The SG505 is an outstanding very low distortion state variable oscillator SVF developed by Bruce Hofer while he was at Tektronix before being a co-founder of AP. His use of the SVF inspired most of the circuitry in the THD analyzer I designed (Audio article available on my website). As described below in another post, JFET Q1620 amplifies and inverts and amplifies the error signal at the positive input to U1520. This enhances the net error signal applied to the op amp and further reduces the already low distortion of the 5534. The feedforward/feedback arrangement of R1521, R1423, pot R1510 and R1525 comprises what is often called a Baxandall volume control, in which the positive input of the op amp is usually grounded. The distortion-reduction circuit of Q1620 is just for further reducing distortion of the output buffer stage, and is not necessary if the already-low distortion of the 5534 is acceptable (good enough for more than 90% of applications). If not the right JFET is chosen, this circuit may not function correctly and actually increase distortion. You can disable this circuit and still get excellent distortion results. You can disable it by just removing the JFET from the circuit. See if doing this makes the circuit work properly. If you do so, you should probably short R1524 to keep the positive input of the op amp at a low impedance to ground, as normally done in the Baxandall volume control. Note that in general use, the Baxandall volume control provides a good approximation to a logarithmic gain vs. pot rotation using a simple linear pot over a very wide range of gain, making it excellent for use as an audio volume control.
Cheers,
Bob
Thank you Bob and Alex for the insights.
Now that we understand the purpose of the JFET, the next step is to find a replacement for the 151-1025-00 that is used. Unfortunately, I cannot find any details about it, nor for the original source, the SPF3036. It would greatly help if somebody would be kind/brave enough to measure the Gate voltage and the Drain voltage in the circuit or better yet, lift that JFET from any Tek instrument and put it on a Curve Tracer, or as a minimum, measure the Pinch off voltage and the rDS(on) for starters.
My goal is to see how far I can get the original SG505 working without making any changes at this moment, so I'm still trying to get the original output amplifier to work as close to the specifications as possible, before I start to use possible improvements, or use Bob's output amplifier section. 😉
So to continue with that quest, if I may in this thread, here is what my mouse-pal Bud (an ex-LT chip designer) found out while using LTspice for that particular circuit.
Many thanks for the collective insights, it proves the value of this forum.
Paul
Now that we understand the purpose of the JFET, the next step is to find a replacement for the 151-1025-00 that is used. Unfortunately, I cannot find any details about it, nor for the original source, the SPF3036. It would greatly help if somebody would be kind/brave enough to measure the Gate voltage and the Drain voltage in the circuit or better yet, lift that JFET from any Tek instrument and put it on a Curve Tracer, or as a minimum, measure the Pinch off voltage and the rDS(on) for starters.
My goal is to see how far I can get the original SG505 working without making any changes at this moment, so I'm still trying to get the original output amplifier to work as close to the specifications as possible, before I start to use possible improvements, or use Bob's output amplifier section. 😉
So to continue with that quest, if I may in this thread, here is what my mouse-pal Bud (an ex-LT chip designer) found out while using LTspice for that particular circuit.
Many thanks for the collective insights, it proves the value of this forum.
Paul
Hi Paul,
I may have missed something, but what happened to the original Tek JFET? Is it still alive? If so, check its IDSS value.
While it was in the circuit, did you check its operating point, such as its in-circuit drain voltage and thus voltage drop across its load resistor? There may also be a chance that the original part was a selected JFET. IDSS and pinchoff voltage can span a large range for JFETs. In a circuit like this, IDSS is probably more important than pinchoff voltage.
Don't rule out that there is something wrong other than the JFET.
Finally, to make sure there is not a problem of some other kind in the output buffer circuit, I would still disable the circuit by operating with no JFET and see if performance is good.
Make sure that the electrolytic coupling capacitor is the drain circuit is not defective in some way.
Cheers,
Bob
I may have missed something, but what happened to the original Tek JFET? Is it still alive? If so, check its IDSS value.
While it was in the circuit, did you check its operating point, such as its in-circuit drain voltage and thus voltage drop across its load resistor? There may also be a chance that the original part was a selected JFET. IDSS and pinchoff voltage can span a large range for JFETs. In a circuit like this, IDSS is probably more important than pinchoff voltage.
Don't rule out that there is something wrong other than the JFET.
Finally, to make sure there is not a problem of some other kind in the output buffer circuit, I would still disable the circuit by operating with no JFET and see if performance is good.
Make sure that the electrolytic coupling capacitor is the drain circuit is not defective in some way.
Cheers,
Bob