F5X -- the EUVL Approach - The Build Thread

This thread will document build details for EUVL's F5X amplifier. The main thread is here: F5X -- the EUVL Approach

I'll start off the thread with the official BOM and schematics, Circuit board assembly, amplifier initial biasing and bring up. There will be some related articles thrown to help as well. Measurements will be posted as soon as I have time to compile them and there will be information about final wiring and the amplifier case along the way.

Thank you for your patience and feel free to post assembly related comments or questions here. Let's keep this thread focused on assembly or similar issues and maintain other discussions on the main thread.

Enjoy,
Dave
 
Last edited:
EUVL F5X Amplifier Schematics and BOM

Here are the official Schematics and BOM for the EUVL F5X amplifier. These are for the standard version, non-cascoded front end. I will post the cascade schematics and BOM details a little later for those interested.
Dave
 

Attachments

  • FX5_STD_SCH.pdf
    149.1 KB · Views: 4,615
  • F5X_BOM_STD_CFG_PAGE1_1.0D.pdf
    25.2 KB · Views: 2,182
  • F5X_BOM_STD_CFG_PAGE2_1.0D.pdf
    180.5 KB · Views: 1,496
  • F5X_BOM_STD_CFG_PAGE3_1.0D.pdf
    195.2 KB · Views: 1,428
Build Notes - Assembling EUVLs F5X

This is the first in a series intended to detail assembly steps necessary to get your F5X amplifier circuit boards up and running. It is written with the average DIY audio builder in mind and therefore may have more information than necessary for many readers. The steps and suggestions herein are just that. Feel free to use whatever method and materials will work best for you. You will also see some words of warning. Nothing different here than any other DIY project but please be careful when dealing with HIGH VOLTAGES, HEAT, SOLDER, CUTTING, DRILLING, SPACE TRAVEL, BRAIN SURGERY, NEGLECTED SPOUSES, etc… You have been warned.

Parts:

Bills of materials, schematics, and wiring references are included as separate files for revision management purposes. Please refer to these for component procurement and stuffing locations. You will also need matched input JFET and output MOSFET devices. It will be discussed separately to highlight some simple multi-point matching techniques that do not require a curve tracer. This is a well covered subject elsewhere and any questions can be posted in the thread.

The PCB sets will arrive panelized. That is multiple boards are manufactured as a unit and scored/routed so that they can be easily parted once you are ready to assemble them. All the boards necessary to build one mono amplifier channel are combined into a single panel. If you ordered the F5X case and PCBs from the group buy you will also have the power supply circuit board panels that include the filter and regulator PCBs specific to the GB case. These are similarly combined into a single panel and each builds into one mono channel.
 

Attachments

  • IMG_0718.JPG
    IMG_0718.JPG
    497.8 KB · Views: 8,967
  • IMG_0836.JPG
    IMG_0836.JPG
    517.8 KB · Views: 9,399
While the PCBs can be assembled while still panelized I chose to part then into individual boards for the prototype build. The boards must be flexed at the score lines to separate them from the panels. Be aware that the routed edges of fiberglass PCBs CAN AND WILL CUT YOU given the right moment of carelessness. Once separated, the area of the score lines will also likely have some fiberglass splinters or hairs that can be a real nuisance if they inadvertently become inserted under the skin. A piece of sandpaper or a fine file can smooth down any rough edges after separation.

Another consideration is the little “ear” that will be left in the corner of the boards near the slots cut in them. Since the tool is round that cuts these slots, the edges here cannot be square. Simply remove it with a file or hobby knife blade.

You might be contemplating “why panelize the boards? Why not simply make them all separately?” There are three boards for each amplifier channel plus five power supply boards per channel if you bought a case. To tool and build each of the eight PCBs separately instead of just two in panels invites errors and increases costs. Most importantly, it makes logistics simple. One panel per channel in a box and you get everything you need to build your amp the first time!

There is a protection PCB that is not yet available. It will be documented separately alongside the case assembly and wiring in the near future.
 

Attachments

  • IMG_0841.JPG
    IMG_0841.JPG
    183.3 KB · Views: 8,349
Last edited:
Power Supply Assembly:

This is pretty straight forward but included for those who desire a reference. If you are not using these boards you can skip directly to the amplifier assembly. Start with the filters by inserting and soldering the screw terminals if you intend to use these. Capacitors come next. Make sure to observe the polarity of the capacitors when assembling these. You will end up with 4 each of +VE and –VE boards for one stereo amplifier.

Next, install all of the diodes and resistors in the regulator boards per the bill of material list. Observe polarity for the diodes and capacitors. Follow with the relay, terminal blocks, and capacitors. Do not solder the power FETs in yet as that will be done once assembled to the heatsink. You will need two of these once complete.
 

Attachments

  • IMG_0727.JPG
    IMG_0727.JPG
    390.3 KB · Views: 8,279
  • IMG_0731.JPG
    IMG_0731.JPG
    378.3 KB · Views: 8,088
Amplifier Assembly:

Like the power supplies, amplifier assembly is straight forward. First solder the surface mount devices on the bottom of the PCB. It is easier to do this with no components on the top. There is a separate post comming that can help with SMT assembly suggestions.

