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Old 30th April 2013, 01:51 PM   #31
AndrewT is offline AndrewT  Scotland
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There are dozens and maybe hundreds of posts, telling you how to work out the maximum continuous AC current into a resistor load.
There will also be dozens of posts telling you that loading a transformer with a capacitor input filter is VERY DIFFERENT.
P=Vac*Iac does NOT apply to a capacitor input filter.
Quote:
0,33A
should be written as 0.33Aac. it is the AC rating when loaded with a resistor.
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Old 30th April 2013, 06:30 PM   #32
Frarun is offline Frarun  Poland
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Yes of course. I should now that.
Can I assume the phase of impedance is something like 45 degrees ? From calculation it will give 0.33Aac/rt(2) ~= 0.23Aac for resistive part. Is that what I can get to keep the transformer in good condition?

Oh I just realized if I go for more condensers I increase a reactance, and this phase may be even bigger...

Last edited by Frarun; 30th April 2013 at 06:38 PM. Reason: another thought
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Old 30th April 2013, 10:53 PM   #33
AJT is offline AJT  Philippines
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Quote:
Originally Posted by Frarun View Post
AndrewT I measured the 370mA on the AC line.

The transformer model: 'tez 4.0/d 12v 4va' 0,33A (fuse symbol) 0,4A . It is overloaded comparing to nominal 0,33A but not so much (about 10%).

How this is possible your current value is so small on the DC? Is not that just P=UI?
strange that you want to measure that, is your traffo overheating? if not then it is not an issue...
resistors as an ac load has unity power factor...the 370mA you measured will have several components, part goes to heating up your traffo and part just energy storing components...
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Old 1st May 2013, 01:37 AM   #34
Frarun is offline Frarun  Poland
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temperature is OK. It is worm and I can attached hand to it.

Somehow we skipped from big power traffos to small powers. The rules should be the same I think. Nevertheless forget about the ampers for the moment. With this trafo I have:

stby off: 15,8vac -> 19,3vdc (after diode bridge + 1x470uF filter)
stby on: 12,1vac -> 12,5vdc (after diode bridge + 2x470uF filter)

I realized now that this 12,5vdc is not enough for my 12V voltage regulator (because of voltage drop) and I get 11V at the end. I have relays for input switching on 12V coils (works fine with 11V), and TDA8425 which has a minimal supply voltage 10.8V, but typical 12V... I consider this transformer as too small and use as temporary solution. Maybe I will go for 10va to have some reserve.

This all tests are without any audio signal attached, just a PSU observation.
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Old 8th May 2013, 01:20 PM   #35
Frarun is offline Frarun  Poland
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I have another doubt. Is it OK to skip the capacitors near the chip on the power lines, if I have such huge capacitance in the PSU? I am asking, because maybe I will change the heat-sink to something like this on the photo.

I see difficulties to dissipate the heat in my small enclosure. It is probably like 25W-30W heat to dissipate. From my general calculation I need about 2C/W (or even smaller) for the heat-sink per channel (assuming 1C/W for chip case, 0.5C/W between case and heat-sink, and 40C ambient temp). This one on the photo is still small, but gives me more compact area if I put the GC PCBs back-to-back (with two such heat-sinks vertically).

Click the image to open in full size.
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Old 8th May 2013, 04:15 PM   #36
AndrewT is offline AndrewT  Scotland
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Do not omit the MF and HF decoupling from the amplifier.
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Old 8th May 2013, 10:56 PM   #37
Frarun is offline Frarun  Poland
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The 0.1uF decoupling will be in place near the chip. I remember from digital electronic circuits it is good practice to use such decoupling on power lines.

I connected the amp to PSU with serial 60w bulb on 230vac line and soft-start. I am using a second version of PSU here which has a fuses on the PCB (is bigger in size, not sure if final, but I am addicted from Farads now it have 2x15F per rail). No explosion, no strange smell. Little heat on heat sink (still clean-aluminium). No DC offset on the amp outputs. So far so good.
The datasheet shows some low value capacitors which blocks eventual high-freq osculations. I didn't implemented this due to missing parts. There is option on the PCB to add them and not sure if required in my case but maybe better to be on the safe side.
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Old 9th May 2013, 04:13 AM   #38
gootee is offline gootee  United States
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The small cap is necessary (i.e. not optional) for high-frequency bypassing, for stability. It should be connected _extremely_ close to the chip's power pin and load gnd, e.g. within a mm or two. The capacitance value is not as important as having the shortest-possible connections. It's usually best to find a cap that will fit directly across the pins and then use the largest value available in that case size. An X7R ceramic is usually the safest, there, in terms of not accidentally causing HF resonances. But a film cap could perform better. You could try both and see if there is any difference in the chip's temperature. A higher chipamp temperature under similar conditions would probably mean that there was high-frequency oscillation, or even just excessive HF ringing, in which case you would probably want to switch back to the X7R ceramic.

You will definitely also want a few hundred uF (probably at least 470 uF) in parallel with that small cap, so that fast-rising transient currents do not have to try to come through the inductance of the power and ground rails. Try to get the electrolytic cap within a cm or so of the pins.

We could calculate the minimum decoupling capacitance needed, and the maximum inductance that could be tolerated in the decoupling network (calculating either in the time domain with differential equations and transient current specs etc, or, in the frequency domain for the maximum tolerable impedance seen by the power pins in order to have some max delta V due to the worst-case delta I, giving Z = dV/dI, which usually needs to be good out to a few hundred kHz, for a chipamp), which (the max inductance) basically equates to maximum allowable connection lengths plus cap lead spacing. But 470 uF within a cm or so should usually be OK, for a chipamp. But, if you can fit more of those in parallel, even a little farther away, then do it.
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Old 9th May 2013, 04:52 PM   #39
Frarun is offline Frarun  Poland
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I mean 15mF, not 15F in the previous post (this would be huge!)

The 470uF somehow kills my idea with heat-sink on the PCB side (please see a picture). I see I purchased too much universal PCBs for GC where area for decoupling is too big for my case (C10 has 25mm). I will waste some space here. The C8 and C9 is in place. Not so close as you described.
Electronic correctness is more important here. I will follow an advice with 470uF. The impedance will be noticeable between PSU and GC PCBs. The precise calculation will not be necessary this approximated values looks OK.

Thanks.

Click the image to open in full size.

Last edited by Frarun; 9th May 2013 at 04:56 PM.
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Old 9th May 2013, 09:37 PM   #40
Frarun is offline Frarun  Poland
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Finally I made some measurements with connected load (3 x parallel 18R / 20W). I was able to use oscilloscope and in deed the small capacitors was necessary to cancel same oscillations. Between + and - input (220p) cancels some hi-freq oscillations with same amplitude as signal. Second one (10pF connected parallel with resistor in the amp loopback) cancels small 400mV 750khz oscillations on the output.

I disconnected the safety-bulb. The amp sometimes loose the signal if I make an output voltage bigger than 17V p-p. This is like switching 3 seconds output wave, and 3 secs 0V. This is probably chip overheating (?). Later I lost my fuse on -35V rail which was rated 1.5A (delayed one) so I need to get a new one.
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