Some other owner even mentioned "no hum" on his Model 3.
Well, like "no money" depends on who it says, a beggar
or the Finance Minister, there may be many orders of
magnitude in between.
This said, as a rule of thumb "hum" at power amp output
should be below 1 mV preferably and even below 0.1 mV
for very efficient speakers.
Numbers like this are dependent on the particular noise
spectrum also. Pure 50/60 Hz is less obtrusive, 100/120 Hz
a bit more, but most significant are their harmonics, especially
spikes at zero-crossings periodically repeated with 100/120 Hz.
These spikes do not contribute much to a reading with usual
averaging voltmeters but are the most annoying.
Well, like "no money" depends on who it says, a beggar
or the Finance Minister, there may be many orders of
magnitude in between.
This said, as a rule of thumb "hum" at power amp output
should be below 1 mV preferably and even below 0.1 mV
for very efficient speakers.
Numbers like this are dependent on the particular noise
spectrum also. Pure 50/60 Hz is less obtrusive, 100/120 Hz
a bit more, but most significant are their harmonics, especially
spikes at zero-crossings periodically repeated with 100/120 Hz.
These spikes do not contribute much to a reading with usual
averaging voltmeters but are the most annoying.
My reference to hum was contextual, in relation to the OP's observations and the audio/video he posted. I didn't take measurements, so to be more specific, my amp doesn't hum audibly even when turned up to a sound level that would be unsafe to listen to in my 12' x 25' room.
This was not a comment about cause, whether it is capacitors or not - just an owner's comment on the audible noise/hum of a serviced and maintained amp. Besides the PS caps, the trimmers were also replaced, and I have found dodgy trimmers to cause hum and noise problems in some circumstances.
This was not a comment about cause, whether it is capacitors or not - just an owner's comment on the audible noise/hum of a serviced and maintained amp. Besides the PS caps, the trimmers were also replaced, and I have found dodgy trimmers to cause hum and noise problems in some circumstances.
We check power amp output hum with inputs shorted.
What you experience at home this way is largely dependent on the
efficiency of your spekaers, distance and also background noise ..
What you experience at home this way is largely dependent on the
efficiency of your spekaers, distance and also background noise ..
Understood. I was just responding to the OP's comments about audible hum and offering my own observations on audible hum with the same model amplifier. If others want to suggest the design is inherently noisy, what can I say? Just offering my observations which are that my amp isn't audibly noisy.
Last edited:
Yes, I do - Keysight 34461A 6.5 digit meter. Not sure what you are trying to establish, though; that the OP is just observing an inherently noisy amp? I've got no dog in this fight so not terribly motivated to take time away from work to do this.
Does OP have accurate measurements at this time? Is he even still participating in this discussion?
Does OP have accurate measurements at this time? Is he even still participating in this discussion?
Thank you for your input on this, it was super helpful even though I understand it may be speaker dependent.
Anyway, do you happen to remember uF, Voltage rating and brand of the electrolytic caps you used? Are you happy, did you notice a difference?
These are obviously my two options. I'll see how much good caps cost and if it's worth it over a new tech visit (another guy obviously).
And I agree, this amp sounds great and pairs perfectly with my other gear. I mean, it was bought together and there was a lot of back and forth mainly with the speakers. My uncle first went with a pair of Martin Logans that didn't do well with this pre + amp and only then he switched to the 804s.
And yes, sentimental value is important here too.
Just to remind everyone that the hum does go up in volume when the preamp is turned on for some reason, but it does not change volume when the volume on the preamp is turned up or down.
Again, thank you everyone!
I am actively participating.
The measurements I have are the ones posted. They were taken with the DMM posted (UNI-T UT131A). The caps were measured to ground on saturday (inputs not shorted) and the speaker outputs were measured today. Posts #43 and #62 for values.
OP : we can only consider "hum" of the amp alone, without preamp and with inputs shorted.
Otherwise it is a preamp or system problem.
GKT : I am sure the ac measurement taken from Model 3 output would be useful to compare.
Otherwise it is a preamp or system problem.
GKT : I am sure the ac measurement taken from Model 3 output would be useful to compare.
