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Old 12th June 2006, 01:16 PM   #1
elaar is offline elaar  United Kingdom
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Default Power Supply Design for a solid state amplifier

Hi, I was hoping someone wouldn't mind looking at this design and commenting on it's suitability.

I'm making a 140w/8ohm amplifier (Randy Sloanes favourite design in his book using L-Mosfets).

I don't need to say much about it because i've written quite a bit on the design pic.
This is the first time i have built a psu and so am a bit bewildered, I have made this design by basically copying from about 4 different designs I have studied.
Things i was wondering include are the capacitors the best performance values, and what type of capacitor should each one be? Have I also missed out any components that could improve the filtering? Should the capacitors be bypassed, should the diodes have bypass caps and with a 500VA transformer is soft start recommended?

Thank you to those that take the time to look at it and comment.

Thanks,
Andy
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Old 12th June 2006, 01:26 PM   #2
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The ground from the caps should go straight to the chassis ground, as should your speaker returns. Save your HQ ground for signal levels.
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Old 12th June 2006, 01:52 PM   #3
elaar is offline elaar  United Kingdom
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Ahh okay, thanks for the input I will change the design.

I also meant to ask, I have quite alot of copper sheeting, and as the case isn't that large and the toroidal is large, I was firstly going to put the toroidal into an old pc psu case, and use the copper shielding to make a faraday cage for the amplifier pcbs. Should the copper cases then have a route to ground through a cermaic capacitor? (I have seen this done in some amplifiers).

Cheers,
Andy
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Old 12th June 2006, 02:32 PM   #4
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In a slight contrast to Pinkmouse, I advise you use the centre of the caps as your star point, so each bridge has its own connection to the caps 0V, not shared like you have drawn it. Then a wire off this goes to chassis. All your speaker returns go to the star point not the chassis. The amp board will have a connection also to the star point which you will then take input screen/ground from, and any other 0V references on the board. You might wish to also have a 'dirty 0V' trace on the board which uses another wire back to the star point, for zobel and decoupling networks.

You need another cap across the other switch. I would use 4.7nF Y-rated caps for this purpose as they fail safe.

Your input and output sockets should be isolated from the case.

I would not bypass very large caps with very small ones as this creates resonant circuits.

A soft start is essential for 500VA, usually over 300VA you would use one.

I would personally put the fuses in the amp rails not before the rectifiers. Thus each amp is fused safely then. If there is a problem with the bridge or caps the primary fuse will blow, if sized correctly and protected with a soft start.

Up to you on the faraday cage, I'd be surprised if you needed it or to even put the thing in a PC PSU box, all hassle for nothing IMO.
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Old 12th June 2006, 10:34 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
just to offer yet another option on your PSU.

Fit a soft start.
F1 should be in the Live connection from the mains. Reduce the fuse rating to as low as will allow the amp to start up and run at full power. It may blow after a week or two due to fatigue from the continual reheating. If so go up one rating i.e from T3.1A to T4A.
Add a second cap across the 2pole mains switch or fit real RC snubbers.
500VA will support about 300W maximum. Is your amp being designed to suit 4ohm speakers?
Move F2 & F3 to after the smoothing caps. F5A in the supply rails will allow about 10Apk into your speakers. Are you designing for 4ohm speakers? F3.1A (about 50Vpk into 8r) will better suit 8ohm speakers.
Single rectifiers perform just as well as dual rectifiers. Less heating with a dual but more voltage loss.
16.6mF smoothing caps in combination with 8ohm speakers (RC=133mS) give a low frequency roll off -3db at 1.2Hz. This should be about one octave lower than the input roll off frequency and about half an octave below the NFB roll off frequency. Leaving a useable LF -3db = 2.4Hz and -1db =12Hz.
If you use 4ohm speakers all these frequencies should be doubled or you can double the smoothing caps to 30mF to maintain RC=120mS.
Personally I would use 180mS for a bass amp or wideband amp.
The PCB 100nF should be located at the highest current users on the amp PCB to bypass current glitches to ground along the VERY shortest routes.
Consider adding an interference suppression cap from live to neutral, Xrated 47nF to 220nF and surge absorber (or parallel absorbers) to suit your maximum mains voltage.
Consider adding interference suppression from live to ground and neutral to ground, Yrated 2n2F to 47nF.

