F5X -- the EUVL Approach - The Build Thread

My pleasure Patrick.

One final question. Should R14A, R15A, R27A, R28A be 1K ohm as R14B,R15B, R27B, R28B will be determined and replaced after the bias is set?
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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For the cap after the regulator 25V is enough. But only regulator and not CRC.

Before the regulator and after the rectifiers, the voltage is ~20V under load.
But depending on the transformer, the voltage under no load (standby mode or slow start mode) can be 10% higher.
On top of that your local power company is allowed to vary the voltage by some 5%.
All that means we are getting a bit close to the 25V rating.

The 2nd cap of a CRC sees the same voltage as the 1st when no current is flowing.
So I decided to specify both at the same voltage, i.e. 35V.

I myself use 25V.
But I do not want complaints from people that their caps get busted because I did not specify sufficient margin.
So for the public build I have to specific 35V (the next voltage above 25V), for all caps.

You can use 25V, but then at your own risk.
Don't say you have not been informed / warned.

;)


Patrick
 
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To answer Westley's question, you are correct.

R14A should be 1k, R14C should be 1.5k (fixed value PRP resistors).
R14B should be an optional fixed resistor after the amplifier bias is fully trimmed in steady state on heatsink.
You may choose to keep the trimmer for flexibility (for adjusting bias and DC offset).
But for best sonic results, replacing with a fixed resistor is recommended.

Dave has been informed and will correct the BoM accordingly.


Patrick
 
Hi Ihquam,

Yes, they are obscured by the pots in the picture. They are actually located beneath the pot in each location, also described here. The BOM had one additional error instructing to stuff the 1K ohm incorrectly in the “B” position instead of in the "A" position for each of these you question. Also see the EUVL's post just previous for more info.

I was waiting to see if any additional errors were discovered. My apologies if the delay caused confusion. I will post the updated BOM later on this evening.

Thank you and to everyone so far for the feedback. We want to make it correct and easy to understand!

Dave
 
EUVL:

I am working on a PCB layout of the F5X similar to the F5X EUVL layout with a tighter MOSFET spacing of 70 mm x 80mm heat-centers on a 63 mm x 96 mm board. This MOSFET spacing is optimized for 1/2 HeatsinkUSA E007 dimensions, configured in a fan cooled heat tunnel.

The R17, R20, R23, R26 feedback resistors require too much space. Is there performance sacrifice replacing them with Caddock MP850 resistors?

Your design does not use thermistors for bias stabilization. What is your experience with thermal stability of the F5X without them, particularly with the higher bias currents and .22 ohm source resistors?
 
> Is there performance sacrifice replacing them with Caddock MP850 resistors?

They should have enough power capacity not to overheat, assuming you have them on heatsinks.
Rest I cannot answer, since I have not used them. You should check if they are still linear at 1MHz.

I design my thermal concepts properly, so I have no thermal issues without thermistor.
You can read about the thermal design in Linear Audio Vol. 3.

But Nelson knows thermistors better. So maybe he can tell you.


Patrick
 
How is one supposed to unsolder the pots and install alternate drain resistors without removing the board (and mosfets) from the heatsink?


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
 
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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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?

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
 
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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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 :xeye:. 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
 
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.