Hi,
I hope this is the correct section for my question.
I have built a class A JLH amp and it works properly. Voltages and currents are ok. It also sounds awesome
I have just a noise that lasts a couple of seconds when I turn the amp on and after it completely desappears ( amp is totally noiseless).
The noise comes from the speakers and it is not a bump or a huum. It isnt also a noise from the main switch. It is more like a "voltage" noise, as something is going under voltage or like some cap is charging. It seems something is gonna burn out
Is very diffcult for me to explain this in a better way but I thin it is not normal. I checked all the solders and all is ok. Also checked the isolation of transistors and rca/speakers plugs.
Does anybody have an idea about that?
Regards
I hope this is the correct section for my question.
I have built a class A JLH amp and it works properly. Voltages and currents are ok. It also sounds awesome
I have just a noise that lasts a couple of seconds when I turn the amp on and after it completely desappears ( amp is totally noiseless).
The noise comes from the speakers and it is not a bump or a huum. It isnt also a noise from the main switch. It is more like a "voltage" noise, as something is going under voltage or like some cap is charging. It seems something is gonna burn out
Is very diffcult for me to explain this in a better way but I thin it is not normal. I checked all the solders and all is ok. Also checked the isolation of transistors and rca/speakers plugs.
Does anybody have an idea about that?
Regards
The only noise an AC coupled amp should make is a switch on thump at power on and power off.
It is possible you have some form of instability and the amp is passing through some state or condition that causes this.
If it only does it with an input source connected then the first step (I'm assuming you haven't an oscilloscope) is to see if the noise is present with shorting plugs fitted to the amp input.
It is possible you have some form of instability and the amp is passing through some state or condition that causes this.
If it only does it with an input source connected then the first step (I'm assuming you haven't an oscilloscope) is to see if the noise is present with shorting plugs fitted to the amp input.
So you have no noise with shorting plugs and noise when connected to a source component.
It is possible it is some form of instability triggered under certain conditions.
Although it wouldn't directly help with fixing this, it might be useful to know if the problem occurs if the shorting plug is replace by a fixed resistance, say a 1k, then 10k and then a 100k.
It might just give a bit of a handle on what is going on.
Ideally a scope is needed to see what is happening though.
You could also try adding an HF filter to the input such as a series 1k and say 1nF to ground. Again, it might give a clue.
It is possible it is some form of instability triggered under certain conditions.
Although it wouldn't directly help with fixing this, it might be useful to know if the problem occurs if the shorting plug is replace by a fixed resistance, say a 1k, then 10k and then a 100k.
It might just give a bit of a handle on what is going on.
Ideally a scope is needed to see what is happening though.
You could also try adding an HF filter to the input such as a series 1k and say 1nF to ground. Again, it might give a clue.
The volume pot would normally be wired with the wiper (middle pin) to the amp input and the pot ground terminal going to your input ground.
That standard configuration automatically shorts the amplifier input at minimum volume setting and then as the pot is rotated allows the amplifier input to see an ever higher impedance.
So having for the moment just those TWO connections to the pot should see the amp silent for all volume settings.
That standard configuration automatically shorts the amplifier input at minimum volume setting and then as the pot is rotated allows the amplifier input to see an ever higher impedance.
So having for the moment just those TWO connections to the pot should see the amp silent for all volume settings.
Yes it is. I wanna repeat the amp is deadly silent at all volume level, I have that issue just at the moment of powering it on.
I suspect that there is an issue on the input of the amp maybe because it is designed as a power amp.
In the input of the jlh amp we have just a series AC coupling cap and no resistor between the input (before the cap) and ground.
In general a preamplifier has a resistor on the output between out and gnd that will be in parallel with the input of the amp so the amp would always see that load resistor. In this case we dont have it and at volume set to minimum the input is shorted and see no charge resistor.
Maybe this is the issue?
I suspect that there is an issue on the input of the amp maybe because it is designed as a power amp.
In the input of the jlh amp we have just a series AC coupling cap and no resistor between the input (before the cap) and ground.
In general a preamplifier has a resistor on the output between out and gnd that will be in parallel with the input of the amp so the amp would always see that load resistor. In this case we dont have it and at volume set to minimum the input is shorted and see no charge resistor.
Maybe this is the issue?
You could try a series resistor although odd noises were never reported as an issue on the original JLH69.
You're pot should be like this. If you disconnect the 'top' input wire, do you still get the noise, or do you only get the noise at certain settings ?
If you only get the noise at minimum volume then a resistor might help.
If you get the noise at any setting then something else is going on.
If you only get the noise when the top wire and a source is connected then there may be a wiring layout issue (ground return from input sockets incorrect perhaps).
