SOLVED:Catastrophe on an Emotiva XPA-3 Gen 1 amp  - Page 1
Forum section: electronics / repair

author: rfengg
author_rank: Regular Contributor
timestamp: January 29, 2023, 03:20:04 am
content: Hello All,Need your advice on a Emotiva XPA-3 Gen 1 Audio amp.My Emotiva XPA-3 Gen 1, blew its 10A fuses last week. The amp consists of 3 identical amp sections (modules) powered in parallel with +65/-65V and further investigation revealed that the power supply inputs to one of the faulty modules had a dead short across the +65V and -65V rails.I disconnected the power to the faulty module and the rest of the 2 modules were fine which leaves me with a 2 channel amp instead of the original 3 channel amp.On the failed module, what surprised me was that all the output BJT's, i.e., three PNP NJW0302's and three NPN NJW0281's were a dead shortIn addition to this 4 other BJT's as per the attached schematic were also blown (all damaged BJT's marked in red).Could someone help me understand what chain of events could have led to this catastrophe? Would failure in any one of the marked transistors have led to a chain that caused all output BJT's to fail?All the ballast emitter resistors have survived, so is there any other part I should check? I checked all other BJT's on the board and all are reading alrite on a diode test using a DMM.thanks

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 29, 2023, 03:53:58 am
content: Well, it's hard to know which one died 1st but:Q008 Sets the bias/idle current for the 6 output power transistors. If Q008 went open, then massive bias/idle current would flow and those 6 power MJW transistors would go pop! pop! pop! pop! pop! pop!If any ONE of the 6 MJW output power transistors went shorted, then it would make the speaker output swing to one of the 65V rails. Since the amplifier has negative feedback, the 3 opposing output transistors would turn on hard, massive current would flow, and those remaining 5 power MJW transistors would go pop! pop! pop! pop! pop!Same thing goes the Q012 & Q015 pairs. If one of them went shorted, the other would turn on hard, turning on the corresponding 3 power MJW transistors which would fight their opposite 3, massive bias current would flow and those 6 power MJW transistors would go pop! pop! pop! pop! pop! pop!

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 29, 2023, 05:06:03 am
content: The same failure sequence could happen if C031 at Q007 or C017 at Q011 shorted. The corresponding transistors would be driven to saturation. The design seems somewhat brittle.

author: cvanc
author_rank: Frequent Contributor
timestamp: January 29, 2023, 11:06:20 am
content: Good luck with your repair, but may I ask where you got an Emotiva schematic?I've been looking without success for drawings to another box they make (Stealth DC-1 DAC).In general, schematics for this brand are difficult to find. Thanks.

author: rfengg
author_rank: Regular Contributor
timestamp: January 29, 2023, 12:33:32 pm
content: Quote from: cvanc on January 29, 2023, 11:06:20 amThe folks at Emotiva were very helpful.....the didnt have a complete Service manual but they did send me the schematics.......try contacting them via phone on 615-790-6754 between 1:30pm and 4:30pm CST which is what I did......they sent me the schematics via email......super service imo

author: dietert1
author_rank: Super Contributor
timestamp: January 29, 2023, 02:12:37 pm
content: These amplifier designs are not 100 % reliable, as a bass speaker or a speaker with built-in passive xover represents an inductive load. If you do the calc, you will need to absorb about 2 KW of power (if damping is 4 Ohm) and a power BJT already hot will take at most 100 or 200 W or so to die. So instead of three one would probably need 10 pairs of output transistors to improve. Or reduce supply voltage.Of course when using an amplifier with "normal" music of variable output level, it may last a long time, but technically speaking it is unsafe. Maybe worthwhile checking fan operation.Regards, Dieter

author: rfengg
author_rank: Regular Contributor
timestamp: January 29, 2023, 05:09:56 pm
content: Thanks for all your inputs.....agreed that the B+ voltages of 65V each may be a bit too high but then having all the 6 NJW output devices fail short which can each handle 15A each is crazy.@Kim Christensen : What would be the feedback back that " the 3 opposing output transistors would turn on hard"?Any info available on the web that would explain the working of the attached schematics especially the negative feedback path?thanks

author: dietert1
author_rank: Super Contributor
timestamp: January 29, 2023, 06:48:27 pm
content: Maximum collector current isn't the criterion, but the SOA (safe operation area). In Figure 2 of the NJW0281 datasheet you can see that at 130 V it will stand about 2 A collector current for about 10 msec. This is the situation when you decelerate fully accelerated bass cone by reverting voltage from +65 to -65 V or vice versa. If the speaker has 4 Ohm resistance, you need about 32 A, so that gives 16 transistor pairs and they should be "cold" before you try. As the deceleration isn't a rectangular but a triangular power pulse it may survive with 8 pairs - when cold!Regards, Dieter

