Hi guys,
I bought my uncle's setup about a year ago. It's as follows:
I had the preamp checked and everything was fine. I also had the amp checked and asked a tech to change the bias pots. I then adjusted the bias to around 35-40mv iirc which means the heatsinks are now running at around 45ºC each per Nelson Pass advice.
Everything's good and it sounds really nice, but I have a hum problem since the beginning that the tech said he couldn't find at the store. Now, this is my first setup with a dedicated solid state power amp so I don't know if any kind of hum is to be expected or if it should be 100% silent.
A few things:
I made a video showing the hum with the preamp + amp powered on. It sounds a lot more louder in the video than it actually is because I put my phone just next to the speakers.
Forté Audio Model 3 Hum - YouTube
I'm sorry if there's a way to embed YouTube videos, but I don't know how.
Now, I'm no expert in electronics, but I'm super comfortable with a soldering iron. I'm not saying that I'll be the one fixing this thing, but if you tell me I need to change a couple of capacitors, as long as I know where to get good ones (I'm in Europe), I might just go for it.
If not, I'll need to find a different tech because the last one said he couldn't find any hum
Thing is, I need your opinion about this. What could it be, is it possible to narrow it down to a component? I'll say again, the amp sounds really really good, but I don't know if it could sound even better. The fact is, a hum is there, so unless it is to be expected, something is not right.
Thanks in advance!
I bought my uncle's setup about a year ago. It's as follows:
- Amp: Forté Audio Model 3
- Preamp: PS Audio 6.1
- Speakers: B&W 804 Matrix
I had the preamp checked and everything was fine. I also had the amp checked and asked a tech to change the bias pots. I then adjusted the bias to around 35-40mv iirc which means the heatsinks are now running at around 45ºC each per Nelson Pass advice.
Everything's good and it sounds really nice, but I have a hum problem since the beginning that the tech said he couldn't find at the store. Now, this is my first setup with a dedicated solid state power amp so I don't know if any kind of hum is to be expected or if it should be 100% silent.
A few things:
- The amp has a mechanical hum that is kinda strong as soon as I power it up and in a matter of seconds fades out to being just there. It's audible if I put my hear next to it, but not as loud as it is when I first turn the amp on. Please note that the hum fades out in 2 seconds;
- The speakers have a hum too when the power amp is connected. It seems like the same frequency of the mechanical hum, but I can't be sure. It's close though;
- The hum may be heard through the speakers even if there's nothing connected to the inputs of the amp;
- The hum gets louder when the preamp is on, like it's amplifying it. But it does not go up in volume if I turn the gain up on the preamp. And it's also independent of source;
- I can't really hear the hum when I'm sitting down around 2.5/3 meters away from the speakers, but if it's completely silent and I get close to them, it is noticeable.
I made a video showing the hum with the preamp + amp powered on. It sounds a lot more louder in the video than it actually is because I put my phone just next to the speakers.
Forté Audio Model 3 Hum - YouTube
I'm sorry if there's a way to embed YouTube videos, but I don't know how.
Now, I'm no expert in electronics, but I'm super comfortable with a soldering iron. I'm not saying that I'll be the one fixing this thing, but if you tell me I need to change a couple of capacitors, as long as I know where to get good ones (I'm in Europe), I might just go for it.
If not, I'll need to find a different tech because the last one said he couldn't find any hum
Thing is, I need your opinion about this. What could it be, is it possible to narrow it down to a component? I'll say again, the amp sounds really really good, but I don't know if it could sound even better. The fact is, a hum is there, so unless it is to be expected, something is not right.
Thanks in advance!
Thank you so much for your reply!
I actually thought 45ºC (which results in 35-40mV) was actually the factory spec. It isn't?
Just for context, the tech that changed the bias pots didn't know how much to set it at so he set it at the bare minimum. As soon as it stopped distorting, he left it. So I got it back set at 5mV (measured by myself). It sounded a bit harsh to me and it wasn't getting hot at all so I searched the forums and found a couple of posts (including from Nelson Pass) saying that each heatsink should measure 45ºC which resulted in 35mV-40mV (this also checked out because I found another post from someone saying that he remembered the correct measurement to be around 40mV.
Anyway, unless you know exactly what the factory spec is, I can't test it, but I do know that the hum was there both at 5mV and it still is there now. So unless somewhere in the middle makes the hum stop, I don't think the issue is there.
