I have an NHT SubTwo with a history of blowing outputs.
Read more about it's history here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=43199
That was two years ago, and now I have another blown output. This one occurred when I was moving the sub a few inches to reach an antenna wire behind it. The sub was in standby mode at the time, upon being moved I heard the two 10" drivers get shorted to rail voltage, followed very shortly by the fuse pop.
Damage is a failed original (non-counterfit) 2SC3281. The previous failure two years ago was one of the 2SA1302's.
I am wondering if there is a problem in the jack for the interface cable to the controller, and that moving it caused a connection failure. I'll be inspecting it carefully.
But, alas, on to the reason for this post:
Now that the 2SC3281/2sa1302 combo is no longer available, I am looking at replacing all four outputs with the newer 2SC5200/2SA1943 devices.
Question 1: Does anyone sell these in matched sets? The amp design has no emitter resistors, so I'd really like to install a matched set.
Question 2: If nobody is currently selling these as matched sets, is there a relatively benign way to match transistors? I don't mind buying a dozen or so of each and building a small circuit to match them, and my DVM has a hFE test mode as well. I don't have access to a curve tracer, though.
Question 3: are there now any better output devices on the market that would be a drop in replacement for this design? (see schematic in thread linked above)?
Question 4: any suggestions on what's causing them to fail? Poor amp design is my #1 suspect.
Read more about it's history here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=43199
That was two years ago, and now I have another blown output. This one occurred when I was moving the sub a few inches to reach an antenna wire behind it. The sub was in standby mode at the time, upon being moved I heard the two 10" drivers get shorted to rail voltage, followed very shortly by the fuse pop.
Damage is a failed original (non-counterfit) 2SC3281. The previous failure two years ago was one of the 2SA1302's.
I am wondering if there is a problem in the jack for the interface cable to the controller, and that moving it caused a connection failure. I'll be inspecting it carefully.
But, alas, on to the reason for this post:
Now that the 2SC3281/2sa1302 combo is no longer available, I am looking at replacing all four outputs with the newer 2SC5200/2SA1943 devices.
Question 1: Does anyone sell these in matched sets? The amp design has no emitter resistors, so I'd really like to install a matched set.
Question 2: If nobody is currently selling these as matched sets, is there a relatively benign way to match transistors? I don't mind buying a dozen or so of each and building a small circuit to match them, and my DVM has a hFE test mode as well. I don't have access to a curve tracer, though.
Question 3: are there now any better output devices on the market that would be a drop in replacement for this design? (see schematic in thread linked above)?
Question 4: any suggestions on what's causing them to fail? Poor amp design is my #1 suspect.
Nick78,
www.ampslab.com click on the component tab at the top he sells them 5 complimentary pairs for $35.00 and he accepts paypal.
www.ampslab.com click on the component tab at the top he sells them 5 complimentary pairs for $35.00 and he accepts paypal.
I think what is needed more is to match the same sex devices rather that the complimentary pair. The Vbes of the same sex devices should be matched, preferably at their operating current, to force equal sharing of current. For a subwoofer, matching compliments would not be as high a priority.
I'd say replace them all. Consider OnSemi's MJL3281/1302... or even the 4281/4302. Consider replacing the driver transistors as well.
Looking at the schematic in the other thread, I'd say that amp is junk. Using two different types of transistors in the LTP sounds like a no-no to me. Similarly, a diode based bias network stinks, and NO EMITTER RESISTORS is just insane - no wonder this amp blows outputs!
Find out what rail voltages are used. I'd say 42V, perhaps 48V for two pairs of outputs.. If it's more than that I'd be surprised... then build a better amp in there!
I read in the other thread that this is not really what you want to do, but to be honest, the design is so poor that it's just going to keep totalling transistors, and costing you money.
Looking at the schematic in the other thread, I'd say that amp is junk. Using two different types of transistors in the LTP sounds like a no-no to me. Similarly, a diode based bias network stinks, and NO EMITTER RESISTORS is just insane - no wonder this amp blows outputs!
Find out what rail voltages are used. I'd say 42V, perhaps 48V for two pairs of outputs.. If it's more than that I'd be surprised... then build a better amp in there!
I read in the other thread that this is not really what you want to do, but to be honest, the design is so poor that it's just going to keep totalling transistors, and costing you money.
