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Old 13th February 2004, 01:07 AM   #101
Prune is offline Prune  Canada
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No, no, I mean I replaced the transformer with a 24V center tapped one (12-0-12V) and replaced VR1 and VR2 with the LM317/337 regulators (these can be gotten at Digikey for example). Then you need to adjust R1, R2, R3, R4 to get 9.5 to 10V; see the datasheets for the LM regulators for how to setup the output voltage. I used pots instead of fixed resistors to fine tune the voltage, then replaced them with resistors; when adjusting the power supply there should be some load or the regulators won't function right. Make sure to get it right before you hook up to the DAC board, you don't want to fry anything.

I have a short length of three wires going from the power supply to the DAC board. The three pin Molex connector on top is the only one that I connect to ground; since the ground planes are connected under the DAC, more ground connections are useless and can cause ground loops. And of course the + and - 10V are connected here.
The other two Molex connectors only need the +10V connections. For best results you can use separate regulators for each, but I didn't bother. I just used a CLC filter toe separate them from the analog +10V, as in the right side of this thing (C4, C5, C6 and FB1 ferrite from Digikey), one set for each of the two digital +10V connections. Works fine, no noise.

Also, since transformers sometimes slightly vibrate or buzz, you should put dampers between the transformer and it's seating. Ground the transformer case to the chassis. Ground the little separate ground plane at the digital input to the chassis.
The transformer center tap is connected to the power supply ground, and should be connected to the earth ground on the power connector, and so should be the chassis.
If you use the unbalanced outputs you may get ground loops. The best way to avoid it is by separating the earth/chassis ground from the transformer/power supply/signal ground with a 10 ohm resistor (paralleled with two diodes pointing in opposite directions which can handle large currents in case of a fault, and a 0.1uF non-electrolytic capacitor to pass high frequencies). In this case, unbalanced RCA outputs would use signal ground, and balanced XLR shield/ground pin would use chassis ground.

For rectifiers use any high speed/soft recovery diodes. It helps even more to add a Zobel network between the transformer's 24V secondaries: about 0.1uF non-electrolytic capacitor in series with about 100 ohm resistor (don't put capacitors across the diodes as some do, as that only slows the ringing frequency but does not dampen it).

One more thing: use low-solids flux, such as Flux Pen - Formula 951 No-Clean from Digikey. In the end I washed the PCB with hot alcohol and then filtered water to get rid of all the rosin from the solder; it's very effective.
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Old 13th February 2004, 06:04 AM   #102
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I have a different design idea, much less effective that yours. The transformer I have has 5 wires, all electrically connected to each other, so the secondary is tapped in 3 places. I planned on using two wires that would supply the right amount of voltage the regulators can handle. I wont be center tapping. For the moment, it's not connected to earth ground. The DC output of the bridge is connected to 2 470uF 200V caps and two 1500uF 35V caps, all in parallel with the DC output. So I have a transformer, bridge, and filter caps. From there, I planned on connecting teo 10V regulators and an LM337, both their inputs connected in parallel with the DC output of the bridge. I was planning on connecting the one 10V regulator output to the +10V inputs to drive the digital supplies and the audio supply for the CS8420. This is with no filtering other than the caps. I planned on connecting another 10V regulator to the DC output of the bridge, near the filter caps, and an LM337 and it's supporting circuitry to drive the 10V rail input on the DAC PCB and the balenced line receivers. Please tell me why this super simple design is very bad for this DAC. What you propose requires more and more expensive parts, and will not go good with my budget.

The only difficulty I see in soldering is the regulators and resistor networks. It makes me wish I had some PACE equipment to solder with, but I guess my weller will have to do. I've soldered SMT packages with it by tacking the IC down, and flowing solder all over the pads and leads. I then use solder wick to absorb the excess solder and remove the bridges. This methos is very flawed, yes, but is the best I can accomplish with my current soldering iron. I know not to apply too much heat, and am very careful with that. I suppose flux will help a great deal, so I'll buy some.

Thanks.
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Old 13th February 2004, 05:36 PM   #103
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What is the voltage and VA rating of your transformer? What do you mean all 5 wires are connected? The secondaries should be insulated from the primaries; you better not be using an autotransformer -- that will allow all the noise from the mains to get in your powersupply -- don't do it!
I don't think that's enough filtering capacitance. Capacitors are cheap and fraction of what the SMD parts cost, so I don't know what you mean about budget. It makes no sense to build a good quality DAC and then not match that in the power supply. For a few dollars you can get all the parts of the setup I proposed from Digikey along with the rest of your order. The LM317T for example is 63 cents, and a 25V 6800F cap is under $3. It's already pretty basic and cutting any more corners is a bad idea.

