Time for my update on the SissySit R.3 I built. It went together great, using a typical dual mono linear supply which provides about 22.25 VDC per rail under load. Housed in a 4U / 400 chassis. Only about a 25 degree C heatsink temp above ambient.
Overall, very happy with the sound, but I no longer feel confident in providing comparisons to my previous Pass amps as I am one of the "lucky" ones who had significant worsening of my tinnitus after my Covid immunization.
I am running between 1.7 and 1.8 A Iq. About 190mv across each 0R11 resistor. DC offset easily to about 0.
I am wondering if any change to my level shifter resistors is indicated.
R L
V across 120r resistor 3.555 v 3.531 v
Thf 51s Grid to out 3.702 v 3.690 v
IRFP 9140 Grid to out 3.860 v 3.851 v
Thanks to ZM, NP, and all here for the help and opportunities!
Overall, very happy with the sound, but I no longer feel confident in providing comparisons to my previous Pass amps as I am one of the "lucky" ones who had significant worsening of my tinnitus after my Covid immunization.
I am running between 1.7 and 1.8 A Iq. About 190mv across each 0R11 resistor. DC offset easily to about 0.
I am wondering if any change to my level shifter resistors is indicated.
R L
V across 120r resistor 3.555 v 3.531 v
Thf 51s Grid to out 3.702 v 3.690 v
IRFP 9140 Grid to out 3.860 v 3.851 v
Thanks to ZM, NP, and all here for the help and opportunities!
Last edited:
give me just another value ( keep it at now set stable 136mV across 0R11) - Ugs of SIT itself
I presume you're having 120R in R115 position?
quick calc sez that we can expect something as -2V4 there, but better to confirm that with measurement, I mean - better to get exact value
Last edited:
preliminary ( and most likely it'll be good enough as cure) - decrease value of R115 to 82R
whatever you have in drawer, from 68R to 82R will do the job
as your measurements are showing, you have pair of SITs with relatively small ( absolute value ) of Ugs, so level shifting resistor must be optimized
whatever you have in drawer, from 68R to 82R will do the job
as your measurements are showing, you have pair of SITs with relatively small ( absolute value ) of Ugs, so level shifting resistor must be optimized
Bad luck on this SIT, I guess. I do have some others, but thought these 2 were the best of the bunch.
I went and ordered the 82R resistors, and some pin sockets to make it easier to change this value, if needed some more.
Will I want to swap R115 on the other side, or its ok since we are in the envelope for that SIT already.
I want to have the pin sockets in hand before I take the board off the heatsink, and a beach weekend, so it will be first of next week before I do the swap.
I went and ordered the 82R resistors, and some pin sockets to make it easier to change this value, if needed some more.
Will I want to swap R115 on the other side, or its ok since we are in the envelope for that SIT already.
I want to have the pin sockets in hand before I take the board off the heatsink, and a beach weekend, so it will be first of next week before I do the swap.
it isn't bad luck, yet, no reason to expect worst 
when you get resistors - yeah , put sockets ; when done, just solder resistor in socket
as I said before - it's hard to cover all possible combinations of Ugs values
in fact - it is easy , exactly with optimizing R115 value - it is there with exact purpose
if other channel is behaving, just leave it as is; value of R115 is having no role in AC domain ( music, signal)
edit - taking some care, you can do the swap without removing pcb
if needed - clip R115 pins close to body, then heat with solder gun and pull pins with tweezers
soldering socket pins is even easier
when you get resistors - yeah , put sockets ; when done, just solder resistor in socket
as I said before - it's hard to cover all possible combinations of Ugs values
in fact - it is easy , exactly with optimizing R115 value - it is there with exact purpose
if other channel is behaving, just leave it as is; value of R115 is having no role in AC domain ( music, signal)
edit - taking some care, you can do the swap without removing pcb
if needed - clip R115 pins close to body, then heat with solder gun and pull pins with tweezers
soldering socket pins is even easier
Last edited:
Managed to tilt the big transistor to access R115. It crumbled in my hemostat clamp, and each end removed fine. I had to be careful I starting the pin sockets, as the tails are long. Here is the mounted 82R.
Success! Easily got the bias up over 200ma. I lowered it to 180, with successful offset to near 0. Going to let it cook for a few hours for temp equilibrium, then do final settings. Same for other side tomorrow. Going to install a new switch, as a blew of the LED on the old one. Then put the bottom back on, and I’ll be playing the amp by this weekend.
Thanks again ZM! 🙏🙏🙏
Success! Easily got the bias up over 200ma. I lowered it to 180, with successful offset to near 0. Going to let it cook for a few hours for temp equilibrium, then do final settings. Same for other side tomorrow. Going to install a new switch, as a blew of the LED on the old one. Then put the bottom back on, and I’ll be playing the amp by this weekend.
Thanks again ZM! 🙏🙏🙏
Attachments
Just started doing the 2 hour bias refining on the “good” side. (both good now actually) I’ll try and get those Ugs measurements done tonight before turning it off.
I did mount the feet and secure the bottom before starting it up this afternoon. It should be ready to drag up tomorrow evening and listen too.
I did mount the feet and secure the bottom before starting it up this afternoon. It should be ready to drag up tomorrow evening and listen too.