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Old 15th January 2013, 06:22 PM   #231
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Quote:
Originally Posted by bcmbob View Post
Daniel, on post #220 you are showing cement resistors. Was there a reason for that other than that's what you had on hand? The Mouser links you sent me are not.
Ceramic/Cemented resistors in values Larger than 0.25R are unfortunately inductive. But, the item in that link is 0.24R, and can work fine. By the time the pins go down through the perfboard to reach the rail, it may be 0.27R and probably still just fine. It can't handle the power up surges all by itself, but the diode takes the brunt of the surge.


I'm planning to upgrade mine to 4 or 5 of "metal oxide 3w" per rail: MOS3CT631R132J KOA Speer | Mouser as specified by the schematic in post 218, and installed as specified at post 225.

4 paralleled 1.3 ohm 3W metal oxide resistor makes: 0.33 ohms 12 W
5 paralleled 1.3 ohm 3W metal oxide resistor makes: 0.26 ohms 15 W

SO, you see that the schematic at post 218 has a high power, low cost, non-inductive resistor that can be value tweaked without disassembly.
The metal oxide resistors don't have a big surge tolerance; however, again the diode takes the brunt of the surge.

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The above resistors measure current, but resistors don't really measure voltage, and music signal is highly dynamic, so that any resistor setting is always simultaneously too much and not enough. CRC Resistors react to dynamics, especially bass dynamics, by dumping them like throwing the baby out with the bath water. Fortunately, the bypass diode limits the voltage losses.

This limits the crc resistor's rail sag to a voltage setting of your choice.
Fast silicon diode is about 0.45v (use with small amplifier)
6a and 10a standard diodes are about 0.6v (use with medium size amp)
Series pair of fast silicon diode is about 0.9v (use with big amplifier)

The TDA7294 is inherently slightly bass shy due to the inbuilt softie current limiter protection. The bypass diode helps by "reinforcing" the rails during higher power bass. For example replay at 1 watt with the bass set to match human ear curve is more than enough surge to throttle a 144va transformer. So if you built a subwoofer amplifier's power supply, you wouldn't want CRC resistors throwing the bass away. Anyway, the bypass diode is for good bass; and, the resistors are for pretty midrange and treble.

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Another form of rail fluctuation that can be controlled is noise peaks. On a built power supply, bridge rectifiers provide about 1V to 2V worth of noise that is able to charge up the caps on a lightly loaded or unloaded supply. This is impressive as a percentage. All those little music details you'd want to hear are also a tiny percentage. I believe that we can clear them up by putting an RC from "~" to "~" at the bridge rectifier. Rather than just imagination, less noise really does mean less charge, and should show up as at least 1V less output, measurable on an unloaded power supply. The DC side of the bridge rectifier can only be snubbed only a little bit without dulling the audio. However, the AC side (transformer secondary) of a bridge rectifier can be snubbed a LOT without dulling the audio, and the parameters are easy--make a benefit without overheating a resistor.
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Last edited by danielwritesbac; 15th January 2013 at 06:34 PM.
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Old 15th January 2013, 06:45 PM   #232
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Dan, snubbing the secondary will not affect rail voltage at all. This is because the snubbing is only at treble and ultrasonics and the charging pulses that do all the work are just 120Hz and its harmonics. The 120Hz rectifier noise will still be fully present, just the diode snap won't be ringing the trafo like a bell.

Furthermore, what you're measuring with your ESR meter is probably the inductance of the power resistor leads, not lead resistance. Thicker leads result in marginally less inductance. Flat leads are even better.

If the resistors do require such thick leads, does this really matter unless the rest of the supply has thick wire? If you think it matters (which is unlikely in this application) why not solder the diodes directly across the resistors at the start of the pins?

Last edited by keantoken; 15th January 2013 at 06:49 PM.
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Old 15th January 2013, 07:12 PM   #233
bcmbob is offline bcmbob  United States
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As my PS reading and investigation continues, I came across these two tools. Could either be useful for visualizing some of the ideas being discussed here? It's that old - picture worth a thousand words - thing. Even if not they look like fun

WEBENCH Power Designer

Duncin's PSU Designer II
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Last edited by bcmbob; 15th January 2013 at 07:15 PM.
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Old 15th January 2013, 08:23 PM   #234
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Quote:
Originally Posted by keantoken View Post
Dan, snubbing the secondary will not affect rail voltage at all.
On an unloaded power supply, my voltmeter reads 1v to 2v lower figures after snubbing a bridge rectifier.
Similar to what Mark Houston did here.
But I did it like this, same as Technics circa 1975:
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File Type: jpg Block Bridge ala 1975.jpg (60.4 KB, 225 views)
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Old 15th January 2013, 08:42 PM   #235
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Dan, it could be EMI fooling the meter. I've anguished my meters several times on oscillating amps and so on. Try hooking the meter up with 10k resistors in series with the probes, and a 100n cap or so across the probes.

BTW that's not snubbing, just capacitive bypass or decoupling.

Last edited by keantoken; 15th January 2013 at 08:44 PM.
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Old 15th January 2013, 08:47 PM   #236
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There is another measure:
If I remove those caps from the Technics integrated receiver (amplifier+tuner), the amplifier heatsinks will get fire hot and the tuner can only receive 1 station, badly.
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Last edited by danielwritesbac; 15th January 2013 at 08:51 PM.
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Old 15th January 2013, 09:27 PM   #237
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Those rectifiers must be emitting TONS of EMI. I would not trust a meter here. Even so a volt or so rail difference should not matter; One should not expect the mains voltage to be exact and the Technics designers would not have made this mistake.

Perhaps without the bypass the rectifier area resonances were at high enough frequencies to stomp the AGC? In any case the bypass won't remove resonances but it will lower their frequency and localize the diode snap to a smaller area.

Last edited by keantoken; 15th January 2013 at 09:33 PM.
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Old 15th January 2013, 09:55 PM   #238
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Back to semantics, however one might choose to say it: lower noise = lower voltage.
I expect some sort of voltage decrease after successfully applying a noise filter.
The digital multimeter may be imperfect for this task.
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Last edited by danielwritesbac; 15th January 2013 at 10:22 PM.
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Old 15th January 2013, 10:27 PM   #239
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Dan, are you measuring AC voltage or DC voltage? It should not change DC voltage.

There is a fun and easy way to adjust the snubber I just used. Take an inductor and probe it with the oscilloscope. Use a trimmer or pot as a series resistor; it may resonate with your scope and you'll need the trimmer to tune out the resonance (parallel trimmer should work better, but I didn't try it). I used a 50k pot. Put the magnet near a CRT monitor or something that make RF pulse noise and adjust the pot so that there is no resonance shown on the scope (make sure the resonance is higher in frequency than the resonance you are trying to measure, or it will mask the measurement). Put the inductor near the trafo. You'll see the integral of the rectifier charging pulses. There will be slanted sections between flat sections. The slanted/distorted sections are the charging pulses. There will be a bounce or resonance as they snap to the flat sections. Adjust the snubber for the most benign bounce behavior. My 3.3u snubber cap is still too small; I cannot make the R small enough without creating another 2KHz resonance. But a 2KHz resonance is more tolerable than a 14KHz resonance which causes sibilance and disrupted imaging and general harshness.
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Old 15th January 2013, 10:53 PM   #240
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So, a "~" to "~" snubber for KBPC1004, can be using an RC made with a 2.2uF cap series to a 50 ohm variable resistor. That's great! Thanks!
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Last edited by danielwritesbac; 15th January 2013 at 11:00 PM.
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