Half-Wave Rectifier Question - Page 1
Forum section: electronics / projects

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: February 11, 2018, 01:27:00 am
content: I've got an audio application (TOSLINK->DIR->SRC->DAC->Headphone Amplifier) that I've been working on.The power source to the board is 12VAC input (from wall-wart transformer).I have a positive half wave rectifier with bulk capacitor, and a negative half wave rectifier with bulk capacitor. This is to generate (thru LDOs) +-12V for the headphone amplifier.However, I then have several other LDOs for DIR/SRC/DAC in the system. This means my positive and negative bulk rails aren't going to have symmetrical loads.Is there any benefit to have a second positive half wave rectifier and bulk capacitor for the LDOs supplying the DIR/SRC/DAC?

author: retrolefty
author_rank: Super Contributor
timestamp: February 11, 2018, 01:53:17 am
content: I'm pretty sure one can't get filtered 12 VDC from a half wave rectifier supplied with 12 VAC while drawing any useful load current.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: February 11, 2018, 01:55:33 am
content: To clarify, 12VAC goes thru half-wave diode into 4700uF bulk cap, leading to nominal 12*sqrt(2)=~16VDC, which is then LDO'd down to +12V. Same for negative rail, except diode and bulk cap are flipped around.

author: SeanB
author_rank: Super Contributor
timestamp: February 11, 2018, 12:32:15 pm
content: With 16V DC and a 5V at highest digital load best is to put in a buck converter to give 8VDC to the LDO regulators, and a little extra LC filtering on the input to the LDO regulators. That will make the load more symmetrical, leading to a lower DC componebt of the transformer core current.Better though is to simply full wave rectify the 12VAC, take the 16V supply and use a forward converter and transformer to get a pair of 18VDC rails for the amplifier, and another buck converter for the LDO regulators instead. That way your power amplifier can have a relaxed PSU CMRR reducing mains hum, the LDO's run cool, you have increased headroom on the power amplifier side and more importantly the PSU transformer has no DC component reducing the available power and increasing core loss as it is no longer sitting biased off the centre of the BH curve. Bonus is you can run it off 12VDC direct as well. Extra bonus is you can have a standby mode that turns off all the power stages aside from the digital side, and it will come up from standby fast as well.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: February 11, 2018, 01:10:15 pm
content: Part of my design goal was to avoid switching converters; not out of lack of familiarity, but more a desire to keep board-level frequencies to a minimum. I have already done thermal analysis on the LDOs, and am operating well within margins.I just had a thought; keep the half-wave rectifiers only for the amplifier rails, which we assume are mostly symmetrical. Add a full-wave rectifier for everything else. That should keep the mains to 12VAC transformer fairly balanced.

author: SeanB
author_rank: Super Contributor
timestamp: February 11, 2018, 01:41:15 pm
content: Going to be hard to have a ground reference between the digital logic side and the amplifier then, unless you have 2 secondaries on the transformer, and in that case simply using a full bridge with centre tap will work better. The half bridge centre is one end of the transformer winding, and using a bridge rectifier this rail will be swinging from rail to rail at mains frequency.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: February 11, 2018, 04:13:14 pm
content: Good point. I’ll investigate the center tap method.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: February 11, 2018, 04:34:21 pm
content: With a full-wave rectifier, using the center tap as ground reference, is there still any concern about transformer saturation if the positive and negative rails are greatly imbalanced? If not, why?

author: StillTrying
author_rank: Super Contributor
timestamp: February 11, 2018, 04:47:34 pm
content: Quote from: Evan.Cornell on February 11, 2018, 04:34:21 pmBoth+ve and -ve mains cycles charge both the +DC and -DCat the same time, so it impossible to be imbalanced even if one rail is OC, it just becomes a center tapped 2 diode rectifier.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 11, 2018, 04:52:07 pm
content: You don't need +-12V for most headphones. A single 12V rail split to +5/-7 should be sufficient. In fact, mobile devices generally run the headphone outputs from 1.8V or less.https://github.com/NiHaoMike/OpenDAC-HD/blob/master/OpenDAC_sch_dig.pngAlso note that at typical listening volume, the signal level to a pair of headphones is on the order of -30dBm or less.