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Old 12th February 2005, 10:39 PM   #31
Stabist is offline Stabist  Slovenia
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Hmm - after few days I'm more dissapointed than not

The thing is - apparantely I have big problems with my PSU - I mean - todaj I wanted to listen to some music - but something strangely occured - the bass speaker membranes just moved in and out (with no sound) - well - then it settled down - I've tried to turn one record - and it again started over and over again - like something would lost the control ...
So I decided to measure voltages - the "heater" had only 5,9-6V instead of 6,3 - so I decided to measure AC - ONLY!! 208-210V ... OK I had enough of it and I'll decided to wait to get a more proper voltage - so at evening I tried again - this time AC was around 225-230V ... But quite soon it started again ... So I'll decided to investigate a little more - I've discovered our laundry washer is on and that my electrical radiator has A BIG influence ... Also turning the TV from shut down to STBY etc results in some very loud and "visual" pops in my speakers ...
And AND ALSO the most scarry one - I switched off the preamp with volume knob opened - something bzzzzzd in speakers and then it pushed the membrane "IN" and it stayed there until I "closed" the volume knob .... ?!?!
SO I guess I have quite some work to be done!

So apparently I'll need to filtrate voltage somehow - so can anybodiy enlighten me a bit further what that grid filter is??

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Old 12th February 2005, 10:58 PM   #32
SY is offline SY  United States
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New MC preamp is comming alive
Welcome to the world of turn-on and turn-off nasties. That's something tube preamp builders have to live with- but it's very easy to incorporate a circuit that shorts the outputs to ground until warmup and does the same at shutdown or in the case of a power interruption.

The cone pumping is a classic symptom of motorboating- it means that your circuit is coupling unintentionally through the power supply, a positive feedback situation. You'll need to investigate separating out each B+ feed and bypassing them individually. An active regulator can be a GREAT help here.

Line filtering is a separate issue; there's been a lot of discussion about it, and some searching of the forum will yield great dividends. Morgan Jones does a nice treatment of this and the turn-on/turn-off issues in his book, FWIW.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 14th February 2005, 01:43 PM   #33
Stabist is offline Stabist  Slovenia
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Thanks SY.

I'll do a little redesign in future - hmm - interesting - today (monday) everything works more than OK But during the whole weekend - there was no way to use my preamp ...

I've done some "research" - it just happens that our house apparently is the last in the "row" of local electrical grid ... So that could explain a lot about nonstable AC in our house

I intend to use a bigger trannie - apparently the one used now has to big voltage fall under load to provide enough high voltage for regulator to have 6,3V - even today, when AC voltage is 230+V - I still have only 6V at the ouptut od PSU ...

I also intend to fiddle around with GND connections - apparently also the GND from the arm is somehow "loose" - if I move that connector a bit - the hum rapidly increases/decreases ....

So quite some stuff to be done ...
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