1) rail voltages as rated on the side of the toroidal are 95.6V. There is an additional portion of the schematic (which I did not post in the other thread referenced above) with "downconverters" on each of the rails. This amp is supposedly the same as in the Sunfire VT-3. More explanation here:
http://www.diyaudio.com/forums/showthread/t-103684.html
The short short version: the downcoverters are used to keep the voltage across the output devices at ~ 6V to keep the outputs in their safe operating area. so it's a variable power supply of sorts.
the complete schematic is here:
http://home.comcast.net/~jhidley/SubTwoi_Amplifier.pdf
2) i've already ordered the 5200/1943's as replacements from digikey, ordered a dozen or so of each. Any ideas/suggestions on how I should go about matching? is there a test rig schematic floating around out there somewhere?
I realize that this may not be the ultimate end-all be-all amp, but it works fine normally so if i can get a few more years out of it, that's what I'd like to do. It has never died/blown while in use... kind of odd and yet funny.
http://www.diyaudio.com/forums/showthread/t-103684.html
The short short version: the downcoverters are used to keep the voltage across the output devices at ~ 6V to keep the outputs in their safe operating area. so it's a variable power supply of sorts.
the complete schematic is here:
http://home.comcast.net/~jhidley/SubTwoi_Amplifier.pdf
2) i've already ordered the 5200/1943's as replacements from digikey, ordered a dozen or so of each. Any ideas/suggestions on how I should go about matching? is there a test rig schematic floating around out there somewhere?
I realize that this may not be the ultimate end-all be-all amp, but it works fine normally so if i can get a few more years out of it, that's what I'd like to do. It has never died/blown while in use... kind of odd and yet funny.
Well, I guess this topic is dead. I was able to match two pairs of transistors for the outputs. I installed one of the non-matching outputs in the amp temporarily to make sure she would fire up and not blow the matching pair. unfortunately, it again blew the same Q13 or Q14 output. I traced back in the schematic and discovered full rail voltage (+130V) on the gates of Q22/Q23/Q24, so they are blown as well and who knows what else that drives them upstream. Failure at this extent is probably beyond my ability to fix.
Any suggestions on what kind of amp to build around a nice toroidal that has 95.6-0-95.6 outputs and also 19.3v outputs?
Any suggestions on what kind of amp to build around a nice toroidal that has 95.6-0-95.6 outputs and also 19.3v outputs?
This is some kind of Class H topology. Obviously if the power supply part has died, then it's going to be a complex fix.
*edit* That explains it all - Sunfire is a Bob Carver company. This guy is known for doing very clever rail-switching amplifier designs. I retract my "amplifier is junk" comment from earlier posts
In this whitepaper, he describes this amplifiers topology.
The problem with a transformer rated that high, is a conventional amp is going to be massive, with a lot of outputs, and a big heat sink. So sadly, it looks like this is junk unless you also want to go to the hassle of buying a new transformer.
*edit* That explains it all - Sunfire is a Bob Carver company. This guy is known for doing very clever rail-switching amplifier designs. I retract my "amplifier is junk" comment from earlier posts
In this whitepaper, he describes this amplifiers topology.The problem with a transformer rated that high, is a conventional amp is going to be massive, with a lot of outputs, and a big heat sink. So sadly, it looks like this is junk unless you also want to go to the hassle of buying a new transformer.
no international windings to speak of, and by the looks of the toroidal transformer, I'd have to completely unwind it to break the secondaries in half for four 47.8's.
I'm becoming tempted to remove those three MOSFETS in the (+) downconverter on the schematic, then test upstream to see if everything looks ok. I wonder if I would get lucky enough to just replace them and nothing else would blow.
I'm becoming tempted to remove those three MOSFETS in the (+) downconverter on the schematic, then test upstream to see if everything looks ok. I wonder if I would get lucky enough to just replace them and nothing else would blow.
Hi Nick78,
That's a tough problem. It would be a shame to scrap the sub because the amp was dead.
If it was mine, I suppose I would probably get a new toroid with a more usable secondary voltage. Then build a well-documented, well respected u-brew amp circuit like Quasi's (or others). Or if I had plenty of disposable income (whatever that is) maybe get a kit circuit, which are usually more expensive.
