Question - How can there be speaker destructive DC on the outputs of an amp?

Thread Starter

jansen2810

Joined Feb 21, 2024
10
Question - How can there be speaker destructive DC (f.e. 30v) on the outputs of an dc coupled amp with the output transistors still intact. Is that possible?
 

Thread Starter

jansen2810

Joined Feb 21, 2024
10
Is there a capacitor between the amplifier's output and the speaker / speaker connection?
No. The amp is direct coupled. I got a woofer blown up by an amp, but the outputs are still ok. It sounded like an oscillation problem, high pitched sound from the tweeter (T27) which survived, but the woofer (B200) was dead.
 

MrChips

Joined Oct 2, 2009
30,821
No. The amp is direct coupled. I got a woofer blown up by an amp, but the outputs are still ok. It sounded like an oscillation problem, high pitched sound from the tweeter (T27) which survived, but the woofer (B200) was dead.
There is a fault in the amp.
 

Thread Starter

jansen2810

Joined Feb 21, 2024
10
Is the 30v under an 8 ohm load or open circuit? If the amplifier can put 30v across a speaker, then probably a transistor has failed.
Open circuit. 30v was only an example . No blown outputs, hence my question why the woofer (B200) could die. High frequentie?
 

Alec_t

Joined Sep 17, 2013
14,329
So, 8 Ohms, 25W peak.
30V across 8 ohms = 112.5W. So an amp fault which left 30V DC across that speaker would burn out the speaker coil.
 

nsaspook

Joined Aug 27, 2009
13,307
So, 8 Ohms, 25W peak.
30V across 8 ohms = 112.5W. So an amp fault which left 30V DC across that speaker would burn out the speaker coil.
Yep. In the bad old days we used inline speaker wire fast-blow fuses to protect vintage speakers connected to higher powered modern amplifiers.
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DickCappels

Joined Aug 21, 2008
10,186
There is probably a cross-over network for the speakers. In that case, it is unlikely that DC could get to the tweeter.

At this point MrChip's diagnosis in post #6 looks like it has a high probability of being the correct one.
 

Ian0

Joined Aug 7, 2020
9,835
There is a fault in the amp.
The cause is generally a bad connection on the long-tailed pair transistors, if the input transistor loses its bias resistor or either loses collector or emitter connections the VAS transistor is turned fully on or fully off and the output goes to one or other rail.
Alternatively, the VAS transistor could have failed. It's easy to underestimate how much the VAS transistor dissipates.
A badly designed overload protection stage can destroy the VAS transistor.

*VAS = Voltage Amplifier Stage. The intermediate stage between long-tailed pair and output stage.
 

Thread Starter

jansen2810

Joined Feb 21, 2024
10
There is probably a cross-over network for the speakers. In that case, it is unlikely that DC could get to the tweeter.

At this point MrChip's diagnosis in post #6 looks like it has a high probability of being the correct one.
I can understand that. The tweeter is very much alive, the woofer is not. My question is, where comes the DC from? The outputs are not burnt and the amp is still functioning, not very well. I see an oscillation on the scoop from 4kHz upwards. Could the woofer have died because of a high frequentie?
 

Thread Starter

jansen2810

Joined Feb 21, 2024
10
The cause is generally a bad connection on the long-tailed pair transistors, if the input transistor loses its bias resistor or either loses collector or emitter connections the VAS transistor is turned fully on or fully off and the output goes to one or other rail.
Alternatively, the VAS transistor could have failed. It's easy to underestimate how much the VAS transistor dissipates.
A badly designed overload protection stage can destroy the VAS transistor.

*VAS = Voltage Amplifier Stage. The intermediate stage between long-tailed pair and output stage.
That's a good one! Thanks.
 

Ian0

Joined Aug 7, 2020
9,835
You mention oscillation. Has it gone unstable? If so, it can oscillate at a high frequency (>40kHz) and that often results in a DC shift of the operating conditions if the slew rate is higher in one direction than the other.
 

Thread Starter

jansen2810

Joined Feb 21, 2024
10
You mention oscillation. Has it gone unstable? If so, it can oscillate at a high frequency (>40kHz) and that often results in a DC shift of the operating conditions if the slew rate is higher in one direction than the other.
Thanks! I'll keep that in mind
 
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