For soldering the SMD parts, it helps to use the DIY hot soldering iron. Get a cheap desoldering iron from Ratshack, pull off the suction bulb and attach an $10 aquarium pump to it with a piece of silicon tube (both from a pet store), unscrew the tip and stuff some fine steel wool (just enought not to block air flow too much), and replace the tip. I think this was discussed in the wiki pages here at some point. The hot air heats the part more evenly, increasing the number of pins you solder at a time and decreasing the chance of frying it. And again, don't use regular flux, use Digikey part number KE1804-ND.
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Old 13th February 2004, 06:10 PM   #104
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You guys are scaring me. Try not to electrocute yourselves.
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Old 13th February 2004, 11:22 PM   #105
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Hmm..

The transformer has a primary side, two wires, connected to mains. The secondary side, has five wires. All are electrically connected, according to a meter. All have different voltages, but the most voltage is at, of course, the 1st and 5th wire. It's not an autotransformer. A center tap would be half the voltage, but I don't think it's tapped at the center, as I couldn't find half the voltage of wires 1 and 5. I suppose I'll redraw the schematic and order the parts. Would a toroidal transformer be better than a standard "E" core transformer? Simply buy a 24Vrms center tapped secondary mains transformer, preferably toroidal? Replace the regulators with with the LM317 and LM337 as you said? Can I also use a positive 10V regulator instead of the LM317? The ones I found are rated at half an amp. And how do I use the other power supply schematic? Just use the components that seperate the analog and digital power supplies on the output of a modified pawliw.gif? And what about the coils? Do you have a ballpark inductance?

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Old 14th February 2004, 04:33 PM   #106
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There is no VA rating on the transformer, what should I use? Also, would it be better or worse to use a fixed positive 10V regulator as opposed to an LM317?
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Old 14th February 2004, 04:56 PM   #107
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Yes, use a 24V center tapped. Should be 12V between the tap and each of the other secondaries. Oversized transformers are recommended in audio, so go for 50-60VA. Torroidals are smaller and often used in power amps, but they have higher bandwidth. I'd use an EI with an electrostatic shield between primaries and secondaries. For the L1 and L2 coils you can use Digikey M9229-ND. Half amp for the regulators is too low. Again, oversize. Use LM317T and LM337T which are 1.5A and under $1 each at Digikey. For the other schematic, yes, just use the CLC network (C4, C5, C6 and FB1), one set for each of the two digital supplies. For FB1 you can use Digikey M2310-ND.
It helps to have an EMI filter before the transformer. My IEC power receptacle has a built in capacitors and common mode choke filter. Or you can build your own. Also, put a half amp fuse on the hot line. Normally less current is drawn, but at turn on while the power supply capacitors are being charged smaller fuses may blow.

For the fixed regulator, it's fine, but I think if you intend 7810 or similar, it's more noisy than the 3XX so I wouldn't.
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Old 14th February 2004, 05:51 PM   #108
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Prune, you said for the CLC network, use one set for the two digital supplies. You want me to connect two of these networks in parallel with the 10V output? This way I'll have to digital supplys for the two on the PCB, and two positive 10V analog supplies, for the analog supply on the PCB and the op amp circuit?
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Old 14th February 2004, 06:25 PM   #109
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No, it's one analog supply. It's three way split: one just wire (analog), the other two are CLC (the first C is shared at the split point, with an LC for each digital wire). What opamp circuit?

PHP Code:
......+--------------------- +VA
      
|
      
o----~FB~------o---o-- +VD1
      
|              |   |+
      |             === ===
      |              |
C5 |C6
10V 
--o-~FB~-o---o---)---)-- +VD2
      
|+     |   |+  |   |
     ===    === ===  |   |
      |
C4    |C5 |C6 |   |
GND --o------o---o---o---o-- GND(all
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Old 14th February 2004, 07:18 PM   #110
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Oh, I understand now. I can definately build this. The op amp circuit I was talking about was this one. I want to drive that with the 10V rails.

Secondly, the transformer. Digikey has some open frame transformers from Hammond. They are mains transformers, 24V center tapped secondary. There is no VA rating, just a current rating. One is rated for 2A and one for 4A. Multiplying the secondary voltage and the current rating of the secondary, I get 48VA for the 2A transformer and 96VA for the 4A transformers. Part number for the 2A is HM547-ND and the 4A is HM548-ND. I'd like to use open frame transformers so I can connect an insulated fuse holder and EMI filter. I only planned on using the hot and neutral lines from the mains, and not the earth ground, because I don't have a chassis yet. Once I find one, I will use earth ground.
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