F5X -- the EUVL Approach - The Build Thread

[EUVL]

Thanks Dave.



Patrick

[fitzfish]

My pleasure Patrick.

Originally Posted by Westley

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

[samoloko]

Originally Posted by EUVL

We made a mistake with the BoM.

The cap should be Digikey P6921-ND, Panasonic ECO-S1VP223DA, 22000uF 35V.

If you do want to use the TS-HA series, you need to use P6668-ND 15000uF 35V for the first C after the rectifiers,
and P6591-ND 22000uF 25V for the second C in wither CRC or CRegC.

Patrick

why 35V cap Is necessary with 16 Vac TX output
why not 25V cap

[EUVL]

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

[EUVL]

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

[lhquam]

Fitzfish: I have been conparing the main board layout in post #8 #F5X -- the EUVL Approach - The Build Thread with the schematic in post #4.

I cannot find resistors R14B, R15B, R27B, and R28B. The appear as 1K ohms in the BOM and the schematic.

[fitzfish]

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

[lhquam]

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?

[EUVL]

> 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

[lhquam]

Fitzfish:

How are power and output connections made to the board? The hole diameters appear to be about right for a #6 screw, but there isn't enough space for a nut. Are metal standoffs or spacers used here?