Keep the safety earth and make it PERMANENT. No option!
Keep the disconnecting network from safety earth to Central Star Ground.
Consider adding a ground lift switch in parallel to the other components. You will probably never need it.
Connect your PSU common direct to Central Star Ground. Do not move the Central Star Ground onto the PSU common. It can be adjacent if it suits your PSU and amp locations (see later advice).
Connect Amp PCB power ground to Central Star Ground.
Connect all dirty returns on the PCB to the power ground. These are decoupling caps, Thiel network or Zobel if not located on the speaker terminals and bypass caps.
Connect all clean returns on the amp PCB to the signal ground.
Connect PCB signal ground to PCB power ground using 10r.
Connect RCA input return to PCB signal ground. RCA return should be isolated from chassis.
Connect signal ground to Central Star Ground.
Connect speaker return direct to Central Star Ground.

Connection advice:-
Keep the PSU rectifier/s flow and return, transformer feed and centre tap, smoothing cap flow and return as compact as possible or alternatively keep the loop area in each circuit small by combining wires or twisting. Run a short thick wire from a PSU common to the Central Star Ground , alternatively connect a bolt through the the PSU commons and nut it down tight. On the OTHER SIDE of the nut attach all the high current returns and attach a further nut. Finally attach the signal returns to the bolt and nut tight. The bolt and intervening nuts act as VERY SHORT wires but keep the high currents of the charging side separate from the power returns and the next nut keeps the high currents of the power returns separate from the signal ground reference. Any other dirty returns (relays etc) can be conneted to the wire from CSG to the disconnecting network
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Old 13th June 2006, 07:11 AM   #6
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Lots of good advice from Andrew, with my only disagreement being this:

Quote:
Originally posted by AndrewT
Single rectifiers perform just as well as dual rectifiers. Less heating with a dual but more voltage loss.
Single rectifiers can perform well, but they can never match a dual setup. In a high power setup (anything over 300VA I would say) then I would always use dual rectifiers because of their advantages. The load up the transformer more efficiently (so it runs cooler/you can squeeze more power out of it) but most importantly they allow individual returns and a true star ground, which will result in a higher quality ground.
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Old 13th June 2006, 11:28 AM   #7
elaar is offline elaar  United Kingdom
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Hi, many thanks for all of the great replies, they're very helpful.

I'll draw up a modified diagram showing in detail how i think all of the common and signal grounds should connect up from what you've said (I've had to read some of the replies multiple times and I think i understand it).

The design in question uses the (now a bit tricky to get hold of ) 2SJ162/2SK1058 L Mosfets, this can do 140w(@8ohms) with 55v rails and 200w(@8ohms) with 68v rails witout any circuit alterations.

I'm aiming at the 140w with 55rails, because:
a) I'm using this as a learning exercise and have no need for a very powerful amplifier (I already have 2 huge power amps), infact I would have been happy with 60w amplifier, but...
b) This is sloanes favourite amplifier and so in my eyes is a good one to do.
c) I already have a 40v+40v 500VA toroidal.

This means I should get a fraction under 60v (59.8v roughly) with no load, which is good because I have 8 good 10000uF capacitors already which are only rated at 63v. They won't have a particularly long life with that high a voltage but 100v ones would be too expensive for me currently.

AndrewT: that is one of the reasons why I have used dual rectifiers, just to purposely drop the voltage that little bit more. Also to answer your question about whether it will be used for 4ohms, the answer is probably not. I rarely use 4 ohm speakers, and even in the off chance I do like i said the amplifier will never be used at high volumes so i think it would be okay.

When it comes to soft starts, I was wondering what method people generally design? thyristor soft-start with an output muting relay, or paralleled power resistors and relays? I was thinking of incorporating a small 30-50VA transformer into the case to run auxillary circuits, maybe opamps, clip detection circuits etc.. What if I connect that transformer directly to the power switch and use power from that to activate the relay (maybe with the use of a 555) which will short paralleled resistors to the 500VA transformer?

