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Old 19th July 2010, 05:22 PM   #1
panduro is offline panduro  Denmark
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Default power resistor value ohm an watts??

Hi all, knowing and not so knowing.

Subject power resisters.

ive seen schematics, some with no power resisters, some with fuses instead, and some with powerresistors.

The power resistors/fuses are often placed: one before the main toroid(higher watt and ohm rating than the ones after the elec. caps), one from +v to gnd and one from -v to gnd.


this confuses me! do power resistors act as fuses?

can somebody please explain the use of them and why the one near the toroid has a higher R/ohm rating than the front ones?


best regards from the hopeless one

panduro
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Old 19th July 2010, 05:57 PM   #2
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The power resistors across (in parallel with) the power supply capacitors (from V+ and V- to GND) are to bleed off the voltage that the capacitors store when the amplifier is powered off. It stops you from getting a shock if you open the amplifier and bridge the capacitor contacts. They are a high value so that a smaller wattage (and hence a smaller physical size) resistor can be used. Also, because of the lesser current passing through a higher value resistor, this keeps heat levels down. The resistor does not act as a fuse because it is in parallel with the capacitors.

To explain what the resistor/fuses before the main toroid do, it would help to know whether they are either resistors or fuses, what side of the transformer they are on: primary-mains side, or secondary-lower voltage side and whether they were in series or parallel. If you could also give their values, this would also help. Power resistors before the main toroid are usually for some kind of soft start circuit that limits the current supplied to the transformer on turn on, and stops nuisance fuse blowing.

Jack
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Old 19th July 2010, 06:26 PM   #3
panduro is offline panduro  Denmark
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Hi Jack, and thanks for the reply.

schematic: http://www.ulriklund.dk/files/psu4.jpg

I think the value there are:

main side 17w 9r, sec side 1r, 17w!

so you Are telling me that the powerresistor and the 220nf cap is a soft start?

There is a 580nf cap in series with the owerresistor in the sec side, is that also part of the "bleeder"?

If the power resistor is a bleeder, will it have an impact on the sound if i put a 5w 1r instead of the 17w 1r?

sorry or the daft questions, just trying to find my way of understanding it use.

best regards

panduro
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Old 19th July 2010, 08:09 PM   #4
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Hi, thanks a lot for providing the schematic-it make things much clearer .

I'll start at the left of the schematic on the mains input side and work my way across. From looking at other circuit diagrams and their descriptions, I can probably safely say that the capacitor on the input across the contacts of the switch is a kind of spark suppressor (may also be called a snubber, I think??? Don't trust me on this one )-it prevents the switch contacts from arcing when opened. Then you've got the fuse which will blow if the mains transformer tries to draw more current than it is meant to. This will save any further damage and any dangerous situations where mains voltage may become present on the chassis. Then the capacitor and resistor in series with each other between live and neutral, forms some kind of filter which will stop any frequencies above the mains frequency (50 or 60hz) from passing into the transformer and inducing noise onto the secondary.

On the secondary side of the transformer, you have the two fuses that will protect the transformer secondary if the bridge rectifiers, filter capacitors or the circuit being powered has a problem and draws too much current. I'm not quite sure about the purpose of the two 100nF capacitors but I think there are discussions about whether these are necessary and what effects they have.

After the rectifiers, you have the main filter capacitors totalling 20,000uF per rail and then the smaller 100nF bypass capacitors to filter high frequency noise. Finally, you have a RC (resistor capacitor) filter that will filter out RFI (radio frequency interference) from the power rails. In my previous post, I did not have enough information so assumed this RC circuit was a bleeder, I was wrong . Infact, this power supply does not have any kind of bleeder on the supply rails.

Depending on what you are powering, some parts may not be necessary. For example, if you were using a small <300VA transformer, the capacitor across the switch contacts may not be needed, same goes for the 9R resistor and 220nF cap. The 100nF caps before the rectifiers could also be done away with along with the output RC filter. All of this depends on the load you will be powering. If it was a circuit that required a very clean and stable power supply without any noise, then it's best to leave the RC filter in as well as the 100nF caps before the rectifiers. Instead of the RC filter on the primary, you could use a filtered IEC inlet that will take care of filtering RFI and provide you with a nice solid connector to plug the power lead into. Also, if the circuit you were powering draws a lot of current, then you should consider keeping those parts in too.

For more information, you could have a read of Rod Elliot's power supply design article here. This might explain some of the things I couldn't .

