SymAsym - "The Sequel", AAK's PCB Builders Thread - Page 2 - diyAudio
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Old 20th January 2010, 01:22 PM   #11
ilimzn is offline ilimzn  Croatia
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You might try a setup with a CCS. Basically, connect G to ground, D to the the same positive voltage you find in your application, and S trough a CCS to a negative supply (-10V or so). The CCS can even be a simple JFET CCS set to half the tail current in the input differential. Once you conenct your DUT, measure the voltage between G and S with a high impedance voltmeter (regular DVM will do fine). Match for the same voltage. This method is quite accurate but requires a dual power supply (+V, -V) - you can also use it to pair small MOSFETs, just insert a 1k or so resistor in the gate line to prevent oscillations.
Andrew's method is more accurate for finding pairs because it takes thermal coupling into account, though.
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Old 20th January 2010, 02:02 PM   #12
AndrewT is offline AndrewT  Scotland
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It's not just the thermal coupling.
Testing/measuring as a pair allows Vgs to be equal, Vds to be equal (if Vdrop is equal), Id to be equal, Pq to be equal, Tj to be equal, Tc to be equal. All the things that are very difficult to control accurately are held equal. It's a real comparison of DUT and REF. When you find a pair, you know you have a pair.

Trying to find pairs or sets by measuring each individually leaves all the difficult to fix constants at haphazard values. There is no way to guarantee that the conditions for each single DUT are the same as any previous DUT, that day, that week, last year.
I have noticed that the device pair of DUT and REF can change if a door is opened and changes to cooling effect on one side that is different from the other side. No draughts is an important part of obtaining the equality during the testing.
regards Andrew T.
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Old 20th January 2010, 02:09 PM   #13
ilimzn is offline ilimzn  Croatia
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Default Second stage simplified

Here is an idea for a simple thermally balanced (all transistors in the diffamp and current mirror dissipate virtually equal power) second stage for the SymAsym:
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File Type: gif Thermally_balanced.gif (5.2 KB, 960 views)
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Old 20th January 2010, 02:22 PM   #14
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I used AAK's method before, no audible problems. Is it a matter of principle to use totally matched semiconductors, something between the ears and not outside the ears ?
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Old 20th January 2010, 03:04 PM   #15
AAK is offline AAK  United States
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I've been using the circuit in post #10 for years and works great. I found after doing some research that John Curl recommends a similar circuit for matching Jfets except he grounds the gate and source. I added a source resistor in order to drop Idss so that the jfets can be matched near their actual operating current.

I've attached some pics of the new assembled board for reference.

Note the direction of Jfets Q1&Q2 is different that how it's labeled on the board. You'll find more information in my assembly instructions document found in Post #1.

Attached Images
File Type: jpg CIMG1228.JPG (302.3 KB, 986 views)
File Type: jpg CIMG1232.JPG (305.4 KB, 945 views)
File Type: jpg CIMG1234.JPG (272.9 KB, 881 views)
Love when you can, Cry when you have to, be who you must, that's part of the plan.

Last edited by AAK; 20th January 2010 at 03:22 PM.
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Old 20th January 2010, 06:57 PM   #16
lgreen is offline lgreen  United States
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Default Ordering Parts

The following parts are not availabe at Mouser (they are backordered), so lets talk substitutes.
I have given part numbers that I believe would be good substitutes, would these be ok?


606-4300F1LC (L1, Red LED for + PS indicator)
substitute in
606-4300F5LC (use green instead of red)

651-1904383 (TB1, terminal block for Audio in)
substitute in
651-1872392 (2 pos labeled term block slightly higher profile)

71-RN60D-F-681 (R23, R24 681 ohms, 1/4 W, 1%)
substitute in
71-CCF55-681 (CCF series 681 ohms 1/4W, 1%)

505-MKS2-4.7/50/10 (C25, optional, Wima 4.7uF 50V metal poly cap)
substitute in
871-B32522C475J (Epcos poly film cap 4.7uf 63 V, not sure it fits).

Non Matching Parts.

Is part 757-2SC4793 really 757-2SC4793(F,M) ?
Is part 757-2SA1837 really 757-2SA1837(F,M) ?

