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Old 27th December 2016, 07:16 AM   #1011
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Richard Murdey
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No, JFETs are out of the picture as the circuit needs the precise emitter-base pn diode reference voltage of the BJTs to correctly set and stabilize the bias currents throughout the rest of the circuit.

****

Working/prototype BOM attached. (Unity 1.0e)
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File Type: zip pcb-unity-10e-bom.zip (150.2 KB, 14 views)
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Old 27th December 2016, 10:58 PM   #1012
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Please see the blog post for the most recent BOM. There are a couple of edits from the version posted above.

The boards are on order. I should have them mid-January.
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Old 28th December 2016, 04:26 AM   #1013
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or...

we could just go back to a simple drop-in replacement for the older boards, with the regulator built in.

I like this, since it means I can listen to the new version with minimal fuss.
Attached Images
File Type: png pcb-sapphire-40a-brd.png (19.9 KB, 275 views)
File Type: png pcb-sapphire-40a-sch.png (21.3 KB, 220 views)
File Type: png Sapphire-40a.png (197.5 KB, 218 views)
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Old 28th December 2016, 05:40 AM   #1014
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Quote:
Originally Posted by rjm View Post
or...

... a simple drop-in replacement for the older boards, with the regulator built in.

I like this, since it means I can listen to the new version with minimal fuss.
I like the sound of that, too!
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Old 28th December 2016, 09:59 AM   #1015
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Glad to hear that because I've gone ahead with this version! (4.0b)

Here's all the info. (For download: Gerbers, LTSpice, and BOM)
Attached Images
File Type: png pcb-sapphire-40b-brd.png (20.2 KB, 204 views)
File Type: png pcb-sapphire-40b-sch.png (20.6 KB, 203 views)
File Type: png Sapphire-40b.png (197.6 KB, 65 views)
Attached Files
File Type: zip sapphire-40b.zip (111.8 KB, 18 views)
File Type: asc Sapphire 4.asc (5.7 KB, 14 views)
File Type: zip pcb-sapphire-40b-bom.zip (155.3 KB, 15 views)
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Last edited by rjm; 28th December 2016 at 10:11 AM.
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Old 29th December 2016, 12:04 AM   #1016
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b1 BOM fixed several part numbering errors, added some notes and alternates
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File Type: zip pcb-sapphire-40b1-bom.zip (155.8 KB, 18 views)
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Old 10th January 2017, 05:11 AM   #1017
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Sapphire 4.0b boards are back from fabrication. Also Unity (H) and the various voltage regulator boards.

2 oz copper, 2-layer, black mask, 80x100 mm.
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Old 12th January 2017, 06:48 AM   #1018
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BOM revision 4.0b2

I made some adjustments to the resistance values of the output stage to lower the output impedance a bit. It's about 3 ohms now, vs 7 ohms for the b1 configuration.

R11,12 465 ohms -> 221 ohms
R17-20 10 ohms -> 4.7 ohms

R1 is made 1k, though this is a rather arbitrary value.

R2, R3 and C2 - the gain setting components - have been adjusted slightly. Please refer to the table in the BOM Gain Calculator page. The defaults are now 2k2, 22k, and 10pF respectively. Since this is a current feedback circuit, these components are somewhat of a pain to finesse, but the simulation can be helpful to understand the relationships. Reducing R2 cranks up the open loop gain. This is required to obtain useful bandwidth at high closed loop gain (i.e. circuit gain), but is relatively unstable so a higher value of C2 is needed for compensation. For low closed loop gain R2 can be relaxed since there is plenty of available bandwidth. Less compensation is required here. The closed loop gain is approximately R3/R2 though it is slightly more complex than that in reality especially for high values of R3.
Attached Images
File Type: png sapphire 4b2.png (123.1 KB, 78 views)
File Type: png pcb-sapphire-40b2-sch.png (20.7 KB, 52 views)
Attached Files
File Type: asc Sapphire 4b2.asc (5.8 KB, 6 views)
File Type: zip pcb-sapphire-40b2-bom.zip (155.9 KB, 7 views)
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Last edited by rjm; 12th January 2017 at 07:02 AM.
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Old 12th January 2017, 12:40 PM   #1019
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Did the smoke / operating point test.

One board is perfect, the other had -2 V offset that fell out of the range of the trimmer R6. I found the problem was the Zener diodes, which were not well matched so the positive and negative rails were 0.25 V different. If R6 is 1k that only gives 0.1 V of adjustment, not enough to compensate.

This can be solved by matching the Zeners to closer tolerance, but the basic failure here is R6 was too small a value. Increasing R6 to 5k allows about 0.5 V adjustment, which should cover just about any eventuality. You can use 10k if you want.

Since I don't have any 5k trimmers on hand, I'll match some Zeners tomorrow and replace the oddball.

In point of fact I matched the transistors well enough that the trimmer is hardly needed, removing it would actually improve the situation in this instance since the offset is introduced into the input by the power supply through the trimmer itself.
Attached Images
File Type: jpg IMGP5330s.jpg (270.1 KB, 80 views)
Attached Files
File Type: zip pcb-sapphire-40b3-bom.zip (156.1 KB, 16 views)
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Last edited by rjm; 12th January 2017 at 12:44 PM.
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Old 13th January 2017, 05:08 AM   #1020
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Bench notes, DC operation:

My roll of "12V" Zeners seem to come in two flavors of breakdown voltage. Of seven tested, three were 11.5~11.6 V, while four were 11.2~11.3 V. Odd but that's what I have to work with.

Using one of each type for D1,2 means |(V+) - (V-)|= 300 mV and the center of the trim pot moves 150 mV away from zero. For a 1k pot as R6, the adjustment range is 100<->200 mV i.e. insufficient as zero is out of the trim range. For a 5k pot the range is -100<->400 mV, which is ok.

To fix my board though all I had to do was swap out the 11.6 V D2 Zener for an 11.3 V one to match D1. Now 0 mV it is in range of the 1k trim. In my case it was easier to match Zeners than order a new trim pot, but as I mentioned last post the BOM is updated to increase R6 to 5k.

The output offset of both boards (configured for 26 dB gain with R2 = 475R and R3 = 22k1) after trimming is +/-50 mV cold and +/-10 mV warmed up. The trim setting is so close to zero though it is unclear if the offset adjustment is even needed. If 50-100 mV output offset voltage is acceptable it can probably be left out in favor of a simple 47k resistor to ground, but ultimately its probably safer to keep the trim option.

The bias current is about 40 mA, and the heatsinks barely get warm to the touch, so there is plenty of thermal headroom if higher bias currents are desired. This is accomplished by increasing R11,12 from 221 ohms to 332 ohms or even 475 ohms.
Attached Images
File Type: png pcb-sapphire-40b3-sch.png (20.7 KB, 82 views)
File Type: jpg IMGP5281s.jpg (357.1 KB, 88 views)
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Last edited by rjm; 13th January 2017 at 05:18 AM.
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