Jack

Last edited by jackand08; 19th July 2010 at 08:19 PM.
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Old 19th July 2010, 08:40 PM   #5
panduro is offline panduro  Denmark
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Hi again, and thanks for the very explaning reply.

I have read Rod Elliot's power supply design article, but it didnt really sink in.

its for a 4 channel amp, with either dual toroid 800va, 2 x 40v sec or a huge 2500va, 4 x 40 sec single toroid. 2 of the channels will be powering dual subwoofers running down to 2r. 2 x 300w 4R, 2 x 300w 2r

I have a filtered and fused iec outlet Id like too use, can i still use that with the spark supresser?

last: I have some(4) 50W 10R power resistors. would if change anything soundwise if i use that instead of the schematic value on 17w 9r.

thanks for your very good replies, they help me alot!

best regards

panduro
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Old 20th July 2010, 07:19 AM   #6
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Hello again. I agree with you that Rod's article is quite long-winded and sometimes can take a while to digest.

If I were you, I would stick with the two 800VA transformers as two of these will draw much less power from your mains (around 1600VA) than a single 2500VA transformer. Even with two 800VA transformers, you WILL need some kind of soft start to limit the current going to the transformer on startup and stop it from blowing fuses.

Also, having dual transformers in such a high power setup can improve channel separation as each channel has its own independent supply. For example, if you used one transformer for all channels, if heavy bass was playing in the subwoofer channels, this may draw power away from the two other channels and make the power rails sag. This would reduce the power output of the other channels and possibly make them clip or distort.

With an 800VA toroid, because you will NEED to use the soft start, there is not much point in leaving the spark suppressor in because the soft start will limit the current through the switch and transformer and reduce the likelihood of arcing. Rod Elliot has a soft start design here which many people have used and are happy with.

With regards to the 50W 10R power resistors, I'd stick to the values in the schematic as these values form a tuned filter that takes out any noise or interference from the power supply above a certain frequency. Also, replacing a 17W resistor with a 50W one is complete overkill as the 50W resistors could be used on something else.

I was wondering, what amplifier are you using? Also, have you tried building an amplifier before and if so, what type? The only reason I am asking this is because the high voltages and currents available in the power supply, even on the "lower" voltage side, are extremely dangerous and could kill you a few times over .

Jack
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Old 20th July 2010, 04:48 PM   #7
panduro is offline panduro  Denmark
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Hi Jack,

I appriciate, all your advice and concern!

nope, i've never built anything but speakers and subwoofers, so its a complete virgin experience for me.

I dont want to make it to easy on myself, so i picked 41hz modules that i have to solder myself.
tripath 3020 chips

2 times stereo modules, 4 channels!

http://shop.41hz.com/shop/item.asp?catid=13&itemid=45


Regaring softstart. I have a softstart from 41hz.com that im modifieng with 2 x 5A ntc resisters so it will delay the inrush power further. then ill leave the spark sup. out of the circuit.

I thought that using seperate sec lines would nearly accomplish the same as independent toroids, when it comes to voltage sagging.

That schematic from previous post isnt one 41hz.com recommend for their amps(they recommend their own kit), its just one i heard good things about before and think that the article about solid state power supplies sound reasonable.
Solid State Power Amplifier Supply Part 3

i know its possible to use the 50 1r power resistor in something else(just not making anything else right now). I got them when buying som other components, and they followed for free.
But if it matters if its 50 or 17 watt, i will buy the 17w instead. I just had the ideer that it was really the resistance that meant something and that it coudnt be worse to have a higher power rating.


With 24 10.000uf and minemum 1600va toroidal power there better be, no seriously, i know its lethal and should be taken very seriously.

best regards

panduro
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Old 20th July 2010, 05:47 PM   #8
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Thanks a lot for providing me with the extra information .

Regarding the resistor, I suppose that if you had the part to hand, whilst it is overkill, would work fine even though it is 1 ohm larger. Just make sure it is actually 10R as in one of your posts you say it is 10R and then 1R.

For the power supply, because you are not using the supply from 41hz, you will have to use a pre-regulator before the onboard 5 volt regulator as it will blow or get extremely hot if you use it with voltages above around 30V. A suitable pre-regulator design can be found here. This will step down the main DC voltage to something the regulators can handle. The kit also needs a +/-10V power supply to power the gate drive for the MOSFETS.

Using separate secondary lines would help things to a certain extent but you can't beat having individual transformers, especially when the amps are to be used for subwoofer duty where the power draw is high constantly. This would make less power available to the other amplifiers and reduce their transient response. Having separate transformers also prevents any coupling between the subwoofer and main amp channels.

