Iz top quality, so you not worry!

[Th3_uN1Qu3]

Ah, the wonders of China. Cheap, high quality mass-produced goods.

This here amplifier claims "100W+100W". Over an unspecified load impedance, for an unspecified length of time and with an unspecified amount of distortion. It has been quoted to me as 40wpc real power. Most i could get out of it on a 4.3 ohm dummy load was 60W, one channel driven. So the 40wpc claim seems to be true - from here to "100W+100W" there's a long way.

Just look at this thing. Tiny transformer, tiny heatsink, and a tiny fan (60mm) making not so tiny noise. Actually, the fan noise is pretty much on par with the volume of the music coming out of the amp... and the fan runs maxed all the time. That is, unless the amp is turned up loud, when the fan will slow down because the transformer voltage drops significantly. So, we have a fan that runs fast when you don't need it. Brilliant! Main filter caps are one taller than the other. On both of them it says 4700u 35v though. Which btw is not only too little capacitance, but also with the crap regulation of this transformer they will blow on high line if the amp is left idle.

Output transistors are 2SD1047. Which would be enough for the job, but that heatsink is really flimsy. If the fan seizes they will most definitely go boom as there is no thermal protection. The drivers are 2SD667/2SB647, and the small signal transistors are 2N5401/2N5551. When delivering 60W on one channel the rails sag from +/-30v to +/-26v which is pretty poor performance.

So how does it sound? Well... cheap. The input sensitivity is too high, leading to a healthy amount of background hiss and hum. It goes into clipping at less than 1/3 volume, with a consumer audio source (laptop). And they call this "professional"? The volume pot turns too easily. The mike input adds even more hum when engaged. Lovely.

This kind of thing retails here for $70. You'd be much better off getting a vintage amp for that price. You may need to swap out some caps in a vintage amp, but on this pile of trash you'd have to rebuild the whole thing to get something useful out of it. I don't really care about its current guts tho - the plan is to throw a couple Class D modules and a SMPS in there.

[mockingbird]

The Chinese will leave no corner unturned when finding ways to cut costs. Even if it is a massively disproportionate ratio of savings vs quality, for instance a fraction of a cent for a better part that will increase the quality of the overall product ten-fold - they will still cut costs.

In the late 80s, I had a pair of shoes from Chinatown that was made out of cardboard.

[Th3_uN1Qu3]

Cardboard shoes? Nice.

I proceeded to make this thing a little less craptastic. I swapped out the ridiculous looking 4700uF 35v caps for a pair of 10000uF 35v. Result? Same power output hence the limit is clearly the transformer, however the background hum it had before is now gone. The 4700uFs, like all the caps in this unit, are branded "Paisan". Maybe they misspelled "Poison"?

I also fixed the most annoying aspect - added a temperature control for the fan. Some peace, finally. The fan is made by Adda btw. I forgot to take pics - will do when i feel like taking it apart again. Gotta love the notice on the back - there's some more but the first one is killer: "This fan is for ventilation use." Duh!

[mockingbird]

"Paisan"? Sounds Italian. I like the design. Simple. Easy to work on.

[dumpystig]

No surprise there - that's a pretty big chassis with fuck all inside

[Th3_uN1Qu3]

Just found out something new as i hooked up both speakers to it - one of the channels is inverted, but this wasn't marked on the connectors. So it can be bridged too. We'll see how much it can put out...

But i suspect there's another reason why this was done. When both channels are working at the same time at the same volume and one is inverted, this effectively cancels out the ripple current in the capacitors. So that those pathetic caps it had would actually make it past warranty.

[b700029]

I'm surprised it even has a toroidal transformer.

And those tiny transistors with their heads dipped in thermal paste and butted against the heatsink...

[Th3_uN1Qu3]

Originally Posted by b700029

I'm surprised it even has a toroidal transformer.

Well, the other model didn't. And that one claimed "500W+500W" at the same price, with an MP3 player with USB and SD card included. The MP3 player was mounted so sloppy, i'm not sure you could even fit an USB flash drive in without opening the case and aligning the thing.

Originally Posted by b700029

And those tiny transistors with their heads dipped in thermal paste and butted against the heatsink...

That's the bias servo. It keeps a low distortion level and thermally compensates the power transistors so they don't go into thermal runaway and explode. Such a thing usually has its own dedicated slot in the heatsink (which those have too), but in this case the heatsink is so thin that they had to resort to such tricks to make them stick. Anyway, there's a whole bunch of small signal transistors on there which is pretty surprising - it can get a lot more crude than that. If i get bored enough someday i might lift the schematic off this thing.

[kaboom]

Originally Posted by Th3_uN1Qu3

That's the bias servo. It keeps a low distortion level and thermally compensates the power transistors so they don't go into thermal runaway and explode

I wonder where they 'learned' the bias servo trick.... And a sure sign of a 'cheepie' is that beautiful excessive gain!

The thing that looks like a heatsink- just how are we gonna get the heat from those transistors evenly spread out?? That 'angle' they're attached to must get smokin' hot while the other side stays stone cold!

What did they cut that heatsink with, anyway? An axe?

At least they're tryin'- they greased the micas...

Even the dual BTL TPA3122 PWM amp I built would stomp that thing.

[Th3_uN1Qu3]

Originally Posted by kaboom

The thing that looks like a heatsink- just how are we gonna get the heat from those transistors evenly spread out?? That 'angle' they're attached to must get smokin' hot while the other side stays stone cold!

