Five transistor version of the low distortion sine-wave oscillator

Out of curiousity, I upped the currents through Q1A and Q1B by about an order of magnitude (R27 down to 27k, R17 down to 22kk and R28 down to

Essentially, their incremental resistance has dropped by an order of magnitude, and the ripple on the gain-control signal produces less voltage excursion.

Is that allowed Bill? I thought that making component changes to see if the circuit works better was design by evolution?

Did you mean R20? I don't see R27.

I think the only way forward with this circuit would be to build and test it.

I'd do a first prototype with everything through hole except LT1679 and NSS40301MDR2G.

I'd also put four more resistors in series with each 68k (maybe reduce them to 56k) for the four diodes so I can make the current pulses in the four diodes exactly equal. And add a capacitor (100n min) to ground where the resistors join.

And use the remaining LT4167 (two quad packs) as an output buffer so that whatever is connected to the output doesn't disturb the operation of D10.

I didn't make the change to see whether it worked better - I did it to see if I'd correctly understood what it was doing. The fact that it made it work better was incidental.

I did indeed.

Agreed.

Why?

Why? I can see an argument for removing all the 68k resistors so the current being fed through R11 is as high as possible, with the smallest possible ripple. There is a risk that the diode current will turn off fast enough to drive them into snap-recovery, but it is remote.

Increasing the 68k resisitors reduces the effect of the tolerance on the forward voltage drop through each diode, but choosing diodes with a closer tolerance on the forward voltage drop would be a better way to go. The 1N914 doesn't seem to have one at all.

The Infineon-BAS3007ASERIES diodes at least specify 350mV typical and

Adding more phase delay along the feedback path and make the settling time even longer.

What D10?

Changing almost anything in this circuit in LTSPice changes the residual harmonic levels. Assuming the same is true in reality I'd want to be able to change components easily.

Changing to schottky diodes changes the distortion but not always down. So I'd want to be able to make changes easily on a real prototype.

One of us doesn't care if he has to wait 5 minutes for the purest sinewave. The other seems to put higher priority on the circuit settling in a few seconds. I think we'll just have to differ there.

The one that's D14 in your circuit.

I never had much trouble changing surface mount parts.

There are lots of different Schottky diodes. If I remember right they don't do step-recovery, which might help. The first thing I'd go for would be a tight tolerance on the forward voltage drop.

You might find that in a quad diode pack. I used to browse Farnell catalogue for that sort of stuff. Searching their data-base doesn't work as well.

The best I could do was the BAS40-05 common cathode dual from Nexperia and Infineon. That offers 250mV min, 310mV typical and 380mV max at 1mA, and looser tolerances at higher currents. Within the part the two diodes are matched to better than 20mV.

What I actually want is critically damped - dead-beat - settling. Having a long period while the amplitude is ringing down is evidence that the circuit designer doesn't know what they are doing. This isn't the first time I've called your attention to this.

If John May didn't bother it's probably not worth doing.

More to the point, it's easier fit complex layouts around dual op amps rather than quad packages, and you'd be better off using the LT1678.

"Edward Rawde" snipped-for-privacy@invalid.invalid wrote in message news:vtlm0g$21p4$ snipped-for-privacy@nnrp.usenet.blueworldhosting.com...

As a first prototype I'd build the circuit below using only through hole components except where through hole is either not available or not desirable such as ceramic capacitors. Resistors may be done with pads so that through hole resistors can be soldered on in such a way that they're rather easier to remove than an 0402.

In recent years I've been asked if I can repair equipment such as a music keyboard. If the internals are surface mount and the problem isn't an obvious one (eg wrong power adapter) I will generally decline to try. Find someone who can get the dried up surface mount electrolytics off without damaging the board, if it hasn't been damaged already by leaking electrolyte. There may well be equipment which can do it but I don't have it.

The second prototype for this circuit would be all surface mount.

Unless I win a lottery I won't be either building it or figuring out what I need to test it.

