I use the amp on my front midranges and tweeters, and they are very clearly audible from the seating position.
I have measured as much as 1.75V of dc offset at power down which yes, the speakers can handle that but it's indicative of a malfunctioning amp.
It seems like a touch more delay to switching the j108 off would benefit the power on thump, but how to turn them on faster at power down?
I have measured as much as 1.75V of dc offset at power down which yes, the speakers can handle that but it's indicative of a malfunctioning amp.
It seems like a touch more delay to switching the j108 off would benefit the power on thump, but how to turn them on faster at power down?
There is a resistor between the EF pair that drives the muting circuit and the gates of the j108s. If you remove that resistor to ensure that the jfets are never switched off, does the amp get quiet?
Actually no.
With speakers connected as normal between output and secondary ground it is quiet at power up. There is a slight pop at power down.
When it is connected between the bridging terminals it has a stronger thump at power up, but quiet at power down.
When I shorted the legs on the j108 earlier, I did not have a speaker connected bridged for that test.
Shown in the attached picture I have lifted one leg of R139 lower right of the optocoupler. Gates of all j108's are measuring ~3.5mV.
With speakers connected as normal between output and secondary ground it is quiet at power up. There is a slight pop at power down.
When it is connected between the bridging terminals it has a stronger thump at power up, but quiet at power down.
When I shorted the legs on the j108 earlier, I did not have a speaker connected bridged for that test.
Shown in the attached picture I have lifted one leg of R139 lower right of the optocoupler. Gates of all j108's are measuring ~3.5mV.
Attachments
Is there only one jfet per channel? The older amps with the ceramic boards had two jfets per channel. Could doubling the jfets quiet the amp?
I don't know if I will explain this well but,
With that resistor disconnected;
The measured DC offset of the speaker connected as normal isn't as bad as before at power up but its about as its been previously at power down and the load of the speaker returns it to zero fairly quickly.
The measured DC offset of the speaker connected in bridged mode is as high as 3.5V at power up when it thumps now, and it does not return to zero very fast after power down. Both bridging terminals have ~.350mV on them to secondary ground, but zero volts between them.
With that resistor disconnected;
The measured DC offset of the speaker connected as normal isn't as bad as before at power up but its about as its been previously at power down and the load of the speaker returns it to zero fairly quickly.
The measured DC offset of the speaker connected in bridged mode is as high as 3.5V at power up when it thumps now, and it does not return to zero very fast after power down. Both bridging terminals have ~.350mV on them to secondary ground, but zero volts between them.
There is a group of eight j111 at the bottom center of that pic that I do not know how they are being used, but I do not believe they are part of the muting circuit.
Otherwise, yes it appears as though there are only one jfet per channel.
Otherwise, yes it appears as though there are only one jfet per channel.
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In the earlier test of shorting legs of a j108, it was only one and the specific output associated with that jfet was tested.
Now that all j108 are muted, is when I noticed the behavior in bridged mode.
Now that all j108 are muted, is when I noticed the behavior in bridged mode.
Did you try shorting both of the bridges channels' jfets?
Are the j111s working with the switch sw1 on the back end of the board?
Are the j111s working with the switch sw1 on the back end of the board?
Re-connecting the resistor and shorting two jfets for a bridging pair of channels did not mute the thump.
SW1 is on the crossover board and is for combining the inputs. Switching it did not seem to change any voltages present on any of those j111's.
On the bottom side there were 330pF capacitors connected to the Drain and Source of the j108s. Switching those to 470pF made no change.
Looking at the schematic for the 6800.2, would increasing the 1uF 50V capacitor near the 4n25 to something like 10uF increase the time before muting is released?
SW1 is on the crossover board and is for combining the inputs. Switching it did not seem to change any voltages present on any of those j111's.
On the bottom side there were 330pF capacitors connected to the Drain and Source of the j108s. Switching those to 470pF made no change.
Looking at the schematic for the 6800.2, would increasing the 1uF 50V capacitor near the 4n25 to something like 10uF increase the time before muting is released?
C37? That would delay on and off unless you connected a diode across R107.
Does the power LED go off IMMEDIATELY when you remove the remote voltage?
With R107 out of the circuit and the noise still present, I don't think the jfets are able to mute the audio.
Was the amp every quiet?
Does the power LED go off IMMEDIATELY when you remove the remote voltage?
With R107 out of the circuit and the noise still present, I don't think the jfets are able to mute the audio.
Was the amp every quiet?
The power LED at power up starts off Red then goes Green in about 1 second. Power down it goes from Green to Red then off a little slower than that with it staying Red longer than Red at start up. At the point where it becomes Red at power down is where the thump is heard.
Yes it was once a quiet amp. It is currently the only PPI I have that does not use the ceramic driver boards.
Yes it was once a quiet amp. It is currently the only PPI I have that does not use the ceramic driver boards.
At thins point, I don't the control of the muting will make a difference if, when there is no voltage on the jfet gates and they're doing all they can to mute the audio, they still don't make the amp quiet.
Do you remember anything that coincided with the noise starting? Or was it simply there, all one day?
Do you remember anything that coincided with the noise starting? Or was it simply there, all one day?
I agree the noise is happening outside of the jfet's control.
This amp has had all its electrolytic capacitors changed to Panasonic FC, the heatsink compound cleaned and replaced and Kapton tape used for insulator. It has then been on a shelf for eight or so years until about a month ago when I put it into use again. It worked correctly last I remember all those years ago but not now.
This amp has had all its electrolytic capacitors changed to Panasonic FC, the heatsink compound cleaned and replaced and Kapton tape used for insulator. It has then been on a shelf for eight or so years until about a month ago when I put it into use again. It worked correctly last I remember all those years ago but not now.
What happens if you double the jfets on a channel?
Caps replaced recently or 8 years ago?
Do you think you would have noticed this 8 years ago?
Caps replaced recently or 8 years ago?
Do you think you would have noticed this 8 years ago?
I don't think I have extra j108's, so pull two and just put them leg for leg over another pair to test on that channel?
Yes, these caps are eight years old now. Probably have about a total of ten hours of total use now.
Yes, I would have heard this then too, my test speakers at the bench here haven't changed since it's been set up, well before then.
Yes, these caps are eight years old now. Probably have about a total of ten hours of total use now.
Yes, I would have heard this then too, my test speakers at the bench here haven't changed since it's been set up, well before then.
Yes. Just parallel them.
Can you find a resistor leading into the j108 drain that you could lift to see if the problem is possibly coming in before the 108s?
Can you find a resistor leading into the j108 drain that you could lift to see if the problem is possibly coming in before the 108s?
Parallel J108's make no difference on either of two channels tested.
Lifting the resistors connected to the J108 drain, on the same two channels did reduce the level by quite a bit, to almost tolerable levels.
Lifting the resistors connected to the J108 drain, on the same two channels did reduce the level by quite a bit, to almost tolerable levels.
What is between the preamp board connector (board still out) and the resistor you removed that could introduce the noise?