5 pairs of outputs on +-120V...

[megajocke]

Anyone else than me that thinks this is a stupid idea? I found it in a cheap Chinese made "Pro" Amp - five pairs of 2SC5200/2SA1943 VS rails idling at +-120V... Guess who wins! I'm not surprised these amps blow up within a week or so. It is claimed to be able to drive 4 ohms... The filter caps are underrated though, maybe this is what is depended upon to save outputs.

The sad part is that there is a tap at half of this on the power transformer - going Class H would have been easy, and if they did the five pairs would probably have been more than enough.

[wg_ski]

This is where Pflame Linear got its well-deseved reputation. And they did the 700 without the benefit of modern Toshiba transistors.

If they did use an extra tap for class H, it might have been enough. The QSC 4050 uses two extra taps and a bit higher voltage. They seem to be reliable enough that way, but I'd still use more transistors (and another gain stage). Personally I just hate the idea of running out of Hfe, but that tends to protect transistors, too.

[nigelwright7557]

Originally posted by megajocke
Anyone else than me that thinks this is a stupid idea? I found it in a cheap Chinese made "Pro" Amp - five pairs of 2SC5200/2SA1943 VS rails idling at +-120V... Guess who wins! I'm not surprised these amps blow up within a week or so. It is claimed to be able to drive 4 ohms... The filter caps are underrated though, maybe this is what is depended upon to save outputs.

The sad part is that there is a tap at half of this on the power transformer - going Class H would have been easy, and if they did the five pairs would probably have been more than enough.

I find some of the cheaper designs amazing.

I have bought a couple of so called high power amps off ebay only to open them up and there is very little inside.
The heatsink is tiny and so are the output transistors.
I connect them up and what is supposed to be hundreds of watts sounds like 50-100watts.

This is why I stick to my own designs now, I know what goes into the design and what the actual power out is, instead of something that is grossly over rated.

[megajocke]

Yeah, but then they were metal cans at least...

I was thinking of the QSC RMX2450 (http://www.qscaudio.com/support/libr...es/RMX2450.pdf), which has essentially the same power supply voltages. Four pairs and Class H in this one.

Using a 230V transistors with 120V (this was idle with high line though) rails in class AB like in the amp I saw seems a bit close to me, even if a suitable number of pairs was used. The fan speed control was interesting too - a thermistor connected to a TO220 transistor or maybe voltage regulator which was hot glued to the main circuit board. Of course it had came loose and were dangling around inside...

nigelwright: These amps actually have a quite large transformer and massive heatsinks. There are room to install more output devices, but the manufacturer was probably a bit too cheap the day these were manufactured. The power supply voltage is about right for a 1kW in 4 ohm amp and it can probably succeed in delivering that. Well, at least for a couple of minutes or so...

[nigelwright7557]

Originally posted by megajocke
Yeah, but then they were metal cans at least...

Funny you should say that, I ripped out the insides of them and used them for my own amps.....

[megajocke]

I were refering to the TO-3:s but you have a point there, useful chassis. These amps I looked at had REALLY nice chassis actually. Thick steel, nice connectors, gain controls, big heatsinks and a big toroid.

[nigelwright7557]

Originally posted by megajocke
I were refering to the TO-3:s but you have a point there, useful chassis. These amps I looked at had REALLY nice chassis actually. Thick steel, nice connectors, gain controls, big heatsinks and a big toroid.

With these pro looking boxes I can get away with using my amps with my disco.

I usually use PC cases for the bigger amps which can look a bit suss ! I have people ask me where the monitor is for my PC when asking for requests !

The PC cases are useful as they have fan mounts and mains sockets on them which I can utilise.

I darent ask how much it would be for a custon enclosure design.

I can pick up flat case PC enclosures for £20 off ebay.
Thye are sturdy enough for the huge toroidal transformers I use and big enough to fit in everything else.

[EWorkshop1708]

I can't see how they expect only 5 pairs to deliver into 4 ohms on such high rails at 4 ohms! Especially with the Toshiba transistors being only 150W, and Motorola/Onsemi make much better ones that can handle more power; at least you can replace the blown Toshibas with better parts.

I have ~350W into 4 ohms, and also use 5 pairs, (MJL4281/4302) but that's only +/-75V rails at idle, and even though I have fans and heatsinks, there's still quite a bit of heat to shed. However, 5 pairs sharing only 350W is easy on the transistors.

I wonder if it must be common practice for cheap amps to abuse those Toshibas!
Years ago, I worked on an old Fisher amp that used the same Toshibas as well. It was 70V idle rails, only one pair per channel, 150W/8 ohms each channel. Which only equals 2 pairs @ 300W @ 4 ohm

[luka]

Hi

Why not? Who said at full power?

[AndrewT]

Hi,
if the sink is kept cold and the bias is set low due to adopting either CFP or high value Re, then the amps will deliver reliable power into 8ohm speakers for domestic use (not parties) on wideband unclipped music signals.
The 4r0 spec is there only to impress the car audio boys.

But don't try them on subs or bass only speakers and don't use them on anything less than 8ohm.