Hopefully you won't mind having a quick look at the grounding picture/modified circuit I will post a bit later to make sure I have understood all of your advice correctly.

Many thanks,
Andy

PS sorry, the picture should not include F2+3, they are present on the amplifier pcbs, so that's that part sorted.
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Old 13th June 2006, 01:06 PM   #8
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Soft start by relay and resistors, powered off aux supply should be fine. A simple RC charging network and transistor would do the job.
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Old 13th June 2006, 03:13 PM   #9
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Quote:
Originally posted by richie00boy

I would not bypass very large caps with very small ones as this creates resonant circuits.
creates a circuit which will resonate if there is a sharp di/dt -- as in a digital circuit. analog is a bit different and there some things definitely to be avoided --

the bypass capacitors across the reservoir cap prevent radiated EMI (from the rectifiers) and inductively coupled EMI and RFI (from the line connection) from making its way onto the positive and negative rails.
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Old 14th June 2006, 04:49 AM   #10
Shredly is offline Shredly  United States
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I would regulate the power supply; 140W at 40V should be well under 4A, even with 20W of overhead dissipation, and Pedja Rogic's regulated design here will actually give better performance than all those huge capacitors as well as giving you 5A to play with, which means your PSU would be capable of delivering 5A on each rail, for a total output of 400W to power your 140W RMS continuous output amplifier, which surely will peak at under 300W; thus giving you a safety factor of 33%. With a 500VA transformer, you'll have plenty of safety factor there as well, 25%, and you can probably save some money and time using a regulator instead of trying to use mondo caps to filter the ripple out; those big caps are expensive, and the LM338s are about $5 US each, compared to probably $15 or more for those great big 10,000uF caps. ST makes the 338 too, if you prefer European. In addition, the LM338 regulator will probably do a better job than caps; the ripple rejection ratio is 75dB with a $2 tantalum 10uF cap bypassing the ADJ terminal to the local negative; if greater rejection is desired, you can go as high as 20uF, but National Semiconductor states that additional rejection is minimal above 20uF. You'll need to adjust the value of the 2.2K resistors from the ADJ terminals of the 338s to the local negative, to get the voltage you need. In case it's not clear, by "local negative," I mean ground on the positive half, and negative on the negative half. You should have a 3300uF or 4700uF cap to provide the peak voltage to the regulator, where you now have two 3300uF and a 10000uF.

Here are the design considerations for those resistors:

Assuming your toroid puts out +/- 40Vrms, and has two secondary windings, both of which are normal (output measured RMS, and providing two secondaries) for toroids, you should be getting 56.57VP (Vrms x 2^0.5), so 55VDC might be a bit high; you would have the dropout voltage of the regulators to deal with, and the drops across the regulator diodes. The dropout is 1.3V maximum, and the diodes will drop 0.4V to 0.7V depending on the type. You'll most definitely want to check to make sure that there is adequate voltage drop (1.3V absolute minimum, and many designers would recommend at least twice this, in case of temporary voltage drops from the mains) across the regulators; if there is not, they can drop out and the ripple rejection will be defeated. If you design with 2.6V, your maximum conservatively designed value will be (56.57 - ((2 x 0.7) + (2 x 1.3))) = 52.87VDC. I would recommend you shoot for 52V, which will slightly reduce your power output; you'll get an output of 132.4W RMS continuous. Your drop across the regulators is now (2.6 + .87) = 3.47V. Dropping this 3.47V, and sending the masiumu 5A, the regulators will dissipate 17.35W; there are 20W heatsinks readily (and cheaply!) available for TO-3 casings, and this would be reasonable design.