I begin through-hole assembly with all of the ¼ watt resistors. Insert them and bend the leads slightly outward on bottom to hold them in place. Flip the board over and solder. To make the next step easier I go ahead and clip the leads after each solder step. Keep in mind that the 33R resistors R18, R19, R24 and R25 are from matched sets and should be inserted into the correct locations as sets of three. Install the resistors for the optional cascades now if you intend to use them. If you are not using cascades or intend to use add them later solder jumpers in place of Q7A and Q10A pins 1 to 2. Solder a jumper in place of Q11A, and Q14A pins 1 to 3. I use leftover leads but check them with a magnetic because those from capacitors are often steel and less desirable than copper.

Next, install the 220R 1W resistors on the board. Be careful not to mix up R21 and R22 into the matched 220R resistors sets in locations R17, R20, R23 and R26. You can elevate any of the resistors a little if you like. I chose not to.

If you are contemplating using the optional cascodes I suggest you install the capacitors C1 and C2 now. Install the pots and transistors. Be aware that static can damage the transistors. Last, install the source resistors. It is beneficial to install these spaced above the board so that the leads are accessible. This will help with probing when you initially bias the amplifier. Around 0.080" (2mm) is a recommended minimum.
 

Attachments

  • IMG_0761.JPG
    IMG_0761.JPG
    739.8 KB · Views: 2,706
  • IMG_0739.JPG
    IMG_0739.JPG
    767.6 KB · Views: 2,496
  • IMG_0738.JPG
    IMG_0738.JPG
    745.4 KB · Views: 2,784
For the initial bias setting it is necessary to configure the board as “grounded X”. This essentially separates the two halves of the amplifier into two individual amplifiers for adjustments. To do this, cut 4 small loops out of leads left over from assembly and insert them from the bottom of the board.

The little loops will help later when you need to remove them to reconfigure for floating X. It is necessary to install them from the bottom because the input board will cover this area shortly. Solder them in place and trim the top side leads. Two jumpers are also necessary at location J14 and J15. I used mil spec solid 16 AWG (1.5mm) Teflon insulated wire. Make the jumpers as shown and use spacers to elevate them from the surface of the PCB to prevent shorts. The Teflon jacket conveniently worked for spacers in my case.
 

Attachments

  • IMG_0752.JPG
    IMG_0752.JPG
    273 KB · Views: 2,205
  • IMG_0743.JPG
    IMG_0743.JPG
    313.5 KB · Views: 1,129
  • IMG_0746.JPG
    IMG_0746.JPG
    306.4 KB · Views: 1,307
Input Board Assembly:

Solder the input board components and trim the leads. If you are using the mute relay board, also assemble it at this time.

I used a copper standoff I made for the center ground conductor. EUVL specifies a brass standoff. Assemble the input board onto the standoff and center it over the matching holes on the main board.

The BOM calls for wire to connect the input board to the main amplifier board. Insert the wire through the input board and amplifier board one at a time and solder on both boards. Three wires are necessary on each end, two 20 AWG (0.8 mm) and one 16 AWG (1.5 mm). These will extend through the mute board on the end opposite the input pads if you are using it. Just leave some distance between the mute and input boards.

Don’t forget to install the source resistors and make sure to keep them as sets if yours are matched. For ease of setup when we bias the amplifier I suggest you install these elevated off of the board by at least 2mm. This will allow us to probe them with clip leads during biasing to measure the actual current across each half of the amplifier.
 

Attachments

  • IMG_0832.JPG
    IMG_0832.JPG
    390.9 KB · Views: 2,300
Don’t install the output devices yet. In part two we will cover initial testing of the input stage and the adjustments necessary to get the bias close before installing the output devices.

Your amplifier boards should now look something like this. We'll do some wiring for testing and bias up the amplifiers in the next installment. Until then stay tuned for some other helpful details.

Dave
 

Attachments

  • IMG_0829.JPG
    IMG_0829.JPG
    452.7 KB · Views: 3,847
Build_Notes - Soldering Suggestions

The PCBs for the F5X are made from high quality FR4 PCB material and feature heavy 2oz copper construction. Because of this and the inherent size of many of the components there is quite a bit of thermal mass to overcome when soldering. I like to have those nice solder filets on both sides of the boards that the pros get with a wave solder machine (even if mine never really look that good). I do prefer evenly wetted solder pads and pins on both sides of the PCB if possible and to this end I do what the machines do - I preheat. You can do this at home with a hair dryer, heat gun or oven. ALL OF THESE CAN BURN YOU AND/OR YOUR POSSESSIONS IF CARELESSLY USED - SO BE CAREFUL.

First, make sure to use an 800F/ 425C soldering iron for all soldering. Gently warm the boards up top and bottom with the hair dryer or heat gun to the point where they are hot to the touch BUT NOT SO HOT AS TO BURN YOU. You can instead place them in an oven or toaster but make sure the temperature is SET TO and DOES NOT EXCEED 70 degrees C or 160 degrees F.

