RE: With a bit of assistance from here and elsewhere ... I believe I have the answer. ...

Posted by andyr (M) on September 5, 2024 at 22:11:22

In Reply to: Can someone explain these THD measurements from LTspice? posted by andyr on August 18, 2024 at 23:16:45:

I drew up a simple gain stage in LTspice - here:

... and wondered why, with high input signal levels, I was getting very high THD in LTspice.

It would seem that the culprit causing the high THD levels is the use of 2SK369s in the above circuit - this causes the Source voltage to be down at 120mV, if I set R4 so that 6ma passes through each jfet.

With a 250mV peak input signal ... I need to have the 'volume pot' (R3/R15) set so that it passes a signal level which is, say, 50mV peak at max.

An improvement (in the simulation, at least) is to replace the 2SK369s with 2SK170s. These have lower gain - so R1 has to be increased to get the same output signal level - but the Source voltage for the same 6ma current through each jfet, becomes 460mV ... producing a significantly lower THD level.

So thanks for your help.

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Follow Ups Full Thread

Topic - Can someone explain these THD measurements from LTspice? - andyr 23:16:45 08/18/24 (15)

RE: With a bit of assistance from here and elsewhere ... I believe I have the answer. ... - andyr 22:11:22 09/05/24 (0)

Where's all the noise and distortion coming from? - Geoffkait 08:07:36 08/21/24 (0)

RE: Can someone explain these THD measurements from LTspice? - 13th Duke of Wymbourne 10:27:23 08/19/24 (6)

RE: Thank you DoW ... - andyr 16:49:40 08/19/24 (1)

Why? Physics or Maths depending how you view it - 13th Duke of Wymbourne 19:31:25 08/19/24 (0)

And do what Tom suggests, plot the drain current to be sure the swing is symmetrical - 13th Duke of Wymbourne 10:35:38 08/19/24 (3)

RE: Sorry - can't quite see how to plot the Drain current ... - andyr 17:05:06 08/19/24 (2)

RE: Sorry - can't quite see how to plot the Drain current ... - 13th Duke of Wymbourne 19:52:36 08/19/24 (1)

RE: Yes, that plot *is* the voltage across R1... - andyr 20:00:55 08/19/24 (0)

RE: Can someone explain these THD measurements from LTspice? - tomservo 10:20:49 08/19/24 (5)

RE: Thank you, Tom ... - andyr 16:58:42 08/19/24 (4)

RE: Thank you, Tom ... - tomservo 09:59:12 08/20/24 (3)

RE: "when you change the resistor values" ... - andyr 15:57:11 08/20/24 (2)

RE: "when you change the resistor values" ... - tomservo 05:37:21 08/21/24 (1)

RE: "what do you want to build?" ... - andyr 15:30:30 08/21/24 (0)

Follow Ups

RE: With a bit of assistance from here and elsewhere ... I believe I have the answer. ... - andyr 22:11:22 09/05/24 (0)

RE: With a bit of assistance from here and elsewhere ... I believe I have the answer. ...

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