I have a DIY PSU for a small amplifier. I am using a 25.2v 2a CT transformer. Using a full wave bridge, I obtain two rails. Each rail has 4,400uf of filter capacitance.
To test the PSU under load, I have 4 ohm speakers connected and driving it with sine waves. I selected a low frequency (20 some Hz) that would cause the most voltage sag. Amp was driven into clipping slightly.
Idle voltages (estimated 100ma current)
AC side: 28.0v, DC side: 35.0v
Under max load conditions (can't measure current without cutting wires)
AC side: 25.7, DC side: 29.4v
I have been told that I should select a larger transformer, but I argue that since the loaded AC side is above it's rated voltage, the trafo is fine. The dynamics of music would never load the PSU like the test anyway.
Thoughts?
To test the PSU under load, I have 4 ohm speakers connected and driving it with sine waves. I selected a low frequency (20 some Hz) that would cause the most voltage sag. Amp was driven into clipping slightly.
Idle voltages (estimated 100ma current)
AC side: 28.0v, DC side: 35.0v
Under max load conditions (can't measure current without cutting wires)
AC side: 25.7, DC side: 29.4v
I have been told that I should select a larger transformer, but I argue that since the loaded AC side is above it's rated voltage, the trafo is fine. The dynamics of music would never load the PSU like the test anyway.
Thoughts?
The question seems to be whether it's OK to pull 5A RMS out of a 2A trafo. The answer seems obvious---no, unless you're trying to fry the trafo---so I'm not sure I'm understanding the question properly. If this is really what you're asking a I've found good rule of thumb is to choose a trafo with twice the VA rating of the peak load; when operating close to (but below) the VA rating the core tends to get a bit warm for my preferences.
25.2 VAC should yield 34.4 VDC. 34 VDC should be 17 AMPS peak into two 4 ohm loudspeakers.
The reason your meter reads the transformer output voltage as 25.7 under load is that it is not an accurate meter! It does not do true RMS.
However a real loudspeaker at full power will double it's impedance. so if you want to be cheap get one with an 8 amp rating.
I do not want to go into an actual analysis of output versus AC current rating as that is not for this level of question and the information is easily found these days.
The reason your meter reads the transformer output voltage as 25.7 under load is that it is not an accurate meter! It does not do true RMS.
However a real loudspeaker at full power will double it's impedance. so if you want to be cheap get one with an 8 amp rating.
I do not want to go into an actual analysis of output versus AC current rating as that is not for this level of question and the information is easily found these days.
what are your max load conditions?
DC current through a resistor load,
or
Sine-wave test signal through power amplifier into resistor load,
or
music signal through power amplifier into resistor load,
or
music signal through power amplifier into speaker load?
music power vs sine wave power
Design the PS to supply the rails "stiffness" to support the speaker load at the lowest frequency at the highest level of your music.
If it sags at 30 Hz appreciably and not at 10 KHz by using test sine waves, what will that sound like with music playing into the speakers load?
Transformers are rated by the average heating rise above ambient, your criteria should be related to XFMR regulation at peak levels for the longest durations expected.
Design the PS to supply the rails "stiffness" to support the speaker load at the lowest frequency at the highest level of your music.
If it sags at 30 Hz appreciably and not at 10 KHz by using test sine waves, what will that sound like with music playing into the speakers load?
Transformers are rated by the average heating rise above ambient, your criteria should be related to XFMR regulation at peak levels for the longest durations expected.
I bought some non inductive resistors (4 Ohm) for a better test. Here are the results both channels driven just before clipping. I tried a few frequencies and the results were the same at each frequency (as expected with the resistors).
