Cause of audio amplifier failure?

"Adrian Tuddenham"

** Sheer luck. ** Those two diodes ought to have been included from the start - to suppress the back emf transient from the speakers when if the mute function is engaged at the same time as the amplifier is under high drivel levels. Such transients are well capable of destroying output devices by reverse breakdown.

Why no output Zobel network ? ( ie 100nF plus 8 ohms in series across the speaker line ) - I seriously doubt the stage is reliably stable at HF without one. Excessive heat due to such oscillation will kill the BJT output devices fast too.

Also, tell me what happens if you remove the negative side DC rail fuse during operation ??

Fried voice coils??

..... Phil

I doubt the toe piston solenoids had anything to do with the problem if they were not electrically associated with the amp or its power supply. Furthermore added diodes D53 and D54 don't do anything because the voltage on the speaker rail will never exceed the power rails and these diodes will never forward bias. They're not needed.

I suspect the problem lies in the mute circuit. Transistors TR53 and TR54 are current sources for the input differential pair and the drive circuitry for the output devices. The two currents are set by the voltage between the

30 volt rail and the "Z" point. It's critical that this voltage be controlled, known and free from external influences such as noise, spikes, etc. Muting occurs by removing this voltage causing currents from the current sources to collapse, removing drive and allowing the output to go to zero.

Since the "Z" point goes off campus, we have no clue what the external mute circuit does but it's likely that they problem lies there. For openers, the current setting voltage should never come from outside the amplifier, undefined. That voltage should be locally defined by a zener or current mirror and should simply be turned on and off with a shunting transistor thus isolating the current sources from the outside world.

Noise, spikes, transients, or wrong voltages on the "Z" point could cause the problem. Also look for the amplifier oscillating especially during the muting transition or during mute. Unstable high frequency oscillation could be the culprit. Good luck.

"Bob Eld"

** Shame that is 100% wrong.

The vast majority of BJT power amps made have such diodes fitted to the output - for a damn good reason. That reason is that a dynamic speaker CAN and WILL deliver a very large back emf voltage spike if ever drive to the output stage is suddenly disabled.

There are two common scenarios where drive is suddenly disabled:

1: The use of a VI limiting circuit that shunts drive current away from the output stage when a hazardous ( to the output devices) combination of voltage and current is sensed in the load.

1. The sort of muting circuit use in the OP's schematic.

In both the above, the usually low output impedance of the amp instantly becomes very high and allows any stored inductive energy in the load to " kick back " into the output stage.

You need to read up on the topic:

...... Phil

There are circumstance where they might be needed, but I didn't think any of those applied here (See my reply to Graham)

I didn't show the rest of the circuit, but the 'Z' wire goes to the same point on an adjacent card which contains the bass power amplifier driving stage. The 'Z' wire and the +ve 30v rail both run parallel and are about 1.5" long with soldered connections both ends; both are inside the same die-cast box.

The 'z' wire is well decoupled by C53 and this discharges relatively slowly when the muting operates. It is difficult to see how a fault on tha 'z' wire would blow only Tr61 and Tr63.

I have now put up is a full description with circuits at:

Before I put the amplifier into service, I switched the circuit in and out of muting many times on the bench, with and without many types of load. It withstood all sorts of deliberate abuse and didn't show any tendency towards instability during transitions. The failure appears to have occurred after the muting condition had been established for several seconds (possibly minutes).

Thanks, I feel as though I need it.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

I agree that, in hindsight, I should have included them; however none of the usual reasons for having them seem to apply in this case.

The muting action is very slow, taking nearly a second to fully mute. I'm sorry that this wasn't apparent from the diagram I provided, so I may have misled you on that point. I have discussed the other possible reasons for those diodes in my reply to Graham.

The full circuit is at:

I have tested it on various loads and it shows no tendency towards instability. Some amplifier designs include a Zobel network, others don't; what is the factor which decides this?

Under full drive with a shorted output, the calculated junction temperature never rises to an unsafe value because each transistor is in close thermal contact with a sensing diode which can trigger a shut-down through the same muting circuit. I really thrashed this amplifier (thermally) on the bench before it went into service and it always shut down long before there was any risk to the output devices.

