Success!
Changed the pots for 1K and managed 1V across R10 then tried P2. Couldn't get the voltage up; flashes of 0.2, 0.3 then 0.00V proved very depressing. Then I thought to check the multimeter probe connections. One was loose. Holding the contacts together and suddenly up came 1V.
'Spoilt the sheep for a penny worth of tar', was an expression that came to mind.
I'll check to see whether I needed the 1K later.
Thanks for all the help but 'you can lead a horse to water...' and all that!
Chris
Changed the pots for 1K and managed 1V across R10 then tried P2. Couldn't get the voltage up; flashes of 0.2, 0.3 then 0.00V proved very depressing. Then I thought to check the multimeter probe connections. One was loose. Holding the contacts together and suddenly up came 1V.
'Spoilt the sheep for a penny worth of tar', was an expression that came to mind.
I'll check to see whether I needed the 1K later.
Thanks for all the help but 'you can lead a horse to water...' and all that!
Chris
Has any one experienced heating problems with R10 and R11, spice simulation shows around 1/2 watt dissipation on R10 and R11. Also if I look on data sheet for 2SK2013 and 2SJ313 ID-VGS curves show 0.1A to 0.2A for vgs of around -2.5v and 2.5v respectively. Spice shows around 144ma to 150ma IR10 and IR11. So 0.144 X 22 =3.16 volt drop 3.16 X.144 = 0.456W. Data sheet (2sk2013) shows VGS OFF 0.8-1.6V. So where am I going wrong or just plug in a 3 w resistor. If this has been addressed in some other post please let me know.
R3=0.036V
R4=0.029V
P3 wiper is set a little towards R3 (for the 'air').
Reading this it occurs to me that the voltages for R3 and R4 should be equal for P3 set to middle.
Once this is achieved and R10 and R11 are set for 1V.
Once this has been accomplished does one alter P3 for 'air' or otherwise whilst leaving P1 and P2 alone or does one have to alter P1 and P2 every time P3 is altered?
R4=0.029V
P3 wiper is set a little towards R3 (for the 'air').
Reading this it occurs to me that the voltages for R3 and R4 should be equal for P3 set to middle.
Once this is achieved and R10 and R11 are set for 1V.
Once this has been accomplished does one alter P3 for 'air' or otherwise whilst leaving P1 and P2 alone or does one have to alter P1 and P2 every time P3 is altered?
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You can also vary the bias current of Q1 and Q2. You measure the current by reading the voltage across R3 or R4. With 10 ohm, the voltage wants to be 0.08 volts for 8 mA of current. You can play with this, but keep in mind that the dissipation of this part is rated at 400 mW,
and for this circuit 10 mA will represent around 200 mW. These JFETs were chosen at Idss figures of 10 mA (Idss is the current that flows when the Gate and Source are at the same voltage). If you get lower values, you may want to consider higher values for P1 and P2, say 1 Kohm.
So I'm getting lower than 8mA so that explains why I needed P1 and P2 at 1K.
They are best matched, but P3 can be adjusted to compensate for some mismatch.
I didn't match them. In order to do so do I alter P3 to get equal voltages across R3 and R4?
and for this circuit 10 mA will represent around 200 mW. These JFETs were chosen at Idss figures of 10 mA (Idss is the current that flows when the Gate and Source are at the same voltage). If you get lower values, you may want to consider higher values for P1 and P2, say 1 Kohm.
So I'm getting lower than 8mA so that explains why I needed P1 and P2 at 1K.
They are best matched, but P3 can be adjusted to compensate for some mismatch.
I didn't match them. In order to do so do I alter P3 to get equal voltages across R3 and R4?
THe imbalance that you have somewhat described is part of the characteristic tha determines the distortion that is created. In fact the SJ and SK part have different responses to incoming both incoming signals as well as DC bias. P3 allows you to wipe this difference away or accentuate it. Wiping it away, you get minimal distortion, allowing more inequality and you can manipulate response to favor a higher but more natural distortion. I think
Thanks buzzforb.
I'm not sure what I should be doing though for minimum distortion. I have measured the resistance across P3 in situ and it shows 16R. I have set the wiper to measure 8R each side, however, the wiper doesn't look in the middle.
At this setting R3=0.034V and R4=0.03V.
Does this look OK or should I be aiming (to achieve min distortion) for R3 and R4 voltages to be equal?
I'm not sure what I should be doing though for minimum distortion. I have measured the resistance across P3 in situ and it shows 16R. I have set the wiper to measure 8R each side, however, the wiper doesn't look in the middle.
At this setting R3=0.034V and R4=0.03V.
Does this look OK or should I be aiming (to achieve min distortion) for R3 and R4 voltages to be equal?
There is a range there that is affected by more than just Vgs. It depends on Vds as well as individual fet characteristics. The datasheet is only a generic although close approximation of the response you should expect. With matched fets, the paramaters left that could affect your outcome are Vds and dissipation. That being said, according to the datasheet, you look to be very close to the expected current at your stated Vgs.
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Te only way you are going to be ale to tell about the distortion is with equipment. You have to trust your ears as to what you like best
I think you are right. I have just removed them and have been testing their Idss ranges. Expected range found for the K170 (beteeen 6-10mA) but the SJ74 have all been coming in at around 2mA even though they are marked 'BL'.
Can't work it out.
Any suggestions?