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 29, 2023, 09:30:23 pm
content: Quote from: rfengg on January 29, 2023, 05:09:56 pmIn this Amplifier, the negative feedback path is via R028 (From the speaker output back to the differential input) The AC voltage gain is set by the ratio of R028 to R018.Suppose the DC voltage at the output was too high. This would increase the current fed to Q002's base via R028 turning it on more. Because Q002 and Q001 are configured as a differential pair, this will make Q001 to turn off which turns Q007 off, which lowers the voltage at the base and thus emitter of Q012, this also lowers the voltage on the bases, and thus the emitters of Q026, Q013, & Q014...The opposite (On vs OFF) happens with the mirrored circuitry below (Q004, Q003, Q011, Q015, Q025, Q016, Q017)Note: When I say on and off, I mean On = passes more current.. Off = passes less current... Fully On/Off would be clipping in an audio amp, but it's easier to explain that way.To understand these types of circuits better, look up:"differential amplifier" (Q001 & Q002 for example)"constant current sink" (Q010, R026 & D003)"push pull audio amplifier"

author: coromonadalix
author_rank: Super Contributor
timestamp: January 29, 2023, 09:53:41 pm
content: Only time i saw this was an biais circuit going bezerk, it created a thermal runaway and almost everything went poofNot going as far, on a Sony Str-av1070 surround sound system, i had one pecliar problem : started fine, but after a very long time it began to distortThe surprise was to find a missing resistor in the biais circuit ?? yeah totally missing, never soldered from factory

author: CaptDon
author_rank: Super Contributor
timestamp: January 30, 2023, 02:33:28 am
content: dietert1's math is off a bit. You'll never see the kind of dissipation mentioned when controlling speaker excursion based on the stated 'damping factor' of the amplifier. To a basic degree it is the negative feedback path that helps with damping and you would not encounter a cone which was pushed to the +65 volt excursion requiring damping to the degree of needing the full -65 volts applied to generate the amplifier's stated damping factor. A 1KW amplifier may have a damping at 200 or so when measured at 1 watt but it will be a different story when pushed to levels over 500 watts when the base drive would be cut back by circuits like the emitter swamping resistors and current foldback protection designed to limit maximum output device dissipation. On the other hand, it would be interesting to have a look at the circulating currents in a system consisting of a pair of 18" drivers wired in parallel in a huge scoop cabinet with a 4 square foot exit mouth when driven with square waves at about 40Hz. Would be interesting to see the current waveform overlaid on the voltage waveform. B.T.W., If one of your output devices shorted all devices on the opposite side usually fail as the amplifier tries to correct the D.C. offset and the devices overheat and die. Generally I find one dead device on one side and all devices dead on the other side. Maybe that channel went into ultrasonic oscillation due to an unstable load and overheated the devices until destruction. Some of the early AMPEG solid state 'Bass Transducer' units were known to do that. They suddenly would sound really bad and then blow up about 1 minute later. I scoped them at about 100Khz of self oscillation and the output transistors got so hot in about ten seconds they would damage your fingertips!!!

author: JeremyC
author_rank: Regular Contributor
timestamp: January 30, 2023, 03:25:29 am
content: Quote from: rfengg on January 29, 2023, 03:20:04 amI’m big fun of ROTEL gear from 1980/90s (pure 2 channel). I learned about EMOTIVE in ~2009.I bought EMOTIVA preamp and power amp at Audiogon … it was nice surprise for the money what I spent.Coming back to the topic, did you get chance to the check the C031 and C017 capacitors as Tomorokoshi suggested earlier?

