In post 62 the ac "hum" numbers are higher. Last digit of the DVM, uncertain.
What does the DVM show with input leads shorted in ac mode and same range ?
What does the DVM show with input leads shorted in ac mode and same range ?
Hi Dastinger,
Thank you - got fallen limbs cut, branches and leaves removed and most subsurface drains cleared so only a little more work tomorrow.
My unit has Kemet 33,000uF 63V replacement caps.
It's been out of rotation for about 6 months, so it was actually good to open it up and check bias and offset, and get ACV output measurements. Unfortunately, my left channel has 30mV offset which displeases me, and there is no offset trimmer.
Anyhow, my output ACV is 0.233 left channel, 0.211 right channel. My speakers have 87dB efficiency. I didn't connect them to the speakers tonight - no time - so can't give an up to date observation of audible hum.
Thank you - got fallen limbs cut, branches and leaves removed and most subsurface drains cleared so only a little more work tomorrow.
My unit has Kemet 33,000uF 63V replacement caps.
It's been out of rotation for about 6 months, so it was actually good to open it up and check bias and offset, and get ACV output measurements. Unfortunately, my left channel has 30mV offset which displeases me, and there is no offset trimmer.
Anyhow, my output ACV is 0.233 left channel, 0.211 right channel. My speakers have 87dB efficiency. I didn't connect them to the speakers tonight - no time - so can't give an up to date observation of audible hum.
I'm not sure if something's being lost in translation, but just to clarify... readings in mV with no inputs shorted and speakers connected:
Left:
AC - 0.6 mV
DC - 15.5 mV
Right:
AC - 0.8 mV
DC - 20.0 mV
Readings in mV with both inputs shorted and speakers connected:
Left:
AC - 0.2 mV
DC - 16.7 mV
Right:
AC - 0.2 mV
DC - 21.0 mV
To my ears, hum is just the same both with inputs shorted or not.
PS: When I tested with no inputs shorted, there was nothing connected to the inputs.
Thank you so much for the readings and caps information. I'll look for the best place to get them here and think about replacing mine. If you ever get the time to check for audible hum, I'd be thankful. My speakers have exactly the same 87dB efficiency so this could help. FTR, I hear the hum when close to the speakers and, obviously, getting my ear next to them is when it becomes really obvious, like in the video posted in the OP.
Oh sorry, something was indeed lost in translation. Yes, if I short the leads on the DMM it displays 0.0. I always do that before measuring to check if everything's ok.
.. in the most sensitive range used for your measurements, I hope.
Your result and that of GKT is about 0.2 mV speaker output "hum"
if the readings are correct. This is a very low number.
With "open" inputs you arrive at 0.6 / 0.8 mV, a difference of 10 dB
or more, but claim the audible result is the same. This makes the
reading doubtful.
An ac voltmeter and a scope are required. This way it it will be proven
that the four big caps are not the culprit here by looking at the noise
waveform.
Your result and that of GKT is about 0.2 mV speaker output "hum"
if the readings are correct. This is a very low number.
With "open" inputs you arrive at 0.6 / 0.8 mV, a difference of 10 dB
or more, but claim the audible result is the same. This makes the
reading doubtful.
An ac voltmeter and a scope are required. This way it it will be proven
that the four big caps are not the culprit here by looking at the noise
waveform.
Yes, most sensitive range. It's one of those DMM that chooses the range automatically, but there is an option for V and another for mV. I worked with mV.
And yes, a scope would surely help and tell us what's wrong, but I don't have one. So I'll either go ahead and replace the caps just for how old they are (30 years +/-) and see if that helps or I'll go ahead and find a new tech which would probably cost me a ton and mean that I'd be without this system for who knows how long.
Thank you for all your help.
Used scope and ac vom are cheaper than the caps and can be sold after.
My bet is the cap replacement will change nothing (if internal wiring will
be exactly the same after).
My bet is the cap replacement will change nothing (if internal wiring will
be exactly the same after).
Hello dastinger,
I want to share my experience several years ago with a Forte Model 6 Amplifier.
