Can you explain on my dodo level, what measurement made his SITs not within proper range? (#486) I'm thinking or getting another set or two from Pras and I want to make sure to request proper numbers....or rather proper range.
Thanks,
Russellc
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as proposed on beginnings of SissySIT thread (see measurement setup , hilarious sketch!) , use 22-24Vdc PSU capable of 3A minimum ,and measure your Precioussss ones - if Ugs is anything less (ignore minus sign for "less") than -3V7 , SIT is good for SissySIT
meaning - if you pair usual mosfet with Ugs in range of 4V , there is still 300mV window where bjt part of optocoupler can be squeezed in**
even better if you stumble on mosfet with greater Ugs (say 4V5) - even more space for silly bjt
if you're buying from Pras - just let him know that SITs are for Sissy , that's enough
NB that Pras upped his prices - 70 greenies for random SIT , 75 for measured one
though , I'll start nice and cunning action these days : Send all those no good SITs to OPLDF and we'll take care of them!!
** -4V-(-3V7)=[0V3]
meaning - if you pair usual mosfet with Ugs in range of 4V , there is still 300mV window where bjt part of optocoupler can be squeezed in**
even better if you stumble on mosfet with greater Ugs (say 4V5) - even more space for silly bjt
if you're buying from Pras - just let him know that SITs are for Sissy , that's enough
NB that Pras upped his prices - 70 greenies for random SIT , 75 for measured one
though , I'll start nice and cunning action these days : Send all those no good SITs to OPLDF and we'll take care of them!!
** -4V-(-3V7)=[0V3]
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db -another possibility:
can you measure exact voltage between mosfet's gate and source ?
if it's not superhigh , there is possibility to find another ones , with superhigh Ugs , then we'll have needed space for opto
point is lowering that Iq , at least to 2A , so no need to worry about longevity of mosfet ..... it's pretty hard working fella now ....
I have 3 pairs of SITS from Pras close to yours Russelc and if I understand ZM correctly, we should be fine. The 'most negative' value you would want is something closer to 0 than -3V7 (-3.7v). We are near -3V0 so as long as we find mosfets -3.3V or better there is enough of a window for 'bjt to squeeze in' (300mV).
That's funny because Pras was complaining in an email to me that everyone was asking for devices close to -4V0 or 'higher' and he was left stuck with all these devices with only a very small minority of them measuring that high.
Again, if I understand correctly, dbel84 has SITS which are close to -4V0 so window is too small but he is in range close enough to where he can try the DEF circuit without the need for the optocoupler. If I'm way off base here then I'm going to need to order more from Pras as well.
you know , damn thing is working as Babelfish M25 , where mosfet gates are 8V apart** .... so , I always wrote that pretty much every SIT will work , as long Ugs is not greater of -3V7 (greater in absolute value , so ignoring negative prefix)
and even then , if you have borderline specimens - if you choose mosfets with Ugs in range of -4V5 , it'll work
** N mos gate is 4V-ish above sources , while P mos gate is 4V-ish below ......
NB that verticals are sort of always 4V-ish , can't remember did I ever saw (frankly didn't cared much when exact value is of no importance) lesser than 4V , when armored in FW amp format (223V-ish rail , 1A5-2A Iq)
and even then , if you have borderline specimens - if you choose mosfets with Ugs in range of -4V5 , it'll work
** N mos gate is 4V-ish above sources , while P mos gate is 4V-ish below ......
NB that verticals are sort of always 4V-ish , can't remember did I ever saw (frankly didn't cared much when exact value is of no importance) lesser than 4V , when armored in FW amp format (223V-ish rail , 1A5-2A Iq)
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I bet that these are Mein Bruder Generg and bunch of Misfits (read- Semiconductor Mafia) , having idea of True DEFiSIT with Tokins
though - I can't see or imagine any difference between True DEFiSIT and thingie having energized 3m3 cap between gates ...... outputs are knowing that their gates are connected , but to them it's irrelevant how long is stick between their gates , as long stick is solid
Added Indra's patch to regulate the voltage to the SITs - voltage to THF51S is 20V and this drops the current to 1.5A in DEFiSIT mode. If I pull the jumpers and go back to SissySIT function - I can bias the current back up to 1.95A ( Indra recommended 10% above norm )
is rock solid for many hours at this operating point , heatsinks only get to 40 degC and it is still sounding magical
At these operating points Vgs of SIT is 4.06 .
Plan is to look for FETs with high Vgs or maybe buying another pair of SITs which are better matched to this amp design, but as it is working and sounding great - going to just enjoy it for a short while. I quite like the idea that there is the option to convert to DEFiSIT - could not hear any obvious difference but speakers are not optimal as this is still the test system.
..dB
is rock solid for many hours at this operating point , heatsinks only get to 40 degC and it is still sounding magical
At these operating points Vgs of SIT is 4.06 .
Plan is to look for FETs with high Vgs or maybe buying another pair of SITs which are better matched to this amp design, but as it is working and sounding great - going to just enjoy it for a short while. I quite like the idea that there is the option to convert to DEFiSIT - could not hear any obvious difference but speakers are not optimal as this is still the test system.
..dB
and thank you all again 
there is no such thing as having too much SITs
however , you wrote (in #354) that your SITs are around -2V6 , while dB's are in range of -4V ( and were that for too long - unnoticed
)
hope you'll get your source issues sorted soon , so you'll be able to hear Pumpie and Sissy together , in full blast
however , you wrote (in #354) that your SITs are around -2V6 , while dB's are in range of -4V ( and were that for too long - unnoticed
)hope you'll get your source issues sorted soon , so you'll be able to hear Pumpie and Sissy together , in full blast
Very good, SIT's on the way from Pras and I think for my Cinemags I need to wait for a production run. Now I only need a case and power supply to figure out.