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Old 14th January 2013, 04:47 AM   #221
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Quote:
Originally Posted by keantoken View Post
......I found snubbing the 14KHz resonance made the biggest impact on sonics. .......
Is it possible for you to put up a pic of the normal / snubbed resonance ? Where are you measuring ? Directly at trafo output ?
Great idea for using a 'sniffer type' coil to 'hear' the difference !
Thanks.
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Old 14th January 2013, 07:44 AM   #222
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Let me describe my test setup. My signal generator is a Tektronix FG504. My scope is a Tektronix 465B. Neither are in great condition but they work. I blew the internal fuse on my FG504, which I cannot find a replacement for, so I replaced it with a carbon comp 3.3R resistor. Coax termination and attenuator calibration may be slightly off, but it's still flat within the audio band.

Rail resonances and trafo resonance since they are typically parallel impedances, are damped by low resistances and should be measured with a high impedance signal source. So I use a 1k carbon comp resistor in series with the SG output. I use carbon comp because I'm distrustful of film resistors with a coil-shaped track. This still doesn't matter at 14KHz, but I have also measured resonances in the 1MHz-40MHz range where it does. Rail resonances typically have impedance below 2R so a standard 50R SG impedance will do fine for identifying resonances.

When I do the test I leave everything about the supply as is, and the ground on the secondary side is floating. The decoupling and everything on the secondary side of the rectifier affects the secondary resonance. I connect the SG and scope ground together at one side of the seconary and SG output and scope input at the other side of the secondary. Then I sweep the frequency, looking for peaking.

The voltage at this point can't get too large or the rectifiers will switch on. It is resonance in the off-state that we are measuring.
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Old 14th January 2013, 10:20 AM   #223
bcmbob is offline bcmbob  United States
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Daniel, the PS in post #220 looks straight forward and easy to construct. What is the expected nominal DC output of that configuration?

I too am finishing a discrete project that requires ~ 35 -50 VDC, and it would be great to be able to use the "old gal" as a bench PS for both the TDA and the discrete. Is that a possibility with this design?
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Old 14th January 2013, 07:18 PM   #224
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Quote:
Originally Posted by bcmbob View Post
Daniel, the PS in post #220 looks straight forward and easy to construct.
For dual secondaries transformer, it would have been a lot easier with a pair of KBPC1004 bridge rectifiers instead of the discrete mess of 8 diodes.
Quote:
Originally Posted by bcmbob View Post
What is the expected nominal DC output of that configuration? I too am finishing a discrete project that requires ~ 35 -50 VDC, and it would be great to be able to use the "old gal" as a bench PS for both the TDA and the discrete. Is that a possibility with this design?
Yes, I'm doing the same thing.
This is derated for voltage so that caps last longer:
With 50v caps, about 9vdc~35vdc per rail.
With 63v caps, about 9vdc~48vdc per rail.
Or you could use 80v rated capacitors or 100v rated capacitors if you want to.
See capacitor datasheet for more information on the amount of hours it may last at a given voltage.


Meanwhile, back to the topic of an RC (or RCR) from "~" to "~" at the KBPC1004 bridge rectifier, and I haven't given up on the prospect of "muffle the AC, not the audio."
Quote:
Originally Posted by gootee View Post
Again, capacitance does not snub. . .
NOTE: The resistor, alone, is all that is needed to prevent or damp-out the ringing (or reflections, as the case may be). But if power dissipation in the R would then be too high, a C is added in series with the R, so that only the unwanted frequencies cause currents in the resistor. (And that is the only reason there's a capacitor in a snubber.)
Best explanation of a snubber, ever. And, quite doable!
If I understand correctly, the parameters are to do something beneficial, without getting the resistor too hot. Well, that's not complicated at all. Thanks!!

P.S.
I think we should notice some sort of benefit, such as less heat output at the amplifier heatsink or clearer audio; and with a typical TDA7294, cleaner power will probably do both benefits.
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Last edited by danielwritesbac; 14th January 2013 at 07:25 PM.
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Old 15th January 2013, 05:05 AM   #225
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Default The resistors weren't defective.

Measuring for hours to find non-defective resistors, but the problem was layout. Apparently any amount of excess resistor pin length changes the crc resistor value. Need to bring the copper rods Up through the board and against the sides of the big resistors to minimize pin length. Conveniently, the resistors stack up vertically. I'll probably mount the bypass diode trackside (underneath) so that I can adjust either voltage (diode) or current (resistor) without having to disassemble both.

P.S.
Digital multimeter was helplessly measuring its own test leads; however, ESR meter can be zeroed to negate the test leads and it resolved that the resistor values were reliant on practically zilch for pin length. So, the big search for resistors was over. All of the resistors were fine. But, I need to go to the garage for some nice thick romex solid copper.
Anyway, without some attention to detail, the prospect of using multiple resistors in parallel for lower inductance can be self-defeating if there is excess resistor pin length involved.
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Old 15th January 2013, 07:08 AM   #226
Jay is offline Jay  Indonesia
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I have failed many amplifier builds this last few weeks. For my own discrete designs I can accept it. But for chip amps!? I couldn't even solve the problem!!!? Or may be because I didn't expect failure, I didn't have the motivation to fix it.

SK3875 (Sony Gainclone) - Failed on both channels, no troubleshooting, thrown away after failure to produce sound.

LA4450 - Failed one channel. From quick troubleshooting I thought it was chip issue. Not too good anyway.

SK18752 (Sony Gainclone) - High voltage on output, both on single supply and dual supply application. I'm pissed.

TDA2030A - No sound on both channels. Geez.

TDA2009 - Using IBM laptop power supply and local bypassing (A-la Amp Camp Amp#1), powerboating, kewwl %$#%@

LM4766 - Half way to finish. Too tired of the previous failures

TDA7294 - Originally it was working just fine for about 2 days. Then I built another amp. Then I wanted to compare the new amp with the TDA7294, but no sound!! As if the amp is muted. But if power supply is disconnected, after a few minutes, there is a burst of signal flowing through the speaker. I replaced the caps one by one but same symptoms happened. Arggh... please help... what could be the problem with the capacitor symptom...?
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Old 15th January 2013, 11:23 AM   #227
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Perhaps an adjustable voltage power supply of some sort used for test but glitches every so often releasing the maximum voltage? Or, perhaps music source has strong offset or high power HF output? Or perhaps some randomly occurring short at the speaker?

Need to try that TDA7294 with big caps series to the speaker (if there was previously dc offset or dc offset surge and you block that, it may play). Definitely need to try a different speaker. Need to check out the power supply or try a different power supply. And need to try a different music source. Light bulb test might also be informative. Got led's on both power supply rails?--those can also be somewhat informative.
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Old 15th January 2013, 11:51 AM   #228
Jay is offline Jay  Indonesia
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Thanks Daniel. I have tried and checked all of those. I just thought that the capacitor discharge symptom might ring a bell? The amp doesn't amplify anything until a few minutes after the power cord is unplugged, but only plays music for a few seconds, then off slowly...
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Old 15th January 2013, 03:26 PM   #229
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Jay, the TDA7294 has a parasitic diode from output power pins to frontend power pins. Is it possible your Vs supply has higher voltage and so is heating up the package and tripping thermal protection?
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Old 15th January 2013, 04:43 PM   #230
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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.
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