I picked up two new identical 'Professional' stereo power amplifiers for cheap. They are items which have shipped to customers, found to be DOA, and returned for replacement.
I figured that if I can't fix them, they'll be good for their trafo's (1200va!) and cases.
They are supposedly rated at 385Wrms into 4ohms per channel, and the trafo is 2 x 67vac.
There are several blown output devices, which is where my concerns are, they are both the same make and model amplifiers, yet one uses 2 pairs of Sanken 2SA1695/2SC4468 per channel and the other amplifier uses PMC-Sierra made 2SA1294 and ISC made 2SC3263.
I've tried to contact the manufacturer with regards to the schematic, to see what it specifies on there, but they never replied.
Going by the Sanken datasheets, the 1294/3263's are the 'better' devices, and I would guess the more suitable.
Now, I have some 'free' onsemi NJW0281/302, so before I go ordering 8 pairs of 1294/3263's, would the onsemi's be a suitable replacement?
I figured that if I can't fix them, they'll be good for their trafo's (1200va!) and cases.
They are supposedly rated at 385Wrms into 4ohms per channel, and the trafo is 2 x 67vac.
There are several blown output devices, which is where my concerns are, they are both the same make and model amplifiers, yet one uses 2 pairs of Sanken 2SA1695/2SC4468 per channel and the other amplifier uses PMC-Sierra made 2SA1294 and ISC made 2SC3263.
I've tried to contact the manufacturer with regards to the schematic, to see what it specifies on there, but they never replied.
Going by the Sanken datasheets, the 1294/3263's are the 'better' devices, and I would guess the more suitable.
Now, I have some 'free' onsemi NJW0281/302, so before I go ordering 8 pairs of 1294/3263's, would the onsemi's be a suitable replacement?
A transformer voltage of 2x 67V AC sounds quite high for a 385W / 4 ohms amplifier. Are you sure it's correct?
Maybe they are rather rated 385W/8r and 600W/4r!
Anyway 2 pairs of output transistor sounds not much... 😱
No, trying to do 600W or even 400W in class A/B with just two pairs of 100W Pd (2SC44689) devices or fakes isn't going to work very reliably or even at all.
You need to know why the output transistors blew.
It could be fitting new output transistors fixes the problem or it could be the circuit has faults futher back which might just blow up the new transistors on power up.
Bias problems will kill multiple output transistors.
Personally I have found most often the amp has blown due to a short and replacing the output transistors worked fine.
On other occasions the fault has been in the ltp, vas or bias circuits.
Sorry, I was a little out, it's 350w into 4 ohm, 270w into 8 ohm and i'm quite sure about the transformer voltage - it's printed on the label stuck to it.
I know, I think that's why they blew!
Again, I think that's why they blew, the design doesn't appear to be the best thought out.
Don't worry - i'll be replacing lots of components on these PCB's. They are mostly constructed with poor quality components, which could have attributed to the early death of them. The drivers for one are blown beyond all recognition, as well as several resistors and diodes have just vaporized.
Another reason I'd like to try to use the onsemi 281/302 is that they are rated at 150w power disssapation, instead of the original devices 100 or so watts.
This is a link to the amplifier we are talking about here:
https://dreammachine-radio.nl/imgstage-line-sta750-stereo-pa-amplifier-p-466.html?language=UK
The only way you'll make the amplifier safe is to give the poor thing a decent amount of output transistors so it can dissipate power & still not cook the components. Amps can get away with enough transistors on an output stage that can dissipate double the output power, in this case you are looking at 700W+ of transistors per output stage (350W RMS x 2).
However they'll last a lot longer by doubling this to 4 times the output power, or 1400W of transistors per channel. Take it from me that with an output power of 350W RMS that isn't a great deal 🙂
As yours is a PA amp (& i'm glad i bought a Mackie M3000 & not a stage line amp 😀) then look to fit 4 x 100W NPN & 4 x 100W PNP transistors as a minimum.
It sounds like your amp has a cheap power supply, IE low storage capacitance & the PSU sags badly under even a moderate load.
However they'll last a lot longer by doubling this to 4 times the output power, or 1400W of transistors per channel. Take it from me that with an output power of 350W RMS that isn't a great deal 🙂
As yours is a PA amp (& i'm glad i bought a Mackie M3000 & not a stage line amp 😀) then look to fit 4 x 100W NPN & 4 x 100W PNP transistors as a minimum.
It sounds like your amp has a cheap power supply, IE low storage capacitance & the PSU sags badly under even a moderate load.
Its quite a juggling act to get this right without spending a fortune.
There is the basic transistor power rating to consider.
Then there is derating depending the tempertaure they run at.
Heatsink size comes into the matter here.
Then you have to consider the transistor is only conducting for half a cycle.
Then are you going to use a fan(s).
I find I can get away with twice the transistor power rating per output pair with a good heatsink and a couple of fans. I run a mobile disco and this has worked reliably for three years having done this.
Hi Ben I don,t know if your amp internals look like the photo posted by Aldotin.
But I bought a cheap chinese amp (Warrior DA800) on ebay recently that looks remarkably similar internally.
