I have tinkered with the amp yesterday. I have made the star ground area even smaller (it is about 0.5cm^2 now). I have 4 wires coming to the star ground point (2 from the rectifier bridges and 2 going to the ground boards).
For a moment I though that a lose input ground solder point was an issue; resoldered but no change.
Tried to short the inputs: dead quiet.
Shall I look at the preamp? It is B1 with the original pcb designed by Nelson.
For a moment I though that a lose input ground solder point was an issue; resoldered but no change.
Tried to short the inputs: dead quiet.
Shall I look at the preamp? It is B1 with the original pcb designed by Nelson.
Hi,
Recently I looked again at my "start ground" which was about .5cm^2. I have made it into a real "start ground" ie one 3mm screw with 4 cables meeting there (2 from the bridges and 2 to the amp board). It has helped. I was even initially happy. Still there is some hum if it is dead quiet in the listening room. It is not a problem when the music is playing.
Aiming for a perfect solution I experimented bit. As mentioned above there is no hum with inputs shorted. Also there is no hum with no input cables. When I connect the cables the hum appears. It does not matter if the preamp is on the other end of the cables or not...
I am puzzeled... Not really know what to try next...
Recently I looked again at my "start ground" which was about .5cm^2. I have made it into a real "start ground" ie one 3mm screw with 4 cables meeting there (2 from the bridges and 2 to the amp board). It has helped. I was even initially happy. Still there is some hum if it is dead quiet in the listening room. It is not a problem when the music is playing.
Aiming for a perfect solution I experimented bit. As mentioned above there is no hum with inputs shorted. Also there is no hum with no input cables. When I connect the cables the hum appears. It does not matter if the preamp is on the other end of the cables or not...
I am puzzeled... Not really know what to try next...
If the amp's wiring still is as shown in post #110, I would seriously consider tidying it up. Separate signal from mains, twist wires, minimize length and loop area, cross at right angles. And keep the connecting wires to the output fets as short as possible, especially the gate connection and gate stopper resistor.
Hi,
I've measured about 15mV p-p with my toy scope (Hantek 6022be) Not sure how accurate this is as this seems to be just about the noise level of the poor thing. I kind of I persuaded myself that I see some periodic pattern in the signal so I probably indeed see the hum.
I've measured about 15mV p-p with my toy scope (Hantek 6022be) Not sure how accurate this is as this seems to be just about the noise level of the poor thing. I kind of I persuaded myself that I see some periodic pattern in the signal so I probably indeed see the hum.
I guess I need to revise the wiring, still I thought a shielded signal cable should help me a lot.
15mVpp should read around 5.3mVrms when measured with a AC voltmeter set to resolve 0.1mVac.
5.3mVac as the output noise is a terrible value. I don't/won't accept 0.5mVac for hum and noise at the output using the 199.9mVac scale of my DMM.
I expect a good build using a good amplifier to show 0.0mVac on my DMM. The 0.0 averaged reading means less than 0.05mVac. Less good assembly might measure 0.1mVac, or flicker towards 0.2mVac.
0.05mVac is 40dB less noise and hum than 5mVac.
5.3mVac as the output noise is a terrible value. I don't/won't accept 0.5mVac for hum and noise at the output using the 199.9mVac scale of my DMM.
I expect a good build using a good amplifier to show 0.0mVac on my DMM. The 0.0 averaged reading means less than 0.05mVac. Less good assembly might measure 0.1mVac, or flicker towards 0.2mVac.
0.05mVac is 40dB less noise and hum than 5mVac.
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Thanks for that. I think my DMMs are not up to the task. My one DMM goes to 1.999Vac only, the other one is even worse. It shows indeed 5-6mV so you might be right. Then 5min later it shows 10-11mV (or somewhat less with the power of the amp off). This seems to be with the specs of the meter which is 0.8% in 2V Vac range. An NAD304 I have on loan shows also rubbish values. The NAD doesn't have the noise issue.
Need to get better measurement equipment I guess...
