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Old 9th December 2011, 07:36 AM   #11
zdr is offline zdr  Belgium
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It would be great if someone could post an ultimate output matching tutorial for dummies (like me), or point to one on the net?

I have 20 of each, and just by measuring hfe with dmm, they seem to be quite off. NPN from 66-105, PNP varying between 125-139.
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Old 9th December 2011, 09:10 AM   #12
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Hello Christoph,

I did some rough hFE matching yesterday, and the results looked promising.

In one of his posts, Mihai writes that the output transistors should be matched for both hFE and VBe to stay in Class A operation
as long as possible without overdriving (correct word?) the bias.

Maybe I will solder a small circuit this weekend and measure the VBe of my transistors.

METAL and I exchanged our views on Mihai's PCB yesterday.
Find enclosed his current layout.

Click the image to open in full size.

http://www.abload.de/img/lookedthroughhfvn7.png

(Andrew: I will try and use the forum's attach function in future!)

There are several issues METAL and I found regarding Mihai's layout:

1) Either the reworked layout will include a "mature" PSU on the right side (allowing for caps with 35mm diameter for example,
integrating a "C - L/R - C - filter", why not the rectifier diodes as well?) or it will be completely removed from the PCB
2) The reworked PCB will include EMI protection (sorry for the typo in the image)
3) The current layout does not give sufficient space to the Zobel capacitor
4) There are 4 capacitors near the pre-drivers that do not fit in this position.
In some of Mihai's pictures of his PCB I see that he is using big F WIMA caps on this position, but the EAGLE .brd-file gives only room
for 10 x 3mm MKS3 caps with a grid of 7.62mm
(By the way: can anybody send me the current schematics with values? I found a picture of the schematics in Mihai's original thread,
but this picture does not correspond to the EAGLE schematics file!)
5) The frondend (on the left) shows a lot of unused space. The PCB can therefore made be smaller.
6) I myself do not like resistors being mounted vertically! I will - for example - try and use MPC74 - typ emitter resistors!

Mihai's PCB layout is quite good, for sure, but it can be improved!

METAL and I will now check how easily these modifications can be made and if they will introduce any stability issues.

I am a "straight-forward, passionsless" DIYer - and: I am not a millionaire.

I like to do a layout, solder an AMP and get it running.

I will not buy 25 NJL transistors of each NPN- and PNP type, just to get 6 hFE- and VBe - matched pairs.

METAL and I will do our best.

Best regards - Rudi_Ratlos
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Old 9th December 2011, 09:46 AM   #13
koldby is offline koldby  Denmark
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I have a complete amplifier, with the original PCBs from Mihai in a nice Chassis and 24 extra output NJL pairs for sale .
PM me if interested.
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Old 9th December 2011, 12:16 PM   #14
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Quote:
Originally Posted by Rudi_Ratlos View Post
1) or it will be completely removed from the PCB
5) The frontend (on the left) shows a lot of unused space. The PCB can therefore made be smaller.
My 0.02 EUR: The presence of certain amount of large C near the power drivers helps to cope with instantaneous peak signal loads better.

Existing layout allows to use a heat sink with large flat back surface and the power drivers parallel to PCB, PCB itself parallel to heat sink. I wanted to build two mono blocks with power transistors perpendicular to PCB squeezed between two smaller heat sinks in a construction, resembling medium size UPS (dimensions, not functionality :-) ). However I will not obstruct the initiators of this GB in any way ;-)
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Old 9th December 2011, 03:30 PM   #15
gk7 is offline gk7
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Default 2SK170 matched

It would be nice if the layout would allow to use a LSK389
Linear Integrated Systems - Press Release
instead of two matched 2SK170s. The LSK389 is available
and factory matched. (And you can be sure not to buy
counterfeits.)
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Old 9th December 2011, 04:20 PM   #16
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Rudi_Ratlos View Post
Maybe (I hope so) some of the HighEnders of this forum (for example Andrew T., whose recommendations I appreciate very much) or somebody,
who already built this outstanding AMP, or even Mihai himself, will give METAL and me some hints, how to even improve the layout
Two suggestions:

Enlarge the access holes for mounting Q9,13,15 (original schema, 2SA/SC parts). Make them large enough to pass the mounting screw and grommet. Trying to place the screw and grommet/washer from underneath the board is difficult.

Size the front end supply and align the mounting holes so that it can be stacked over the front end with stand offs. Would even be nice to have the power connection between front end and front end supply done with aligned connector.

Sheldon
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Old 9th December 2011, 06:42 PM   #17
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Gentlemen,

I took an hour or so to rework the PSU on the right side of the PCB (the PSU will now accommodate power-caps with a diameter of 35mm)
and the positive output rail.

Click the image to open in full size.

http://www.abload.de/img/top_layerkj3xk.png

Compare it to the negative power rail output (which is unchanged).

Do you see the differences, how "easy-going and smooth" everything can be, if you just spend some time
and draw an EAGLE-package for the famous FUTABA MPC74 emitter-resistors, redraw several EAGLE polygons, ...?

It causes me nightmares to see some resistors being mounted vertically, some of them being mounted horizontally,
some of them being mounted with an angle of 315 (or so) degrees, ...

In my eyes: everything shall be done straight-forward.

But - also in my eyes - the layout contributes only a small amount of the success to this project.

What does really concern me is: how important is HFe- and VBe - matching of the NJL-transistors for this project?

I will now solder a small circuit, match my NPN NJL* transistors and will give you the results tomorrow.

Best regards - Rudi_Ratlos
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Old 9th December 2011, 08:39 PM   #18
Luke is offline Luke  New Zealand
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Hi Rudi,

if you look at Mihai's board,

RMI-FC100, a single stage audio power amplifier

it appears that the weight of the board is suspended by the transistors mounted on the heatsink. I imagine long term this may cause reliability problems. Finding room for 6 mounting holes on each board may be beneficial.
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Old 9th December 2011, 08:49 PM   #19
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I think we can make the PCB larger and mount the screws on top of the PCB, to secure both, the PCB and transistors to heatsink, but this might increase the PCB cost as well.
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Old 9th December 2011, 09:05 PM   #20
AndrewT is offline AndrewT  Scotland
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The PCB already has plenty of empty spaces to drill mounting holes. Don't make it any bigger.
In my opinion one should be striving to make it smaller, particularly much shorter.

The transistors mounted on the flat back heatsink provide 42 legs for mounting the PCB to the sink.
I cannot see where reliability comes into it. Have you tried moving the PCB relative to the sink?
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