You're pot should be like this. If you disconnect the 'top' input wire, do you still get the noise, or do you only get the noise at certain settings ?
If you only get the noise at minimum volume then a resistor might help.
If you get the noise at any setting then something else is going on.
If you only get the noise when the top wire and a source is connected then there may be a wiring layout issue (ground return from input sockets incorrect perhaps).
Attachments
Yes I have the noise only if the top wire is connected to the source. Noise is there at any level of volume when i turn on the amp.
The amp is "quasi-dualmono" because i use a trafo with two separate secondary wirings. So, i have 4 wires coming from the trafo. (25-0 and another 25-0). Each secondary wire goes to a bridge rectifier (one for channel). From there I go to the +/- of the PS ( one for each channel) and frome PS I go to amp Vin +/-. From the opposite side of the amp board i go to +/- of the speaker banana plugs on the rear panel. From rca input plug i go with +/- to the input/gnd of the potentiometer and from there i go out from out/gnd of the pot to +/- signal input on the amp board. This is separately for each channel.
Gnds on the potentiometer are put together only for input and output of the same channel. Gnd on pot are separated dor the two channels. Rca plug gnd is isolated drom chassis and separated for the two channel
The amp is "quasi-dualmono" because i use a trafo with two separate secondary wirings. So, i have 4 wires coming from the trafo. (25-0 and another 25-0). Each secondary wire goes to a bridge rectifier (one for channel). From there I go to the +/- of the PS ( one for each channel) and frome PS I go to amp Vin +/-. From the opposite side of the amp board i go to +/- of the speaker banana plugs on the rear panel. From rca input plug i go with +/- to the input/gnd of the potentiometer and from there i go out from out/gnd of the pot to +/- signal input on the amp board. This is separately for each channel.
Gnds on the potentiometer are put together only for input and output of the same channel. Gnd on pot are separated dor the two channels. Rca plug gnd is isolated drom chassis and separated for the two channel
Be clear
So no wire and no noise at any pot setting.
And only when the wire is connected do you get the noise at any pot setting.
If so then you need to look at the grounding. Try and get one channel right first.
Make sure the speaker return goes to the power supply ground for the relevant channel. Make sure the input ground goes the correct input ground on the PCB, not the power supply ground. As a mono amp that channel should be silent and work correctly.
The same applies to other channel used in isolation.
As dual mono construction there should be no ground continuity between the two channels apart from where the two input grounds finally meet.
Have you inadvertently created a ground loop via safety grounding ? That might need ground lift resistors to isolate the power supply of each amp from mains ground if so.
So no wire and no noise at any pot setting.
And only when the wire is connected do you get the noise at any pot setting.
If so then you need to look at the grounding. Try and get one channel right first.
Make sure the speaker return goes to the power supply ground for the relevant channel. Make sure the input ground goes the correct input ground on the PCB, not the power supply ground. As a mono amp that channel should be silent and work correctly.
The same applies to other channel used in isolation.
As dual mono construction there should be no ground continuity between the two channels apart from where the two input grounds finally meet.
Have you inadvertently created a ground loop via safety grounding ? That might need ground lift resistors to isolate the power supply of each amp from mains ground if so.
Correct. No wire and no noise at any volume setting.
With wire, noise yes at any volume setting.
The safety gnd (main earth) only goes to the chassis. No gnds of the amp in that point. Main earth is connected only to the chassis. The front metal part of the potentiometer is in contact with the chassis and of course with the main earth.
Gnding is a bit different because spkr return goes to the gnd point of the pcb amp, and not to the supply gnd.
On the amp pcb I have an unique gnd path which is connected to PS gnd, input gnd and spkr gnd.
I suspect you were right talking about an oscillation period the amp pass through when powering up.
With wire, noise yes at any volume setting.
The safety gnd (main earth) only goes to the chassis. No gnds of the amp in that point. Main earth is connected only to the chassis. The front metal part of the potentiometer is in contact with the chassis and of course with the main earth.
Gnding is a bit different because spkr return goes to the gnd point of the pcb amp, and not to the supply gnd.
On the amp pcb I have an unique gnd path which is connected to PS gnd, input gnd and spkr gnd.
I suspect you were right talking about an oscillation period the amp pass through when powering up.
You must always have the 0.5uF (or other suitable value) present. Without and the DC conditions of the stage would be incorrect.
As a check... if you have no source connected you should also have no direct continuity between the two amplifier grounds. The amplifiers should be separate. Check and confirm with a meter.
You should also try connecting the speaker return back to the power supply, not the to the PCB
As a check... if you have no source connected you should also have no direct continuity between the two amplifier grounds. The amplifiers should be separate. Check and confirm with a meter.
You should also try connecting the speaker return back to the power supply, not the to the PCB
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