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Old 9th January 2013, 08:07 PM   #181
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Default Simple power, wasn't so simple.

Hey smart guys, I need your estimates for these additions:

Cap and resistor values for RC (or RCR) across each DC rail, and located near the bridge rectifiers.

Cap and resistor values for RC (or RCR) across each transformer secondary winding. X2 caps can be used if desired.

Data:
1). With this capacitance, I believe that one may assume 24.5+24.5vac dual secondaries transformer, probably toroid and an average 270va (range 240va to 300va), but the exact model of transformer used by the constructor cannot be predicted, so please aim for "mild" RC values that will in all cases be better than omission.
2). Given the habits of diy audiophiles the exact model of bridge rectifier diodes used by the constructor cannot be predicted (could be terrible MUR, could be fantastic Fairchild Stealth, might be KBPC1004), so due to vast variance, there is no snubbing of individual diodes.
3). The remainder of the available data is shown in the attachment below. Since the last time you saw it, some simplistic RF filtering has been added, capacitance has been upgraded to support a "normal" range of transformers and the resistors have been upgraded to non-inductive types. Now it needs well estimated RC's added before-and-after the bridge rectifiers, and I couldn't calculate the values for them. Help?
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File Type: gif TankAndSmoothingNosag-DM.gif (21.8 KB, 313 views)
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Last edited by danielwritesbac; 9th January 2013 at 08:22 PM.
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Old 9th January 2013, 10:35 PM   #182
KSTR is offline KSTR  Germany
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I typically use a total series R of 1x..3x the transformer winding DC resistance. With your scheme of diode//resistor I'd put (part of) that resistance in front of it to have some lossy resistance in series to the diode's high capacitance, and increase the parallel R to about 1R or even more (and perhaps two series diodes), depending on which specific soft-sag characterisic looks best for the application.

EDIT : Snubbers accross primary and/or diode bridge are probably best found empirically and by measurement. One may not need them, though, when series resistances provide enough losses to act simultaneously as snubbers.

Last edited by KSTR; 9th January 2013 at 10:41 PM.
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Old 9th January 2013, 10:48 PM   #183
gootee is online now gootee  United States
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Quote:
Originally Posted by danielwritesbac View Post
Hey smart guys, I need your estimates for these additions:

Cap and resistor values for RC (or RCR) across each DC rail, and located near the bridge rectifiers.

Cap and resistor values for RC (or RCR) across each transformer secondary winding. X2 caps can be used if desired.

Data:
1). With this capacitance, I believe that one may assume 24.5+24.5vac dual secondaries transformer, probably toroid and an average 270va (range 240va to 300va), but the exact model of transformer used by the constructor cannot be predicted, so please aim for "mild" RC values that will in all cases be better than omission.
2). Given the habits of diy audiophiles the exact model of bridge rectifier diodes used by the constructor cannot be predicted (could be terrible MUR, could be fantastic Fairchild Stealth, might be KBPC1004), so due to vast variance, there is no snubbing of individual diodes.
3). The remainder of the available data is shown in the attachment below. Since the last time you saw it, some simplistic RF filtering has been added, capacitance has been upgraded to support a "normal" range of transformers and the resistors have been upgraded to non-inductive types. Now it needs well estimated RC's added before-and-after the bridge rectifiers, and I couldn't calculate the values for them. Help?
Daniel,

If you are talking about snubber networks, to damp unwanted resonances that might be excited by the diode turn-off, there is NO good generic recipe, because the resistance that does the damping needs to be set equal to the characteristic impedance of the specific resonant LC tank that needs to be damped i.e. Z = sqrt(L/C). And the L is probably/usually/often from the parasitic inductance in the transformer secondary, while the C might be from what the diode junctions become as they turn off.

The resistance does the actual snubbing. The C is there so that only the unwanted frequencies cause power to dissipate in the R.

Both values need to be determined AFTER the circuit is built, using a procedure similar to: paralleling film caps with electrolytic caps .

