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Chip Amps Amplifiers based on integrated circuits

why no dc protection
why no dc protection
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Old 14th August 2018, 11:57 AM   #11
scottjoplin is offline scottjoplin  Wales
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You could short them simultaneously
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Old 15th August 2018, 06:11 AM   #12
goodguys is offline goodguys  United Kingdom
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Sorry, the ebay search is rubbish. Here is the number again along with a few others hopefully one of them will work. 272854006702 123153716819 142385939188 272961379262


Is there anything that can be done to lower the chance of dc output, a weak power supply can cause clipping if underpowered for the job, but is there anything else.


The transformer i am wanting to use for this project is the multicomp mcfe500/25. 25 volt dual secondaries 500 va. If i were to hook the two secondaries together would that give me 1000va. What do you think of multicomp, are they 'audio' grade



Many thanks
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Old 15th August 2018, 07:55 AM   #13
Samsara is offline Samsara  France
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It's 2x250VA = 500VA, not 2x500VA.
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Old 15th August 2018, 08:11 AM   #14
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by goodguys View Post
Sorry, the ebay search is rubbish. Here is the number again along with a few others hopefully one of them will work. 272854006702 123153716819 142385939188 272961379262
Is there anything that can be done to lower the chance of dc output, a weak power supply can cause clipping if underpowered for the job, but is there anything else.
The transformer i am wanting to use for this project is the multicomp mcfe500/25. 25 volt dual secondaries 500 va. If i were to hook the two secondaries together would that give me 1000va. What do you think of multicomp, are they 'audio' grade
Many thanks

NAIM NAP250 modules. The 2x25Vac transformer will leave you with some 2x37Vdc when rectified and with idle loading. Fine with 8 Ohm speakers. The NAP250 modules should be 80W in 8 Ohm. Do you use 8 Ohm speakers? 500VA transformer leaves good margins. Do not put your secondaries in series, you need to use them individually to generate the -/+37V symmetrical supply voltage. As far as I can see, the Multicomp transformer is shielded - even better.
I see no problems unless you use 4 Ohm speakers at high power levels.
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Old 15th August 2018, 07:15 PM   #15
BrianL is offline BrianL  United States
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why no dc protection
Neurochrome just came out with a protection board that uses MOSFETs and it appears to have little or no effect on distortion. The search tool should get you to the threads about it on this site.
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Old 16th August 2018, 12:09 AM   #16
goodguys is offline goodguys  United Kingdom
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The recommended voltage for the amp boards if 40volts, the 25 volt transformer should give 37.9 volts, a little under the recommended, the next size transformer up will give me 44 volts, what is best, to undervolt or overvolt slightly.


Are four ohm speakers to be totally ruled out with these amp boards? Can you please expand on your concerns.
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Old 16th August 2018, 06:36 AM   #17
goodguys is offline goodguys  United Kingdom
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If the speaker impedance drops from eight to four ohms the current will double, but with the large 500va transformer the power supply should be able to supply this, or am i wrong, because the recommended transformer is 300va so plenty of headroom there.



Just to clarify, when i buy a 500va transformer is the manufacturer saying the va is 500va per secondary or 500 va as a total of all the secondaries combined. And if i were to connect the two secondaries together would i get 1000va, and if i were to form a centre tap, as suggested earlier, would the va stay at 500.


Many thanks.
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Old 16th August 2018, 06:57 AM   #18
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by goodguys View Post
The recommended voltage for the amp boards if 40volts, the 25 volt transformer should give 37.9 volts, a little under the recommended, the next size transformer up will give me 44 volts, what is best, to undervolt or overvolt slightly.
Are four ohm speakers to be totally ruled out with these amp boards? Can you please expand on your concerns.
It is so that amplifiers are designed to work within a certain supply voltage range, for this amplifier board +/-15V to +/-40V. On top of that, an amplifier has a maximum peak output current it can supply. This peak output current is often not specified but only implied through the maximum output power in a load with a certain impedance (here 80W in 8 Ohm). 80W in 8 Ohm corresponds to a peak output current close to 4.5 Amp. Thus, this amplifier can handle 4.5 Amp (peak) at least. If we assume the limit to be 4.5 Amp that would be 40W in a 4 Ohm speaker.

Then the synthesis: The amplifier has to be operated within its supply voltage range, else you risk damaging the amplifier permanently. No over-voltage! Within the supply voltage range it is so that the more supply voltage, the more output power you can get out of the amplifier. As the output power increases with the square of the supply voltage (about), half the supply voltage leaves one quarter of the output power. Your amplifier can deliver around 20W with a +/-20V supply voltage.
You can use a 4 Ohm load as long as the maximum output current is not exceeded. We do not know the exact current limit, we only know it is above 4.5 Amp (peak). So, you can get at least 40W in 4 Ohm, perhaps more. It also depends on how much current the power supply can deliver. With a 4 Ohm load, the heating of the amplifier will be considerable higher due to a higher voltage drop across the output transistors.

The supply voltage must not exceed 40V even in idle mode. +/-38V is a fine choice leaving a bit of margin for variations in the grid-voltage. As you load the amplifier, the supply voltage will sag, perhaps to +/-34V at full power. That will decide your maximum output power. This is an inherent phenomena using a non-regulated power supply.
Your transformer is 500VA and 2x25Vac. That means you have 10A (RMS) of current, thus 5A (RMS) for each channel. 5Arms is enough for 100W in 4 Ohm, IF your amplifier can handle good 7A current peaks AND you can cool the amplifier sufficiently.

If you know you will always use a 4 Ohm load, you simply choose for a lower supply voltage (and higher current capability). Stability wise an amplifier may work best with a particular loading - often this is 8 Ohm.

The above is valid for class AB amplifiers like the NAP250. For class D amplifiers it is slightly different as they can convert power.

Reasonably clear?

Last edited by FauxFrench; 16th August 2018 at 07:13 AM.
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Old 17th August 2018, 07:23 AM   #19
goodguys is offline goodguys  United Kingdom
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Thanks for the reply, much appreciated.
Very clear - Thank you.
Do you know how to put together the +/- 37 volt power supply for this amplifier. I made one for my quad amp which uses +/- 50 volts but do not know if it will be exactly the same for this amp.
For the quad amp i converted a dual secondary transformer into single secondary with centre tap. After the transformer there is a bridge rectifier, then two 10kuf capacitors.
The circuit diagram went like this: Positive from rectifier to positive of first capacitor, then to positive of amp boards.
Then negative from rectifier to negative of second capacitor, then to negative of amp boards.
Then i connected the negative of first capacitor to the positive of the second capacitor, then took this to the centre tap.
Would the nap250 boards bear any similarity to this.

Many thanks
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Old 17th August 2018, 08:46 AM   #20
FauxFrench is offline FauxFrench  France
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I will give you the link to a schematic:
Dual 90V/10A Unregulated Power Supply - Electronics-Lab
In this design I would replace the 4700uF/100V capacitors with 6800uF/50V capacitors and remove (short circuit) the two fuses (F1/F2) at the output.
The bigger the capacitors are, the less ripple you have on the supply rails. But, the bigger they are the more you stress the transformer and rectifier bridge at start-up and with very large capacitors you may need a particular inrush-current limiting circuit.
The two fuses protect the power supply against mistakes made during tests and very rare component break-down in the amplifier. If I use such fuses, I do it after the rectifier and before the capacitors. However, the fuses at the output increase the intended low impedance of the supply rails with the resistance of the fuses.

Last edited by FauxFrench; 17th August 2018 at 08:49 AM.
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