why no dc protection

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Hi And thanks for reading this thread.


I am going to be building an amp using this board, Ebay item number 272679086995
This has been done by members of this forum and on other forums as well. But why do diy'ers not use dc protection for the speakers. Most commercially made amplifiers have dc protection but i very rarely see dc protection being used on finished projects displayed by users of this forum.
Why is this? Are manufacturers being excessively cautious. Is it as long as you are careful with an amplifier dc protection is not needed.

I have had a quad 405 and 405-2 amp and these are generally regarded as being very stable, indeed many people have used these quad amps for over twenty years without a problem and these have no dc protection.
What do you think about employing dc protection on this latest project of mine. Is it worth doing. Do you use dc protection, if so what do you prefer.
If i were to employ dc protection, would this degrade the sound quality.


Many thanks.
 
Didn't find it on Ebay. Anyway, DC-protection:

If the protection relay is of good quality, it will do no harm. If the protection circuit is well designed, it will reduce the risk of harming your speakers. DC-protection is in general a good facility.

The DC protection circuit will not improve sound, just leave the sound as it is. DIY's are often most interested in the sound and with careful handling the risk of DC is small.

The reason why many commercial amplifiers are equipped with DC-protection may be of legal reasons: If your (the manufacturers) amplifier causes damage to a customers loudspeakers due to a DC fault and nothing was done to prevent this damage, you may be held liable for that damage as manufacturer. A clear incitement to add a DC protection circuit.

Yes, adding a DC protection circuit is useful as long as the relay(s) is good quality. My relays are bought and just awaiting my priority to implement the protection circuit. But, also I am most interested in the amplifier sound......
 
Perhaps such a relay does exist, but i have never been able to find one.

I find these audible in a most unpleasant way, even when new. With age they only become worse, especially if they routinely disconnect the load with a signal present.

Hope the weather has cooled down somewhat in Cascais - it has here (Pyrenees).

For the relays: overrated contact current specs, more contacts per channel and a reasonable reaction time. Contact wear can be a problem.
 
Only if it's included in negative feedback loop...if it isn't it will raise thd numbers somewhat (this has already been discuses and measured).

Interesting but not without consequences.
That will make it difficult for class D amplifiers.
For class AB, we like the output voltage to settle before engagement. Do you then propose a temporary or partial feedback loop to control the settling?
 
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
 
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.
 
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.
 
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.
 
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?
 
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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
 
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.
 
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