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How to measure bias voltage on Acoustat
How to measure bias voltage on Acoustat
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Old 25th January 2014, 05:47 PM   #11
tyu is offline tyu  United States
diyAudio Member
Join Date: Jul 2009
Thats not mine.....i did rinse...... at one time i thought the pic was ok......
use air blow out the dust......!...........no more water for me....
It Not dust..........most of what you see ...dark in the water is bias coating comeing off..Live an learn.............
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Old 29th January 2014, 06:50 AM   #12
moray james is offline moray james  Canada
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Location: Calgary on the Bow
Acoustat had a coating resistance of about 50K ohms so washing off coating is not a bad thing remember that Quad are many meg ohms so the higher resistance will keep the charge stable on the diaphragm and not let voltage run around to jump off at the closest stator with a leak in the dielectric. Must my 2 cents on that topic. Best regards Moray James.
moray james
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Old 30th January 2014, 08:30 PM   #13
tyu is offline tyu  United States
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Join Date: Jul 2009
So when the Panels being driven hard....what happens....the bias is pull off the cothing....used up?...it being driven off?...an has to catchup?
Is this why i am finding that lowering the bias feeder res. in the 121 stock is 500meg.....to 30meg that there is less... arkover an less stress on the tranfoumers an res. in the interface?....An Amps geting HOT.....

I have done the same on some of my MLogans bias.... drop the stock 60meg...20meg ....an thay sound like my Acoustat now.... some thing i have never heard....out of any ML panel!
go fig....

I see it like 1/4" pipe feeding a tank....so i just put in a 1" pipe...now the panel output ... can keep up with how had there being driven....NO booging?

Tell me why droping the MEGs.... is a bad thing...?

As always... thanks for your time an any info on ESLs...Bias..
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Old 31st January 2014, 02:39 AM   #14
pforeman is offline pforeman  United States
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Tyu, add to your observations of decreasing the resistor value, the additional complicating findings of people who suggest adding a large value choke between the resistor and the coating on the membrane.
That will increase resistance, but only by a little (relatively). And should impede the flow of current onto the membrane if the charge is reduced from playing (or just with time).
Was that you that recommended that Moray James? That was a while ago.
Do you still a recommended this tweek?
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Old 31st January 2014, 01:26 PM   #15
geraldfryjr is offline geraldfryjr  United States
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Join Date: Feb 2008
Location: Jackson,michigan
Adding the Choke Makes sense to me.

The reason is that as the diaphragm is moving I have found that this does modulate the bias level a bit.

Having a choke in line will stop this A.C. content from feeding back into the supply therefore allowing the supply to maintain a constant level.

This is one of the reasons I have chosen to build a feedback circuit to regulate my HV supply.

Just the slightest amount of current draw can significantly drag down the bias voltage by as much as 5% or even 10% or more, especially if the supply only has a 60Hz refresh rate.

If the panel has absolutely no leakage then the size of the resistors should have no effect on the bias charge on the diaphragm.
But they will help stop the A.C.content from modulating the bias supply.

I have proven this in tests with my scope monitoring the Bias Voltage.
Putting a resistor on the ground side of the bias supply feeding the transformer also reduces this effect even more.

I am not sure how much this would effect the THD but it may have a bit of results on the IMD factor.



Last edited by geraldfryjr; 31st January 2014 at 01:28 PM.
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Old 31st January 2014, 02:08 PM   #16
tyu is offline tyu  United States
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Join Date: Jul 2009
Will the choke dose help....
Moray was the one that came up with the Choke Mod!

I have been Diying tube amps OTL an push pull....an have found that chokes have there owen sound to add......most chokes Ring......in tube amps...i am finding that to get the sound i have been looking for over 40 years is to use the choke on the front end only.. way less ringing..... but Not the output tubes....not saying chokes dont work...but you better have a well made Choke!....$15-20..... not going to cut it....just like the ESL bias...it is the sound of the ESL...better parts in the bias well give better sound!....

An yes i have know dout that the THD must go up when lowering the Res. in the bias...but the sound..May be it the more THD that makes the sound better..
I now think it not raising the bias. so much....to get better output...
We may even get by with less bias....but get the res. to about 20-15meg...
One thing this Mod has done for the ML.... i have been in a lov-hate with for ever.....
is move there sound so close to the sound of my Acoustats....that the logans have moved to front place...even giveing my Apogees stages... a run for the money...an the Output....50 tube watts driven them with eezzs............

You dont know if you dont go.....you can always add megs....have fun...it all just about geting the best sound....from your ESL
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