PSU voltages
AC voltage quiescent: 27.7vrms
AC voltage loaded: 25.0vrms
DC voltage quiescent: 34.5v (+/- 17.25v)
DC voltage loaded: 27.6v (+/- 13.8v)
Amp voltages
AC signal at output w/ 4 Ohm: 6.15vrms (9.5 watts)
AC signal at output w/ no load: 6.30vrms (same signal level as with the load)
PSU voltages
AC voltage quiescent: 27.7vrms
AC voltage loaded: 25.0vrms
DC voltage quiescent: 34.5v (+/- 17.25v)
DC voltage loaded: 27.6v (+/- 13.8v)
Amp voltages
AC signal at output w/ 4 Ohm: 6.15vrms (9.5 watts)
AC signal at output w/ no load: 6.30vrms (same signal level as with the load)
Note) something seems wrong with your amp or test (something is limited before the output stage) b/c you said w/ no load with +/-17 volt DC rails, the amp puts out only 18 Vp-p max? I would expect to see 28V p-p
then with load same clipping point 9 Vp / 4 ohm = 2.2 A peak
1st/ check your set up with gains and expected drive levels.
1) post schematic for amp? with test equipment used
2) It would be useful to O scope the DC supply under load a)/ report ripple voltage Vp-p and b)/ avg DC w multimeter.
then with load same clipping point 9 Vp / 4 ohm = 2.2 A peak
1st/ check your set up with gains and expected drive levels.
1) post schematic for amp? with test equipment used
2) It would be useful to O scope the DC supply under load a)/ report ripple voltage Vp-p and b)/ avg DC w multimeter.
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I did not change the signal when I went from loaded to no load. I just wanted to see the amp as a voltage source and since the output changed very little, it is a pretty good one.
With 9.5 watts out and considering dissipated power, the PSU is in no danger of burning up. However, my focus has changed since I'd like to get 20 watts of output. The amp is a two channel TDA2040 based circuit right off the datasheet. With that PSU, I'm not going to get 20w per channel. Time to look into a better supply.
Something does seem wrong even with +/- 13.8 volt rails, It seems I should be getting more output swing before clipping. At least 12 watts, I'd think.
Here's another approach. You say you'd like 20W out, but this is an arbitrary number of no real significance, and there's speaker efficiency to consider too.
Is it loud enough and does it sound OK and is the trafo getting hot if you run it for a good while (representative of what it is likely to encounter at high volume in practice)? If it sounds OK, and it's not alarming to the touch, then OK.
w
Is it loud enough and does it sound OK and is the trafo getting hot if you run it for a good while (representative of what it is likely to encounter at high volume in practice)? If it sounds OK, and it's not alarming to the touch, then OK.
w
That is a good point. My speakers are high efficiency, large ported cabs with mid and high horns that I had to tame. They get "tinnitus" loud!
However, I plan on making a small portable amp that will see various speakers. Having the extra head room seems desirable.
The TDA2050 seems to provide a better output swing at a given PS voltage, but I need to confirm with tests. That and a better power source should get me where I want.
for a target maximum output of 12W+12W a transformer with a VA rating from 24VA to 48VA will work. But I recommend that you always use >100VA for power amplifiers and I prefer 160VA as the minimum for a toroid to obtain a reasonably low regulation of about 6% to 8%.
For your higher target of 20W+20W the transformer becomes a 40VA to 80VA (at a slightly higher secondary voltage). Again I would apply the minima of >100VA and preferably 160VA with lowish regulation. Small EI transformers have enormous regulation. An EI designed for specifically low regulation (instead of cost and temperature) is no longer small.
For your higher target of 20W+20W the transformer becomes a 40VA to 80VA (at a slightly higher secondary voltage). Again I would apply the minima of >100VA and preferably 160VA with lowish regulation. Small EI transformers have enormous regulation. An EI designed for specifically low regulation (instead of cost and temperature) is no longer small.
I have a split bobbin EI core transformer out of a 250w class D Sub that blew up. The transformer is good. The actual core measures 76 x 63.5 x 40.5mm. It appears to be around 100 VA.
Secondary (CT) measures 100 volts. The open design should allow for easy unwinding and rewinding a secondary of appropriate gauge wire. Unloaded volts per winding is .392.
