Re:Build your own professional-grade audio amp on the sort-o...
by unwind on Sunday November 08, 2020 @04:39AM
(#25024435)
This level of free electronics review is awesome, thanks from a software guy with more solder than sense.
Glad I'm not into power or audio electronics. :)
Re:Build your own professional-grade audio amp on the sort-o...
by tuatoru on Sunday November 08, 2020 @05:10AM
(#25024604)
I hope I haven't put you off for life! :)
It can be fun and rewarding to buid your own.
Your amp will be fine as long as you use it for lounge-room listening. Just don't use it in pro situations, e.g. a wedding reception or large party (50+ people). Please!
(But if that cap across the transformer secondary is only DC rated, not AC, you can expect it to explode. Might take a couple of years, if you're lucky. I had one last five years before I learned about "X-class" mains rated capacitors.)
Re:Build your own professional-grade audio amp on the sort-o...
by Hydraulix989 on Wednesday November 18, 2020 @01:36AM
(#25133813)
- There is only one lug, see photos.
- Polyester caps are AC rated
- Transformer is 300VA
Safety: -
The mains safety earth (chassis bolt) should only ever have one lug: the mains input earth. Put a separate bolt an inch or so away from the mains safety earth bolt for the chassis connection of the bridge centre ground. (People have killed themselves doing repairs by ignoring this, and having faulty mains wiring.)
Fuses on the transformer secondaries are recommended if the transformer is expensive. If the bridge rectifier or an amp output stage fails, it'll almost certainly fail short, and the transfomer will heat up until its internal thermal fuse melts -- or if there isn't one, until it catches fire.
Likewise fuses on the DC rails will help protect the expensive smoothing capacitors from amp failure. (Yes, the TPA3251 has built-in protection - but that might also be damaged by the surge that caused the output stage to fail.)
I hope the polyester cap across the transformer secondary is X2 class.
Other points:-
A snubber across the mains switch will reduce arcing on switch-off and prolong the life of the switch. 10R-50R 3W in series with 100nF, X2 class, will do it. (The wattage rating is a proxy for both voltage rating and surge current rating.)
From the scale it looks like the transformer is a 120 or 160VA model. That's seriously undersized for an amp module rated at 350W (circa 550 VA with a traditional mains-frequency transformer). Transformer secondary fuses are strongly indicated.
OTOH, going up to a 500VA toroidal transformer means you'd need a soft-start circuit to prevent your house circuit breakers popping on start-up, from the initial transformer saturation inrush current.
I'm amazed you don't get RF interference with all that length of unshielded wire on the input side. I just have to leave one inch unshielded to be plagued with EMI - unless I use filters on the input terminals (100R / 220pF RC filter).
You don't have DC blocking on your volume control. I guess none of your sources have any DC offset. If not, changing the volume will be scratchy.
References: -
1. Bob Cordell, Designing Audio Power Amplifiers.
2. Doug Self, Audio Power Amplifier Design 6e.
3. Rod Elliott, sound-au.com. "Designing Linear Power Suppplies part 2" https://sound-au.com/power-supplies.htm
4. Idem, "Transformers - The Basics, part 2". https://sound-au.com/xfmr2.htm