Improvements for old linear power supply - should be fairly straightforward! - Page 1
Forum section: electronics / projects

author: tophat
author_rank: Newbie
timestamp: June 05, 2020, 01:41:42 am
content: Hello everyone,This is the first post to this forum. I am currently working on an audio amplifier with a power supply as shown below.It is different to the schematic because someone has doubled-up R58 in parallel so it is now approx 2R3.The power resistors are dissipating quite a lot of heat - as would be expected in this design!I was just wondering if anyone had any thoughts about why the circuit was designed like this in the first place, using the resistors and the specific differing values of electrolytics, etc.Would I be correct to increase the capacitance and use a better quality of electrolytic in general, to avoid having to use the series resistors?Many thanks for any thoughts on this, cheers , Jim

author: ArthurDent
author_rank: Super Contributor
timestamp: June 05, 2020, 02:22:30 am
content: The heat has to go somewhere so the designers decided to lower the heat dissipation in the regulators by putting a 5R8 ohm resistor before it. The two capacitors and the resistor in between the two caps form a crude pi filter that reduces the ripple somewhat. Apparently someone though the 5R8 dropped the voltage a little too much and reduced its value. Without knowing if they also made other changes that increased the load on the regulators, I wouldn't dare guess why they made that change.

author: themadhippy
author_rank: Super Contributor
timestamp: June 05, 2020, 02:33:55 am
content: Daves video explains wot the resistors doing,you could try without the resistor or maybe try the ideas in the video

author: ArthurDent
author_rank: Super Contributor
timestamp: June 05, 2020, 03:11:17 am
content: "Daves video explains wot the resistors doing,you could try without the resistor..."If you're saying it is a capacitance multiplier, that is incorrect. All four regulators are used as out of the databook regulators. What is before the regulators is just voltage dropping and filtering.Bypassing the resistor will cause the first positive regulator to run hotter.

author: themadhippy
author_rank: Super Contributor
timestamp: June 05, 2020, 03:24:26 am
content: QuoteWhat im saying is at @7.20 he explains what the resistors are upto,and the problems they cause.

author: bdunham7
author_rank: Super Contributor
timestamp: June 05, 2020, 03:30:58 am
content: It's Captain Overkill ripple control. Is this for the main output driver supply or just for the pre-output stages? You could try a straight wire in place of the resistor and see if you have any hum in the outputs.EDIT: I thought I recognized that. It is the pre-output and control power supply for an old Bryston amplifier, isn't it? What is the issue? Is it just the positive side getting hot? I'd want to know how much current is being drawn--this is not a high-current section of the amplifier and current demand should be modest. Can you also check the AC output of the transformer, perhaps with both R57 and R58 installed and then removed?

author: tophat
author_rank: Newbie
timestamp: June 05, 2020, 03:53:40 am
content: Thanks for the replies guys, I'll have a further think. This is to supply quite a few low voltage supply rails, it is a high quality AV processor.My main concern was the heat dissipation from the resistors as it appears to be causing visible damage to nearly components. However the unit is not faulty as such. I am servicing it.The unit is definitely old enough to benefit from fitting modern electrolytic capacitors which I will be doing. As mentioned above, this seems an odd design choice having the resistors and also doubled up there.Thanks all .

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 05, 2020, 10:47:23 am
content: You may safely lower the resistance value and simultaneously increase a capacitance a bit.For example you may change 4R7-2200uF to 3R3 or 2R7 and to 3300uF or 4700 uF.You may safely increase C12 and C17 up to 6800 uF (up to C6, C9 value).My way to lower PCB temperature is to put larger power resistors (if there is enough place), for example I'll use 2x10R 2W resistors in parallel instead of one 4R7 2W (but the amount of heat is the same in such way).

author: tophat
author_rank: Newbie
timestamp: June 05, 2020, 01:04:36 pm
content: Thanks - it's a Bryton SP1.7.I have not done any work to this yet because of waiting for them to supply any information, only visual inspection, I have not done any live testing .However this is probably where I will start, as you and others say with experimenting with the resistor and capacitor values.I can measure voltage, current, ripple and hum on the output and see what's going on.It is possible there is a fault that is causing excessive current draw on one of the rails, so I'll check for that first.I was intending to follow best practices when it comes to design of the supply, as it is obviously not complex, but obviously much better components are available now than when it was made so therefore it makes sense to think about what should be used instead of simply copying the original design. I am fairly sure that the ripple can be reduced to original figures without resorting to the same choice of filter resistances but obviously that's one for a little experimentation.Thanks all

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2020, 02:05:38 pm
content: Quote from: Vovk_Z on June 05, 2020, 10:47:23 amIncreasing C12 / C17 is Ok and maybe a good idea. At the same time one could reduce C6 / C9, especially if the AC voltage is relatively high.The resistors shift some of the power loss from the regulators U5 and U7 to the resistors. If C6/C9 are not very large the resistors would even lower the overall loss and improve the power factor and reduce peak currents. For the resistor one may want a larger form factor if the space allows.

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2020, 06:31:47 pm
content: The resistors can serve other functions besides redistributing the power dissipation away from the regulators:1. The additional RC section helps remove reverse recovery noise from the rectifiers.2. The series resistors act as fusible links for safety in the event of a short.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 05, 2020, 07:31:36 pm
content: If you would consider a better, but more invasive improvement: I would add a separate, lower voltage transformer and rectifier for the purpose of feeding the +/-5 volt regulators instead of feeding them from the +/-15 volt rails.

author: tophat
author_rank: Newbie
timestamp: June 05, 2020, 11:24:58 pm
content: QuoteThis is closest to my current line of thinking.QuoteI think the unit would catch fire if the circuit protection was based solely around these resistors... not sure what advantage there would be considering them a possible fuse, I can only think of disadvantages...QuoteThere are already two separate transformers, one for the analogue preamp stages and the other for the digital stages. Three would be an improvement yet again but I don't think there is enough space in the chassis, and if there were enough space I would be concerned about the added heat - something which is already on the edge.Thanks very much for everyone's inputs. It is good to bounce ideas around