It's not that bad, the heat does get spread out... eventually. Admittedly it does take a while, heck it even takes a while for the heat to reach the fan control thermistor which i put between the middle transistors so it's between both channels. On that angle there's differences of like 6 deg C over a distance of ~3cm, that's just silly.

[kc8adu]

all this reminds me of the craptastic infinity ps12 on my bench.
gotta post pics of that one.
if i hadnt seen the label i would have thought it was a "white van scam" unit.

[kaboom]

Originally Posted by kc8adu

if i hadnt seen the label i would have thought it was a "white van scam" unit.

Let's just say I got a set of them for free after a 'sucker' found out ya couldn't return them...

Attempting to imitate Dahlquist, they put "Dahlton" on their chrome-plated-plastic "look at me" emblems.

Cheap drivers, horrible boomy response. A 10" driver, side-firing, is a woofer according to them. OK, fine, but they didn't filter the mids out. So much for 'non-localizable bass.'

The best part is the bi-amping "feature" that never isolates the woofers from the tops, so if you attempt to biamp, you'll blow your amps!

http://web.archive.org/web/200703021...om/dahlton.htm

And people say the old Cerwin Vegas were bad- gimme a break.

[weirdlookinguy]

I love me a good pair of Cerwin Vegas, fuck all the haters.

[lti]

The Peerless drivers I used in my computer speakers are "white van" quality. There is almost no response below 300Hz unless they are installed in a sealed enclosure and the impedance is low enough that they can kill an amp that is not rated to drive speakers with an impedance lower than four ohms. Even in a sealed enclosure with an amp that is rated for a 3 ohm load, they require a bass boost to produce decent sound.

I can't see how mounting the components on the fins of a heatsink works very well.

[Th3_uN1Qu3]

What size enclosure? Sounds to me like you need a bigger one.

As for the heatsink... yes it doesn't work all that well, i mentioned that already. However it does keep the devices from blowing up and that's enough.

[smason]

Originally Posted by weirdlookinguy

I love me a good pair of Cerwin Vegas, fuck all the haters.

My dad was a bit of an audiophile, not one of those buy fancy cables guys, just loved good quality gear and could really hear.

When I was 17 I bought a pair of Cerwin Vega speakers. They were reasonably priced, and I liked them.

My dad rolled his eyes and mumbled something about wasting my money.

I hooked them up to my Pioneer SX-650, or was it my Sansui 6700? Anyway I cranked up Boston (first album) and was enjoying it, when my dad barged in.
I expected him to turn it down, and complain, but instead, he closed the door, cranked it up,and said "Holy Shit!, those are damned good speakers for the money"
I remember the slogan on the box: "Loud is beautiful, if it's clean"
And they were.

[mariushm]

Couldn't resist posting this.... it looks just too much like hardware porn:

http://www.eevblog.com/forum/index.p...cseen#msg82610

attaching pictures from that thread here as well... not claiming they're mine...

With a massive power capability and the transformer from a power station its perfect for music or doing some spot welding

RMS continuous at 8 ohms, at or below rated distortion, 20Hz to 20kHz 500 Watts (250/CH)
RMS continuous at 4 ohms, at or below rated distortion, 20Hz to 20kHz 800 Watts (400/CH)
Total Harmonic Distortion: at or below rated power from 20Hz to 20KHz 0.5%
Intermodulation Distortion: at or below rated power, any combination of two frequencies, 20Hz to 20kHz 0.5%
Hum and Noise: 100dB
Basic Amplifier Input Sensitivity and Impedance: 1.5V - 50k ohms
Damping Factor: 40

GENERAL:

Dimensions:
Width: 18"
Height: 7 - 3/16"
Depth: 18 - 1/2"
Weight, unpacked : 38KG

[Th3_uN1Qu3]

@ mariushm: Never knew Bose actually made something this good. Nowadays they're just living off the hype.

I bothered to lift the schematic too. Reason for the volume knob problem can be readily seen - the gain is x22 while x10 would have been more than enough. With 1V RMS input the output voltage from the preamp is 2.8v RMS, and keep in mind that the max this thing can put out is 16V RMS, so x22 gain is completely pointless... In fact, x7 would have been enough but i was out of 7k5 resistors so i used 10k ones for x10 gain. Now at least i have a semi-usable volume control. I was in for another surprise tho - the LED meter isn't wired at the output of the amp but at the output of the preamp/EQ... after the first 1k resistor you see in the schematic. So now the blinkenlights no longer reflect what's going on with the sound - they max out long before the amp distorts. Ah well... i'll fix them some other time.

The output offset is also pretty high at -104.2mV left, 109.2mV right channel.

As for the inverted phase thing, it wasn't the amp at fault (no it isn't set up for bridging). One of my woofers was wired the other way round in its box. Oopsie.

[lti]

At least that amp isn't as bad as this.

[Th3_uN1Qu3]

I've done some improvements which brought the offset to damn near zero (btw, it was negative on both channels, i missed a minus over there). I also removed the front PCB, found the input dividers for the LED meter and wired trimpots in parallel, swapped out the lousy 4558 opamps for TL072s, and now i'm also replacing the scratchy volume pot (which was the reason i got it cheap in the first place) and throwing a couple green LEDs in there - since there is a clear plastic trim with room for LEDs around the volume knob anyway, why not?

Will come back with moar pics and updated schematic.

As for the amp in the video... less than 5Wpc for something that's powered off 12v is just pathetic. It's rather trivial to get 15-25wpc from 12v without boosting the supply voltage in any way, car radio amp ICs have been doing it for ages.