If you haven't had much experience soldering, I would suggest that you got a pre-assembled version of the kit as it has quite a few surface mount parts and connections to be made on both sides of the board.

As a first project though, even though you have built speakers and subwoofers before, I would really try some more simple and lower power amplifiers first. Whilst you may have all of the theoretical knowledge, building something that works properly and is not bodged can be very difficult, especially with the high powers involved in this project. Also, if anything went wrong, without experience fixing and troubleshooting smaller amps, you would have a pretty hard time trying to fix it. Yes, support is available on this forum but it is quite limited.

If I were you, I would try a chipamp using an LM1875 or LM3886 first. These can provide ample power output for a home system and in the process of building them, you will pick up skills along the way. You could try Rod Elliot's P3A which is highly regarded and sounds good. You should buy his boards though as layout is very important. Alternatively you could try the Symasym Amplifier or Carlos' DX amplifier. Both of these have freely available circuit diagrams and PCB layouts simplifying the build.

By the sounds of it though, with the high power requirements, you want a PA amplifier. For this, the best solution is to go out and buy a proper PA amp with the specs that you require. There are plenty of them out there but make sure you go with a respected manufacturer.
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Old 21st July 2010, 02:52 PM   #9
panduro is offline panduro  Denmark
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still appriciate all the advice

Regarding the resistor, I suppose that if you had the part to hand, whilst it is overkill, would work fine even though it is 1 ohm larger. Just make sure it is actually 10R as in one of your posts you say it is 10R and then 1R.

its because i need 2 powerresistors on the sec side pr channel and one pr prim side.
prim side value: 9R 17w- thats the one id replace with a 10R 50W thats in the closet(have no use for it elsewhere)

the sec sides, i dont have, will get them in value stated above 1R 17w.

For the power supply, because you are not using the supply from 41hz, you will have to use a pre-regulator before the onboard 5 volt regulator as it will blow or get extremely hot if you use it with voltages above around 30V. A suitable pre-regulator design can be found here. This will step down the main DC voltage to something the regulators can handle. The kit also needs a +/-10V power supply to power the gate drive for the MOSFETS.

I have a seperate 220v, 2 x 12v sec, 2x 1,5A toroid to use as "extra" supply.
Havent gotten around to figuring out what i need other than the small toroid to the -+10volt supply.

I will read the link( http://sound.westhost.com/project102.htm)a couple of times and se if it makes me wiser


If you haven't had much experience soldering, I would suggest that you got a pre-assembled version of the kit as it has quite a few surface mount parts and connections to be made on both sides of the board.

Actually its not to bad with the smp's, the real problem is that the info on partsplacement isnt good enought for an amatour like me. getting some help from the 41hz forum and from an electronic engenier friend of mine(althought he hasnt be working with electronics since he had his diploma 10years ago)

If I cant make it, ill prob buy some assempled modules, but i like doing it myself, dispite of the vertical learning curve.


As a first project though:

I know, but i dont have any interest in small power amps, got enought amplifiers as it is, (nakamichi pa-5 e2, 2x holfi power amp) but wanna build one myself.

been told that before, but im not gonna listen to reason as the modules are being solderet and i got almost everything i need fore the psu.

yep id like a lot of power, but not at the expence of having to get a commercial pa amplifier.

best regards

panduro


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Old 21st July 2010, 06:47 PM   #10
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Hi again !

All of the stuff about the resistor is fine now, completely sorted .

Even though you may not have any interest in small amplifiers, it is always a good idea to start at the bottom and work up. Although you have many other low power amps, these were made in a factory and configured there, eliminating almost all of the hassle-all the end customer has to do is plug everything in. Not really that hard compared to designing and building a whole amp. By starting with a small amp, it helps you to understand the basics of building and troubleshooting an amp. When you move up to much larger power levels and complicated topologies like Class-T (as used in the 41hz amp) things such as troubleshooting can get waaaaay more complicated. This is due to the fact that there are many adjustable controls and a large portion of the circuitry is contained in an IC.

At the end of the day, it is up to you completely. If you feel comfortable with trying something quite large and complicated-not to mention the fact that it is dangerous-then go ahead and do it. Personally though, I would like to get a bit more experience first before I try something like that and depend on other forum users to solve problems. BTW, I am in no way trying to put you down here, just explain the complexities of such a project.

Jack
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