power supply conversion - Page 1
Forum section: electronics / beginners

author: alfajim
author_rank: Newbie
timestamp: March 07, 2015, 08:48:38 am
content: Hi there, I am a newbie to this forum so you will have to pardon my off the wall questions, but I am not a newbie to electronics digital and or analogue.This will be a two part request. First is has anyone done a measurement on noise from a laptop power supply specifically a Toshiba branded 15vdc 5amp 75w version. I watched the youtube video on detecting noise on a power supply and finding that a cheapo ps plugged into her mains was the culprit.Second part and the reason behind my asking is I am planning to modify a phono pre amp I picked up it is non op right now but I am going to do a series of mods to it to improve its musical output. The unit uses a 12vac 750mamp wall wart for its external power supply. It then has a voltage doubler dual half wave rectifier ps on the board into a pair of 7818/7918 regulators with a 18vdc + &- rails with a 0vdc in the center.One of the mods done previously is to remove the on board ps and convert it to batteries, a 6v and 12v, two of each in a series parallel system there by supplying the necessary 18vdc and 0v.center. My thought was to use a pair of laptop power supplies to feed the main board with 15vdc in the same connection points as the battery guys used. That way I could get clean dc with out all the mess and fuss of batteries. It was pointed out to me that the laptop ps's would not work as they would be switching supplies and there would be a great amount of noise produced and that the TT cartridge would pick it up and there would be terrible noise into the amp's and therefore into the speakers. I have seen the inside of the Toshiba power suplies and they look to be very well filtered so that is why the first question. If these are switching supplies and noisy how does a laptop cope with it? I also thought to tie in in front of the regulators and change them out to a lower output dc ans the op amps used only require + 5v min and there are no high power items on the board.One of the posters on VE that is one of the mod folks suggested a alternate dc supply a DIY unit with a 25vdc output and a different tie in to the main board it has interesting possablities.Any suggestions here? with out schematics? Not sure how to do that any way. For info it is a Cambridge 640p pre amp with both MM and MC cartridge inputs.Jim

author: Paul Price
author_rank: Super Contributor
timestamp: March 07, 2015, 10:33:16 pm
content: Switching power supplies are not usually noisy, some go bad and are noisy.The noise from a switching power supply is ultrasonic >30 KHz, and so you wouldn't be able to hear it anyway.High frequency noise is much easier to filter out than low frequency noise, smaller bypass capacitors/inductors are both small in value and size to do the filtering.If I needed to get the 640p back working, I would make something similar to the original +/- 18 V supplies, using a two half-wave rectifiers running off a 24 VAC transformer and 7818 and 7819 regulators.A 24V transformer to power a low-power device such as a per-amp shouldn't need much current, so the size of the AC power transformer needn't be too big or expensive. You probably don't need more than 100mA per supply.The size of a small 24V transformer would be much smaller than a 18V laptop brick, and the brick only gives you one positive voltage supply. A big disadvantage of a linear supply using a power transformer is you must make sure the transformer is well-away from the magnetic phone cartridge or you will hear a lot of hum pickup from the transformer.You can download the 34063 datasheet which explains the circuit below, even gives you a printed circuit pattern to build it.http://www.alldatasheet.com/datasheet-pdf/pdf/392304/ONSEMI/SC34063A.htmlIf the preamp is going to anywhere near a magnetic pickup on your record player, build the power supply in a box that lies on the floor below and run three wires up to the preamp to minimize hum and noise from the power supply.if all I had was a 18V laptop brick, I would use the +18V output for the positive supply and use a 34063 IC (see below with --18V) output for the negative supply. ll you gotta do is replace R1 with a higher valued pot (2K) and adj. the output voltage to -18V.

author: alfajim
author_rank: Newbie
timestamp: March 08, 2015, 12:42:04 am
content: Quote from: Paul Price on March 07, 2015, 10:33:16 pmPaul thanks very much for the reply. Your suggestion for the 24v transformer and rectifier supply is pretty much what a member on VF suggested and how to tie it in to use the 7818/7918 regulators and some of the filters to power it up.As for having just one laptop brick I actually have a number of them as I have been doing a laptop program with one particular Toshiba laptop with all the same hardware and software and a Spanish keyboard. I had an opportunity to pick up a quantity of them so was aquireing the laptops and hardware in lots so also got a lot of power supplies.So my idea was to use two bricks with two sockets wired to the board pos, neg, pos, neg so the pos, neg in the center became 0 volts, that's how the battery guys did and say it really makes a difference in the sound out to the main amp.

author: Paul Price
author_rank: Super Contributor
timestamp: March 08, 2015, 01:27:47 am
content: Only one problem, the power switch for the two 18V bricks..they don't have one.365 days * 24 hrs * $.08 /Kwh * about 5W*2 bricks = Buys a pizza or two?

author: alfajim
author_rank: Newbie
timestamp: March 08, 2015, 08:27:16 am
content: Quote from: Paul Price on March 08, 2015, 01:27:47 amHeHe, there is no power switch for any of these pre amps you plug it in on all the time. they figure so little draw it doesn't matter on the bill.

author: Seekonk
author_rank: Super Contributor
timestamp: March 08, 2015, 11:40:40 am
content: Sorry, but I'm having trouble with "do a series of mods to it to improve its musical output" and a wack a doodle wall wart power supply setup.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 08, 2015, 04:34:03 pm
content: Most audio electronics mods are voodoo-magic with little basis in reality anyway, I doubt it makes much difference. Its better than magic stickers.

author: alfajim
author_rank: Newbie
timestamp: March 08, 2015, 05:57:51 pm
content: Quote from: Nerull on March 08, 2015, 04:34:03 pm@SeekonkJust for giggles folks take a trip or to vinyl engine and check out the topic Cambridge 640p thread. There is a lot more to these mods than "magic stickers"

author: Seekonk
author_rank: Super Contributor
timestamp: March 08, 2015, 06:41:26 pm
content: I was involved in high end audio way back. I knew Saul Marantz. He told me he answered his home door on day and a man stammered out "Saul Marantz, you need me" as if he practiced it 100 times. That man became his chief engineer. As much as he tried, he never did improve on the Marantz tube preamp. His first venture into electronics was trying to make a car radio work in the house by making a power supply. By training, Marantz was a graphic artist. No wonder his stuff looked good.Another friend of mine bought a new system every month. I asked him once what he was going to do when he found the perfect system. He replied, "I'm going to turn it off." Best advice ever!

author: alfajim
author_rank: Newbie
timestamp: March 08, 2015, 09:16:26 pm
content: Quote from: Seekonk on March 08, 2015, 06:41:26 pmI am assuming you were the fellow at the door?