tailor output voltage of power supply
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 23rd April 2001, 08:15 PM #1 Evaas   diyAudio Member     Join Date: Apr 2001 Location: Montreal QC Hi All I'm building a cross of a Pass Aleph 4 and 5. Anyhow I want a rail voltage of 30 to 35 V DC. For my filter I was just planning on using tons of caps. However, I don't have a transformer yet, but I found a good deal on 33v transformers. If I just use the caps, it would be around 46V. My question is can I use a choke to limit the voltage? Whats the formula to calculate the voltage drop. Also, how does this change when its used in a pi filter (i.e. how is an inductor fed by the transformer different from an inductor fed by a cap which is in turn fed by the transformer) Finally, what is the power loss for such an inductor. I want an efficient psu, and don't want to waste any more power then I have to. Thanks for any help you can give, as I've searched the site and found nothing to help so far. I also want to brush up on my formulas, but I'm afraid that they'll be sold out before I figure it out on my own. Evan
 23rd April 2001, 08:51 PM #2 GRollins   diyAudio Member   Join Date: Feb 2001 Location: Columbia, SC Evan, All things being equal (they never are), a choke input will give you something on the order of .9x your secondary voltage. Not quite what you had in mind. Granted, you can scale the inductor, but... Keep in mind is that high current inductors are hard to come by. Another point is that the current draw of the circuit will pull down the rail a bit. How much? Depends on the current capability of the transformer vs. the current draw. Nelson has stated that the Aleph circuit is pretty scalable. You might consider running your circuit at the higher voltage, but pulling back the idle current to keep device dissipation under control. Or you could go ahead and shoot for the 100W target, since the Aleph 2 uses a 45V rail. Grey
 23rd April 2001, 09:21 PM #3 Evaas   diyAudio Member     Join Date: Apr 2001 Location: Montreal QC Grey, when you say .9x the secondary voltage, I assume you mean the RMS voltage. That would be 33*.9 = 29.7 V. A little low for me, but it might be ok. For my design, I want to be able to idle as much as 1A bias through my four output transistors. I will add the ability to change the bias so that it will produce less heat in the summer, and because I'm not exactly sure of my surplus heatsink's thermal resistance (done some approximations based on area and other ones that are similar). I think my target will be more like .7A but I want to leave a little margin. So... at .7A and 35V, each transistor is dissipating 24.5W. I'd go up to 35W each, but thats pushing it and I'm not sure how hot it will get. My problem is that I'd prefer to reduce the voltage (and output power) than to reduce the bias current, which will have a bigger impact on the sound quality. Therefore I don't want to go above 35v. Well ok, so if I use 46V rails and .7A bias, the power per device would be 32W, which is still acceptable. However if it runs too hot then I'll have to lower the bias. If I use .5A (the lowest that any Aleph uses) then I'll dissipate 23W, which I'm confident will not be too hot. However, the only designs that bias at .5A also use six devices, which serves to reduce the distortion that is addes with lower bias. If I add more devices then I increase my power again... So basically I'd be ok using 46V rails if I could be sure of my heat sink. Oh well maybe its time to actually test the darned thing... oh yeah.. I'm also leaning more towards Aleph 5 specs (which I think is around 33v) as NP claimed that had the punchiest bass, and I do enjoy that sort of thing Thanks again, Evan
 23rd April 2001, 09:29 PM #4 Evaas   diyAudio Member     Join Date: Apr 2001 Location: Montreal QC oops! Looks like I did more spewing of the mouth than thinking.. if the choke gives .9 times the secondary voltage, does refer to the rectified voltage? if there's still some sinusoidal component (which there is, but I don't know how much), then the fully filtered component would be higher. Or does that .9 refer to the voltage after filtering?
