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Old 29th December 2012, 07:43 PM   #1341
davada is offline davada  Canada
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Quote:
Originally Posted by dimitri View Post
David,
ExpressPCB will accept proprietary output format from their free soft, the board size 3.8 x 2.5 inches only,
33each - normal gerber files from your CAD, e.g. FreePCB; with 33each you are limited with max size 60 sq. inches - but this is pretty large PCB.
My board is quite a bit larger than this. I'll take a look at their other offerings.
I don't what to go to surface mount.
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Old 29th December 2012, 07:46 PM   #1342
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
I like expressPCB a lot. Its not real cheap but the boards are good quality and its easy to share the source files. And plenty quick. They arrive just as you thought of the other changes you want to make. I would not use them for leading edge stuff (you can't anyway. . .).

However Canada shipping may make it lots less attractive.
I would do a shipping hop for this. Send to a friend in US and regular post to Canada.

For a final board I don't mind going the expense. But one change or mistake really bothers me.

A home brew board... hack away.

My bother in law just gave me an HP 2840 he no longer uses at his business. This thing is huge and takes up a lot of floor space.
I'll use this for the transparencies. My Samsung laser just wasn't cutting it.
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Last edited by davada; 29th December 2012 at 07:50 PM.
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Old 29th December 2012, 08:03 PM   #1343
davada is offline davada  Canada
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Default A lamp multplier for SVO

Here is the basic multiplier design.

The multiplier is designed around the lamps resistance curve because we have little control of this. R4 is chosen to center the gains and inverting crossover about the lamps curve.
R6 and R7 set the non inverting attenuation and R4 the inverting attenuation.
The multiplier operates below unity gain. Because the resistance values are low we can take advantage of using one of the ultra low noise op amps available. I chose the OPA1612 because of it's low noise current spec and outstanding distortion spec.

One obvious problem with injecting a DC control voltage or current into a lamp is the introduction of voltage offset appearing at the amplifier's output. Simply adding a blocking capacitor led to phase shift problems within the multiplier which interfered with the oscillator loop. Using two lamps as shown solves this problem. However it is near impossible to match two lamps well enough to keep the offset to a reasonable amount.
Because of this a DC servo loop is added to take care of the last remaining bit of imbalance. The servo forces the current in each lamp to balance, near eliminating
the offset at the multiplier's output. The lamp drivers are a balanced configuration and offer cancellation of ripple and other signals riding on the DC controls signal.
This cancellation takes place at the summing node of the lamps and depends on the current balance of the lamps. The lamp drivers see signal at their outputs through the lamps. Both noise and distortion can be reflected back through the lamps into the multiplier.
For this reason low noise low distortion op amps are chosen.

I have tried other lamps for stabilizing an SVO but the 7220 lamp seems to work best.

The multiplier has been tested with an SVO from 10Hz to 100KHz with a manually set DC voltage at the lamp driver input. I am still working on a suitable ACG for this multiplier. And I need to isolate the performance of the multiplier. I have still yet to determine the TC of the lamps. This is important to know to set an appropriate TC for an AGC. As far as I can tell the AGC TC will have to be ten times longer than the lamp TC.

The distortion performance of this multiplier with an SVO is really quite impressive and well worth the extra complexity of the driver circuitry.

There is more refinement for this multiplier that I haven't included here which has to do with linearizing, (not sure that's a word) the multiplier gain constant. But this is a matter of an additional stage and will be addressed separately.

I've included a cheesy LTspice schematic and the plot of the lamp's curve.
The 7220 has a cold resistance of about 70 ohms.


Cheers,
Attached Images
File Type: png Lamp Mult sch.PNG (53.0 KB, 134 views)
File Type: png 7220 curve.PNG (66.8 KB, 134 views)
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Old 29th December 2012, 08:06 PM   #1344
davada is offline davada  Canada
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Default Lamp curves

Here is a zipped file of the xls.

I'd also like to point out that lamps are not the only element that can be used with this multiplier configuration.
thermistors, RDT's, possibly thermocouples or any other heating element like the LT IC mentioned etc. can be used.

One possibility is cancellation of 3rd H of a thermistor?
Signal feedback is possible through the driver amplifiers.

Have fun.

Cheers,
Attached Files
File Type: zip Lamp 7220.zip (4.2 KB, 19 views)
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Last edited by davada; 29th December 2012 at 08:19 PM.
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Old 29th December 2012, 08:42 PM   #1345
davada is offline davada  Canada
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Default SVO with multiplier perfromance

Here is a screen shot I took back in March of this year.

Dick Moore loaned me his EMU0204 and Twin T notch filter to try out while he was on holiday for a couple of months. Okay he's retired so maybe not holiday.


