Modding a high power amp...

[NUTTTR]

Hi All,
This topic HAS come up in the past, however i'm looking for advice on this situation
There is 2 "main" problems i'm looking to alleviate!
1. The amp sounds "bad" on main speakers, it is a "cheap" pro audio amp and mostly built well, but the pre-amp stage (i'm guessing) really sucks badly!!! It goes from Cannon's into a board (that looks like it was bought from a $2 shop!) with NE5532's (2 of them) to drive the preamp and obvious gain adjustments go to the front panel... I understand the NE5532's aren't that bad and when setup right are quite good, however the sound is SO stale and dry that it's not actually listenable!!! I use it bridged to a sub and it sounds very average... From the NE5532's it goes into 2N5551's and 2N5401's (pairs obviously and there's 2 lots of them) Then into 2SD669 and 2SB649 into 2SA1942 and 2SC5200 (there's 5 pairs of them in each side (channel A 5 pairs, channel B 5 pairs).... There is a 1500VA (output) transformer hooked up to this which has +-78V on the DC side (after 10,000uf filter caps (2 of them))... The amp supposedly does "1200w" into bridged 4 ohm... Now, the capacitance of the filter caps needs to be increased in my books as it's not enough... However would that affect sound quality? Sorry to post such a long post, however, would the pre-amp section (i can post a picture if that helps?) affect the output more? Like i said the whole preamp could have come from a $2 shop to be honest, it has the cheapest of cheapest parts on it and they don't look too excellent!!! What would be the first port of call? I'd really like to modify this!!!

2. Power output - into 4 ohm bridged i've measured 89v across the outputs (it was obviously clipping!!!) It does get hot quite fast (15 seconds it's getting very hot but heatsink is still touchable), it is fan forced (which i will be upgrading too) but looking at possibilities of adding another torroid and having one p/s per channel - realistically i will only load it down to 4 ohm bridged, however the headroom there is very important as it's for home cinema use and needs to develop at LEAST 1200wrms continuous (preferably!)... I don't mind about heat, heat is easily enough fixed with fans and most amps in home cinema don't get continuous output, more pulses so it will cool enough to not have an issue... Can anyone help me with this? I've looked into adding more outputs, this is not easily done (no room and obvious problem of doubling up BJT's - not easy and potential to blow a lot of chips!)... I'm really after more power and more stable, i feel the power supply isn't holding up it's end of the bargin to start with - even the internal wiring gets quite warm (i think it's 14ga - each channel has 154v DC on the chips (to ground) so it has the voltage!!!) Wiring is an upgrade and i think the capacitance needs to go up by 4x and maybe add another torroid? Can someone help me with some ideas for stability? Thanks very much in advance!!!
Aaron

[richie00boy]

I would say you are well on the right track. You estimate for the PSU caps sounds about right. Maybe just a 2kVA toroid in place of the existing one rather than adding another one.

I wouldn't worry about how naff a board looks, it's the sound that matters. If it's got NE5532s on then it's a good start. Maybe you could change some signal path caps for better ones, but I doubt you will notice much to be honest.

Maybe have a look at the bias for the amp. It could be set really low.

[psychosteve]

my oppinions on things like this, and i deal with a lot of pa gear and some REALY cheap nasty stuff at that, is

A, driven within limits any amp should sound fine if its the corect one for the job, given speakers that are up to the quality you want to listen at.

B, yes up the caps to at least 15 to 20 thou.

C, upgrade the pre amp components, all the resistors and caps, resistors should be .5 watt metal film, maplins do a bag of 1500 varied values for under £20. Caps the choice is yours, there are plenty of poly caps designed for audio, but that choice is another thread all together.

D, do you need a pre amp? is there a conection for pre out? and pre in? try making a conection between pre and power amp, and feed in line signal, that will show if its the pre or power amp thats sounding bad.

E, bridging should give double voltage swing, (i think!) so even 70 volts should give 140 before clipping, id check the pre amp!!!!

this is just a few thoughts, and hopefully they will help, see you later, steve.. ..

