"300W" ebay amplifier 3055/2955

[df_genius]

Has anyone bought and built one of these?

DIY Transistor Stereo Power Amplifier 300W TIP3055 MJ2955 Amp board PCB | eBay

FYI the schematic is the last picture in the listing

I bought one just to see if it works and probably to use as a spare amp for comparison purposes. I'm using some transistors I had lying around, so BC212L, BD139, TIP41C/TIP42C drivers and TIP35C/TIP36C outputs. So far it seems to function, no outputs fitted yet but the front-end works. It does seem to have a higher background hiss level than the NCC200's I normally use. I will be mounting the three bias diodes on the heatsink for thermal compensation.

I can't find a matching schematic anywhere, but I guess it's a typical 1970's or so design from the transistors in use. Does anyone recognise the design? Any cheap/obvious mods we can make?

I think the "300W" comes from adding very optimistic power outputs at 4 ohms of both channels together. Although if using 3055/2955 you're limited to +/-30V rails, so you might get 50W per channel at 8 ohms and 90 at 4 ohms out of it at most

I intend to use it at +/-40V rails with the TIP35C/36C outputs so I might get 70W per channel at 8 ohms. Yes I know I can get more but I'd like to use the spare parts I have and am aware it's not hi-fi, for a fiver I just wondered if it works.

 

[jacques antoine]

At 300W, it fully qualifies as an 'ampliFIRE' as stated on the PCB !

Jacques

 

[bimo]

View attachment 466126At 300W, it fully qualifies as an 'ampliFIRE' as stated on the PCB !

Jacques

Edit:
In my country, it called "OCL150". I built it about 20 years ago and still populer.
You better build like this one: http://anistardi.wordpress.com/2014/10/14/perkutut-amplifier/

 

[nigelwright7557]

Be careful if you buy modern MJ2955/2N3055.

I got hold of a 1980's Maplin 225WRMS amplifier with no output transistors.
So I bought in new transistors.
The amplifier oscillated very badly.
When I tested the new transistors on a semiconductor analyser they had much greater gain than the originals datasheet. I had to increase my VAS capacitor to stop oscillation.

 

[df_genius]

Thanks for that, did get some TIP2955 and TIP3055 recently but won't be using them on this build, I will check for oscillation when it is fully built.

Thanks Bimo, I can see the OCL150 circuit seems very similar, can you advise if there are any mods available to improve performance? I plan on adding local decoupling caps to the output transistors, and like I say mounting the bias diodes on the heatsink. I can't seem to find much information in English on the OCL150

 

[sreten]

Hi,

With 3 diodes trying to bias 4 base voltage drops
thermal compensation doesn't matter, its not
enough, so no point moving the diodes. *

Its old and crude, no protection, no catch diodes,
no output networks. Basically the more you ramp
up the power rails the more likely it will expire.

rgds, sreten.

* It is underbiased, only the drivers will conduct at idle.

 

[nigelwright7557]

You really need a proper Vbe multiplier with a trimmer.
You can adjust it to the correct operating point.

 

[bimo]

Thanks for that, did get some TIP2955 and TIP3055 recently but won't be using them on this build, I will check for oscillation when it is fully built.

Thanks Bimo, I can see the OCL150 circuit seems very similar, can you advise if there are any mods available to improve performance? I plan on adding local decoupling caps to the output transistors, and like I say mounting the bias diodes on the heatsink. I can't seem to find much information in English on the OCL150

It is class B. It is no reason build class B amplifier with +-45V PSU. You should modify it become class AB.
It use old transistor, slow (low fT). You should use fast transistor. It is very difficult to achieve low distortion at high frequency if using slow transistor.
I use lead compensation in Perkutut Amplifier to get high slew rate.

 

[df_genius]

Thanks, I'd like to change it to class AB if I can, do you have an idea of how I can apply it to this circuit?

 

[Ian Finch]

....I'd like to change it to class AB if I can, do you have an idea of how I can apply it to this circuit?

You need to add a sub-circuit that has a thermal control element, usually a TO126 transistor that can be attached to the heatsink plus an adjustable setting pot. It's variously called a Vbe multiplier, bias controller, bias generator etc. depending on whose text you read. It determines the thermal stability of the vast majority of class AB amplifiers and it's not an optional or trivial device.

It will also require a lot more than suggestions on this - you need to know what you're doing and how to amend a design as you go or you'll quickly find out what thermal runaway or "ampli-fire" can do to your hours of work and $$.

I suggest you build this 300W disaster as it is designed, if at all, rather than try to polish a turd. It will be noisier and miles below the sound quality of NCC200 amplifiers, even though they date from the 1970s too. You could do worse than copy that schematic if you still want to add a bias controller though. It is based on and much the same as the Generic Naim NAP 140 design. Even so, as a priority, buy either Douglas Self or Bob Cordell's Amplifier design books. They are promoted on their websites or you can simply look them up at Amazon and read the sections on bias and thermal stability thoroughly before attempting this, even with all the helpful suggestions likely on offer here. Also, check Rod Elliotts ESP site, starting here: Elliott Sound Products - Audio Power Amplifier Design Guidelines

Bear in mind that the electronics can be as little as 10% of the amplifier cost which is largely transformer, case and heatsinks. It makes no sense to skimp on the electronics which probably determines what its sound quality will be, regardless of how much is spent on extras.

Buy one of LJM's Ebay kits if you like everything complete and ready to assemble. He has a finished NAP 140 version you could modify as an NCC200 but much cheaper, if you wished too. The kits will be cheaper than anything you can do with locally bought parts and they will at least work with the parts supplied. The specs are also closer to reality than the extreme BS used to rate that OP "300W" amplifier. 60W/8Ω would be more realistic there, with the typical transformers I've seen used.

 

[df_genius]

Thanks Ian, all advice noted.

I've had a few years of amp building, but only "joining the dots" as it were, I was thinking that this being a relatively simple, old amplifier it would be a good starting point for understanding how the topology hangs together, what makes it good/bad and how to improve on it. At the minute this board hasn't cost me over £10 yet for all the board and the parts

I do have the Doug Self book, as you say I do need to go through it.