I may take some pictures if it helps, but everything looks fine and I guess the tech would have found bad caps immediately if there were any, but I don't know exactly what he did other than change the pots.
I actually thought 45ºC (which results in 35-40mV) was actually the factory spec. It isn't?
Just for context, the tech that changed the bias pots didn't know how much to set it at so he set it at the bare minimum. As soon as it stopped distorting, he left it. So I got it back set at 5mV (measured by myself). It sounded a bit harsh to me and it wasn't getting hot at all so I searched the forums and found a couple of posts (including from Nelson Pass) saying that each heatsink should measure 45ºC which resulted in 35mV-40mV (this also checked out because I found another post from someone saying that he remembered the correct measurement to be around 40mV.
Anyway, unless you know exactly what the factory spec is, I can't test it, but I do know that the hum was there both at 5mV and it still is there now. So unless somewhere in the middle makes the hum stop, I don't think the issue is there.
I may take some pictures if it helps, but everything looks fine and I guess the tech would have found bad caps immediately if there were any, but I don't know exactly what he did other than change the pots.
Sorry for the late reply but it was a super busy week. You may find some pictures on the following Imgur album. I couldn't upload here, probably they're too big? I hope they're good enough.
Imgur: The magic of the Internet
And no scope, unfortunately. I took the opportunity to record another video showing the mechanical hum it has when turned on btw. Hope it came through on it. As I said in the OP, it's strong in the beginning and fades out in a matter of a couple of seconds. Even though it might not be noticeable in the video, it does not go totally silent though.
If there is, how could I solve it? Only with one of those super expensive filters?
Here's the video: Forte Audio Model 3 Hum - 2 - YouTube
Does this and the pictures help somehow?
I'll definitely do that as soon as I get the opportunity. I won't be able to test with speakers though since it'd be so much trouble. But I can at least look out for the mechanical hum.
I have a multimeter and experience with handling electronics when I know what to do, but I don't know exactly how to do that. Where exactly should I put the probes to measure DC? And where to measure AC? And yes, I'm aware of the high currents on these.
see picture
park DVM black probe to GND
then step by step, DVM set to big enough range Vdc:
-put red probe to A , read, write down;
-put red probe to B , read, write down;
-put red probe to C , read, write down;
-put red probe to D , read, write down;
then step by step, DVM set to 200mV or 2V range Vac:
-put red probe to A , read, write down;
-put red probe to B , read, write down;
-put red probe to C , read, write down;
-put red probe to D , read, write down;
park DVM black probe to GND
then step by step, DVM set to big enough range Vdc:
-put red probe to A , read, write down;
-put red probe to B , read, write down;
-put red probe to C , read, write down;
-put red probe to D , read, write down;
then step by step, DVM set to 200mV or 2V range Vac:
-put red probe to A , read, write down;
-put red probe to B , read, write down;
-put red probe to C , read, write down;
-put red probe to D , read, write down;
Attachments
Ahahahahah, just saw this today. I'm lucky to have only one friend that knows what a multimeter is.
remedy is simple
either decrease number of friends...... or increase number of DMMs........ drastically
I'm really sorry for taking so long to get back to you, but I was on vacation in the mean time, then work got on the way etc etc.
I finally was able to do the readings and there's something off.
DCV first:
A - 39
B - 78.1
C - 39
D - 78.1
ACV:
A - 85.5
B - 171.8
C - 0
D - 0
So, should C and D be 0 or A and B? What does it mean and is it an easy/cheap fix?
Again, thank you so much!
Vdc readings are okay - showing +/-39Vdc rails and +/-78.1Vdc rails
Vac readings are suspicious
you had 200mVac or 2Vac range set on DMM?
indulge me, and repeat that Vac measurement
it seems as - if both channels are same or approx same heatsink temperature and if hum is present in both channels in same amount - that these big caps are suspicious
of course - all contacts (cap screws, fuse holder snap on connectors etc) must be clean and tight ....
Vac readings are suspicious
you had 200mVac or 2Vac range set on DMM?
indulge me, and repeat that Vac measurement
it seems as - if both channels are same or approx same heatsink temperature and if hum is present in both channels in same amount - that these big caps are suspicious
of course - all contacts (cap screws, fuse holder snap on connectors etc) must be clean and tight ....
When I am debugging hum, I want to eliminate ground loops as a source.
The way to do that is to use a battery powered device known to not have any 60Hz mains hum in it. Like your smartphone with a 3.5mm to RCA jack.