 23rd April 2001, 11:30 PM #6 grataku   diyAudio Member   Join Date: Dec 2000 Location: - Evaas, what drops the DC voltage down is R (resistance). Usually valves amps use chokes with lots of turns of thin wire. It is not uncommon to see 12 henry inductors used in Pi filters with 150 ohm resistance and 200mA current rating. That works well to drop and filter the DC voltage. In your case you can probably find a 50 mH inductor with hi current capacity but low internal resistance. That may be effective as filter but not as voltage reducer. Just get a transformer with the right secondary voltage 2x ~22-24V and be done with it. Then if you want to be cool you can use a high current inductor in a pi filter to get rid of most of the ripple noise. Would have to be 4 caps and two inductors, right? One filter for the + and one for the - rail.
 24th April 2001, 12:10 AM #7 Evaas   diyAudio Member     Join Date: Apr 2001 Location: Montreal QC Ok, so I'm getting the feeling that this is a bad idea. I definitely want to stay away from the restistor and diode ideas, and even the thin wire inductor, as those are very inefficient. However, I thought that inductors that are wound with low resistance wire will not dissipate much heat, but this does not seem to be what you guys are saying. Let me walk through an example: I have a transformer with 8.5VAC secondaries driving a bridge rectifier(assume 2v loss through the rectifier), so the rectifier has 10v peaks. (8.5*1.41-2=12-2=10v). If I then use a large capacitor bank, my dc rail will be close to 10VDC. However, if I use a choke between the rectifier and the capacitors, then it will serve to lower the peak voltage out of the rectifier, and raise the bottom voltage. Whereas the peak was 10v and low was 0v without the choke, the peak my be, say 8v, and the low will be 2v with the choke. Is this basically correct? Furthermore, the area under the curve of the circuit with the choke will be similar to the area under the curve for the circuit without the choke. That is to say that the average voltage in both scenarious will be similar, but it is distributed differently. Based on this, the power dissipated by the choke should be small because in essence, its chopping off the peak voltage and distributing it later on (90deg later) rather than wasting it as heat. While the average voltages may be the same, what matters to me is the peak voltage, since that is what my caps will hold and the peak voltage of the inductor circuit will be lower. Since nobody else seems to think that this is a good solution, can you tell me what I'm missing? as for getting the right transformer, I'd love to Unfortunately in two months, and with trips to all the local surplus stores and all over the web, I still haven't found a good one less than \$70, and I need two because I'm building monoblocks. Grataku, yes I'm thinking 4 caps (actually 4 banks of caps) and two inductors Robert, when you refer to difficulty of driving 4Ohm loads with lower current, you're talking about current limited by the power supply and not about bias current, right? I don't see how bias would affec tthe ability to drive harder loads, but I know it would be a problem if the psu ran out of steam Evan
 24th April 2001, 03:04 AM #9 Evaas   diyAudio Member     Join Date: Apr 2001 Location: Montreal QC Robert, I need to apologize because I looked at my first post and I wasn't entirely clear. When I gave the bias current values, I was talking about current per transistor. So since I'm using four output mosfets, that would be a total of 4A bias per channel; and .7A bias *4 = 2.8A per channel. Also, I'm building a separate power supply for each channel. So lets take your calculation and assume 4A bias per channel, and remove the x2 because I'm only driving one channel with each transformer. so lessee.. 70V x 4A = 280VA double that and get 560. I'm looking at a 750 VA transformer, so I think that should be able to source enough current. I agree with you completely and I want to use a good transformer. Evan
 24th April 2001, 03:36 AM #10 rljones   diyAudio Member   Join Date: Mar 2001 Location: california Evan, If you're going to bias at 0.7 A/device with 4 devices, that would be 2.8A x 70, or about 200 VA. I happen to have an Avel catalog (860-355-4711) right here. They make a 500 VA (25+25 @ 10A), model Y236801. I think you could get by with a 330 VA (25+25 @ 6.6A), model Y236750. The 25+25 should rectify up to about +/- 35 VDC. But if you might bias at 1A/device, then I would go with the first transformer. I would guess about a \$10 to \$15 price difference per transformer between the two models. Robert

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