The only sensible measurement I got is the value with the input cable disconnected 9-10mV (no hum) and 10-11mV (hum) with the cable in. If the diff is any indication of the true value it is still 10x more than your reference.
Need to get better measurement equipment I guess...
The only sensible measurement I got is the value with the input cable disconnected 9-10mV (no hum) and 10-11mV (hum) with the cable in. If the diff is any indication of the true value it is still 10x more than your reference.
Greg,
to assess the hum+noise of the Power Amplifier you should fit a zero ohms dummy input plug, or a dummy ohms that matches the output impedance of your source.
Don't leave the input open. That is not the way it will get used.
to assess the hum+noise of the Power Amplifier you should fit a zero ohms dummy input plug, or a dummy ohms that matches the output impedance of your source.
Don't leave the input open. That is not the way it will get used.
Hi Andrew,
There is no hum with the input shorted (ie I hardly hear anything some 20cm from the woofer; maybe some faith hum with 5cm from the woofer - but that is not how I listen to music ;-) ).
The preamp is B1 as I probably mentioned before.
With the preamp connected the hum is there, pretty much the same when the preamp is on or off. This hum does not appear when I used different power amps.
There is no hum with the input shorted (ie I hardly hear anything some 20cm from the woofer; maybe some faith hum with 5cm from the woofer - but that is not how I listen to music ;-) ).
The preamp is B1 as I probably mentioned before.
With the preamp connected the hum is there, pretty much the same when the preamp is on or off. This hum does not appear when I used different power amps.
Ok. I am getting there!
I followed Rodeodave's advise and twisted my cables (the input cable is still shielded, not twisted). I got to the stage that the noise with the preamp attached but switched off is virtually non existent (you have to listen less that 5-10cm from the speaker). There is some hum when I turn the preamp on. So I guess I should start looking there and trying different preamps. Anyway I will give myself a few hours cool off period and see how it goes.
I followed Rodeodave's advise and twisted my cables (the input cable is still shielded, not twisted). I got to the stage that the noise with the preamp attached but switched off is virtually non existent (you have to listen less that 5-10cm from the speaker). There is some hum when I turn the preamp on. So I guess I should start looking there and trying different preamps. Anyway I will give myself a few hours cool off period and see how it goes.
Have you read D.Joffe's paper on the interference loop when you connect two interconnects to a stereo amplifier?
Your hum could be because you have two interconnects introducing an interference loop.
R0 is HBRR. It attenuates the interference current in the loop and that attenuates the interference voltage in the signal circuit.
But I am slightly confused about why they have shown sgnd separate from gnd (the signal return). I can't find where sgnd connects to the rest of the circuit.
R0 is HBRR. It attenuates the interference current in the loop and that attenuates the interference voltage in the signal circuit.
But I am slightly confused about why they have shown sgnd separate from gnd (the signal return). I can't find where sgnd connects to the rest of the circuit.
Yes I was confused as well. I thought I checked it on the pcb (when I've read the Joffe's paper) and sgnd was also the spkr gnd. Anyway I need to open the case again to confirm.
Meanwhile I've twisted every possible cable inside the B1.
It is very difficult to assess now not being able to measure precisely with my DMM - do I listen to the woofer at the same distance now?
Meanwhile I've twisted every possible cable inside the B1.
It is very difficult to assess now not being able to measure precisely with my DMM - do I listen to the woofer at the same distance now?
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Try listening to the speaker out with a pair of headphones.
My two Koss don't reproduce the 50Hz hum well, (red devils not at all) but the Sennheiser do let me hear hum & noise more easily.
If you need to monitor hum&noise on a supply rail, then use a DC blocking capacitor to prevent blowing up your headphones. A series/parallel set of four 470uF gives a net 470uF that lets the hum get through to 16ohms headphones.
My two Koss don't reproduce the 50Hz hum well, (red devils not at all) but the Sennheiser do let me hear hum & noise more easily.
If you need to monitor hum&noise on a supply rail, then use a DC blocking capacitor to prevent blowing up your headphones. A series/parallel set of four 470uF gives a net 470uF that lets the hum get through to 16ohms headphones.
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