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Old 15th October 2012, 06:47 PM   #1
Jolida is offline Jolida  India
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Default Power Amp Hum !!

I recently bought a Threshold 4000 Power amp. The sound quality of the amp is good, but I am getting a hum from both channels (with & without the inputs connected). It's audible even from my listening position when the music is not playing & also in the quiet passages of the music..
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Old 15th October 2012, 08:42 PM   #2
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Get a clip lead, a wire with an allegator clip on each end. Put a cable in the inputs, and short the tip to ring of the other end. Does the amp still hum? Problem is Inside the amp.
How old is this amp? if relatively new (under 10 years) then possible problem is a dirty connector or a screw on a metal cover not making contact. Reseat the connectors, make sure all metal cover screws have star washers or at least a clean unoxidized lockwasher.
If over 10 years old, definitely over twenty years old, the electrolytic caps can have leaked out the water past the cracked rubber seals and gone low capacity. The ones in the power supply near the transformer are the most likely ones. A switcher supply (little toroid transformers) runs caps even harder and can wear them out quicker. What brand electrolytic caps in the power supply? Panasonic, nichicon, rubicon, sprague, make some long life quality caps. They also make some short life (cheap seal?) ones. Other brands, ???? The dead 3 year old PCAT power supplies I have contain Other brands. The TB Woods motor drives that I have that ran 12 years inside a hot oven motor compartment before anything failed, had nichicon e-caps. The SEMI motor drives that I have that all failed on cold mornings at about 3-5 years of age with a puddle of cap ooze in the motor box of the conveyor (not real hot environment), contained CDE e-cap's. Oh, BTW, Semi is out of business, the conveyor manufacturer is now substituting Yakima motor drives.
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Last edited by indianajo; 15th October 2012 at 08:48 PM.
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Old 16th October 2012, 04:29 AM   #3
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Do you have any instruments or tools or enough safety awareness and education to dig inside and measure voltages and replace components? Even a DC measurement across the output terminals (no signal or speakers connected) is a good start to finding out what ails your amp. It should read somewhere below 50 mV.

This is a fairly big amp, near 35 years old - way past the use-by date for many of the parts, mostly electrolytic capacitors, as Indianajo suggests. Those big electrolytics are what keeps the hum in the transformer, not struggling to power your amplifier, so if they are dried out or blown out, you get lotsa hum and sometimes the whole amp. can go down with them so don't try to use the amp with a load or signal longer than absolutely necessary whilst you routinely replace the main and in fact all the electrolytic capacitors.

It seems half the newb threads on the forum are about replacing the caps in old amplifiers so there will be no shortage of threads to refer to for guidance in choosing caps. I'd say the fault is a major reason they come up for sale in the first place. You can download schematics for parts lists if you need to be sure, yet none of the old stuff will be available new now (which it needs to be) so you will substituting newer, much smaller parts anyway. Likely, it wont hurt to over size on capacitance values up to 100% since the manufacturing tolerance may be that much. Same with voltage ratings which are not important so long as the marked working voltage is reached.

Here's a comprehensive listing from this forum of most Threshold models and references to information you may want.
Threshold Amplifier - Overview and Schematic Collection of all Models
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Old 16th October 2012, 05:03 AM   #4
Jolida is offline Jolida  India
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Quote:
Originally Posted by Ian Finch View Post
Do you have any instruments or tools or enough safety awareness and education to dig inside and measure voltages and replace components? Even a DC measurement across the output terminals (no signal or speakers connected) is a good start to finding out what ails your amp. It should read somewhere below 50 mV.
That being said, I set my Volt Meter to DC & connect no speakers or interconnects to the amp, thereby measuring the DC voltage across the + & - terminals of each channel. Is that correct ?
I had read in one of the posts is that the speakers need to be connected..

