China amps

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Nordskov

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Joined 2007
#1
Hi there.

I need an amp for my passive 8 Ohm HT sub. I think ~200W is sufficient.
For that I need a power supply and an amplifier.
I have found some interesting items on AliExpress:

PS
https://www.aliexpress.com/item/Active-subwoofer-pure-bass-Power-Amplifier-board-400W-30HZ-200HZ/32616239823.html?spm=2114.13010208.99999999.264.iClF4s
https://www.aliexpress.com/item/Fever-active-subwoofer-amplifier-board-Toshiba-two-pairs-of-tube-C5200-A1943-Output-Power-300W/32458033963.html?spm=2114.13010208.99999999.278.iClF4s

Amp
https://www.aliexpress.com/item/350W-Digital-Power-Amplifier-Switching-Power-Supply-36V9A-12V2A-Dual-Output/32562207113.html?spm=2114.13010208.99999999.288.iClF4s
https://www.aliexpress.com/item/500W-amplifier-switching-power-supply-board-dual-voltage-PSU-60V/32657362885.html?spm=2114.13010208.99999999.285.iClF4s

What is the general opinion of these cheap designs?
Its for sub duty, not mains.

Is the 400W rating for the first amplifier bullock, or did I miss something?
With a 35V rail I get 35*35/8 = 153W. 4 Ohms = 306W.

The amplifier specifies a 36V PS. Will a higher voltage PS harm the amplifier as long as I dont crank it (with electrolyttes rated high enough)?

The first amp is rated 400W and is driven by 8 power transistors, the other 300W by 4 power transistors. Should I go for the first just for the number of transistors (less heat pr unit)?

My AVR has 12V triggers. I suppose a simple 230V/12V relay can be used to turn the PS on/off.


Cheers 🙂
 
#4
For a 400W amp 4 pairs of output devices isn't enough. The output devices also need to be spaced apart farther, otherwise the center ones will current hog and the amp will self destruct. A 36V supply won't support 400W. A 36VAC transformer will put you in the 100W range at 8 ohms.
 
#6
You are calculating the power output wrong. Using (Urail^2) /Rload is only valid for squate waves, not very useful. What you want is sine wave power, and since the effective voltage of a sine wave (that DC voltage which makes equal power into a given load) is Urail/(square root (2)), the output power is (Urail^2)/(2x Rload). Half of what you calculated.
Ads for cheap amps often use this trick to claim exorbitant power outputs.
 
#7
Thanks a lot for being specific, although it was not what I wanted to hear 🙂
So those amps output are highly inflated.
What about the SMPS I linked to? Surely it's more simple to design a power supply.

I could use a plate amp, but in this part of the world, you dont get much south of $250.
And I dont want to cut my sub open.
I could go for a PA amp, but the price is too high and I only need mono.
Building one would be an option, hence the PS question, with a good diagram.

@ilimzn.
I get your point in dividing with square root 2, but why 2x Rload?
 
#8
For sub duty I reckon you'll be far better off choosing a classD amp rather than classAB. You won't need such a large power supply due to the increased efficiency and consequently the heatsinks can be fairly modest. Probably will sound better too.
 
#9
If you want a real 400W or even 250W sub amp you will be easily spending $250 on the supply alone. As JonSnell says, you get what you pay for (sort of). The supply is where your power comes from so it's equally as important as the rest of the amplifier. If you use a cheap supply, expect poor results.

The input stage of an amplifier dictates what is required for a supply. If the input design is sound and has good PSSR a simple supply providing enough voltage and current will work great. If the input is very simple and lacking PSSR you need to put a lot of work into the supply to reduce ripple.

If you are looking for the best bang for your buck for a subwoofer amplifier, look at class D designs. I personally hate the sound of them, but they do provide a lot of power for the price and you are less likely to hear any difference in a subwoofer application.
 
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