SVT 7 pro no speaker output

[J M Fahey]

Have you already asked Ampeg?
Do they explain "how the circuit works"? (A staple feature in old/good service manual schematics)
Have you already googled those ICs Datasheets?
You can't repair what you don't understand, and we can't be of much help either, that one being a new , different design.
I'm sure it must be very clever, but I'd *love* somebody familiar with it giving me the grand tour.

[lowell]

I will call Ampeg tomorrow. If they help me and I begin to understand the circuit better I'll be sure to share.

[Jazz P Bass]

Class D Amplifiers

I am new to class D also.
I feel the pain.
Here are some pdf's I pulled off of the net.

[lowell]

Thanks! I will definitely get into those pdf's. I called Ampeg and they said I'd need to call a service center for help on this. I'm not going to call a service center cause I'm sure they'll need to get paid for helping... as I know I would. I'll get into reading those though and I'll get back here.

[Jazz P Bass]

Curious the way they compare Class D to a buck SMPS.

It also appears that you need a low pass filter to "look" at the signal.

[lowell]

Curious the way they compare Class D to a buck SMPS.

It also appears that you need a low pass filter to "look" at the signal.

Yeah that's cool. I'm gonna try that when I get back to work.

[Jazz P Bass]

It looks to me like the signal coming into the power amp should be a normal signal up to
U300 & U17(edit), entering at pin 3 of both IC's.
The PMW part appears to originate at U3, a 555 timer.
As both the signal & the PMW carrier wave enter U300 & U18, I would suspect that all of the "magic" of class D is in these two IC's.
The topology of the output is "Full Bridge."

[MarkusBass]

It looks to me like the signal coming into the power amp should be a normal signal up to U300 & U18, entering at pin 3 of both IC's.

No. If you check the datasheet of irs20955, you'll see that there should be pulse width modulated signal on pin #3 of the IC. The signal if formed by U21 - tripple inverter. After U21 there is a square signal. U21-B provides inverted signal for the second part of the power amp.

The PMW part appears to originate at U3, a 555 timer.

I'm not sure about it. I'd say that 555 in this amp is used only as a muting circuit.

As both the signal & the PMW carrier wave enter U300 & U18, I would suspect that all of the "magic" of class D is in these two IC's.

The "magic" of class D is in U16.

Lowell, if you checked the signal on inputs of U16A, why haven't you checked the signal on the output (pin #1)? And the signal on U16-B output (pin #7)?
Is there 5V on the output of U37? I would also check the D55 (double) diode.

Mark

[Steve Conner]

As to how the audio is "hidden among the pulsating DC" or whatever: you should be able to see the duty cycle of the square wave vary in response to a music input. On an analog scope, the rising or falling edges (depending on your scope trigger) will fuzz and expand horizontally. If it's stuck at 50% then the audio signal isn't making it to the modulator.

Since this is a full bridge, be careful what you do with that scope ground clip: the amp could be going into protection because you're shorting it with the scope.

The above waveform behaviour assumes this is the old fashioned PWM type of amp, and not a sigma-delta, where the duty cycle stays 50/50 and cycles are skipped as necessary. But I think that's unlikely. Yes, old-fashioned, you have missed at least two generations of the things.

A Class-D amp isn't just "similar", it is a synchronous buck converter.

[J M Fahey]

Nice thread.
I think we'll all end up learning about Class D, whether we like it or not.
We'd better !!!!!!!!!!!!!!

[Jazz P Bass]

No. If you check the datasheet of irs20955, you'll see that there should be pulse width modulated signal on pin #3 of the IC. The signal if formed by U21 - tripple inverter. After U21 there is a square signal. U21-B provides inverted signal for the second part of the power amp.
I'm not sure about it. I'd say that 555 in this amp is used only as a muting circuit.
The "magic" of class D is in U16.

Lowell, if you checked the signal on inputs of U16A, why haven't you checked the signal on the output (pin #1)? And the signal on U16-B output (pin #7)?
Is there 5V on the output of U37? I would also check the D55 (double) diode.

Mark

Thanks for the clarification.
So U18B/ pin 7 is the last stage of the normal audio signal. (that is where the squiggle line stops)
I agree the 555 timer is most likely a shutdown/ mute command.

[lowell]

Well now I get NO signal on U16A at all. Not even on its inputs. I have a speaker connected. Yes there is 5v on U37. There is still visible signal on output of U18. I check the D55 double diode and it isn't shorted. Id like to take it out to measure it out of circuit but I'm afraid to stick my iron in there. These smd components are so small and fragile. How the heck do you solder these things back in?

[Jazz P Bass]

Lowell, do you have a signal at U18_B pin 7?
That is the preamp signal.
D57 is a clipper diode. It makes a square wave out of the preamp signal which will then ride on the PMW carrier wave.

[lowell]

Yes I have signal at U18B pin7 but it is distorted. It looks like a sine wave but has spike on the peaks and valleys. Do you mean D55? I guess both D55 and 57 are clipper diodes.

[Jazz P Bass]

I would not worry too much about the spikes.
D55 & D57 will make short work of that.
If you notice on the schematic at U18, that squiggle line ends there.
From there on in the signal is going to be different.
Have you scoped the output?
Have you scoped the output pre output board? (before the filter)
You should be able to see the Hi freq carrier wave.
The audio should be riding on it.