Last edited by Jolida; 16th October 2012 at 05:06 AM.
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Old 16th October 2012, 06:13 AM   #5
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Yes, correct and no, not with speakers or any signal connected (short the inputs even) because you are trying to see what the static, idle or quiescent condition of the amplifier is. If you load it that way you have a abitrary condition, even allowing background noise into the amp, dependent on your speaker.

The replies made on forums can be wrong because some of us are not versed in SS amplifier design or understand operation at all. Others parrot stuff they read but get the story backwards (hearsay). It happens but sometimes the actual test being described does indeed call for a fixed load but that will be for a dynamic test such as power output, stability, bandwidth etc.

So, what do you read?
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Old 16th October 2012, 06:42 AM   #6
Jolida is offline Jolida  India
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Quote:
Originally Posted by Ian Finch View Post
Yes, correct and no, not with speakers or any signal connected (short the inputs even) because you are trying to see what the static, idle or quiescent condition of the amplifier is. If you load it that way you have a abitrary condition, even allowing background noise into the amp, dependent on your speaker.

The replies made on forums can be wrong because some of us are not versed in SS amplifier design or understand operation at all. Others parrot stuff they read but get the story backwards (hearsay). It happens but sometimes the actual test being described does indeed call for a fixed load but that will be for a dynamic test such as power output, stability, bandwidth etc.

So, what do you read?
So to summarise:-
1. I disconnect the speaker cables from the amp
2. I use Shorted RCA pins on both inputs of the Amp
3. I set the meter on DC & check for voltage across the speaker terminals after switching the amp on..!!

Correct ?
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Old 16th October 2012, 07:05 AM   #7
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Yes.
Note though - first up, don't worry about shorting the inputs. Check that the readings are low with the meter range at 200 mV or lowest DC range if it's not an autoranging meter. Then, when you have, turn off the amp and fit the input shorts

Power on again - same readings again, power off and fit speakers and how is the hum now? same, less, more?
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Last edited by Ian Finch; 16th October 2012 at 07:12 AM.
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Old 16th October 2012, 07:12 AM   #8
Jolida is offline Jolida  India
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Quote:
Originally Posted by Ian Finch View Post
Yes.
Note though - first up, don't worry about shorting the inputs. Check that the readings are low with the meter range at 200 mV or lowest DC range if it's not an autoranging meter. Then, when you have, turn off the amp and fit the input short.

Power on again - same readings again, power off and fit speakers and how is the hum now? same, less, more?
So u first want me to check the DC at the speaker terminals without speakers connected but without shorting the inputs.
And thereafter connect the speakers & listen to the hum with the inputs shorted..??
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Old 16th October 2012, 08:08 AM   #9
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Don't try to summarise, it's taking us in circles. Just follow single steps.

1. Set the DMM as above post #7
2. Remove all signal and output connections.
3. Power up
4. Measure DC voltage across output connections. Verify that it is below 50mV or so. If ...not, just record it but don't proceed with further testing.
5. Power down.
6. Short inputs to amplifiers.
7.Power up.
8.Measure DC voltage across output connections.
9.Power down.
10. Connect speakers.
11. Set volume low and power up. Is hum less, same, more?
12. Power down
13. Reconnect amplifier as normal.

Reply when you can
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Old 16th October 2012, 10:06 AM   #10
Jolida is offline Jolida  India
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Quote:
Originally Posted by Ian Finch View Post
Don't try to summarise, it's taking us in circles. Just follow single steps.

1. Set the DMM as above post #7
2. Remove all signal and output connections.
3. Power up
4. Measure DC voltage across output connections. Verify that it is below 50mV or so. If ...not, just record it but don't proceed with further testing.
5. Power down.
6. Short inputs to amplifiers.
7.Power up.
8.Measure DC voltage across output connections.
9.Power down.
10. Connect speakers.
11. Set volume low and power up. Is hum less, same, more?
12. Power down
13. Reconnect amplifier as normal.

Reply when you can
Great. Will do as suggested this evening & post a feedback. Thanks so much
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