author: dietert1
author_rank: Super Contributor
timestamp: January 30, 2023, 03:34:29 am
content: As i wrote before: With "normal" music you stay well below full amplitude on average and there will only be drum beats or so reaching the limit. But modern pop music is terrible in that respect. They do all sorts of digital processing to fit the largest possible sound level into the digital full scale. And once the amplifier runs hot, it doesn't take much to blow it. It's not clear whether those crashes happen with pro 18" speakers as those usually have light cones for best efficiency. Maybe the OP has more details.Regards, Dieter

author: JeremyC
author_rank: Regular Contributor
timestamp: January 30, 2023, 03:52:23 am
content: Quote from: dietert1 on January 30, 2023, 03:34:29 amIt doesn't matter on type of music, it shouldn't cause chain reaction like the OP described.I'm listening to many kind of music classical, jazz ... and hard core UK punk from 1980 at extreme levels... my amps eventual limited output, but never died

author: rfengg
author_rank: Regular Contributor
timestamp: January 30, 2023, 04:53:18 pm
content: Coming back to the topic, did you get chance to the check the C031 and C017 capacitors as Tomorokoshi suggested earlier?Yes I did and both are alrite on a DMM check......have ordered the final stage BJT's from Newark and the other 4 from Ebayas they were available nowhere else and hope to change them out sometime within the next 2 weeks.

author: rfengg
author_rank: Regular Contributor
timestamp: January 30, 2023, 04:57:15 pm
content: Quote from: JeremyC on January 30, 2023, 03:52:23 amI too am a bit confused to how someone would design a 1000$ amp like this with a chain reaction destroying all the final semicons.At my regular day job, I have to perform single and double fault analysis......

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 30, 2023, 06:25:44 pm
content: Quote from: rfengg on January 30, 2023, 04:57:15 pmAs far as I can see this is a pretty common configuration for an audio power amp.As Capt. Don said earlier, if one output device shorts, then all the devices on the other side pass shitloads of current into this shorted device.So either these devices also end up short, or the emitter resistors blow instead, saving them.But it doesn't really save them as nobody would really put these devices back in service after what they've been through.

author: rfengg
author_rank: Regular Contributor
timestamp: January 30, 2023, 06:47:19 pm
content: Quote from: Audiorepair on January 30, 2023, 06:25:44 pmSorry, but I did not understand why you would say its not worth repairing after what they have been thru......its a 1000$ amp......any reason why the fault might repeat with new devices in place?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 30, 2023, 07:01:32 pm
content: Quote from: rfengg on January 30, 2023, 06:47:19 pmThat's not what I said.

author: dietert1
author_rank: Super Contributor
timestamp: January 30, 2023, 08:01:36 pm
content: There used to be professional linear audio amplifiers with sophisticated current limiting that depends on output voltage. The current limit is smaller at small output voltage and full current will only be allowed when output voltage is high and in the proper direction. This safety circuit keeps the output transistors within their SOA under all signal and load conditions. Clipping indicators will light up when the circuit limits audio output.I have seen this in many professional amplifiers and they can be considered safe from a technical view.Some professional amplifiers of 100 W and more use different supply voltages to reduce losses in the output transistors (class H). For example Philips DSS active speakers used to implement this for their bass channel.The circuit presented above does not belong to this group of amplifiers. It can be considered safe for home use - where actual output power never reaches 100 W or more. 100 W into a speaker of 80 dB SPL means more than 120 dB sound level.Regards, Dieter

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 30, 2023, 08:09:56 pm
content: A shorted output device is just one failure mode, and can be catastrophic, taking lots of others with it.Another common failure mode is something else goes wrong and the amp just goes DC.The built in DC protection shuts it down/disables the speaker. and no output devices get harmed at all.

author: rfengg
author_rank: Regular Contributor
timestamp: January 31, 2023, 02:32:54 pm
content: Emotiva has indeed incorporated a dc protect using a UPC1237.Once I get hold of the damaged parts, I am thinking of putting in a current monitor based on the individual emitter resistor V drops and OR them to shut down the PS relay......I believe there was something similar done in an Elektor magazine some years before.Any other ideas on how to keep the output BJT's always under the SOA curve?To their credit, Emotiva has built the amp extremely serviceable but getting hold of the o/p BJT's especially the NJW0281G has been a pain......only Newark seem to have them.Aliexpress and Ebay seems to have tons of them but many are missing the "ON" semiconductor logo.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 31, 2023, 07:11:28 pm
content: Well this is how the big boys do it, might give you some ideas.Page 14 shows the power amp schematic.