After fixing the broken rail fuse, I found that it was not reliable in PA use, as it quickly overheated. I ended up fixing 2 extra fans in the front panel (existing fan was 24V)
I also added some extra capacitors I had in my junk box.
It still sounds fairly poor, but has so far been usable as a vocal monitor amplifier without failure.
But I bought a cheap chinese amp (Warrior DA800) on ebay recently that looks remarkably similar internally.
After fixing the broken rail fuse, I found that it was not reliable in PA use, as it quickly overheated. I ended up fixing 2 extra fans in the front panel (existing fan was 24V)
I also added some extra capacitors I had in my junk box.
It still sounds fairly poor, but has so far been usable as a vocal monitor amplifier without failure.
Some of the cheap ebay amplifiers are terrible.
I bought a 480w amplfier and there was very little inside and what there was a was tiny ! The heatsink was unbelievably small. It did have a fan but it had very little throughput.
I blew it up pretty qucikly and now use the case and transformer as a test bed for my own amps under development.
Hi,
Yes, it's almost identical, except the trafo fills the whole area on the right side, and the heatsink on mine has two fans blowing down.
Thanks for the schematic Aldotin! I can just about make it out, so it should help with resistor values etc.
The power supply capacitors do look quite small, they don't have their capacity printed on them, but I would guess 8200uF or maybe 10,000uF at a push by looking at their physical size and assuming 100v.
Yes, it's almost identical, except the trafo fills the whole area on the right side, and the heatsink on mine has two fans blowing down.
Thanks for the schematic Aldotin! I can just about make it out, so it should help with resistor values etc.
The power supply capacitors do look quite small, they don't have their capacity printed on them, but I would guess 8200uF or maybe 10,000uF at a push by looking at their physical size and assuming 100v.
I spoke too soon about the company not getting back to me... they sent me the schematic, although there is no parts list available for it.
a 2pair output stage fed from 67+67Vac is asking a lot driving 8ohm speakers.
Don't even think about 4ohm speakers.
It MUST be able to drive a 4r0 load, short term at maximum power, without any signs of overheating of any components and without any artefacts showing up on the sinewave displayed on the oscilloscope trace.
I believe that to be classed as an 8ohm capable amplifier, it should be able to drive 2r7 in the short term without losing too much drive voltage.
Don't even think about 4ohm speakers.
It MUST be able to drive a 4r0 load, short term at maximum power, without any signs of overheating of any components and without any artefacts showing up on the sinewave displayed on the oscilloscope trace.
I believe that to be classed as an 8ohm capable amplifier, it should be able to drive 2r7 in the short term without losing too much drive voltage.
Hi,
it was the protection level in the Crimson 1704. At that time ~1978, I just accepted it.
Now I find references to peak currents into severe reactance 8ohm speakers being as high as three times the predicted current into a nominal 8r0 load.
One paper referenced on this Forum indicates that real music recordings into three test speakers, of varying severity reactance, can demand 5times as much current as the nominal load.
The 2r7 test load is just a way to confirm that the amp is capable of delivering that 3times level of peak current without the output voltage collapsing as many amplifiers do. And then listeners complain that the amp sounds strained, distorted, clipped, grainy, sibilant etc.
it was the protection level in the Crimson 1704. At that time ~1978, I just accepted it.
Now I find references to peak currents into severe reactance 8ohm speakers being as high as three times the predicted current into a nominal 8r0 load.
One paper referenced on this Forum indicates that real music recordings into three test speakers, of varying severity reactance, can demand 5times as much current as the nominal load.
The 2r7 test load is just a way to confirm that the amp is capable of delivering that 3times level of peak current without the output voltage collapsing as many amplifiers do. And then listeners complain that the amp sounds strained, distorted, clipped, grainy, sibilant etc.
Do you think i'd be better putting two pairs of 200w rated MJL21193/94 in the output stage instead of my planned 150w NJW0281/0302?
I know that the hfe is quite a bit lower than the original 2SA1294/2SC3263, but do you think the drivers will be up to it?
The drivers i'll be using are 2SA1837 and 2SC4793
I know that the hfe is quite a bit lower than the original 2SA1294/2SC3263, but do you think the drivers will be up to it?
The drivers i'll be using are 2SA1837 and 2SC4793
that is not what I was saying.
I am saying that proper scientific testing of real music into real speakers shows peak currents that are way beyond what you would expect if a nominal resistive load were actually on the output.
8ohm speakers often have a minimum DC resistance in the range 5r0 to 7r0.
similarly 4ohm can range from 2r5 to 3r5.
Look at what that would predict for peak current.
No where near the three times to five times that are often quoted.
In the last paper I read, and believed, the testers chose a very reactive speaker (by reputation) and a normal speaker and what was reputed to be a very benign speaker that was easy to drive.
The testers were surprised to find that all three of these commercial speakers drew peak transient currents that were equivalent to nominal resistance/5
That would be equivalent to 0r8 not 2r5 for a 4ohm speaker.
Last edited:
- Status
- Not open for further replies.