EZ Buy / BOM Import

I have created the following EZ Order text using Al's part list.
Just click "EZ Buy" on the mouser page (at the top) and then the icon for the "BOM Import Tool." Paste the text into the window and the items will get added to your cart; do it 3 times for the 3 parts or all at once if you delete the "part" sections. Then you can remove the parts you already have/ don't want. The 3 sections correspond to the 3 pages of the .pdf parts list so you can follow along (EZ Buy does not let you put in user designators).

Part I

660-CF1/2C123J |4
594-5073NW1R000J |4
283-10K-RC |4
71-CW2B-10 |2
71-LVR3-0.20 |8
71-RN60D-F-22.1 |4
71-RN60D-F-10 |8
71-RN60D-F-100 |4
71-RN60D-F-150 |2
71-RN60D-F-150 |4
71-RN60D-F-261 |2
71-RN60D-F-10K |8
71-RN60D-F-681 |4
71-RN60D-F-22.1K |4
71-RN60D-F-2.0K |4
71-RN60D-F-499 |4
71-RN60D-F-1.0K |6
660-MF1/2CL68R1F |2
652-3296W-1-501LF |2
71-RN60D-F-221 |4
71-RN60D-F-10 |2
MKS2-.1/63/5 |18
661-EKMH630VNN472MQ |8
505-M10.1/250/10 |4
5982-10-500V100 |4
5982-10-500V220 |4
5982-5-300V10 |2
647-UVZ1J101MPD |8
647-UPW1E471MPD |2
505-MKS2-4.7/50/10 |2

Part II

863-MUR1520G |8
863-1N4007G |8
78-1N5245B |4
78-1N5235B |2
757-2SK170BLF |4
863-2N5551G |14
863-2N5401RLRAG |14
863-BC556BG |2
863-BC546BG |2
512-BD13916S |2
757-2SC4793(F,M) |2
757-2SA1837(F,M) |2
757-2SC5200-O(Q) |4
757-2SA1943-O(Q) |4
606-4300F1LC |2
606-4300F7LC |2
606-4300F5LC |6
534-1213 |10
651-1904383 |2
534-3517 |8

Part III

576-0217005.HXP |4
526-NTETP0006 |1
526-TP0010 |2
532-577002B00 |4
571-6409151 |10
534-29314 |8
561-K6.375 |8

No excuse for not getting moving on this now, right?
Retired from DIY (2010) but still lurking now and again. My DIY audio projects-

Last edited by lgreen; 20th January 2010 at 07:03 PM.
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Old 20th January 2010, 10:42 PM   #17
sng001 is offline sng001  Australia
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Default Matching JFET

Thanks for the input.

AAK's method is straight forward, I will try that first to pick two pairs out of my batch of 10 FETs. If I got time, I will try other methods; I need to study & digest AndrewT's method.

- Stanley
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Old 20th January 2010, 10:56 PM   #18
sng001 is offline sng001  Australia
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Default Mouser part no. for C7/8

Originally Posted by AAK View Post
Hi All,

For the BOM I added Mouser part number 505-M10.1/250/10 which is a MKP10which according to the data sheets is better suited for audio equipment.


I did an quick search on the mouser web by entering "505-M10.1/250/10" into the search box, it return two parts:

505-M10.1/250/10 - Polypropylene Capacitors 250V .1uF 10% 6X12X13 - L/S 10mm (stock available = 4121)
505-M10.1/250/10P15 - Polypropylene Capacitors 250V .1uF 10% 5X11X18 L/S 15MM (stock available = 3)

The detailed specification for 505-M10.1/250/10 says that it has got lead spacing of 15mm, which contradict with its description, I suggest to call Mouser to clarify before ordering.

There is a Wima MKP10 capacitor of higher voltage rating, its lead spacing is also 15mm: 505-M10.1/400/5

- Stanley
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Old 20th January 2010, 11:18 PM   #19
AAK is offline AAK  United States
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Hi Stanley,

The 505-M10.1/400/5 looks like a better choice. I agree the lead spacing of 15mm for the 505-M10.1/250/10 looks wrong.

I'll update the BOM.


Love when you can, Cry when you have to, be who you must, that's part of the plan.
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Old 21st January 2010, 11:21 AM   #20
AndrewT is offline AndrewT  Scotland
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all is not lost.
C7 C8 are 15mm pin pitch. If you cannot source this size, a 10mm can be made to fit the PCB.
Look at the photos of the back and front of the PCB.
An extra hole can be drilled in the power trace 5mm from the existing hole.
Then a 10mm pin pitch cap can be made to fit.
regards Andrew T.
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