That is your reference “quiet and ground loop free source”. It may not be the best sound quality. But it is almost guaranteed to be hum free.
Not connecting anything to the input of a power amp may not be a reasonable mode of operation. What purpose does a power amp have when not connected to an input. Some amps will hum with no input. Grounding the input is better. Make a special RCA grounding plug.
Next thing is to work on one channel at a time. Ground one channel input. Connect the RCA from the phone 3.5mm to RCA adapter to the other channel.
If there is hum, use a DVM in AC mV mode and measure the noise. Something > 1mV rms will genenerally be audible.
If it is quiet with one channel connected to the phone source, you are in luck. You have a ground loop problem and not an intrinsic amp noise problem.
Now connect the other RCA to the amp, this will make a common contact between the RCA shields and sometimes, hum will now appear if the amp has a ground loop issue.
For the case that there is hum with both RCA inputs grounded, then the hum is intrinsic to the amp and there is a deeper issue.
Solid state amps should not have audible hum if well designed. As this was a commercial product designed by Mr Pass, I would assume it to be hum free as designed.
Some sources of hum have been mentioned above.
1. DC in your AC mains. This saturates the transformer and can result in hum. This can be addressed with a DC blocker circuit for the mains. Many DIY designs exist to fix this. A couple of big back to back caps and a string of power diodes are generally the parts needed.
2. Non sinuisoidal waveform on your mains (mine look like a series of breadloaves on a conveyer belt) - this generated harmonics in the mains and may pass through as hum.
3. The ground strap or bonding wire from the chassis to the heatsink or protective earth connection on the amp PCB is faulty or not connected to the IEC mains “earth ground”.
4. Failing old electrolytic caps.
5. Some other electrical issue - will require more debugging by experts familiar with the particular amp.
The loud mechanical hum you hear for 2 seconds is the inrush current mechanically moving the trafo at power up due to magnetorestriction and it goes away after the inrush is gone. But not completely. That might be the DC or the non sine AC mains.
A good amp should not have speaker outputs with more than say 0.2mV to 0.4mV rms noise. Ideally close to 0.1mV or less.
I hope that helps.
The way to do that is to use a battery powered device known to not have any 60Hz mains hum in it. Like your smartphone with a 3.5mm to RCA jack.
That is your reference “quiet and ground loop free source”. It may not be the best sound quality. But it is almost guaranteed to be hum free.
Not connecting anything to the input of a power amp may not be a reasonable mode of operation. What purpose does a power amp have when not connected to an input. Some amps will hum with no input. Grounding the input is better. Make a special RCA grounding plug.
Next thing is to work on one channel at a time. Ground one channel input. Connect the RCA from the phone 3.5mm to RCA adapter to the other channel.
If there is hum, use a DVM in AC mV mode and measure the noise. Something > 1mV rms will genenerally be audible.
If it is quiet with one channel connected to the phone source, you are in luck. You have a ground loop problem and not an intrinsic amp noise problem.
Now connect the other RCA to the amp, this will make a common contact between the RCA shields and sometimes, hum will now appear if the amp has a ground loop issue.
For the case that there is hum with both RCA inputs grounded, then the hum is intrinsic to the amp and there is a deeper issue.
Solid state amps should not have audible hum if well designed. As this was a commercial product designed by Mr Pass, I would assume it to be hum free as designed.
Some sources of hum have been mentioned above.
1. DC in your AC mains. This saturates the transformer and can result in hum. This can be addressed with a DC blocker circuit for the mains. Many DIY designs exist to fix this. A couple of big back to back caps and a string of power diodes are generally the parts needed.
2. Non sinuisoidal waveform on your mains (mine look like a series of breadloaves on a conveyer belt) - this generated harmonics in the mains and may pass through as hum.
3. The ground strap or bonding wire from the chassis to the heatsink or protective earth connection on the amp PCB is faulty or not connected to the IEC mains “earth ground”.
4. Failing old electrolytic caps.
5. Some other electrical issue - will require more debugging by experts familiar with the particular amp.
The loud mechanical hum you hear for 2 seconds is the inrush current mechanically moving the trafo at power up due to magnetorestriction and it goes away after the inrush is gone. But not completely. That might be the DC or the non sine AC mains.
A good amp should not have speaker outputs with more than say 0.2mV to 0.4mV rms noise. Ideally close to 0.1mV or less.
I hope that helps.
Last edited: