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Nakamichi PA 5E class A Stasis amplifier hum problem
Nakamichi PA 5E class A Stasis amplifier hum problem
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Old 8th February 2018, 04:11 PM   #11
Mooly is offline Mooly  United Kingdom
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Nakamichi PA 5E class A Stasis amplifier hum problem
Quote:
Originally Posted by Sixtron View Post
The problem appears when i set the bias to 40mah like the instructions say.
If i set it to zero then there is no hum.
When you turn the bias up to 40ma then you start to develop ripple on the rails (which is normal) as more current is drawn.

It sounds like you have a ground loop somewhere that is allowing this ripple current to develop a voltage between the inputs, which then gets amplified and becomes audible.

Problems like this can be hard to find.

Have you owned the amp from new and is it totally unmodified ? meaning that it used to be OK and now it isn't.

Something as simple as a ground lift resistor could be the answer but incorporating it is something where you really need to the amp in front of you to see what is going on.
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Old 8th February 2018, 04:18 PM   #12
Mooly is offline Mooly  United Kingdom
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Nakamichi PA 5E class A Stasis amplifier hum problem
This sounds very like a problem I had on Doug Selfs 50 watt blameless amp many years ago.

Read post #44 and then work back:

3 stage LIN topology - NFB tappings?
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Old 8th February 2018, 04:26 PM   #13
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I bought it used very long time ago. It was working without hum.
I modified the cables to bigger ones and it was still working.
I remember that the Heatsinks had warm temperature in this time.So bias was set.
Then i made a crash with the input plugged in while the amp was on and i had to change two mosfets and one ceramic resistor from the limiter pcb.
When i remember well the hum was getting louder with the time and one day i opened this amp and lowered the bias even if i didn't knew what i was doing in this time.
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Old 8th February 2018, 05:20 PM   #14
Mooly is offline Mooly  United Kingdom
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Nakamichi PA 5E class A Stasis amplifier hum problem
I've no magic answer I'm afraid as it would need a detailed look into how the grounding was configured. I think also that you should probably concentrate on setting the bias correctly and then leaving that alone.

You could try as a 'quick fix' (and with no guarantees) adding a low value resistor in series with the signal ground to one channel. Easiest place to do that is at the input socket. Try a 10 ohm 0.5 watt and see if it improves things.
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Old 8th February 2018, 05:25 PM   #15
stocktrader200 is offline stocktrader200  Canada
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replace the large capacitors as the amp gain stage is fed by these as well as the power stage
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Old 8th February 2018, 06:30 PM   #16
Sixtron is offline Sixtron
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I replaced the large cap's already some days ago.

Mooly i have read and what i understand is that the input and feedback circuit must be connected via 10 ohms to the ground star. Thats all, right ?
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Old 8th February 2018, 06:51 PM   #17
Mooly is offline Mooly  United Kingdom
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Nakamichi PA 5E class A Stasis amplifier hum problem
What I am suggesting you try is this. Just desolder the ground wire from the socket and add a series 10 ohm. It will either make a difference or it wont.

I wouldn't like to advise you to alter grounding within the amp because I'm not familiar with the layout of it all, and it would be easy to cause big problems if something went wrong. Its something you need in front of you to see how its all interconnected and what goes where.
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Old 8th February 2018, 06:56 PM   #18
classAchap is offline classAchap  United Kingdom
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Quote:
Originally Posted by Sixtron View Post
Is it usual that i can measure with a normal voltmeter 274 Volts AC over the plus and minus from the two 47.0000uf filter capacitors measured in one direction only. The other direction 0V AC.
You have to get to the bottom of this 247V a.c. You can make progress in this by disconnecting both channels by removing leads 1,2,3 & 4 labeled here:

A0FACE30-950E-421C-AE10-717CEE166A43.jpeg

Now check d.c. and a.c. voltage across C401 you should see 60V d.c. and a few hundred mV or so a.c. - then check same across C402.

When you have good solid + / - 60V d.c. connect up one channel (1,4 or 2,3)
Twiddle with that channel, setting bias and whatever, when you’re happy disconnect it and do the same with the other channel.

This is a basic binary chop procedure after which you’ll have a much better idea what’s going on.

However, as Mooly (no, Mooly not Molly - sorry about that) says, problems like this can be hard to find - more so remotely.

Oh.. and do resist the urge to change components without good reason.
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Old 8th February 2018, 07:03 PM   #19
Sixtron is offline Sixtron
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30 years old components should give me a reason :-)
You think it is safe to remove this cables like you say and turn the amp on ?
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Old 8th February 2018, 07:10 PM   #20
Mooly is offline Mooly  United Kingdom
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Nakamichi PA 5E class A Stasis amplifier hum problem
The 247 Vac has to be a measurement error or a faulty meter or a bad connection.

If you really did have that AC voltage present then there would be a hole in the ceiling where the caps just exploded through.
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