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Babelfish M25, SissySIT - general building tips and tricks
Babelfish M25, SissySIT - general building tips and tricks
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Old 11th February 2019, 01:29 AM   #441
Zen Mod is offline Zen Mod  Serbia
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Babelfish M25, SissySIT - general building tips and tricks
just another thing ( I even checked now did you connect them SITs properly ) - you got new THFs , or used ones ?
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Old 11th February 2019, 01:42 AM   #442
dBel84 is offline dBel84  United States
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"it can't be Vgs of SITs"

good - I passed along my spare set , so was hoping not to have to buy again


"if you tried increasing value of R18 and R19 (even if I can't figure reason why )"

this is curious and might give insight into what is happening??

when I increase either or both R18/R19

.. the offset gets worse and I am not able to get anywhere near zero

decreasing R19 to 33K has been the best so far - if I reduce R18 too, I can zero but the offset fluctuates a lot

"if you tried decreasing value of R15"

yes - dropped it to 270R and it made no difference that I could tell

"if you tried decreasing value of R16 and R17 ............."

not yet - but this is why I socketed R15/16/17/18/19 - soldering and unsoldering is only good for a few cycles before the board fights back

"I hope you didn't mount Rs (which is strictly for Babelfish M25) ( I can't see them on pics , but better to ask)"

always good to ask - but no, never soldered.

I was only an idiot and soldered D1/D2 initially but they have been removed

"if you don't have too much protection diodes"

not sure I understand - everything that is on the schematic is soldered on the board

"if you read a note about ZD1 and obeyed"

I did indeed

"if you have steady 5V feeding Halls ......."

yes - this I have checked

"then I'm in need of new (spare) towel , to preserve my calm"

I will have to send you a bottle of bourbon

"just another thing ( I even checked now did you connect them SITs properly ) - you got new THFs , or used ones ?"

I bought both new and used - the used were more closely matched with testing VGS -3.17 , so I used them.

Other things I have checked

The Hall output is about 3.3V which is what I recall being posted as normal operation , curiously the 2.5V reference is 1.1V with P1 at zero resistance

the negative rail current is about 2.8A on one channel and 2.4A on the other (this is when it has been cooking for a few hours ) , P1 adjusts both and I can increase this to 3A easily

I will keep working on it , no rush on your part as I have the M2X making lovely sounds for me and although it is not Sissy, it is still very good.

..dB

Last edited by dBel84; 11th February 2019 at 01:48 AM. Reason: added another line
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Old 11th February 2019, 01:53 AM   #443
dBel84 is offline dBel84  United States
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I also bought matched Harris mosfets for this , I forget what the VGS but I have it written down somewhere.
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Old 11th February 2019, 02:01 AM   #444
Zen Mod is offline Zen Mod  Serbia
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Babelfish M25, SissySIT - general building tips and tricks
what's Ugs of new THFs ?

you didn't tried Sissy with them?
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Old 11th February 2019, 02:05 AM   #445
dBel84 is offline dBel84  United States
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VGS -2V and -1.8V iirc , no I never used them and no longer have them. If you think it is the THF51s, I will get another set. They behaved well in the test circuit and are producing clean sound which is why i never suspected them to be a problem

The VGS on the 9240 is close to 4V

..dB
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Old 11th February 2019, 02:08 AM   #446
Zen Mod is offline Zen Mod  Serbia
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Babelfish M25, SissySIT - general building tips and tricks
frankly , I can't be sure of anything

can't find damn logic ........ it's easiest when thing is Dodo , not schizophrenic

though , problem is always in da scull , not in da gadget
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Old 11th February 2019, 02:12 AM   #447
dBel84 is offline dBel84  United States
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I know when the problem is identified , I will be kicking in my scull. stupid I am not but an idiot, always
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Old 11th February 2019, 03:02 AM   #448
Ben Mah is offline Ben Mah  Canada
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Quote:
Originally Posted by dBel84 View Post

decreasing R19 to 33K has been the best so far - if I reduce R18 too, I can zero but the offset fluctuates a lot

Hi dB,

Sorry to hear about problems cropping up again.

How much fluctuation are you getting?

The offset in my SissySit fluctuates too but I just assumed it is due to varying temperature and line voltage. For instance, it was up to about 90mV earlier, and this is after the amp had been on for hours. Now it is at about 60mV. It has been at around 0mV too. I just leave it alone as I believe it is due to varying temperature and line voltage. I have not been concerned since in my case, the offset in both channels change in lockstep. The temperature in my room definitely varies a few degrees over time and the line voltage can be as high as 119V and as low as 115V. It is 115V presently. I do not know what it was when I last adjusted the offset, but it was probably higher as I just measured Iq at 1.6A and I do know that I had set it at 1.8A.

I notice that you have large ventilation holes in your top plate. Perhaps this allows for more variation in operating temperature and therefore more variation in offset?

Also do you have large fluctuations in line voltage?

Of course it could be totally something else that is the cause of your troubles .
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Old 11th February 2019, 07:15 AM   #449
dewdrop is offline dewdrop  Singapore
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Babelfish M25, SissySIT - general building tips and tricks
Will we be pushing it with a 3U chassis?
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Old 11th February 2019, 08:16 AM   #450
indra1 is online now indra1  Indonesia
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Originally Posted by dBel84 View Post
I know when the problem is identified , ...
Can you measure supply voltages at the boards? You may have THS with a bit higher mu so that a bit higher supply voltage results in very high Iq.
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