There isn't a negative DC fuse, the power fuses are on the AC side of the rectifier. I take the point that there could be a DC fault, so I have provided each loudspeaker with its own fuse.

The previous replacement amplifier (a commercial transistorised design), which this one now replaces again, had failed with a short circuit to one power rail and had burned out the bass loudspeaker (an 18" Goodmans from 1954)). Fortunately I was able to make a new former and rewind the voice coil, so it was back in service with very little delay. Because of this, I was particularly careful to pre-empt that possibility in my own design.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

Why is the naming of Tr21 and Tr22 in the base amplifier in figure 7 called Tr38 and Tr39 in the text?

Jon

Actually they aren't named at all, the numbers 21 and 22 are associated with their base-emitter diodes. It would be logical to name them Tr21 and Tr22 and then change the text, so that is what I shall do.

Thank you for spotting that anomaly.

--
~ Adrian Tuddenham ~
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"Adrian Tuddenham"

** That is only how it seemed to YOU - pal ** All of them stupid and WRONG !! ** My god, what a PIG'S BREAKFAST !! ** Proves nothing except you are an incompetent.

The Zobel network need to be there.

Costs SFA so why be a prick and argue about it ?

** WRONG.

Nearly ALL commercial audio amps include them.

** Whether the designer was an amateur idiot or not. ** What load of old bollocks.

What the hell does bullshit like "calculated" and " close thermal contact " actually boil down to ???

Wishful thinking - I bet.

** More off with the fairies style wishful thinking.

Those poor TIP35s and 36s are right on the SOA limit ( 4A at 30 V) so the chips must be glowing hot under short circuit conditions.

** Means they have to have large ratings to survive the inrush surge of the filter caps.

Fire limitation only.

** Damn silly waste of effort.

Large DC voltages push speaker voice coils way out of the magnet gap, exposing them to the air - SO they FRY in few seconds at power levels WAY less than the maker's nominal power figure.

Result ?

Any speaker fuse sized to save a voice coil speaker from DC will easily blow on programme material.

Proper DC protection for speakers REQUIRES either a "crow bar" circuit be fitted across the speaker line or a properly and designed hefty relay ystem - cos large DC current may have to be broken an that is not so simple.

** Oh dear.

More dopey, wishful thinking.

..... Phil

voltage

CAN

the

Thanks. I agree with your other post that a Zobel R-C network is needed on the output to insure a high frequency load at all times.

"Bob Eld" "Phil Allison"

** Zobel ( R-C ) networks fitted across speaker outputs do several things.

1. Provide a known RESISTIVE load to the amp at frequencies up to many MHz.
2. Swamp unpredictable load impedances and resulting phase shifts found in all speakers, x-over networks and cables at such frequencies.
3. Provide unambiguous EVIDENCE of high frequency oscillation being the cause of an otherwise mysterious amp failure - ie the resistor gets smoked.

...... Phil

[Sigh!] Back into the Kill File.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

On a sunny day (Sun, 15 Mar 2009 12:12:54 +1100) it happened "Phil Allison" wrote in :

Actually a 'Zobel' is not always needed, for example the TDA7294 audio amp chip does not need it.

Wow, picture is from Feb 200o, so nine years of 24/7 on (it is always on) should be proof. And my mains and audio wiring does see huge spikes. LOL

"Jan Panteltje"

** That is NOT what the maker's data says.

FUCKHEAD

...... Phil

On a sunny day (Sun, 15 Mar 2009 23:34:58 +1100) it happened "Phil Allison" wrote in :

The 'maker's' datasheet (available from the above link) says: TDA7294

100V - 100W DMOS AUDIO AMPLIFIER WITH MUTE/ST-BY

VERY HIGH OPERATING VOLTAGE (±40V) DMOS POWER STAGE HIGH OUTPUT POWER (UP TO 10 SIC POWER) MUTING/STAND-BY FUNCTIONS NO SWITCH ON/OFF NOISE NO BOUCHEROT CELLS FUCKHEAD

New type of probe you are using?

muting

all

gated

have

Your output transistor protection circuitry design is weak

Can you elaborate on that please? What are the problems I have not forseen and what circuit changes would you suggest to overcome them (apart from adding D53 & D54, which I have now done)?

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk