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Old 11-27-2021, 12:30 AM   #1
eccerr0r
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Default Frying amplifier finals... audio, not rf...

Well for RF transmitters, transmitting without an impedance matched antenna and if the transmitter doesn't have automatic limiting... kiss the final transistor goodbye and chalk one up to something you killed.

But that's not the same way with audio amps for the most part, well, at least the wavelength of audio on copper is so long that you get all of it back anyway but the transistor is fine with it...

So how are people destroying finals in their audio amplifiers then? Heh.

Are people shorting their outputs or something, overdriving output to a too big speaker? Or are these amps just frying on their own due to being in an overly compact enclosure and simply got too hot over time?

Anyone actually kill their audio amplifier output or witness it leaking the magic smoke?

TBH at least as of right now, knock on wood, I have yet to blow an audio amplifier output. Just wondering what it takes to blow an output?

(I haven't blown an RF final/amplifier output yet either yet, likewise knock on wood, but I've gotten both audio and RF amplifiers that had bad finals that need repair... And also honestly I think I envision myself killing an RF final well before killing an audio final...)
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Old 11-27-2021, 05:39 AM   #2
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Default Re: Frying amplifier finals... audio, not rf...

i often help a guy fixing amps - dont get them myself.

the most common cause of final destruction is faulty bits in the pre-drive either overdriving the finals or turning both parts of a push/pull pair on together or overlapped!

it can be anything - last one was just an out-of-value resistor shifting the bias enough to make one transistor drive against the other!!!
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Old 11-27-2021, 05:56 AM   #3
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Default Re: Frying amplifier finals... audio, not rf...

shorted speaker wires . mismatched speakers to amp ..too low ohms . having amp turned up to clipping for too long . then there is faults like idle bias set too high . bad ventilation causing overheating .
i have had amps blow up for no apparent reason . and also for obvious reasons .
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Old 11-27-2021, 09:06 AM   #4
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Default Re: Frying amplifier finals... audio, not rf...

Interesting... but nothing self observed?
Self inflicted wounds is the most frequent cause of failure?
Was wonderring hiw often bias shifts cause enough leakage to self destruct...if cooling is sufficient it should be able to handle it at least for several mA...
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Old 11-27-2021, 09:31 AM   #5
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Default Re: Frying amplifier finals... audio, not rf...

I've killed a few back in the days trying to see if that 2ohm stable car audio amp could give up just one more ohm. A few didn't end well.
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Old 11-27-2021, 10:34 AM   #6
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Default Re: Frying amplifier finals... audio, not rf...

Yow...so I suspect a lot of amps that end up on the repair bench were self inflicted wounds?

I'm surprised that a lot of pre-drivers go bad... or were they too self inflicted?

I guess I did get one amp that failed due to poor QC/assembly. One of the leads on the STK chip broke. I didn't notice it broke until after I debugged down to the STK chip, bought a new STK chip, and I removed the old one. I think there was enough of a stub that I could have wired a jumper and reinstalled the old one, ha...
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Old 11-27-2021, 11:24 AM   #7
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Default Re: Frying amplifier finals... audio, not rf...

it depends on age.
i see a lot of 70's and 80's stuff - drifted values on resistors and caps is common.

there is a french guy who is reverse engineering a lot of stk stuff to create clones btw.
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Old 11-27-2021, 11:44 AM   #8
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Default Re: Frying amplifier finals... audio, not rf...

drift happens ...but enough to cause it to leak too much and meltdown? Still if you have a bias current of 20mA and it goes to 50mA, it should still be able to tolerate the heat as it's still lower than RMS draw at high volume...

I suppose it's best when someone designs the amp using semiconductor voltages like diode drop voltages to regulate against drift.
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Old 11-27-2021, 01:29 PM   #9
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Default Re: Frying amplifier finals... audio, not rf...

it's not heat, if the bias goes out of balance on a push/pull output circuit the transistors drive into each other and create a dead short
if your lucky the amp has seperate internal fuses on the + and - rails.
but a lot dont
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Old 11-27-2021, 01:38 PM   #10
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Default Re: Frying amplifier finals... audio, not rf...

it is most definitely generating heat when both are turned on... Not a dead short but most definitely is drawing power that way. Crossover dissipation to an extreme.

It would just be the rectifier diodes on the power supply that'd die if it was a true short. Which very well may happen if the transistors meltdown... but do they? It'd have to leak a lot to generate that much heat I'd think.

Let's say you have 100mA crossover, at 100V across. That'd only be 10W (actually 5W per) which should be very doable if your transistors are well heatsinked. I'd say you'd need near 300+mA crossover to start getting worrysome, and how much component drifting would be needed to get that when the setpoint was 20mA to begin with?

Last edited by eccerr0r; 11-27-2021 at 01:44 PM..
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Old 11-27-2021, 01:54 PM   #11
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Default Re: Frying amplifier finals... audio, not rf...

the output transistors often go open internally - probably melted bond wires.
and sometimes they crack or shatter the resin casing
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Old 11-27-2021, 09:21 PM   #12
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Default Re: Frying amplifier finals... audio, not rf...

I'm not sure if this belongs here, but here goes......

When I was growing up in the 1970s, my parents bought a LOWREY brand home organ and made me (or convinced me to) take music lessons. If you wanted to practice the organ while people were sleeping, there were headphones and a headphone jack that you could use that would silence the room-filling speakers.

You can guess what happened. One of the wires in the jack was not soldered properly, or it came loose (there was not a generous amount of wire between the jack and the headphones, quality was lacking in many products in the early 1970s). When I heard sound in only one of my ears, I did not realize anything was wrong. Later, after I unplugged the headphones, suddenly the speaker on the left side of the organ's cabinet did not produce any sound.

We had an electronics repair guy (who maybe specialized in organs?) come to the house and fix a few things, including the transistor that fixed our speaker. The organ could achieve pretty much its full potential again.

Guess what happened next? Someone (probably me) blew that same circuit AGAIN by plugging in the headphones with the bad soldering ! I guess I didn't realize that was the cause of the problem.

I tried to get my parents to fix it when we were giving it away to a church several years ago, but my parents wanted me to be quiet -- they didn't want the church to have any doubts. In the end, the church did take it and put it to good use. I don't know if they ever realized the left speaker was dead and the amplifier really could have been repaired (if you could still get the parts/transistors/ICs for it).

Last edited by Hondaman; 11-27-2021 at 09:23 PM..
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Old 11-27-2021, 10:03 PM   #13
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Default Re: Frying amplifier finals... audio, not rf...

yay for design defects and shorts when you plug in phone plugs! As much as I like phone plugs for being generic and ubiquitous, if not designed properly they do short (notwithstanding as that organ demonstrates - even poorer design that even allow the solder lugs to short together while plugging it in...booo!!!).

Kind of funny too, like the Baofeng amateur radio HTs that when you plug in the ⅛" and 3/32" phone plugs for headset, it will trigger the PTT and transmit as you plug it in... talk about overloading the plug pins and designing it such that you must turn off the HT when plugging the headset in.
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Old 11-28-2021, 04:42 AM   #14
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Default Re: Frying amplifier finals... audio, not rf...

i dont blame baofeng for that, it's called a "kenwood" connector for a reason.

jack plugs are shit though, i killed some small psu's in the past that used a 3.5mm jack to connect to the device because of the shorting problem.

the u.k. used to use a big 4pin version for telephones too!!!
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Old 11-28-2021, 10:57 AM   #15
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Default Re: Frying amplifier finals... audio, not rf...

Wonder if that's better or what was used in the US - four real prongs instead of a single prong with 4 contacts ... https://www.oldphoneworks.com/4-prong-plug-male.html

Forgot the last time I saw one of these and whether the prongs matched banana jack/plugs. Figure they do match just like SO-239 and MC4 ...
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Old 11-28-2021, 06:12 PM   #16
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Default Re: Frying amplifier finals... audio, not rf...

that 4pin is o.k. because you wont short the contacts by hot-plugging one.

it's not quite as good as the old french plugs though.
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Old 11-28-2021, 06:34 PM   #17
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Default Re: Frying amplifier finals... audio, not rf...

shorting amplifier headphone jacks by simply using headphones as intended should be fine because they have resistors . well any i have seen have .
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