Babelfish M25, SissySIT - general building tips and tricks

"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 :p

"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 :scratch2:

"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
 
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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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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 :confused:.
 
25.4v loaded at 2.5A is pretty high. If the THS you have are high mu, lowering supply voltage using a variac or a series resistor will allow you to set Iq to lower value. If my suspicion is verified then you have 2 options, use reduced supply voltage or use more suitable pair of THS.
 
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25.4v loaded at 2.5A is pretty high. If the THS you have are high mu, lowering supply voltage using a variac or a series resistor will allow you to set Iq to lower value. If my suspicion is verified then you have 2 options, use reduced supply voltage or use more suitable pair of THS.

Sorry to be thick, how does one know if they have high my SITs? Only parameter I know is that mine from Prass say 2.96 and 2.97 and I don't want to fry them!

Thanks,

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

If you need them back just let me know and we’ll work something out. I still have the other two pair that I got from watanabetomoaki. Haven’t measured any, just hoarded most everything I’ll need for the Sissy.

To be honest, I’m going to finish the F2J I started before I start the Sissy and play with some iron in my DCB1 while ZM puts together my Iron Pumpkin Xmas present that I bought myself.

I keep saying it...I’m almost finished with my dipole bass project and am excited to set it up with the B4 and Oris horns.
 
Sorry to be thick, how does one know if they have high my SITs? Only parameter I know is that mine from Prass say 2.96 and 2.97 and I don't want to fry them!...
You'll need a curve tracer for that. But don't worry, use 22-23V supply and adequate heatsink, you won't fry them, they are tough beasts. Regulated supply is recommended for SITs when line voltage supply varies more than 5% or if you try to push things close to the limit.
 
well reducing the rail voltage does reduce the current - not unexpected BUT the control circuit is still not working , so I am not convinced replacing the SITs will solve the problem in my amp .

There must be something simple I am not understanding , the Iq potentiometer has the ability to increase the current , so logic tells me reducing R15 should reduce the current to enable the pot to increase it once again - but this is not the behaviour I saw previously.

I may end up burning some heat in series resistors to drop the rails to 20V which is what it takes to get 1.8A flowing.

more pondering but not tonight. thanks for the suggestion to try this.

..dB
 
There must be something simple I am not understanding , the Iq potentiometer has the ability to increase the current , so logic tells me reducing R15 should reduce the current to enable the pot to increase it once again - but this is not the behaviour I saw previously...
Not when the output of opto is saturated. Measure opto output voltage between R21 & R20 (pin 3 & 4) to verify, if voltage is < 0.5V then opto output has saturated and no longer able to regulate Iq.
... I may end up burning some heat in series resistors to drop the rails to 20V which is what it takes to get 1.8A flowing. ...
When you are sure reducing rail voltage is the solution you want and since you have a 25.4V rail and only need to reduce supply to the THS, I would suggest a regulator instead. I think a cheap IRF540 or IRFP240 mounted on the heatsink will do the job nicely, off course bigger part has better thermal transfer property. Cut the leads between board and drain of THS and insert regulator so regulator is placed between the Hall chip and THS.
 

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Official Court Jester
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that's bad patch , not solving the problem

though , with rectifier etc. loaded , I'm expecting no more than Vac x 1.25

there are few volts more than that , how's that possible ?

only if donuts are 20Vac , not 18Vac

and one thing more is puzzling - how increasing both R18 and R19 is not working ?