The voltage output of the LM338 is 1.25V(1 + (R2/R1) + Iadj(R2). R1 is recommended to be 120 ohms; Iadj(max) (which is an error term) is 100uA, and Iadj(typ) is 45uA, but delta Iadj is stated as less than 5uA from 10mA to 5A output current, and from 3V to 35V dropped across the regulator; no minimum value is given, and since it is an error term we will set it to zero and 100uA to find our range. The output voltage is thus dependent upon R2. The ideal resistance of R2 for your 52V is therefore:

(1.25V(1 + (R2/120)) + 0uA(R2) = 52V
or
R2 = 120((52V/1.25V) - 1) = 4872 ohms
Refactoring 4.7k into the equation, we get 50.2V, a bit lower than your desired level; but if we go with 5.1k, the next higher 5% value, we get 54.375V, and we're into the dropout danger zone. At this level, 50.2V, you'll get 127W. The error term of 100uA x 120 ohms will give an additional 12mV, for a total of 50.212V; a miniscule increase.

On the capacitors, one thing you should be very aware of is that overloaded capacitors can explode. I have seen a 4700uF aluminum electrolytic cap explode with sufficient force to drive pieces of the aluminum case an inch or more deep into acoustic ceiling tiles ten feet above the bench, and that's at 12V, not 55V. When we made our first linear supplies in school, we were required to use a containment around the bench, and wear a face shield and welder's apron, before applying power to them for the first time. To this day I wear at least eye protection and most often a full transparent face shield when I first put power to a newly built supply. I do not recommend 63V caps in a 55V supply; should there be a power line surge, they would have no extra capacity to deal with it, and could be damaged and even explode. Bear in mind that there is no circuitry in your supply between the secondary of the transformer and the caps that would protect them. Supply surges happen all the time; you must design your supply so that it will tolerate them. Yet another reason to use a regulator; you won't have problems affording the caps that have sufficient voltage to handle surges. Let me put it this way: a 10000uF cap is probably equivalent to a respectable portion of a hand grenade. Think about it.

I would fuse the input to the transformer from the mains on the hot side, if you're running 230VAC (my guess since you seem to be from Britain) then at 2A, slo-blo of course, and fuse the output of the transformer to the rectifier bridges on the hot side at 5A, standard fuse; the LM339 has internal current-limiting circuitry that will protect the load from drawing more than 5A continuously. Make certain that you heatsink the rectifier bridges and the regulator extensively; consider a fan. If you really want to protect your amp, you can add a crowbar circuit to the output of the LM338, which will disconnect the 338 and short the DC line to the amp. I own a 10A 12V bench supply with a crowbar circuit, and can make a schematic available.

The inrush preventer is a good idea. It may preserve your fuses, and will reduce the stress on the entire system, transformer, rectifiers, caps, regulators, and amplifier, as well as your speakers. That giant thump you hear when you turn it on is not a Good Thing.

Safety ground is safety ground; I do not recommend putting anything but wire between the earth connection at the mains input to the facility (by "ground" or "earth," I mean the third wire that goes to the local hard earth ground where the mains enter your house- this is the safety ground- a green wire with a yellow stripe in the harmonized standard, and green in the old British standard) and the chassis; I also do not recommend tying the chassis to the neutral mains connection of course. If possible, a GFCI (you call them "RCDs-" "residual current devices" in Britain) would be a good idea. But remember: RCD/GFCI only protects against current that is lost to a local earth ground; phase-to-neutral (or phase-to-phase, if you happen to be using 3-phase) can still kill you and the RCD/GFCI won't do a dang thing about it, so don't get across a mains circuit. 230VAC can ruin your whole freakin' day, kill you very quickly, and even if it doesn't the burn scars will last a lifetime.

As a general practice, when designing a power supply, ensure that the components have at least 25% headroom for conditions that can cause any sort of problems if they go on for very long, 50% for conditions that will cause damage to the equipment if they go on for more than a second or so, and 100% for conditions that will cause immediate or instant failure of the equipment, or immediate or instant death or injury. The transformer can handle an overage for a little while before it becomes critical; but the rectifier diodes may be damaged and potentially cause damage elsewhere if they are exposed to something that's wrong for more than a second or so. Capacitors, to finish the lesson, can EXPLODE. Watch out.
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