Sure, and it does hum without a preamp connected to the inputs. I was just adding that the hum becomes a bit louder when the preamp is on. It doesn't change, it simply becomes louder. Just in case that helped.
I didn't try to short both inputs (I used the term grounded when I wanted to use shorted, sorry, English is not my main language). I shorted the left input, and connected the phone to the right and vice-versa. Results of that testing are also on post #62 I believe.
Shorting both inputs will not do any damage to the speakers correct? If not, I may test that later tonight.
Does it mean preamp is connected or it is connected AND "on" ?
If this is a new addition to your investigation results it may hint to
a sub optimal ground wiring inside the power amp.
Thanks, I'll do the ABCD measurements with the inputs shorted.
From post #1:
No, the output ac measurements with inputs shorted.
Re post 1 cited above : compare hum with shorted power
amp inputs and then with preamp only as a second step.
Re post 1 cited above : compare hum with shorted power
amp inputs and then with preamp only as a second step.
Hi Dastinger,
Thank you for making the measurements and observations.
When you ground the left and have phone on right, “exactly the same” means that the hum is present on right and left is quiet?
If shorting the input makes the hum go away, that says that the PSU rail caps are not to blame. You can change them simply because, like others say, it’s been 20 yrs and they do age. But I can almost guarantee you that the hum won’t be fixed.
Because the hum is louder when inputs are not shorted and connected to a source, the problem is somewhere in the input stage or a ground loop associated with it.
It would be interesting to see what the mV AC DVM says is the speaker output when the input is shorted.
0.6mV and 0.8mV on my 82.4dB sensitive speakers is indeed audible hum. Audible with ears maybe 0.5m away even.
At 0.2mV rms, ears need to be within 10cm.
It depends on your tolerance to hum.
One more experiment to try is to disconnect the ground from RCA jacks on rear panel (the shield wire soldered to jack from gray cable). Connect the common ground tab on RCA to the PSU ground bus bar via a 10ohm ground loop breaker. I think this is a fairly safe experiment to try but make connections with amp off.
Like this:
Thank you for making the measurements and observations.
When you ground the left and have phone on right, “exactly the same” means that the hum is present on right and left is quiet?
If shorting the input makes the hum go away, that says that the PSU rail caps are not to blame. You can change them simply because, like others say, it’s been 20 yrs and they do age. But I can almost guarantee you that the hum won’t be fixed.
Because the hum is louder when inputs are not shorted and connected to a source, the problem is somewhere in the input stage or a ground loop associated with it.
It would be interesting to see what the mV AC DVM says is the speaker output when the input is shorted.
0.6mV and 0.8mV on my 82.4dB sensitive speakers is indeed audible hum. Audible with ears maybe 0.5m away even.
At 0.2mV rms, ears need to be within 10cm.
It depends on your tolerance to hum.
One more experiment to try is to disconnect the ground from RCA jacks on rear panel (the shield wire soldered to jack from gray cable). Connect the common ground tab on RCA to the PSU ground bus bar via a 10ohm ground loop breaker. I think this is a fairly safe experiment to try but make connections with amp off.
Like this:
Attachments
Last edited:
I'll try to do that tonight. Dealing with some damage and flooding issues from a major rain/wind storm, but it would be a nice distraction afterwards.
Done it. Measurements below. TLDR: Pretty much the same. Hum still there.
Thank you for getting back to me.
To answer your question about the phone + short inputs test: no. The hum would be more prominent on the left than on the right independently of whether the short or phone was on the left or right input. But it did lower it in volume a bit - on both channels. The hum on the right became barely noticeable (it's the channel where the hum is always less prominent anyway).
Now, w/ shorted inputs and speakers connected, hum is still there just the same. Measurements:
Left:
- AC - 0.2 mV
- DC - 16.7 mV
Right:
- AC - 0.2mV
- DC - 21mV
Regarding your last suggestion, I think I won't go through with it, at least for now. It navigates waters that I'm not too comfortable with just yet. I'll think about replacing the caps first and see how that helps.
Sorry about the flood, hope everything is sorted out now. Also, thank you for your help.
I'm not sure if you saw my last post asking for uF, Voltage and brand of the new caps you used on your Model 3. That'd help me a lot.
Thank you all.