxes. These are used on the Locomotives in a panel board somewhere. I grabbed about 15 of them.

My power supply is located about a foot away from my amp. I get a hum all the time whether the CD player is hooked up or not. As I increase the volume the hum gets louder. I took it out this morning and hooked it up to my car battery. It was dead silent, no matter how loud I turned it up. No noise! I do have a 12V regulator that I bought when I made my order for parts but after finding out the power supply would handle 15V I decided not to use it.
What should I do next?
steve

 

[paulb]

If you have access to a scope, you can check the ripple on the supply (make sure you have the amp running, there won't be any ripple if there's no current being drawn).

If ripple is the problem, maybe a capacitance multiplier:
http://sound.westhost.com/project15.htm