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Old 5th March 2014, 01:58 PM   #1
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Default Replacing output transistors in a Lightning Audio B300.4

Hi guys,

I have bought a 2nd hand Lightning Audio B300.4 a couple of years ago and have been using it ever since. A couple of months ago I noticed that the LF channel has a sort of static crackling noises when the amp is on and music isn't playing or the volume is low enough to hear it. It seems to be unaffected by volume, and 'disappears' when the music is playing loudly enough.
It is very annoying and distracting for me, so I decided to open the amp up and see if I can find the source of the problem. Sadly I had no luck trying to find a schematic for this thing, I would appreciate it if someone does and can share it with me.
What I saw when I opened it is that sometime in the past one pair of the power supply MOSFET fried and were replaced. It seem that the original ones were IRFZ46N (the other side) and the parts they were replaced with are ST P80NF. I don't think it's that much of a problem, though.
I kept digging, and it seems that 3 out of 4 pairs of the output transistors were replaced. I found 3 pairs of D44H11/D45H11 and one Toshiba 2SC5198/2SA1941 pair. My guess is that the Toshiba parts are responsible for the LF channel (they are silkscreened as Q1xx series), perhaps they are damaged as well and need replacing, or maybe the gain on the amp is not adjusted correctly.
I was thinking about replacing all 4 pairs with something like FJA4313 & FJA4213 ( http://www.mouser.com/ds/2/149/FJA4313-80706.pdf & http://www.fairchildsemi.com/ds/FJ/FJA4213.pdf ) as they are pretty cheap and seem to have similar characteristics, and are also meant for audio equipment.
Since I'm not an EE and don't specialize in audio amplifiers I wanted to hear some feedback on how I should proceed from here.

On a side note, would you say using a computer power supply is ok for testing out the amp when it's out of the car?
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Old 5th March 2014, 04:38 PM   #2
ishiru is offline ishiru  Indonesia
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so when you push the amp louder, the cracking sound disappeared?..it sounds like cracked soder joints.check and re-flow all solder joints.

yes you can use coumputer supply as long as the PSU could deliver enough power.short the PS-ON (usually green) cable w/ any ground to turn the PSU on.
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Old 5th March 2014, 04:48 PM   #3
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It's a bit hard for me to explain how it sounds, but basically it's similar to static on the radio between stations. Only it's not constant, it gets a bit louder and quieter constantly.
This noise doesn't change with volume, so the louder I play music the less noticeable it is. It's audible between tracks or if I pause the track, and seems to be at the same level no matter what the volume setting is.
I'll look into the cracked joints, but what is your opinion about not having matching output transistors could this affect sound quality?
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Old 5th March 2014, 06:11 PM   #4
rrae is offline rrae  Norway
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Ask Rockford Fosgate for schematic, they may help You.
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Old 5th March 2014, 06:59 PM   #5
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Thanks. I sent them an email requesting the schematic.

Back to my original question, do you think that these transistors I picked will do the job? Fairchild FJA4313 & FJA4213.
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Old 5th March 2014, 07:28 PM   #6
ishiru is offline ishiru  Indonesia
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IMO, unmatched output pairs affects the sound quality but not much.if you're not a perfectionist, unmatched pairs isn't a problem
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Old 5th March 2014, 07:33 PM   #7
rrae is offline rrae  Norway
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U can use PC PSU, but dont forget to use max. 10A fuse inline B+, for safety purpose.

I dont have schematic, i cant say what are the orginal parts for sure.
The power supply fets needs to be all the same, from the same date, batch, so they can share current evenly.
If the amp guts are correct, the amp uses 2n6488/2n6491 for the outputs.
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Old 5th March 2014, 07:38 PM   #8
ishiru is offline ishiru  Indonesia
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Quote:
Originally Posted by rrae View Post
U can use PC PSU, but dont forget to use max. 10A fuse inline B+, for safety purpose.

I dont have schematic, i cant say what are the orginal parts for sure.
The power supply fets needs to be all the same, from the same date, batch, so they can share current evenly.
If the amp guts are correct, the amp uses 2n6488/2n6491 for the outputs.
i forgot to mention the batch.like rrae said, batch and part number should be matched while the electrical properties can be leaved unmatched (ie : HfE, blahblahblah)

and a good computer PSU has built in OCP..we don't know what PSU did you use, so like rrae said, use a safety devices either a resistor, lamp, or fuse.
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Old 5th March 2014, 09:09 PM   #9
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That's a good tip, I guess that a headlight lamp should work well as a resistor. A 60W lamp, will allow 5A through to the amp.
I've got several quality power supplies, the one I planned to use is an FSP 350W unit. It does have OCP and all basic protections in any case.

Here are some pics I took of the amp when I had it open: B300.4 - Imgur
You can see that the previous power FETs went up in flames at some point in time.

@ishiru
I am not an audiophile, although I can appreciate the sound of quality audio gear. I'm a programmer who has lately started toying with electronics and microcontrollers and I do like order. The fact that the parts are actually mismatched in this amp makes me feel that it has to "fixed".

@rrae
What I saw when I looked at the ICs used on the undamaged side were IRFZ46N, the replacement parts are ST P80NF. Are you suggesting that I should replace all 4 FETs with new ones from the same batch?

Now, if you look at Q160/Q161 and the other Qx60/Qx61 you'll see that they are different. Q160\1 are Toshiba 2SC5198/2SA1941 pair. The other 3 are pairs of D44H11/D45H11.
I looked at other Rockford schematics (400a4) and it seems that the Q1xx FETs were responsible for the LF channel.
I would hazard a guess that the Toshibas are the original parts and that they are also damaged and should be replaced.
Now since the Fairchild FJA4313 & FJA4213 are pretty cheap and seem to have similar characteristics to the Toshiba parts I'm assuming they should work fine, and if I'm doing a 'refurb', changing all 4 channels to new matching pairs is not a bad idea.

BTW, where did you get the data about the amp originally using 2N6488/2N6491s as the outputs?
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Old 5th March 2014, 10:06 PM   #10
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Yeah I seen this one before, It comes with 6488 and 6491 devices but had thru holes for larger outputs like say TIP 35 and TIP 36. That is what i put in them anyway for durability reasons. Amp was designed for larger outputs but had factory cut downs installed probably for costs savings at manufacture level.

With all the flash marks you kind of see how they got the name "Lightning Audio " lol lol lol..I would tell you to either use the original factory cut down part of 6488 and 6491 or jump up to TIP 35 and TIP 36 for added durability since they have about 10 amps higher collector rating or almost twice the current handling ability. The D45 and D44 parts are much older obsolete outputs from way back in the day, and are tough to find nowadays since the 6488 and 6491 parts become their replacement in the first place 20 years ago now...

Fairchild FJA4313 & FJA4213 are also 2SC5200 and 2SA1943 check this DIY link out for matched sets possibilities if your thinking about upgrades

bipolar (bjt) transistor families for audio power output stages

Looks like the other repairs replaced the emitter resistors on one side, better check them all just for hits and giggles, as the last repair tech did some really special stuff in there to get the amp working again... hope this helps some, its been 6 years since i saw one of these, but they aren't anything special IMHO.

Last edited by 1moreamp; 5th March 2014 at 10:15 PM.
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