Tom
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Old 10th January 2013, 04:56 AM   #184
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Quote:
Originally Posted by KSTR
I typically use a total series R of 1x..3x the transformer winding DC resistance.
Question(s):
Would you like to raise the resistor value up to 0.33R, and are 9w 0.33R resistors strong enough?
Quote:
Originally Posted by gootee
Both values need to be determined AFTER the circuit is built, using a procedure similar to: http://www.diyaudio.com/forums/chip-...ml#post2828689
This is a beginners power supply, with led's that indicate to stay away from the lethal charge. Even when not aiming the build complexity higher than the moon, it seems possible to improve filtering because this is specifically a stereo power supply for specifically a pair of TDA7294 amplifiers, with specifically transformers from 240va to 300 va range. Although optimal might be nice, I'd settle for practical.
Problem:
I don't have low-and-high examples from which to create a reasonable (albeit slightly imprecise) average for snubbing the transformer secondary windings.
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Last edited by danielwritesbac; 10th January 2013 at 05:13 AM.
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Old 10th January 2013, 09:54 AM   #185
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by danielwritesbac View Post
Question(s):
Would you like to raise the resistor value up to 0.33R, and are 9w 0.33R resistors strong enough?
P=IČR, so a 10W 0R33 is OK for 5 Ampere continuous current. Select a wirewound that has high peak/surge current capability (which is generally the case for most any wirewound) and mount them such as to NOT heat the PCB, that is, with clearance.

For example, I've used 4pcs 5W 0R47's around the diode bridge in a 2x50W active speaker (100W xformer) which gave enough wattage headroom for normal application, playing music signal at home at moderate levels. For a heavy duty laboratory amp which must deliver full spec'd power for any amount of time and take any overload with grace, one should beef up transformer and resistor wattages considerably (as well as increasing supply capacitance and heatsinking of the chips), like 3x...4x.
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Old 10th January 2013, 03:07 PM   #186
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Quote:
Originally Posted by KSTR View Post
P=IČR, so a 10W 0R33 is OK for 5 Ampere continuous current. Select a wirewound that has high peak/surge current capability (which is generally the case for most any wirewound) and mount them such as to NOT heat the PCB, that is, with clearance.

For example, I've used 4pcs 5W 0R47's around the diode bridge in a 2x50W active speaker (100W xformer) which gave enough wattage headroom for normal application, playing music signal at home at moderate levels. For a heavy duty laboratory amp which must deliver full spec'd power for any amount of time and take any overload with grace, one should beef up transformer and resistor wattages considerably (as well as increasing supply capacitance and heatsinking of the chips), like 3x...4x.
r= 0.22ohm ----0.47ohm
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Old 10th January 2013, 10:06 PM   #187
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Quote:
Originally Posted by KSTR View Post
P=IČR, so a 10W 0R33 is OK for 5 Ampere continuous current. Select a wirewound that has high peak/surge current capability (which is generally the case for most any wirewound) and mount them such as to NOT heat the PCB, that is, with clearance.

For example, I've used 4pcs 5W 0R47's around the diode bridge in a 2x50W active speaker (100W xformer) which gave enough wattage headroom for normal application, playing music signal at home at moderate levels. For a heavy duty laboratory amp which must deliver full spec'd power for any amount of time and take any overload with grace, one should beef up transformer and resistor wattages considerably (as well as increasing supply capacitance and heatsinking of the chips), like 3x...4x.
CRC resistors
When I checked the non-inductive resistor prices at Mouser, I was pleased by 4 parallel 1.3 ohm 3W metal oxide when teamed up with a 6a05 for surge current. Will these handle the current of stereo TDA7294? Or do the crc series elements need strengthened?
Question: Bass?
In order to compensate for the decreased bass impact consequences of changing the "crc resistor" value from 0.25R up to 0.33R, I'm quite tempted to use a fast silicon diode in order to get the switch-on point a tiny bit earlier for the "rail stiffener diodes," and the fast silicon is somewhat more quiet. When I try shopping, that results in FFPF06U20STU, FR603-B, and STTH802D types. Are these more suitable than the 6a05?

Rectifier noise
Also yet more research to dump the need of dc side snubbers by swapping to Stealth schottky diodes, but was concerned about the hazards of complexity, and therefore, some sort of prefab is needed. Then I discovered that the dual series package is conveniently same width as the rails, and the "confusion factor" is reduced considerably (there's only 1 package per each transformer output wire). Wow, nice low parts count. Higher surge tolerance and less voltage drop than KBPC1004's.
A new question:
Are 4 of these (datasheet) big enough for the job of stereo TDA7294?

Transformer inductance peak
(In some ways similar to woofer inductance peak)
If a prefab crossover were made for a limited range of similar woofers, the nonspecific filter would remove at least half of the peak, which is a lot better than allowing all of the peak. Much to the point, the end user would be "in the ballpark" prior to attempted fine tuning (if any).
Likewise. . .
Could you measure a 24+24 (or 25+25) 300va transformer for the values of the secondary windings snubbers? I simply need a "better than nothing but not worse than nothing" estimate. Like a rubber bullet, to get a non-specific snubber to work needs somewhat less resistor current in order to accomplish the wider tolerances. The location of the missing filters are at the spots marked RC (in the attachment below).
Question:
In the near-absence of available data, I did find a chart for this application, and the capacitor value in common to most of the likely transformers is 4.7n, and so if this were used, what's a reasonable (but slightly less than optimal current) resistor value to put with it for the RC to snub a transformer secondary?
Attached Images
File Type: gif TankAndSmoothingNosag-DM.gif (21.9 KB, 283 views)
__________________
Tools, Models & Software for DIYClipNipper boostLM1875 TurboPowerful TDA7293 kitTDA7294 pt2pt ♦ My post has opinion.

