Faulty amplifier

tatchoune · 24th August 2010, 04:11 PM

Hi

I got an Audio Research 300.2 amplifier faulty after a big electric shock (lightning).
It did'nt started so I changed the 4 output Fets (cc) and the Tripath TA0105A.
The amplifer starts, but there's is a big HF noise on the output terminals.
When a signal is applied, at mid volume, the signal begin to pump very quickly and the amplifier goes into protection.
All the power supplies are ok. All the diodes and little fet's have been tested and are ok. I did'nt seen any other part burnt.
Can you help ?

Here is a picture of the signal just before the RC output filter : (160 Volts PP, 500nS timebase)

A picture of the noise : (8 volts pp)

An other picture of the noise : (timebase 1µS)

mitch90 · 24th August 2010, 06:54 PM

Hello tatchoune.

Check your LC output filter, both capacitor and coil.

Regards,

MITCH.

tatchoune · 24th August 2010, 07:02 PM

The signal forms are exactly the same on the left and right channel... I do not understand.
I supposed too it was the fault of the rc filter; when I install a 0.22 uF in parallel with the original, the noise decreases !
Could someone post a picture of the residual noise on the speakers terminals with no signal at the inputs ?

tatchoune · 07:50 PM

This pictures shows that the residual noise on the speakers terminals is surely coming from the FETs output, I suppose the RC filter is defect ?

But look at a 1000 hz signal .... how it's bad !

And worse than that, look at the 1000 hz signal on the input (amplifier power off)

And now, the same when the amplifier is on :

mitch90 · 25th August 2010, 04:24 PM

Hello again.

I have noticed that there is quite a bit of ringing on the bottom MOSFET.
Do output transistors get hot without load?

Regards,

MITCH.

tatchoune · 25th August 2010, 04:48 PM

No, they stay cold. Only the output coil warms up (40°C) a little.

mitch90 · 26th August 2010, 06:03 PM

Hello.

Could you please clarify thing or two for me;
1. Did you take waveform snapshots with or without speaker load?
2. Your Amp is based on Tripath topology, which is clocked design with pre-filter
feedback, so if there is no load on the output and if output filter is not damped
with RC damper you will easily see 8V PP output ripple.

Regards,

MITCH.

Tekko · 26th August 2010, 06:14 PM

Thats not a very good design. The output filter should ALWAYS be included in the feedback loop.

tatchoune · 26th August 2010, 06:37 PM

The pictures were taken with a 8 ohms resistive load. But I tested without load and got exactly the same pictures.
I didn't take the risk to connect expensive loudspeakers because when I connect a "sacrified" loudspeaker, I hear a big "tic" (with no signal in the inputs)and I think they will heat then burn. There's no dc voltage on the outputs.
When I was testing the amplifier, I unfortunately made a c/c and the 2 Fet's of the channel died...that's to say that the protection doesn't work.

mitch90 · 27th August 2010, 07:20 PM

Hello.

Could you please describe this 'tic' you are getting from the speaker,
is it speaker making single 'tic' on power-up, or is it ticking while the AMP is running?

One more thing, from your first post, you said that when the AMP operates at mid
volume setting it goes into protection due to the bus pumping, is this correct.

MITCH.

24th August 2010, 04:11 PM	#1
tatchoune diyAudio Member Join Date: Oct 2007 Location: Far from the sea	Faulty amplifier Hi I got an Audio Research 300.2 amplifier faulty after a big electric shock (lightning). It did'nt started so I changed the 4 output Fets (cc) and the Tripath TA0105A. The amplifer starts, but there's is a big HF noise on the output terminals. When a signal is applied, at mid volume, the signal begin to pump very quickly and the amplifier goes into protection. All the power supplies are ok. All the diodes and little fet's have been tested and are ok. I did'nt seen any other part burnt. Can you help ? Here is a picture of the signal just before the RC output filter : (160 Volts PP, 500nS timebase) A picture of the noise : (8 volts pp) An other picture of the noise : (timebase 1µS)

24th August 2010, 07:35 PM	#4
tatchoune diyAudio Member Join Date: Oct 2007 Location: Far from the sea	This pictures shows that the residual noise on the speakers terminals is surely coming from the FETs output, I suppose the RC filter is defect ? But look at a 1000 hz signal .... how it's bad ! And worse than that, look at the 1000 hz signal on the input (amplifier power off) And now, the same when the amplifier is on : Last edited by tatchoune; 24th August 2010 at 07:50 PM.

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24th August 2010, 06:54 PM	#2
mitch90 diyAudio Member Join Date: Sep 2005	Hello tatchoune. Check your LC output filter, both capacitor and coil. Regards, MITCH.

24th August 2010, 07:02 PM	#3
tatchoune diyAudio Member Join Date: Oct 2007 Location: Far from the sea	The signal forms are exactly the same on the left and right channel... I do not understand. I supposed too it was the fault of the rc filter; when I install a 0.22 uF in parallel with the original, the noise decreases ! Could someone post a picture of the residual noise on the speakers terminals with no signal at the inputs ?

25th August 2010, 04:24 PM	#5
mitch90 diyAudio Member Join Date: Sep 2005	Hello again. I have noticed that there is quite a bit of ringing on the bottom MOSFET. Do output transistors get hot without load? Regards, MITCH.

25th August 2010, 04:48 PM	#6
tatchoune diyAudio Member Join Date: Oct 2007 Location: Far from the sea	No, they stay cold. Only the output coil warms up (40°C) a little.

26th August 2010, 06:03 PM	#7
mitch90 diyAudio Member Join Date: Sep 2005	Hello. Could you please clarify thing or two for me; 1. Did you take waveform snapshots with or without speaker load? 2. Your Amp is based on Tripath topology, which is clocked design with pre-filter feedback, so if there is no load on the output and if output filter is not damped with RC damper you will easily see 8V PP output ripple. Regards, MITCH.

26th August 2010, 06:14 PM	#8
Tekko diyAudio Member Join Date: Jan 2005	Thats not a very good design. The output filter should ALWAYS be included in the feedback loop.

26th August 2010, 06:37 PM	#9
tatchoune diyAudio Member Join Date: Oct 2007 Location: Far from the sea	The pictures were taken with a 8 ohms resistive load. But I tested without load and got exactly the same pictures. I didn't take the risk to connect expensive loudspeakers because when I connect a "sacrified" loudspeaker, I hear a big "tic" (with no signal in the inputs)and I think they will heat then burn. There's no dc voltage on the outputs. When I was testing the amplifier, I unfortunately made a c/c and the 2 Fet's of the channel died...that's to say that the protection doesn't work.

27th August 2010, 07:20 PM	#10
mitch90 diyAudio Member Join Date: Sep 2005	Hello. Could you please describe this 'tic' you are getting from the speaker, is it speaker making single 'tic' on power-up, or is it ticking while the AMP is running? One more thing, from your first post, you said that when the AMP operates at mid volume setting it goes into protection due to the bus pumping, is this correct. MITCH.

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