The tough part might be finding a design that will suit your output device mounting configuration.
Another thought... you could buy a plate amp module from Parts Express, like a BASH amp or something. I picked up a BASH 500 subwoofer plate amp for $130 on sale one day.
Any amp retrofits would sacrifice any specific NHT EQ circuits though, so you might have to measure and tweak the sub accordingly.
..Todd
That's a tough problem. It would be a shame to scrap the sub because the amp was dead.
If it was mine, I suppose I would probably get a new toroid with a more usable secondary voltage. Then build a well-documented, well respected u-brew amp circuit like Quasi's (or others). Or if I had plenty of disposable income (whatever that is) maybe get a kit circuit, which are usually more expensive.
The tough part might be finding a design that will suit your output device mounting configuration.
Another thought... you could buy a plate amp module from Parts Express, like a BASH amp or something. I picked up a BASH 500 subwoofer plate amp for $130 on sale one day.
Any amp retrofits would sacrifice any specific NHT EQ circuits though, so you might have to measure and tweak the sub accordingly.
..Todd
Hi Nick78,
That's a nice amp. I know how the more refined ones work (lightstar).
My guess is that the diode / resistor network results in low to no bias current in the outputs. Just a higher E-B voltage to reduce the crossover distortion, it's a class "B" design. You should still match the NPN's and the PNP's, but not across polarities.
Many of the comments up to this point are suspect, but understandable since not many people understand this power supply type.
Hi carlmart,
Hi Todd,
Hi Giaime,
So, you must repair the downconverter first. If you don't, the amp will blow again. This is very important.
I've had to repair downconverters before. Mine had 125 VDC rails and a ton of mosfets in parallel. It can be repaired.
The original failure of the output (this time) probably caused on fet to fail. You may find all shorted or one shorted D-G. That will turn the whole pack on. You need to make sure all those resistors are intact and the drivers (Q8 and Q9) for starters. All resistors between those transistors and your fets must not be open, open is very bad for fets. Replace the group with units of the same run. Check the diodes as well. Fix the outputs at the same time.
I am frankly surprised that some people would recommend replacing the amp instead of repairing the downconverter. It's easier than class D for pete's sake! You can test the amp section from a "laboratory" type dual tracking supply. In this way you can get the amp working before getting into the downconverter. What you can't do is run the supplies up so that it overheats. Make sure the bias section is working. So this is only as difficult as you want to make it.
-Chris
That's a nice amp. I know how the more refined ones work (lightstar).
My guess is that the diode / resistor network results in low to no bias current in the outputs. Just a higher E-B voltage to reduce the crossover distortion, it's a class "B" design. You should still match the NPN's and the PNP's, but not across polarities.
Many of the comments up to this point are suspect, but understandable since not many people understand this power supply type.
Hi carlmart,
Go study some more amplifier theory. Parallel transistor matching is always important. Emitter resistance would have been nice, but at no bias I can see it working. 0.1R would work and Bob is in the habit of using 0.05R.
Hi Todd,
No. Not even close to a reasonable answer.
Hi Giaime,
This from you??? Okay, I'm surprised.
So, you must repair the downconverter first. If you don't, the amp will blow again. This is very important.
I've had to repair downconverters before. Mine had 125 VDC rails and a ton of mosfets in parallel. It can be repaired.
The original failure of the output (this time) probably caused on fet to fail. You may find all shorted or one shorted D-G. That will turn the whole pack on. You need to make sure all those resistors are intact and the drivers (Q8 and Q9) for starters. All resistors between those transistors and your fets must not be open, open is very bad for fets. Replace the group with units of the same run. Check the diodes as well. Fix the outputs at the same time.
I am frankly surprised that some people would recommend replacing the amp instead of repairing the downconverter. It's easier than class D for pete's sake! You can test the amp section from a "laboratory" type dual tracking supply. In this way you can get the amp working before getting into the downconverter. What you can't do is run the supplies up so that it overheats. Make sure the bias section is working. So this is only as difficult as you want to make it.
-Chris
Might have known a Bob Carver design would get Anatech interested
Anatech, I fully retract my "poor amplifier design" comments - they were made before I saw the whole schematic and noticed the Class H powersupply part of the circuit. If you look at it as a standard Class AB schematic alone, it does seem rather suspect, does it not?