author: floobydust
author_rank: Super Contributor
timestamp: January 31, 2023, 07:37:37 pm
content: No bias trimpot or lag comp or stopper resistors does not instill a wise design. I can't take OP's amplifier seriously.It could have an RF pole or other instability and oscillate. I have seen this before in other power amplifiers. Gen 2 or 3 would show if they made changes for that.Many of these audiophile amplifiers are hype and marketing added on to a build which is missing the hardcore pro parts - although the EV P3000 can do sound reinforcement/club duty 24/7 look at the extra parts required for that.OP you could check if Emotiva made critical changes for their later generations, this is the giveaway.Also, it is standard practice to replace transistors one stage ahead of the blown ones because, although they test good, they can be damaged and fail later on.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 31, 2023, 07:48:42 pm
content: This is why I don't repair hi-fi......To elaborate, in the semi Pro/Pro Audio world, you pretty much get what you pay for.In the hifi world you get sold a $1000 dollar amp that is not a $1000 dollar amp, they just told you it was.

author: dietert1
author_rank: Super Contributor
timestamp: January 31, 2023, 09:49:27 pm
content: The P3000 circuit shows current limiting. Output current of each transistor is measured on 0.47 Ohm emitter resistor (R1 .. R6). Then R7 .. R12 get the average current of all six output transistors as voltage. Then there is a 4:1 divider (R188 R187). The the current limit is established using the base-emitter voltage of Q57 as a threshold. It becomes conductive at about 0.6 V. times 4 = 2.4 V, divide by 0.47 Ohm gives about 5 A per transistor or 30 A in total. When it becomes conductive, it clamps the input voltage of the output stage, near the bias circuit. This limiting circuit is present 4x in a stereo amp.To get a current limit proportional to output voltage one can add a resistor from the base of Q57 to Gnd. For example delete R187 and replace it by a 2.2 KOhm resistor in series with a diode from the base of Q57 to Gnd. Then the current limit will be about 1.2 A per transistor near and below 0 V output voltage and 5 A per transistor near 60 V output. At 60 V output the output transistor has only 8 V collector-emitter voltage, so at 5 A it only takes 40 W.Regards, Dieter

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 31, 2023, 10:38:35 pm
content: The R188/R187 divider is shunted by an NTC and fixed resistor, R200 and R189.My simple maths calculate that as the temperature increases, the limit threshold is therefore increased, which seems somewhat counter intuitive. (?)(I presume the NTC is on or near the heatsink)Edit: Er, to clarify, I mean that with increasing temperature comes less limiting, not more.

author: dietert1
author_rank: Super Contributor
timestamp: January 31, 2023, 10:59:32 pm
content: Yes, very strange. Maybe they want to compensate the -2 mV/K temperature coefficient of the base-emitter threashold voltage of Q57.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 31, 2023, 11:01:56 pm
content: Or NTC is a typo for PTC?Or maybe its not actually measuring the heatsink then and it is just a tempco as you suggest.

author: JeremyC
author_rank: Regular Contributor
timestamp: February 01, 2023, 01:07:45 am
content: Quote from: rfengg on January 30, 2023, 04:53:18 pmMost of the LCR meters (including DMMs) are testing the DUT under low voltage. I suggest you replace just in case these 2 caps with quality, 200V+ rated.Quote from: rfengg on January 31, 2023, 02:32:54 pmAnother suggestions:- Don’t trust anything from Ebay, the vendors are famous for faked components , use Mouser, Digi-Key, Newark etc. They maybe little bit more expensive, but you will be sure what you are getting, especial when comes to paired transistors...- Improve the Emotiva’s design and add separate fuse(s) for each channel’s rail. I would start with underestimated fuse like 5A. It maybe frustrating, but it will save your $$$.- Try power on the amp after repair in series with 200W bulb (I mean classic bulb and not an LED) and ammeters in the rails.Good luck and keep posting how it went.

author: JeremyC
author_rank: Regular Contributor
timestamp: February 01, 2023, 01:29:27 am
content: Quote from: rfengg on January 31, 2023, 02:32:54 pmContact Mouser, or Digi-Key, they offer paired transistors.BTW, did you ask Emotiva, they may have solution for you.

author: rfengg
author_rank: Regular Contributor
timestamp: February 02, 2023, 02:20:34 pm
content: Quote from: JeremyC on February 01, 2023, 01:29:27 amYes I did contact Emotiva......they offered a flat $250 for the repair.......I was quite alright with that but the shipping is prohibitively expensive......the amp weighs close to 55lbs thanks to the conventional transformer based PS.