This amplifier is slightly different than yours because it uses IGBT transistors but, I believe the power supply section is basically the same. It’s a single center tapped transformer feeding two bridge rectifiers with 4 screw terminal capacitors. When I received the amp it was in very good condition and was never touched internally. But, there was a low level hum that was audible about 1 foot away from 90dB efficiency speakers. The hum did not change in pitch or volume no matter what preamp or source was connected to its inputs.
I jumped to the conclusion that since it was over 20 years old the electrolytic caps “needed” to be changed to cure the hum. So, I recapped the entire amplifier, large psu caps as well as the local caps on the amp boards. Put the amp back together and powered her up…..No change in hum pitch or volume. Phoewy!!!
At that time I was not savvy enough to have taken AC output measurements, and the amp is no longer with me.
Good luck with your hum-busting Model 3 journey 😉
I want to share my experience several years ago with a Forte Model 6 Amplifier.
This amplifier is slightly different than yours because it uses IGBT transistors but, I believe the power supply section is basically the same. It’s a single center tapped transformer feeding two bridge rectifiers with 4 screw terminal capacitors. When I received the amp it was in very good condition and was never touched internally. But, there was a low level hum that was audible about 1 foot away from 90dB efficiency speakers. The hum did not change in pitch or volume no matter what preamp or source was connected to its inputs.
I jumped to the conclusion that since it was over 20 years old the electrolytic caps “needed” to be changed to cure the hum. So, I recapped the entire amplifier, large psu caps as well as the local caps on the amp boards. Put the amp back together and powered her up…..No change in hum pitch or volume. Phoewy!!!
At that time I was not savvy enough to have taken AC output measurements, and the amp is no longer with me.
Good luck with your hum-busting Model 3 journey 😉
as_audio: what advantage does the ac VOM confer in this case? More sensitivity than dastinger's DMM?
It's been so long since I've used one. I've still got one and though not sure it is functioning properly, I could take measurements of my unit with scope and VOM.
Thanks
It's been so long since I've used one. I've still got one and though not sure it is functioning properly, I could take measurements of my unit with scope and VOM.
Thanks
@dastinger
If you are still looking for a source of caps on your continent you might check Frag Jan zuerst --- Ask Jan First. I'm not sure what values they have, but in the past I purchased F/T and Authenticaps from them.
If you are still looking for a source of caps on your continent you might check Frag Jan zuerst --- Ask Jan First. I'm not sure what values they have, but in the past I purchased F/T and Authenticaps from them.
Re post 110 :
As we can see here the sensitivity of a standard digital voltmeter is not
high enough.
If you use a scope, a look at the hum waveform allows good investigation
of possible hum origin.
Sensitivity of the scope may also not be high enough, in this case the AC
VOM can act as measuring amplifier. A useful model will have an AC output
for this purpose which will be connected to the scope input.
As we can see here the sensitivity of a standard digital voltmeter is not
high enough.
If you use a scope, a look at the hum waveform allows good investigation
of possible hum origin.
Sensitivity of the scope may also not be high enough, in this case the AC
VOM can act as measuring amplifier. A useful model will have an AC output
for this purpose which will be connected to the scope input.
A pair of cheap headphones with banana jacks is useful as a “stethoscope” for listening for hum and hiss at speaker output. I say cheap because a blip may blow them out. The disposable ones airlines give you is good. Although $10 will get you sufficiently cheap ones to use a test device. You can definitely hear hum and hiss at the sub mV level.
Thank you for sharing your experience!
I know the Model 6 was not designed by Nelson Pass, but they share the same architecture so you might be onto something.
Thank you very much! I'll contact them and see what they have for me. Mouser also has a warehouse in Spain I think so that might be a great option.
I'll keep you guys posted.
Sort of. I didn't get my hands inside it until now (other than to look ahah). I tried first to get some advice, but ended up feeling discouraged and searched for a tech that only changed the bias pots and couldn't find any hum so I decided to try again. I didn't even remember I had opened up that thread to be honest or else I would have used it instead of opening a new one.
It's all explained on this topic anyway.