A screen shot of the performance of the SVO at 6Vrms output, 1.072KHz and the multipler. Most of what you see in the spectra is power supply harmonics and some EMU junk. The input to the Twin T is attenuated to 1Vrms (0dBV) and the ARTA is calibrated to 0dBV 1Vrms, The output of the multiplier is at about 64mVrms.


Cheers,
Attached Images
File Type: png SVO Mult.png (52.9 KB, 123 views)
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Old 29th December 2012, 09:29 PM   #1346
davada is offline davada  Canada
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Default AGC for this multiplier.

I still need an AGC for this multiplier. I't has to have a very clean output and as ripple free as possible or what you see in the last post won't be possible.

A THSH would do it but it seems a waist to have a high speed AGC just to have to slow it down again to make it work. This oscillator has no chance of quick settling using lamps so a fast AGC is not needed or will even work. The oscillator settles in less than a second at most frequencies just on the lamp stabilization alone. At higher frequencies it's a bit longer. At low frequencies it's almost instantaneous measured by patients.

What I need is more of an auto level trim but standard servo circuitry tends to fight with the lamps time constant and slows the settling down.

Suggestions? Idea?

Cheers,
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Old 29th December 2012, 09:38 PM   #1347
1audio is offline 1audio  United States
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Its a great solution. Could you take the servo from the main output and cancel the offset through the system?

The time constant of the thermal device will limit the low frequency performance and harmonic generation. Something with a big filament would be better (heavier) but two full size light bulbs may be a little weird in a box. Thermocouples won't work since they generate DC from the heat instead of a resistance change. I looked at Thermistors which could be valid http://www.digikey.com/Web%20Export/...f?redirected=1 however the note indicates a voltage sensitivity which would be all wrong for this app. This may be the most obscure possible solution for stabilizing http://www.amperite.com/assets/Documents/Ballasts.pdf and they are still made. It looks like they could work pretty well. Now you can have tubes in a all solid state circuit. . .
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Old 29th December 2012, 10:11 PM   #1348
benb is offline benb  United States
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Quote:
Originally Posted by dimitri View Post
Interestingly enough, but I prefer to do PCB for every project. 33each.com or miniboard service from ExpressPCB ($17 each)
You can't really get $17 each unless what you really want is three copies of the same board 3.8" x 2.5" - the miniboard service is $51 for three boards, the "best" deal they offer for a board anywhere near that size. They've had the same services at about the same pricing for over 12 years now ... here's November 2000 website showing the miniboard service $59 postpaid (to USA only) verses current pricing of $51 + postage: How much ExpressPCB costs - PC board layout editor for Windows. They must be making money at what they're doing. With so many other services available I would have thought they would have dropped their prices.
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Old 29th December 2012, 10:31 PM   #1349
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Its a great solution. Could you take the servo from the main output and cancel the offset through the system?

The time constant of the thermal device will limit the low frequency performance and harmonic generation. Something with a big filament would be better (heavier) but two full size light bulbs may be a little weird in a box. Thermocouples won't work since they generate DC from the heat instead of a resistance change. I looked at Thermistors which could be valid http://www.digikey.com/Web%20Export/...f?redirected=1 however the note indicates a voltage sensitivity which would be all wrong for this app. This may be the most obscure possible solution for stabilizing http://www.amperite.com/assets/Documents/Ballasts.pdf and they are still made. It looks like they could work pretty well. Now you can have tubes in a all solid state circuit. . .
Hi Demian,

"Its a great solution. Could you take the servo from the main output and cancel the offset through the system?"

I don't follow what you mean by this. Pictures are worth a thousand words...

What is the main output and what is the system?

The servo can be returned to different nodes. It doesn't have to be wired through the drivers. An alternative is to return the servo to the non inverting node of the multiplier amplifier. This saves one inverting amplifier to satisfy the phase requirement.

The low frequency problem you mention are not nearly as significant as with traditional lamp
stabilization. The signals in the lamps are tiny by comparison. In fact because of the DC bias on the lamps there is no AC in the lamps at all. There is only a changing DC current and in phase but offset ac voltage across the lamps.

It's an interesting arrangement that lessens the usual problems found with lamp stabilization.

This is a better way to return the servo. The servo decoupling resistor has to be included with and in parallel with R7.

If you mean to include the servo in the AGC there would be no way to balance the lamps as they would both be seeing the same error signal.

Cheers,
Attached Images
File Type: png Servo.PNG (51.5 KB, 113 views)
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Last edited by davada; 29th December 2012 at 10:40 PM.
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Old 29th December 2012, 10:32 PM   #1350
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
Here is the basic multiplier design.

I've included a cheesy LTspice schematic and the plot of the lamp's curve.
The 7220 has a cold resistance of about 70 ohms.


Cheers,
This circuit will fit easily onto PCExpress size. Lamps off board, of course. Could use a quad IC as well... at least for proof of concept. Such as OPA1644. -RNM

Last edited by RNMarsh; 29th December 2012 at 10:37 PM.
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