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Old 11th April 2012, 08:11 PM   #51
EUVL is offline EUVL  Europe
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All power connection holes are meant for standard M4 standoffs.

Patrick
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Old 14th April 2012, 06:18 PM   #52
lhquam is offline lhquam  United States
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How is one supposed to unsolder the pots and install alternate drain resistors without removing the board (and mosfets) from the heatsink?


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Originally Posted by fitzfish View Post
My pleasure Patrick.

Hi Rick,

The drain resistor values are determined by the operational current of the input JFETs and the Vgs of the output FETs at the chosen output bias current point. If you look here you will find a chart that gives an estimated value of the “final” drain resistors’ value. For any front end operating bias >= 5mA both the 1.5K and 1K resistors need to be installed along with the pot. They are all paralleled. If running less front end current you can install one or the other along with the pot to get it where you need. With the Idss parts suggested and the output device’s typical Vgs the pot is near its highest resistance setting.

The pot actually covers the PCB pads and component area for the “B” resistors so they are mutually exclusive. I’m leaving the pots in for now. Once biased up and balanced I suggest you leave the pots in for at least a while to burn everything in and allow it to stabilize. Check it several times to make sure you are happy with it before even considering swapping them out. If you want to change them do so one at a time to avoid confusion. Remove them carefully so that you don’t thermally damage the element (permanently change the resistance) or move the trimmer. You need to let them cool off (from soldering heat) after removal and measure them. Find close to exactly the same value fixed resistor to install in its place. Check it carefully after changing them and let it warm back up completely to check again once the bias settles.

Hope it helps answer the question. More details forthcoming in the bias build notes.
Dave
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Old 14th April 2012, 07:11 PM   #53
EUVL is offline EUVL  Europe
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For my own proto boards, I removed the PCB from the heatsink to swap the trimmers for fixed resistors.
Easy enough to do without any problems.

But if I were to do this now, I would solder DIL sockets like these at the 3 holes for the trimmers.
You can then just plug in the trimmers and set current as before.
After you are satisfied with the setting, just unplug the trimmers, measure their trimmed resistances, and then solder in fixed resistors.

Of course you can also use these for the fixed resistor holes as well for easy changeover.


Patrick
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Old 14th April 2012, 07:22 PM   #54
lhquam is offline lhquam  United States
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That looks like a good approach.

What is your reason for replacing the pots with fixed resistors? Is there a reliability/stability issue with the pots?

Quote:
Originally Posted by EUVL View Post
For my own proto boards, I removed the PCB from the heatsink to swap the trimmers for fixed resistors.
Easy enough to do without any problems.

But if I were to do this now, I would solder DIL sockets like these at the 3 holes for the trimmers.
You can then just plug in the trimmers and set current as before.
After you are satisfied with the setting, just unplug the trimmers, measure their trimmed resistances, and then solder in fixed resistors.

Of course you can also use these for the fixed resistor holes as well for easy changeover.


Patrick
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Old 14th April 2012, 07:24 PM   #55
EUVL is offline EUVL  Europe
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> Is there a reliability/stability issue with the pots?

The wiper contact is the weak link.


Patrick
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Old 14th April 2012, 08:43 PM   #56
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Patrick is doing a great job of answering questions hours or minutes before I see them. Thanks Patrick!

> How are power and output connections made to the board?

There a few different ways to do this. As Patrick suggested he uses M4 brass hardware. You can use an off the shelf brass screw and nut, or even a standoff like the one required for the center ground connection between the input board and main board. The SAE thread size that the hole supports is 8-32. 6-32 will work as well.

I used two different connections for these when I was biasing up the amp boards vs. the final wiring. For biasing I used binding posts secured with a nut on back in each and made an adapter for the center ground standoff (see picture below). A nut on this center ground connection is just as useful.
I believe Patrick's M4 hardware is just as effective if you use the brass screws at each of the +/- supplies and speaker connections. These can then be used for the final assembly as well.


For the final assembly here I soldered the wired directly into the holes for +/- supplies and speakers, and used fork lugs to the center ground nut for each supply. This is fine to do once everything is biased and stable, but not easy to remove if needed while setting up.

> The hole diameters appear to be about right for a #6 screw

The finished hole size on the board measures 0.162” , or 4.1mm. 4mm or 8-32 hardware will work.

> but there isn't enough space for a nut.

There is, though it is tight. For the PCB I was asked to duplicate Patrick’s layout exactly wherever possible. He used threaded hardware for each. The annular ring in both his prototype and the final PCBs (copper pad on the top and bottom) are reasonable for the metric hardware specified.

I made my copper nuts out of 3/8 inch hex stock. It is tight and the points do extend off of the copper pad on the PCB but careful assembly ensures no shorts. The more common 5/16 inch hex size for 8-32 nuts will fit better.

> Are metal standoffs or spacers used here?

The middle ground conductor requires a standoff. The +/- supplies and speaker connections do not.

I am working to polish off the bias article as we speak…

Dave



- The connection setup I used for initial bias and testing. This is just one way to do it...
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Old 15th April 2012, 05:30 AM   #57
EUVL is offline EUVL  Europe
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I do not use standard M4 brass nuts.
I use M4x5mm through-threaded (female) standoff instead as nut.
They are smaller in size, and will fit the solder pad easily.


Patrick
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Old 15th April 2012, 06:54 AM   #58
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Thanks for the additional information Patrick.

You originally posted it here. Pictures tell more of the story. See here and here. This stuff is spread over way too many threads and at least three years... It is almost impossible to find some of these details even though I thought I knew where to look . Hopefully the links will help everyone.

The brass M4 nuts I have are M4-0.7, 5mm tall and made for a 7mm wrench. They work with the 6.7mm pads on the amplifier board. Perhaps they were originally made as standoffs... Similar 8-32 hardware with 1/4" and 5/16" hex size are available as well.

Dave
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Old 16th April 2012, 01:41 PM   #59
lhquam is offline lhquam  United States
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Can someone explain the purpose of R21 and R22 which connect between the outputs and ground? Is it for DC stability if no speaker is connected? I have seen a similar resistor loads in circlotrons, but I cannot figure out the purpose in the F5X.
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Old 16th April 2012, 04:23 PM   #60
palmito is online now palmito  United States
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Quote:
Originally Posted by EUVL View Post
But if I were to do this now, I would solder DIL sockets like these at the 3 holes for the trimmers.
You can then just plug in the trimmers...
...Of course you can also use these for the fixed resistor holes as well for easy changeover.
Please forgive my ignorance, these DIL sockets have holes big enough for the resistor lead to go in without any solder? Can you give a part number so I know what to look for (for the part knowledge handicapped)? All the ones I've come across have holes that are too small for the resistor leads to go in. Thanks.
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