author: rfengg
author_rank: Regular Contributor
timestamp: February 02, 2023, 02:24:36 pm
content: Quote from: JeremyC on February 01, 2023, 01:07:45 amThanks JeremyC......I found the final NJW's from Newark ($3 each) but the 2 of the other 4 I need i.e., 2SC4793, KSA2073, 2SA1837, KSA940 are hard to come by from Mouser/Digikey/Arrow/Newark.Will try to implement separate fuses as well.......I do use a dim bulb tester with an incandescent 100W in series when I do the repairs.

author: rfengg
author_rank: Regular Contributor
timestamp: March 11, 2023, 02:59:09 pm
content: She finally sings again !!!After waiting for what seemed like an eternity, I managed to get hold of originals all the damaged parts and replacing them has made the channel 2 of the amp completely functional again; channel 1 and 3 were already working fine but the short on channel 2 kept blowing the 10A fuses......A humble thanks to everyone on the forum whose advice was invaluable.Also replaced the caps as per @Tomorokoshi advice, just in case as they were cheap and easily available from Digikey.Cheers to all

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 12, 2023, 06:13:37 pm
content: Quote from: dietert1 on January 29, 2023, 06:48:27 pmSOA (safe operating area) is a critically important specification for analog audio power amplifiers. Unfortunately SOA isn't well understood. The semiconductor manufacturers tend to promote specifications like "maximum current" and "maximum voltage" because larger numbers look good. Therefore one must search through the transistor manufacturer's datasheet for the graph of "Safe Operating Area" to determine the (much lower) allowable collector current for large E-C voltages.Nearly all of the BJT power transistors made for linear audio amplifiers have similar SOA characteristics. Allowing roughly2A per output transistoris a good rule of thumb for all linear audio amplifier circuits withpower supply rails of +/-50V and larger.This explains why well-designed amplifiers intended for "heavy duty" usage have such a large number of output transistors arranged in parallel.Another circuit design known as "Class H" uses multiple +/- power supply rails along with commutating diodes and additional transistors which momentarily raise the +/- rail voltages for large peak audio signals. Carver and NAD were the first manufacturers to popularize this technique. Typical rail voltages are something like +/-30V and +/-60V. For small amplitude signals the main output transistors receive collector voltages of +/-30V. This is below the "2nd breakdown" portion of the SOA chart where the maximum permissible collector current drops rapidly. When a musical peak would be clipped with +/-30V rails, the Class H circuit briefly raises the rail voltages to +/-60V. However, because the instantaneous output voltage is already close to 30V, the actual E-C voltage across each main output transistor never exceeds 30V.Some of the Carver amplifiers with huge power ratings have 3 sets of supply rails: +/-30V, +/-65V, and +/-120V. Again, the higher voltage rails are applied to the main output transistor collectors for only a few msec at a time.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 12, 2023, 06:46:15 pm
content: Quote from: Audiorepair on January 31, 2023, 07:11:28 pmThis Electro_Voice P30002-channel power amplifier has an unusual output stage design where the main supply railsfloatrelative to ground. Theoutput configuration is a "full H bridge."However, unlike most bridged amplifier circuits, one H-bridge output is grounded while the other H-bridge output drives the load. This is why the power supply rails must "float."Each channelof this Electro-Voice P3000 amplifier has24 main output transistorswhich are arranged in 4 groups of 6 parallel transistors each.The total supply voltage of 150V is divided by 4 for each of the 4 sets of output transistors. This is 37.5V for each of the 4 legs of the H bridge. 37.5V is below the "2nd breakdown" voltage of the output transistors.I believe the oldest implementation of this design might have been known as the"circlotron."A stereo amplifier of this design requires two main power transformers (or two isolated secondary windings on a single power transformer) because of this "floating" design. However, only a single bulk energy storage capacitor (or group of capacitors) is needed per channel. The main supply rails aren't +/-75V. Rather this is a single-ended 150V supply rail with no reference to ground.A legendary American stereo power amplifier known as theElectron Kinetics Eagle 7 (or 7A)had a similar circuit topology. In this case there are 12 output transistors per channel arranged in 4 groups of 3 parallel transistors per group.