According to the TDA2050 datasheet, a +/- 18 volt rail should provide 30 watts of output pre clipping power into a 4 ohm load. +/- 15 volts would provide 20 watts, So I think a loaded PS of in the range of +/- 16 or 17 volts would be my target. Looks like I would need a 5Arms PS for two channels, but not sure of the transformer calcs.
Secondary (CT) measures 100 volts. The open design should allow for easy unwinding and rewinding a secondary of appropriate gauge wire. Unloaded volts per winding is .392.
According to the TDA2050 datasheet, a +/- 18 volt rail should provide 30 watts of output pre clipping power into a 4 ohm load. +/- 15 volts would provide 20 watts, So I think a loaded PS of in the range of +/- 16 or 17 volts would be my target. Looks like I would need a 5Arms PS for two channels, but not sure of the transformer calcs.
Does your salvaged EI have a good 110/120Vac Primary?
Does the salvaged EI sound completely quiet when connected to mains with the secondary open circuit?
Does your salvaged EI run cool, or warm, or too warm when the secondary is open circuit?
If all these are OK, then you can wind on a pair of secondary windings for your low voltage supply.
I would allow 3Aac per sq mm of secondary wire cross sectional area.
If you target 100VA and 16Vac+16Vac, then your secondary current is ~3.1Aac and this requires ~1sqmm of wire. This is ~1.1mm diameter, Easy to wind.
If you can get in sufficient turns of a slightly thicker wire then go for that. It will reduce regulation, it will reduce heating of secondary, it will reduce the voltage sag under power.
Try for 1.2mm diam or 1.3mm diam.
If the only problem is a little bit of excess noise when driving the primary only, then this can be cured.
Tell us what you find and then we can guide you forwards.
Can you measure the size of the core area of the middle leg of the "E" ? This should be exactly double the area of the "I".
Output VA ~ 30 * core area^2, where core area is measured in sq inches.
Does the salvaged EI sound completely quiet when connected to mains with the secondary open circuit?
Does your salvaged EI run cool, or warm, or too warm when the secondary is open circuit?
If all these are OK, then you can wind on a pair of secondary windings for your low voltage supply.
I would allow 3Aac per sq mm of secondary wire cross sectional area.
If you target 100VA and 16Vac+16Vac, then your secondary current is ~3.1Aac and this requires ~1sqmm of wire. This is ~1.1mm diameter, Easy to wind.
If you can get in sufficient turns of a slightly thicker wire then go for that. It will reduce regulation, it will reduce heating of secondary, it will reduce the voltage sag under power.
Try for 1.2mm diam or 1.3mm diam.
If the only problem is a little bit of excess noise when driving the primary only, then this can be cured.
Tell us what you find and then we can guide you forwards.
Can you measure the size of the core area of the middle leg of the "E" ? This should be exactly double the area of the "I".
Output VA ~ 30 * core area^2, where core area is measured in sq inches.
don't try to predict the output current requirements, neither DC nor AC. An amplifier replaying audio is a very non steady load and normal current limits do not apply.
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Andrew,
It is quiet. Runs up to about 95F after 15 minutes. Barely lights a 25 watt bulb when I did the inline dim bulb test. Seems fine to me.
Measuring the middle leg volume (Not the full height right? Pretty much the area under the bobbin, if I understand.), I get 2.44 sq. inches or ~178VA.
Thanks for your help so far!
It is quiet. Runs up to about 95F after 15 minutes. Barely lights a 25 watt bulb when I did the inline dim bulb test. Seems fine to me.
Measuring the middle leg volume (Not the full height right? Pretty much the area under the bobbin, if I understand.), I get 2.44 sq. inches or ~178VA.
Thanks for your help so far!
An open circuit transformer should not light a dim bulb.
If you have a clear glass, a 25W filament may just show a tiny bit of red/orange glow, but pretty dim. An opaque glass would hide this.
170VA to 180VA 18+18Vac would be equivalent to ~ 5Aac.
Use 1.4 or 1.5 or 1.6mm diameter wire for each of the secondaries.
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