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Old 30th January 2012, 08:20 PM   #11
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Below is an attachment which shows the problem with the lowpass circuit. left is the original, right is after changing the two 40K resistors to 33k (below 40K previously resulted in oscillation in the sim) and a 220pF cap across the top 33K resistor.

This smooths out the dip and also gets rid of the nastiest part of the phase change. I'd ideally like the phase to remain flat but I think this is much more benign that the first one. I suspect the original oscillations were phase margin related.

Unfortunately I don't understand what causes the deviation from the filter slope and phase change so my "fix" for it is from trial and error. If I actually understood what was going on I might be able to fix it properly. My hope is that because it is so far down that it will be basically not a problem.

I've also attached the latest version of the simulation schematic. Real world implemtation has 100nf caps on the opamp power pins and 1000uF caps on the rails decoupled from the ps with a 1 ohm resistor.

Tony.
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File Type: png synergy_lowpass_phase.png (15.4 KB, 320 views)
File Type: png synergy_schematic.png (33.5 KB, 351 views)
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Old 30th January 2012, 08:34 PM   #12
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Quote:
Originally Posted by Joachim Gerhard
The Pass buffer has lower distortion as the Salas because it has biasing at the expense of two coupling caps.
How does the different bias voltage affect distortion? I don't see why it would make any difference.

I'm not too familiar with the Salas circuit, apart from that it uses a dual rail supply to eliminate coupling caps. Aren't they both biased at Idss?
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Old 31st January 2012, 01:28 AM   #13
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I think what Joachim is saying is that the distortion on the original is higher because of the need for coupling capacitors (ie it is the coupling capacitors that are adding to the distortion). The split rail makes it possible to eliminate the coupling capacitors because the input is not at 1/2 the potential of the power supply, it is at zero volts

BTW I measured the dc offset (I forgot to mention) and it was 0.8mV I can't remember if that was positive or negative. I didn't match the jfets, but I did use jfets from a bunch that I had sorted into 7-7.5 mA idss so they would have been close.

Tony.
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Old 31st January 2012, 09:56 AM   #14
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Quote:
Originally Posted by wintermute View Post
There is a problem with some sort of resonance in the low pass circuit which I've finally been able to reduce the effects of (it was originally at around 20Khz but I've now pushed it out to about 40Khz which is a long way above the 200Hz crossover freq), I haven't retried the sim with the OPA2134's after the mod.
That's why I just build rather than simulate, I find it so much easier to troubleshoot a real world "thing". For crossover type circuits, I just wire in a DIL socket for components that are likely to change, then you can just plug in various values quickly.
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Old 31st January 2012, 10:54 AM   #15
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Hi Al, I've done half the low pass on the breadboard tonight Too tired to finish it, and I know from past experience it is best to stop when I feel tired I will be very interested to see whether the resonance is a figment of the sims imagination or a real world occurrence.

I would be exceptionally happy if it is fictitious, but the irony would not be lost on me, as I spent a lot of time trying to get rid of it

If this circuit is successful I think I will be more motivated to build stuff more quickly, as that will be two successful "designs" under my belt.

I decided to have a crack at learning spice because I lacked the confidence to build something I had dreamt up myself (actually I think the first thing I simulated was SY's acheron which was the inspiration for this circuit!!)

I was scared my circuit would blow up, and wanted some reassurance from the sim that it had a chance of working. Simulation seemed to be the easiest (and quickest) way to test out some ideas to see if they had merit or not, unfortunately my obsessive nature meant that it became an exercise in tweaking that went on for a long time and eventually I was burned out and left it for a long time. I actually bought most of the parts for this back in April 2010.....

I didn't have a bread board when I started to sim (I'm so glad I bought one). My next biggest problem once I have verified it works on the breadboard will be working out the layout for the "final" board. Oh well I'll get there eventually!!

Tony.
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Old 31st January 2012, 11:29 AM   #16
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Quote:
Originally Posted by wintermute View Post
I decided to have a crack at learning spice because I lacked the confidence to build something I had dreamt up myself (actually I think the first thing I simulated was SY's acheron which was the inspiration for this circuit!!)
Found my math mistakes while you were at it? I'll fix them one of these days... since going digital, my motivation for updating that article has dropped to near-zero.
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Old 31st January 2012, 07:08 PM   #17
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hehehe your math mistakes actually resulted in a much better understanding for me because I looked that much harder to try and work out what the hell was going on!!

Tony.
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Old 2nd February 2012, 11:51 AM   #18
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So the lowpass part of the circuit has been realised this evening. I have done the original circuit without the 220pf cap and the 40K resistors just to see how it went compared to the sim. Well it actually peters out at about 15K rather than 20K and the phase (according to holm impulse) is all over the place!! but I'm not sure that it is showing what I think it is...

I'll have to get some 220pf caps so I can try that mod and see what the effect is. I did try to get a 192Khz measurement with my onboard sound, but it was way too noisy and basically useless.

anyway here it is.

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File Type: png synergy_low_and_high.png (55.9 KB, 297 views)
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Old 3rd February 2012, 11:21 AM   #19
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ok so I worked out what was going on with the phase. The impusle was negative and detect time zero was set to highest positive peak

I've done some measurements tonight at 192Khz with the onboard sound. There appears to be some resonance at around 58Khz and 75Khz.

In the sim I had to use 40K resistors in the lp circuit to get stability anything less than that and I had problems, so I decided to try with 33K and 22K.

The graph below has had all curves smoothed 1/48th octave as they were a tad hairy. Blue is with 40K resistors, red with 33K resistors and orange with 22K resistors.

Its obvious that 22K resistors are an issue I think I will stick with the 40K's and hope that the little peaks at 58 and 75k don't give my amp any problems. since they are at around -55db and it will be driving a 10" at 200Hz crossover frequency hopefully not

I've also included a picture of the current implementation which may have a lot to do with the spikes. There has been zero consideration given to grounding, and component leads have not been trimmed. basically a rats nest of poorly laid out components.

I may do a listening test to the one channel tomorrow if I get a chance.

Tony.
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File Type: png 22_33_40K.png (30.5 KB, 234 views)
File Type: png low_high_192khz.png (48.7 KB, 223 views)
File Type: jpg DSC_0473_s.jpg (254.0 KB, 100 views)
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Old 3rd February 2012, 08:55 PM   #20
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Ah Tony,
Good to see you've got back to this project and that the 'gyrators' are showing promise.

If you are still using the cct as per post #11, could you try a couple of things - first, can you get rid of that extra vol pot in the input to the high pass filter and replace it with a nominal gate stopper (ie 22R) and same for that 12k resistor on low pass input?

For vol control, could you just add a pot on output of low pass followed with another buffer?

Also, could you try (in the sim) replacing the main input vol pot of 25kR with a rather low one of 7kR as this is about the load of a Lightspeed vol control, I think.

my Offer of a pcb 'pre-design' is still there - trying to learn the new pcb program "Sprint" to replace my prehistoric Protel (but still works just fine!)

Playing with Stu's 'Acheron' but with Aikido buffers instead of the 'Hereticals'- looks promising ...
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