I'm done with sinewave oscillators except for being curious about how to design a suitable sample and hold circuit in the other circuit JM posted. Use of a comparator to obtain the sampling signal might reintroduce all the harmonics we want to get rid of.

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Edward Rawde 30 March 2025.\nBased on designs by JM and BS. Re-worked by JM, getting rid of half the transistors,\nand again by Bill Sloman, making Q1A/B and Q2A/B close matched transistor pairs\nand adding the cascode transistor at Q3, 12th April 2025.. TEXT -2896 16 Left 2 !.MODEL NSS40301MDR2G NPN\n+is=6.87023e-12 bf=445.496 nf=1.08926 vaf=60.529\n+ikf=7.23313 ise=2.38192e-09 ne=4 br=23.6872\n+nr=1.10701 var=8.89608 ikr=1.25064 isc=1e-16 \n+nc=1.13174 rb=905.334,irb=2.30349e-07 rbm=1e-10\n+re=0.00600548 rc=0.0300274 xtb=1.2219 xti=4,\n+eg=1.05 cje=3.80477e-10 vje=0.912237 mje=0.397194,\n+tf=5.90916e-10,xtf=0.0998483,vtf=7.09172,itf=0.010481,\n+cjc=8.35472e-11 vjc=0.702862 mjc=0.43477 xcjc=0.899998\n+fc=0.414631 cjs=0 vjs=0.75 mjs=0.5 \n+tr=5.64658e-08 ptf=0,kf=0 af=1

Then you haven't changed very many or you have better equipment than I do.

You might still want to equalise the currents because the four diodes aren't driven 100% exactly the same.

There's no need to state the obvious, which is that I don't care if it takes 5 minutes for the circuit to settle. You have it heavily damped, under damped, over damped, lightly damped, critically damped, slightly damped or whatever damped you want. I'm ok with that.

John may did show that separately buffering the signals to the four diodes improves performance. This is similar to equalising the currents through them.

Yes I have to agree that use of the dual version (four packages or five with some unused) would be a good idea here.

Then you don't know enough about what's going on. This low distortion sine wave exercise is - to an appreciable extent - an exercise in keeping audiophools happy, and you need to give them a product that doesn't make them anxious.

Loading the op amps in the ring oscillator with a rectifier diode is going to affect the output current a little - op amps don't have zero output impedance. Buffering everything might be the ideal, but it is extravagant and offers extra interactions which you may have to tame separately.

It isn't. Even if you equalise the current drawn (and they should be very similar) the current is only being drawn during when the sine wave is appreciably positive, and that's going to generate some distortion, though probably not enough to get excited about.

Too true. I once got stuck with cleaning a comparator-heavy board in fairly old Cambridge Instruments electron microscope.

Final test hated it because the comparators tended to talk to one another, and burst into collective oscillation. The original prototype might not have done it, but as the manufacturer improved the parts and built them with smaller dies the design got more and more twitchy.

I worked over the layout to minimise the interactions as much as I could, but while final test liked what I'd done, it merely reduced the problem and made it easier to fix when it did show up.

LOL I'm no audiophool but I do know that you would seize on any opportunity to tell someone else that they don't know what they're doing Bill.

Actually the low distortion sine wave exercise was an exercise in finding out whether you can make a low distortion 1Khz oscillator without using lamps, thermistors, opto devices or FETs as voltage variable resistors. It has nothing whatsoever to do with audiophoolery. If any aspects of the circuits make you anxious then I'd advise consulting a doctor for the appropriate meds.

DO NOT FEED THE TROLL

Like John Larkin, you are fishing for flattery, and resent it when you don't get it.

It's an interesting intellectual exercise, but it wouldn't be all that interesting if there weren't audiophools out there to buy an eventual product.

Nothing that you can see.

More bad advice, and it reflects - once again - your poor grasp of message you think you are responding to. The anxiety was in the audiophool potential customers. I do get mildly anxious from time to time, but it's a rational and appropriate response to imperfectly predictable reality and no doctor (certainly not my admirable GP) would prescribe anything to reduce it.