So we have this MTX Thunder Elite on the bench. It originally had a single shorted output FET. The rest of the outputs measure fine. And drive components all the way back tot he 3120 opto-couplers seem fine as well. Correct supply voltage to all 3120s as well.
The amplifier powers up, nothing gets hot, it appears to be producing clean output, but there are a few strange things I have noticed and Id like to run them by somebody else. Maybe these are normal operation for this amplifier, but nothing Ive seen before.
So first off, with no speaker load connected to the amplifier, once remote power is applied to the amplifier the fans power up, at this time I measure up to 14V DC on the speaker terminals. (Again, no load connected) After a few moments, the fans shut off and a red LED near the Class-D circuitry lights. The DC voltage then goes down to around 200mV and slowly drops from there.
When remote power is removed from the amplifier with no load connected to the speaker terminals, the fans turn on again and remain on for nearly a minute before finally shutting off. ??
Now things change when there is a load connected to the amplifier. When powering up, the DC voltage on the speaker outputs does not climb over 200mV. It remains low and then slowly drops from there.
When removing remote voltage with a load connected, the amplifier powers down normally and the fans do not come on.
As stated before, with no load connected and shutting down the amplifier, the fans would come on. At any point within that first minute or so, if I connect a speaker to the output terminals, the fans will stop. Remove the load and they come back on. Repeat continuously until finally they will not come back on anymore after the minute or so has elapsed.
These seem like very strange issues, but other than the slightly high DC offset, the amplifier seems to be functioning properly in other respects, although I have not done any high power runs or anything with it yet. The speakers do not "pop" when powering up the amplifier either. Even when the output is showing 10+ volts while powering up without a load, if a load is connected at that point, the voltage immediately drops down and only a very small "pop" is heard. Not loud at all.
Any ideas? Or are these normal things for this amplifier to do with no speakers connected to it?
The amplifier powers up, nothing gets hot, it appears to be producing clean output, but there are a few strange things I have noticed and Id like to run them by somebody else. Maybe these are normal operation for this amplifier, but nothing Ive seen before.
So first off, with no speaker load connected to the amplifier, once remote power is applied to the amplifier the fans power up, at this time I measure up to 14V DC on the speaker terminals. (Again, no load connected) After a few moments, the fans shut off and a red LED near the Class-D circuitry lights. The DC voltage then goes down to around 200mV and slowly drops from there.
When remote power is removed from the amplifier with no load connected to the speaker terminals, the fans turn on again and remain on for nearly a minute before finally shutting off. ??
Now things change when there is a load connected to the amplifier. When powering up, the DC voltage on the speaker outputs does not climb over 200mV. It remains low and then slowly drops from there.
When removing remote voltage with a load connected, the amplifier powers down normally and the fans do not come on.
As stated before, with no load connected and shutting down the amplifier, the fans would come on. At any point within that first minute or so, if I connect a speaker to the output terminals, the fans will stop. Remove the load and they come back on. Repeat continuously until finally they will not come back on anymore after the minute or so has elapsed.
These seem like very strange issues, but other than the slightly high DC offset, the amplifier seems to be functioning properly in other respects, although I have not done any high power runs or anything with it yet. The speakers do not "pop" when powering up the amplifier either. Even when the output is showing 10+ volts while powering up without a load, if a load is connected at that point, the voltage immediately drops down and only a very small "pop" is heard. Not loud at all.
Any ideas? Or are these normal things for this amplifier to do with no speakers connected to it?
Some updated details.
The amp is definitely not functioning normal. It will play ok at low volume, but the power supply makes some noises (sizzling?) Doesnt seem to be a normal torroid squeal or anything. When you attempt to turn the volume up via gain or headunit, the output distorts.
If you let the amp play for a while at low volume, after a while the output will remain clean even at high volume, and the power supply will no longer make strange noises.
Once you shut the amp off and let it completely dischareg again, the noise and the distorted output return.
We are not familiar with this design so any help would be great, I have read the notes in Perrys tutorial on the 7801 which is a similar design (smaller), but havnt been able to find the cause of the issue.
The amp is definitely not functioning normal. It will play ok at low volume, but the power supply makes some noises (sizzling?) Doesnt seem to be a normal torroid squeal or anything. When you attempt to turn the volume up via gain or headunit, the output distorts.
If you let the amp play for a while at low volume, after a while the output will remain clean even at high volume, and the power supply will no longer make strange noises.
Once you shut the amp off and let it completely dischareg again, the noise and the distorted output return.
We are not familiar with this design so any help would be great, I have read the notes in Perrys tutorial on the 7801 which is a similar design (smaller), but havnt been able to find the cause of the issue.
14v Battery bank. Current limiter (Headlight) It does not make any weird noises with the limiter in-line, but when hooked to direct power is when it starts to make noise int he power supply.
Our bench power supply isn't regulated.
With no load connected, running through the current limiter, voltage drops in to the 9.xx v range.
With no load connected, running without the current limiter, voltage stays at around 14.4v
LED400 stays lit with or without a load.
If RCA's are not connected, all lights on the control board that is connected to the main board stay lit with red LED's. When RCA's are connected, the LED's flash a purplish color.
With no load connected, running through the current limiter, voltage drops in to the 9.xx v range.
With no load connected, running without the current limiter, voltage stays at around 14.4v
LED400 stays lit with or without a load.
If RCA's are not connected, all lights on the control board that is connected to the main board stay lit with red LED's. When RCA's are connected, the LED's flash a purplish color.
Much of the noise is likely from an under-damped regulator circuit. The noise stops when the voltage is lower because the duty-cycle goes to the maximum possible duty cycle (regulator inactive). If the voltage drops far enough, that could account for the distorted signal.
Even with full sized wire running to it, if you try to make more than a few watts, the output starts popping and making noise. So it definitely isn't due to voltage drop at the input to the amp.
Do you have any ideas for why it will make popping sounds through the subwoofers whenever it gets over a certain (low) volume?
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