Need help for first project in a long time. (Headphone amplifier) - Page 1
Forum section: electronics / projects

author: NoradIV
author_rank: Contributor
timestamp: October 15, 2017, 04:35:26 pm
content: Hello guys,I am making my first project in a long time.So, I am building aTPA6420based headphone amplifier.So far, I purchased and assembled the unit. I have purchased an enclosure and a 25.5V transformer with a centertap.The picture below is where I am currently at.On the rear is a RCA connector salvaged from a TV. I plan on getting a IEC320 for the power connector, and I also plan on securing the board to the case with screws. I am only at assembling right now.At this point, I have 4 questions.First, should I install fuses (pretty sure I should). Where? (Between transformer and board, between transformer and AC?)Second, should I add a shield? If yes, where?Third, I noticed that when I disconnect a transformer, a rather large spike of voltage occurs. This thing is rated 12-20 VAC. Should I get a 30Vac clamp? How can I build such a thing?Should I switch the transformer itself with my switch, or just the board? (The switch is rated 250Vac 15A). Note: I live in canada. Main is 120V here.Thanks for your time!

author: jaycee
author_rank: Regular Contributor
timestamp: October 15, 2017, 06:20:47 pm
content: Put a fuse between the transformer primary and the live/hot power input. 100mA T type will do. Your power switch should switch the primary side, be in the hot line, and after the fuse. However if it's that toggle switch you need to make sure you sufficiently insulate those terminals. Personally I think that is too close to the board. Consider using a small rocker switch either on the rear or the side of the unit instead.No need for the voltage clamp - it looks as if the board includes its own voltage regulators. You should also be OK without shielding, but use screened cable for your RCA jacks.

author: Audioguru
author_rank: Super Contributor
timestamp: October 15, 2017, 09:26:49 pm
content: The ebay Chinese seller speeky zee Engrish poorly. Welding? Restaurant? Cattle?I wonder why they say this low power amplifier needs a 30W power supply and I wonder why you bought a 50W transformer. 2W would be fine.

author: NoradIV
author_rank: Contributor
timestamp: October 15, 2017, 10:13:40 pm
content: Quote from: Audioguru on October 15, 2017, 09:26:49 pmBecause, as specified by the title, it was my first project in a while. Also, I went for what was available in the store. I had the option between 10 and 50 watts. Went for larger since 10w was in the low range.You are probably right and a much smaller transformer (10W 36V) would have done it. Stupid me, I went with what was specified by the seller.Maybe should I reverse engineer the board next time?Quote from: jaycee on October 15, 2017, 06:20:47 pmThe TPA6420 says 1.5W. I do see a L7815CV and a L791515CV. So 10 watts should be enough (100mA works then).From what I can find, T type are fast blow, am I correct? There is 4x4300µFElna RJDcapacitors at the input of this board. Won't those blow it?Not arguing with what you said, simply trying to learn. The board's power input appears to go through 2 100pf caps, then a diode bridge, then the 4300µF caps, then a combo of L7815CV and L7915CV. The caps are rated 25V, theregulatorssays 36v max in absolute maximum ratings.Now, I have no experience with transformers, but I have a 48V centertap 6.7A transformer here, and when I disconnect it, I had a peak going all the way up to 168V on my multimeter.Wouldn't that damage anything?

author: jaycee
author_rank: Regular Contributor
timestamp: October 15, 2017, 11:05:19 pm
content: Quote from: NoradIV on October 15, 2017, 10:13:40 pmT is "time delay" type, aka Anti-surge. F is fast blow.Quote from: NoradIV on October 15, 2017, 10:13:40 pmNo - the reason you are seeing this spike is because the transformer is unloaded, and your meter has a high input impedance. You are seeing a flyback spike as effectively the transformer becomes an inductor. However there isnt enough actual energy to do any damage.

author: NoradIV
author_rank: Contributor
timestamp: October 16, 2017, 03:13:48 am
content: Alright, thank you very much!