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Old 1st February 2007, 10:43 PM   #21
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cvllier-

I am unclear on one point. Is there no connection from the transformer's secondaries (I assume it is center tapped) to ground at all? or are the secondaries attached to the neutral. When I did that, I got increased hum in my amp.

I know that people often disconnect the earth ground to get rid of hum -- I just don't know if it always works.

Did you check to see if there was any ripple in the signal ground? You can plug your 'scope into an outlet that is on a different circuit than the Babblefish is on, and see what is on the scope. If there is no ripple I would guess that is not the problem, but since you saw 1 mv of ripple on the input shorted to ground -- it has to be coming from somewhere.

JJ
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Old 2nd February 2007, 01:13 PM   #22
cviller is offline cviller  Denmark
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Quote:
Originally posted by Zen Mod
leave it for few days; do some other thing,then (we) try again......
Perhaps that would be wise. I just redid the bridges to MUR3020 on a decent heatsink, but no luck there either.

Quote:
I am unclear on one point. Is there no connection from the transformer's secondaries (I assume it is center tapped) to ground at all? or are the secondaries attached to the neutral. When I did that, I got increased hum in my amp.
I use to bridges and connect center point to gnd on psu.

Quote:
Did you check to see if there was any ripple in the signal ground? You can plug your 'scope into an outlet that is on a different circuit than the Babblefish is on, and see what is on the scope. If there is no ripple I would guess that is not the problem, but since you saw 1 mv of ripple on the input shorted to ground -- it has to be coming from somewhere.
I have shorted the inputs on the board and I have no signal wire out of the board, so I guess that is not the case. The ripple I saw was on the inverted input of the jfet. It is the feedback signal from output.
Perhaps my negative feedback is not working properly... could that be the case?
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Old 2nd February 2007, 04:55 PM   #23
cviller is offline cviller  Denmark
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Just tried to insert 100k in place of R1 on aleph 30 - not much improvement.

Now I'll take a break to listen and enjoy, cos it sounds marvelous, except for the 100Hz buzz.

I removed two transistors on one channel to see if it was mismatch issue - but it only reduced the buzz because less current was drawn. Now I'm listening to 1.5...
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Old 2nd February 2007, 07:30 PM   #24
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Cviller-

You wrote
"I have shorted the inputs on the board and I have no signal wire out of the board, so I guess that is not the case. The ripple I saw was on the inverted input of the jfet. It is the feedback signal from output.
Perhaps my negative feedback is not working properly... could that be the case?"

The ripple I was looking for here would be caused by an out of phase hookup of your two bridge rectifiers. This possibility is briefly mentioned in the Rod Elliot article I referenced previously.
Having no signal wire out of the board would not matter in this case -- it would be coming through your short to ground. (I am assuming it is shorted to signal ground.)

I think an out of phase hookup would show up at twice the AC mains frequency.

I don't think it would be anything to do with the feedback resistor--it is just a resistor, right?

How did you determine it was coming through the feedback connection?

JJ
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Old 2nd February 2007, 08:08 PM   #25
cviller is offline cviller  Denmark
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jupiterjune, the article you are referring to describes a dual +/- supply. Mine is only a single +/- supply.

I have drawn a quick schem of how my supply is wired. The two bridges are made with 8 mur3020 (dual diodes).
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Old 2nd February 2007, 08:23 PM   #26
Zen Mod is offline Zen Mod  Serbia
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cviller -just try this arrangement where 33K resistor is splited and middle point is filtered via cap;
one thing is still not clear to me;
I tried exact circuit ,same as Steen ( my man ).....without any troubles.............



anyway ........I agree......be sure that secondaries from your two xformers are wired in phase,same as that you wish to make rectifier with just one graetz bridge,with common ground ........

steen....any comment?
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Old 2nd February 2007, 08:53 PM   #27
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Steen used insane capacitors - he could filter out a weeks power down...
but I'll changing the phase and try the trick with splitting the 33k tomorrow.
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Old 2nd February 2007, 09:02 PM   #28
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Quote:
Originally posted by cviller
Steen used insane capacitors - he could filter out a weeks power down...
but I'll changing the phase and try the trick with splitting the 33k tomorrow.

check phase of xformers first ;

that's rule of thumb and common sense that phases in supply must be coherent
it's better to cure culprit.....
anyway- that 33K is connected to minus supply exactly to achieve better CMRR.........comparing to "usual" practice of connecting it to gnd ......it's better like this-at least in my mind

you can also try to put 470n or even 1uF instead 220n there.........later.........when you check phase
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Old 2nd February 2007, 09:40 PM   #29
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Quote:
check phase of xformers first ;
How can that change much when the input to the bridges are symmetrical? Perhaps I'm missing a vital point here.

Could my problem simply be solved by connecting the secondaries and only use one bridge?
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Old 2nd February 2007, 10:04 PM   #30
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Quote:
Originally posted by cviller


How can that change much when the input to the bridges are symmetrical? Perhaps I'm missing a vital point here.

Could my problem simply be solved by connecting the secondaries and only use one bridge?

read this slowly,same as I'm typing....slowly...... joking ,off course.........

problem is-when you have two independent secondaries in anti-phase - that ripples from both side can be .......how to define this.....in anti-phase too.............so you don't have superposition of plus and minus ripples in your actual load ; instead-you have extraposition of ripples (is that proper word?)

that is the case when you have two bridges ;
in case that you use just one bridge you must connect secondaries in correct phase ,or your supply will be without voltage .........burni,burnie,burnie ............


to be honest- I didn't even think of posibility that secondaries aren't in phase that's grande no-no .........
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