Last edited by danielwritesbac; 10th January 2013 at 10:09 PM.
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Old 11th January 2013, 05:28 AM   #188
gootee is online now gootee  United States
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Quote:
Originally Posted by danielwritesbac View Post
CRC resistors
When I checked the non-inductive resistor prices at Mouser, I was pleased by 4 parallel 1.3 ohm 3W metal oxide when teamed up with a 6a05 for surge current. Will these handle the current of stereo TDA7294? Or do the crc series elements need strengthened?
Question: Bass?
In order to compensate for the decreased bass impact consequences of changing the "crc resistor" value from 0.25R up to 0.33R, I'm quite tempted to use a fast silicon diode in order to get the switch-on point a tiny bit earlier for the "rail stiffener diodes," and the fast silicon is somewhat more quiet. When I try shopping, that results in FFPF06U20STU, FR603-B, and STTH802D types. Are these more suitable than the 6a05?

Rectifier noise
Also yet more research to dump the need of dc side snubbers by swapping to Stealth schottky diodes, but was concerned about the hazards of complexity, and therefore, some sort of prefab is needed. Then I discovered that the dual series package is conveniently same width as the rails, and the "confusion factor" is reduced considerably (there's only 1 package per each transformer output wire). Wow, nice low parts count. Higher surge tolerance and less voltage drop than KBPC1004's.
A new question:
Are 4 of these (datasheet) big enough for the job of stereo TDA7294?

Transformer inductance peak
(In some ways similar to woofer inductance peak)
If a prefab crossover were made for a limited range of similar woofers, the nonspecific filter would remove at least half of the peak, which is a lot better than allowing all of the peak. Much to the point, the end user would be "in the ballpark" prior to attempted fine tuning (if any).
Likewise. . .
Could you measure a 24+24 (or 25+25) 300va transformer for the values of the secondary windings snubbers? I simply need a "better than nothing but not worse than nothing" estimate. Like a rubber bullet, to get a non-specific snubber to work needs somewhat less resistor current in order to accomplish the wider tolerances. The location of the missing filters are at the spots marked RC (in the attachment below).
Question:
In the near-absence of available data, I did find a chart for this application, and the capacitor value in common to most of the likely transformers is 4.7n, and so if this were used, what's a reasonable (but slightly less than optimal current) resistor value to put with it for the RC to snub a transformer secondary?
Transformer secondary's leakage inductance will depend on how it was manufactured. But ONE particular 25VRMS/300VA transformer I can get a number for has 43.3 uH there.

The snubber resistor value should be set to

Z = √(L/C)

I was thinking that the parasitic capacitance was dependent on the diodes. And it will also depend on the layout, etc.

But IF you were to use 43.3 uH and 4.7 nF, then the optimal snubber resistance would be sqrt(43.3u/4.7n) = 96 Ohms .

For the series capacitance in the snubber network, you should use 4 to 10 times the parasitic capacitance. Using 10x would give a little better damping by the resistor at the expense of more power dissipated in the resistor.

The series capacitor's value would be in the range of 18.8 nF to 47 nF, for your case.
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Old 11th January 2013, 07:06 AM   #189
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My secondary has a 14KHz resonance. I used a 1.5u/22R snubber for this. I found there was a surprising increase in image depth after adding the snubber. I could not find any ultrasonic resonances which I falsely remembered but there was a resonance at 14MHz that was totally unaffected by the snubber.

This does not agree well with your analysis or mine. I discovered that mains impedance affected resonance greatly (I have a mains filter+switch and a variac), but the region several Khz and up was mostly unaffected and tunable via RC. I think trafo resonance may be highly dependent on external impedances, so it may not be practical to employ a secondary RC if a mains filter is not also used.
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Old 11th January 2013, 12:29 PM   #190
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For the sake of argument (did I just say that ? ) the power supply redjr used in his build hasn't received any comment. As there have been two approaches discussed, the basic kit upgrade and/or the new advanced PCB/PS, does anyone have an idea/recommendation or two on that unit they can share?

SMPS300R
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