[AndrewT]

Hi
1200w into 4 ohms bridged is putting a lot of load on the output stage and it's drivers.
It is equivalent to 600w into 2 ohms. The voltages on the rails indicate you should have about 200w to 250w into 8 ohms. So test each amp into 4 ohms and see how close to power doubling you get. Check temperatures. Usings temps and currents check the output stage SOA against manufacturers data. If all this is OK then proceed with very big heatsinks and try testing into 2 ohms going through the checks again. If your power supply, smoothing and heatsinks are upto scratch then you may get 600w or 700w into 2 ohms.
good luck
Andrew T.

[jacco vermeulen]

I think the comlementary of the 2SC5200 is the 2SA1943, not the 2SA1942.

The 5200/1943 have a maximum dissipation of 150W/ 15A, a bandwidth of 30mHz., not bad for a PA amplifier i think.

With 5 pairs you have 1500 maximum dissipation per channel, bridged 3000 watts.
Why not through out all the PCB's, add another transformer and go for a decent front end design ?

[anatech]

Hi NUTTTR,
I think there are serious driver problems here. There is no way a 2SD669A/2SB649A will drive that output stage into a 4 ohm load at those power levels. Bridged is out of the question. By chance is there an output pair acting as drivers for four paralleled output pairs? You should also check the standing bias current. The amp may not have bias given your description of the sound.
Any way you can post a schematic, or make & model?

-Chris

[sam9]

Pro gear may well have protective circuitryat both the input and output has an adverse impact on sound quality. It's there because on the road prospects of "helpful" folks plugging the wrong thing into the wrong socket must be considered. There may also be compression, peak limiting, or "soft clip" cicuits in there that are not likely to help with audiophile goals. If you could dig up a schematic, it may be possible that someone here can take a look at the possability of just bypassing some features that useless to you a=or worse.

BTW, the transistors you mentioned are pretty good (as in almost nothing available to "upgrade to"), so I think you should leave those alone unless you can get your hands on the schematic. You can do Plug-n-play with the opamp but don't expect too much - there even some who swear the NE5532 has yet to be improved on -m arguable but it's still pretty good.

[thanh]

Any way you can post a schematic, or make & model?

oh! yes ! I have agreement with you

[Eva]

I've worked in live sound performances for some time and my experience is that even badly designed PA amplifiers are far more perfect than loudspeakers or rooms. Your problem is probably related to these two last items. Don't mess with amplifiers prematurely

[NUTTTR]

*whoops* thanks muchly for picking up my typo, it is a 1943

And anatech - sorry - i have also missed something else too (it was late!) 2SA1941 2SC5198 is inbetween the D669 and the outputs Sorry, that was kinda cruial!!!

The PCB's themselves seem ok quality (the amp part anyway) and i have added wiring to the back of them (from memory, i've done this on several amps...) and it's all 12ga on the board + tracks so it can handle current now!

As for brand - it was an "OEM" style unit made by soundking and it is an SKAA1200E version... I'll have to post photo's, schematics (AFAIK) i can't get hold of and no one has!! Very very simple design on the output section!

http://nutter.caraudioaustralia.com/...s.php?album=20

There's pictures in there!!! If you move the mouse over the pictures it will have the file name "big" is ~300-500kb each "SM" is about 100-200kb each... Can someone look there and give me some advise on it? Would appreciate it greatly!!!

1500w per side? Excellent! That's what i wanted to hear!!

Potentially there may be no bias? not sure i've tried measuring it, unless my multimeter is stupid, i have yet to get a measurement!!!

There is only protection on the output in the form of a relay on either side... That's it, no input protection at all

Speakers and room are great they normally run off a Peavey PV-8.5C which they sound unreal on and it sounds just as good as my H/K home theatre amp! However it's almost not listenable with the other amp

Thanks for the great help folks!
Aaron

P.S. The big resistors in those pics are 0.47ohm 5w with 3 legs (centre is common)... i'll go ahead and try and measure bias again!