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Old 27th April 2013, 04:52 AM   #21
gootee is offline gootee  United States
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Quote:
Originally Posted by Frarun View Post

<snipped>

Regarding capacitance. After some more reading about GC I see there ware a versions with only 1000uF per rail (with 200VA transformer), so this is all subjective thing ...

<snipped>
Not quite. The capacitance determines what you can say the rated max output power is. For a given rated max power, there is a minimum capacitance, below which the amplifier will clip for a sine signal at or above the rated max power.

Remember that the load, for each rail of the power supply (with a class AB amplifier), is the amplifier in series with the speaker, not just the speaker. The dropout voltage is the minimum voltage between the amplifier power pin and the amplifier output pin.

The rail voltage minus the ripple voltage minus the amplifier clipping (i.e. dropout) voltage must be greater than the peak output signal voltage. Otherwise, the ripple voltage will intrude down into the voltage space that is normally occupied by the amplifier, and will gouge ripple-shaped chunks out of the signal voltage waveform.

Attached is a spreadsheet that calculates the minimum required capacitance in two ways. One way assumes the absolute-worst-case signal, i.e. DC at the peak level of a sine output, for a given rated maximum output power. That way gives a "bulletproof" minimum capacitance that will never allow clipping, even for every non-sine signal. The second way uses the maximum RMS sine voltage, instead of the peak. If you need to minimize the capacitance needed, the second way should usually be "good-enough".

Also remember that the system will probably be somewhat more than 50% efficient, i.e. somewhat less than HALF of the power will be wasted, as heat. So I do not believe that 1X the rated maximum output power would be suitable, for a transformer VA rating. I would always want to use AT LEAST 2X the rated max amplifier output power, for the transformer VA rating.
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File Type: zip Res_Cap_Reqmnts_Gootee_xlsx.zip (81.7 KB, 11 views)

Last edited by gootee; 27th April 2013 at 05:03 AM.
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Old 27th April 2013, 09:43 AM   #22
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by Tony View Post
for a 150va traffo a soft start circuit is not really needed....if you have a power switch rated for at least 10amps.
150VA on 220Vac needs T625mA or T800mA fuse.
Why use a bigger fuse and a bigger switch to save the cost of the soft start?
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Old 27th April 2013, 10:53 AM   #23
AJT is offline AJT  Philippines
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Quote:
Originally Posted by AndrewT View Post
150VA on 220Vac needs T625mA or T800mA fuse.
Why use a bigger fuse and a bigger switch to save the cost of the soft start?
because switches are much simpler and cheaper for a newbie.....and fuses are likewise dirt cheap...
a DPST switch similar to the one shown below is rated for 8amps...
besides, the dc resistance of this size of traffos are high enough to allow switches only imho...
Click the image to open in full size.
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Old 27th April 2013, 06:31 PM   #24
Frarun is offline Frarun  Poland
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First of all I totally forgot about the fuses, and rated power on for eventual switch. I would like to control the power-amp stage PSU with relays, to have a stand-by mode. This GC will be extension for pre-amp built many years ago. I see I need consider some high current topics yet .. So Tony you say there will be some about 10Amp kick at the start...

I faced another design problem.. pre-amp is powered from a low voltage transformer (will be replaced with 150VA), and there is lm7805 an lm7812 voltage regulators. I made a test with my new transformer which gives about 35V DC (this is maximum input voltage for this chips) and they go really hot! I need to consider a separate mini -PSU for that..

Quote:
Originally Posted by gootee View Post
AT LEAST 2X the rated max amplifier output power, for the transformer VA rating.
I would like to have such ratio, but I have a limited space for transformer in my case. The space for capacitors are limited as well... I fight with the space, and will see If I can get about 2x40W continuous power output...
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Old 27th April 2013, 11:22 PM   #25
AJT is offline AJT  Philippines
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Quote:
Originally Posted by Frarun View Post
.. So Tony you say there will be some about 10Amp kick at the start...
it will be only a few cycles, energy content is low and nothing to worry about...

Quote:
Originally Posted by Frarun View Post
I faced another design problem.. pre-amp is powered from a low voltage transformer (will be replaced with 150VA), and there is lm7805 an lm7812 voltage regulators. I made a test with my new transformer which gives about 35V DC (this is maximum input voltage for this chips) and they go really hot! I need to consider a separate mini -PSU for that..
you do, you were trying to lose about 23volts on the regulator so you are wasting power that way....what is wrong with the old setup? if you have space i would leave it alone...

Quote:
Originally Posted by Frarun View Post
I would like to have such ratio, but I have a limited space for transformer in my case. The space for capacitors are limited as well... I fight with the space, and will see If I can get about 2x40W continuous power output...
i have no doubt that you can get that power with a 150va traffo..
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Old 29th April 2013, 07:10 AM   #26
Frarun is offline Frarun  Poland
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The old one PSU was a transformer (50VA) with some kind of stand-by module. This transformer is too big to put together with the new one. The stand-by module is something which maybe will be redesigned. Of course if I go in that direction to enable/disable power-amp, but your suggestion with the ON/OFF switch is really attractive too (less work to do).

Please look at the picture. Now when I know the PSU condensers are required to provide 'solid' voltage for power-amp, this makes even less space for everything. (on the right area the old trasformer, the soft-start module and capactitors module). I am thinking to maybe put some modules vertically. No doubts I can't do anything with connectors (module on the backplane). It is too much adjustments at this moment (even if this upgrade with GC is a great fun I have somehow limited time for that).

If you can see, there is no any fuses and power-cord is without 'grounding' (and would be nice to have some space the socket). Even if the safety is first, I will think about the cord later.

I need to change the approach to heat sink as well, and probably cut this on two parts, to have more compact area. The GC will be put on the left because there is better ventilated area. But this is another story ...

Thanks for any suggestions. This PSU is still some kind of challenge!

Click the image to open in full size.

Last edited by Frarun; 29th April 2013 at 07:33 AM. Reason: /problem with image/
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Old 30th April 2013, 02:58 AM   #27
gootee is offline gootee  United States
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You could bolt the heatsink to the outside of the case, and mount the devices against the inside of the case, where the heatsink attaches.

That would probably also work better, in terms of getting rid of heat.
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Old 30th April 2013, 07:58 AM   #28
Frarun is offline Frarun  Poland
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This is more classical case. There is a black cover. The photo 'describes' this better. I will stick with heat sink inside.

I started test with the small transformer for pre-amp with nominal 12V 4VA. It is funny how voltage drops when I take current. Normally with stand-by off (takes about 30mA) the AV voltage is 15V, and when I enable pre and display which takes about 370mA the AC on the transformer drops to 12V.

One more notice. I needed to drop down a back-light current for LCD from 100mA to about 60mA with serial resistor, because the situation with voltage drops was even worse and I couldn't really get 12V after regulator. Anyway this is not a super stable solid PSU. Maybe I should think about bigger condensers like in case of power-amp toroid. Now I have 2x 470uF, and could exchange this to 2x1000uF. I feel every voltage counts..

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Old 30th April 2013, 10:33 AM   #29
AndrewT is online now AndrewT  Scotland
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The maximum continuous DC current from a capacitor input filter connected to a 4VA 12Vac transformer is ~167mAdc.

370mAdc is massively overloading the transformer. The internal heat will build until you turn down that current draw.
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Old 30th April 2013, 01:47 PM   #30
Frarun is offline Frarun  Poland
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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?
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