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Old 15th February 2014, 04:31 PM   #1
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Default Dad's away so I can play

DSIR9643.JPGDad has almost given up on this DIY F5 build, he's the expert but I'd like to see what I can do with it.

He's scrimped on the PSU and has gone for a single PSU for the two amps. I've got no money so I'm just trying to see where the problem might be.

The PSU is a single 500VA transformer driving a CCRCCC PSU.

C1 is a 56000uF cap then all the following caps are 4700uF so the total C on each rail is some 79500uF.

There is small hum from the amplifier. What sort of ripple from the PSU would cause this?

Due to the PCB design it is very difficult to short the inputs to eliminate input pick-up but the input leads are unshielded twisted pair.
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Last edited by KatieandDad; 15th February 2014 at 04:36 PM.
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Old 15th February 2014, 05:10 PM   #2
ishiru is offline ishiru  Indonesia
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why not try to short the input on the RCA plugs?

have you tried to unplug the input to see if it's not a ground loop?

nice tube preamp btw..
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Old 15th February 2014, 05:38 PM   #3
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Quote:
Originally Posted by KatieandDad View Post
Attachment 399977There is small hum from the amplifier. What sort of ripple from the PSU would cause this?
If it is hum, like a pure 60 Hz, it's most often from some transformer or mains wiring getting too close to input circuitry.

If it would be a buzz with a rattling sort of character, that would be a ground loop, some parts of the circuit sharing some supply return wiring.

If it's hum try moving mains and transformer wiring around to see if it makes a difference.

Jan
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Old 16th February 2014, 01:48 AM   #4
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Try loosening the bolt that holds down the transformer and then rotate it to see if that gives you any joy. Nelson regularly suggests this when there are complaints of hum.
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Old 16th February 2014, 03:56 AM   #5
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The F5 has limited PSRR (hum rejection).

So, after shorting the inputs at the RCA jacks (don't do this any way other than with an RCA plug with a shorting wire, and with the amp OFF before you plug it in - even more so if you solder or try using a screwdriver or bit of wire to short) you'd want to measure the ripple (hum) at the OUTPUT of the amp - both channels, and compare the values. Also compare the values of the bias current. See if the channels have the same hum or if the one with more bias has more hum or not.

Next, measure the ripple (AC) on the + and the - rails. See if that figure compares to the output hum, same, equal or less?

Report back.

_-_-


PS. the fast, effective and cheap fix for PS ripple would be to throw a choke between the two cap banks.
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Old 16th February 2014, 05:16 AM   #6
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Quote:
Originally Posted by bear View Post
The F5 has limited PSRR (hum rejection).
The output noise is about 100 uV unweighted with 70 mV ripple on the supply.

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Old 16th February 2014, 02:10 PM   #7
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OK, with shorting plugs on the RCA inputs it's virtually silent, so it's a 0V/Ground issue.

I know in all Dad's gear 0V and Mains Ground are not connected together, all have heavy star 0V configurations with the star centre being close to the main supply.

As I said earlier, the hum isn't too bad, it's just there in the background and is annoying. It is more of a "Buzz" than a hum.
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Old 16th February 2014, 02:13 PM   #8
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Pas Mal.

Wondering what the effect might be, if any, of poorly matched input jfets (like a bogus J74 with a real K170), and mosfeters with rather different Vgs thresholds where a lot of offset has to be dialed in to get DC balance? So, does improved device matching and less offsets improve output noise?

Which, suggests a ground loop issue in the OP's system...
...and the actual ripple needs to be measured in the OP's system too.

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Old 16th February 2014, 02:15 PM   #9
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Quote:
Originally Posted by ishiru View Post
why not try to short the input on the RCA plugs?

have you tried to unplug the input to see if it's not a ground loop?

nice tube preamp btw..
That was an early version of the Impasse. It's evolved slightly since then. I uploaded that photo by mistake and can't seem to get rid of it.
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Old 16th February 2014, 02:57 PM   #10
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Quote:
Originally Posted by KatieandDad View Post
OK, with shorting plugs on the RCA inputs it's virtually silent, so it's a 0V/Ground issue.
Try shorting one input and connecting the other to the preamp. If there is
no noise, then likely the ground loop consists of the pair of input cables
themselves picking up the radiated noise from the transformer.

Rotating the transformer can often help that a lot. After that it is usually
an internal layout issue and you can start by connecting the grounds together
between the input connectors.

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