The main reason we've been suggesting replacement amps here, is that it seems like nick78 is not confident enough in his skills to do extensive troubleshooting and repair. It might well have been simpler for him to construct some regular AB amp than to repair this one.
Anatech, I fully retract my "poor amplifier design" comments - they were made before I saw the whole schematic and noticed the Class H powersupply part of the circuit. If you look at it as a standard Class AB schematic alone, it does seem rather suspect, does it not?
The main reason we've been suggesting replacement amps here, is that it seems like nick78 is not confident enough in his skills to do extensive troubleshooting and repair. It might well have been simpler for him to construct some regular AB amp than to repair this one.
Hi jaycee,
I saw and understood where you were coming from. No problem. Heck, the nameplate is different and until I saw the posted schematic (thank you nick78!) I had no clue what it was, but did have a sense of foreboding. Knowing what it was and having to figure the Lightstar out on my own, I felt I could demystify it for everyone. That's what I had hoped.
Anyway, at the supply voltages this amp runs at, only a very competent design would hang together. I think you will agree on that. This isn't little amp territory at all. The bias circuit isn't really an active type as there is not supposed to be any continuous current to control. Like a class D amp, it relies on some inductance (from the woofer VC) to filter out the hash.
-Chris
I think this is more of a Vic Richardson design. Bob had left before the Lightstar design was completed. I dealt directly with Vic at the time. BTW, Vic is one heck of a nice guy and I'd love to talk to him again. He's a good engineer.
I saw and understood where you were coming from. No problem. Heck, the nameplate is different and until I saw the posted schematic (thank you nick78!) I had no clue what it was, but did have a sense of foreboding. Knowing what it was and having to figure the Lightstar out on my own, I felt I could demystify it for everyone. That's what I had hoped.
Anyway, at the supply voltages this amp runs at, only a very competent design would hang together. I think you will agree on that. This isn't little amp territory at all. The bias circuit isn't really an active type as there is not supposed to be any continuous current to control. Like a class D amp, it relies on some inductance (from the woofer VC) to filter out the hash.
-Chris
You say you have full rail voltage att the gates of the mosfets. If you look at the schematic you see that the gate drive is referenced to their source. If they are shorted drain to source it will make full rail voltage appear on the gates too, still with 0V across G-S and the drive circuit. If the gate is shorted too it probably has taken out at least the gate resistors and the drive transistors, maybe not more.
You should be able to check the preregulator without the output stage, say using a HIGH power resistor or something else appropriate, instead of blowing outputs if it doesn't work. It should be fixable, but might not be something for the average DIY:er... take care, those voltages are lethal. 200VDC between rails. Yikes!
You should be able to check the preregulator without the output stage, say using a HIGH power resistor or something else appropriate, instead of blowing outputs if it doesn't work. It should be fixable, but might not be something for the average DIY:er... take care, those voltages are lethal. 200VDC between rails. Yikes!
this thread has been educational for me, I didn't realize how "special" of a design this amp is.
In another thread/forum, I found that NHT is selling off old inventory, and part of what they are selling is the same amp board from some other subwoofer speaker system (VT3's I believe) so I'm going to pick up one or two of those and replace my amp boards.
After that's done, I think i'm going to make an attempt to repair the original board. The more i stare at the schematic, the more I think "it can't be THAT bad". I have enough basic skills that if I approach it methodically and work through the schematic carefully, I should be able to accomplish this. It will probably end up an educational experience.
Yes, the rails are dangerous. +130 / -130 gives nearly 300V differential. Got to make sure to discharge the PS caps before I work on it....
In another thread/forum, I found that NHT is selling off old inventory, and part of what they are selling is the same amp board from some other subwoofer speaker system (VT3's I believe) so I'm going to pick up one or two of those and replace my amp boards.
After that's done, I think i'm going to make an attempt to repair the original board. The more i stare at the schematic, the more I think "it can't be THAT bad". I have enough basic skills that if I approach it methodically and work through the schematic carefully, I should be able to accomplish this. It will probably end up an educational experience.
Yes, the rails are dangerous. +130 / -130 gives nearly 300V differential. Got to make sure to discharge the PS caps before I work on it....
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