Increasing the board temperature before soldering improves the wetting and flow of common solders. They boards will cool down pretty quickly so it pays to have as many components inserted as is reasonable for each heat cycle. DO NOT hold the heat gun close to the PCB, YOUR SKIN, ARMS, WINDOW TREATMENTS, etc. as it can and will burn whatever is in front of it. A hair dryer is more forgiving but can still burn you or start a fire

If all else fails you can always forgo heating and solder a handful of points then flip the PCB over and solder the same points from the top while they are still warm.

Happy soldering…
Dave
 
Build Notes - Suggestions for Surface Mount Soldering

When soldering surface mount components I use a simple method that can be done without magnification, at least for my tired eyes. Place a small amount of solder on one pad. Hold the component in one hand with tweezers and the iron in the other. Reflow the solder previously applied to the pad and slide the component into the solder with the tweezers. Hold the part as you remove the soldering iron because surface tension can take the part along on the tip of the iron as it is pulled away. Now solder the other pad. If you apply too much solder use de-solder braid. Make sure to remove the braid and the iron together. If the solder in the braid cools while still attached to the surface mount component or pad it will likely all come off when you pull the braid away. This is painfully true for small pads and components.

This technique works with most passives or axial components with two connections. If you have a small active device like an IC or transistor I place a small amount of solder on one pad and set the part over this solder mound. While holding the part in place with tweezers, reflow the solder and allow the part to drop down into it. Make sure the part is aligned correctly on the other pads and remove the iron.

Keep in mind that holding an iron to a small component too long will eventually destroy it.
 

Attachments

  • IMG_0734.JPG
    IMG_0734.JPG
    419 KB · Views: 3,671
Build_Notes - Device Matching

The F5X features a symmetrical input stage built with audio grade JFETs - the 2SJ74BL and 2SK170BL parts from Toshiba. These feature low noise, high transconductance and are advertised as complementary parts. It is necessary to match parts for both the input and the output stage. This is intended to pull relevant data from various posts in the main thread into a single reference. All links are published at the end of this group of posts.

You need two matched quads, one for each amplifier. A “quad” consists of two J74s and two K170s. The J74s are Idss matched to one other, as are the K170s, but the match between J74s and K170s need to be a bit different. Patrick (EUVL) tested the transconductance curves of some of each and found that they are not perfectly complimentary. The curve shape of the J74’s transconductance often does not match the K170’s. The first picture is an example measurement from Patrick that shows what one might find for a typical complementary pair of identical Idss devices. He also found that selecting for a specific Idss current difference and degenerating the J74s brought them better in line to a true complimentary pair.
 

Attachments

  • 2SJ74%20Mismatch.png
    2SJ74%20Mismatch.png
    2.5 KB · Views: 3,104
Finding quads in the same range that he uses may be challenging unless you have a large number of parts to select from. In the table below Patrick lists calculated values for the degeneration resistors at various Idss combinations. This is a good place to start with selection based on the datasheets curves for the J74 and K70. Because every transistor is slightly different these numbers will not be exact. Select your parts to configure the front end bias at 5mA or higher (the yellow line and below in chart). A lower value here will dictate changing the drain resistor combination on the amplifier board. You can pre-select your parts with a Rdegen of zero ohms to see what you have, then change the 5 ohm resistor in the matching fixture to the calculated value from the chart. My quads ended up with the best match using 6.8/7.8mA Idss selected parts with 4R degeneration for the J74s.
 

Attachments

  • F5%20Frontend%20Bias.png
    F5%20Frontend%20Bias.png
    32.3 KB · Views: 3,944
Patrick has already described a three point matching method and circuit. Here is one implementation of his JFET circuit. Jumpers J1, J2 and J3 allow easy changing of the degeneration resistor values in circuit. You can optionally change the single 5 ohm resistor and jumper into multiple steps with additional jumpers across 1, 2, and 3 ohm resistors to find the best matches and Rdegen value.
 

Attachments

  • JFET_matching.png
    JFET_matching.png
    81.8 KB · Views: 3,050
The F5X can use a number of output devices. IRF, Vishay, Fairchild, and other parts will work. If you have followed the thread for long you will note that Patrick chose Toshiba parts and a different combination of supply voltage, source resistor and bias from the original F5 circuit. Simple matching at 2A with the transistor case at 60C will do for the build as described. Use a large thermal mass and heat it in an oven as described in Patrick’s note. Make sure to use silicone or Teflon insulated wires to connect the devices in the oven to the power supply and meters outside. These insulations will not melt at these temperatures.

Doing a simple 3 point curve match will offer performance improvement over single point VGS matched parts. I purchased four “matched” sets of MOSFETs and then further selected them with a multi-point curve match. It is interesting that the best matches at the 2A “static” Vgs point were not necessarily the best curve matched sets among the parts I have.
 

Attachments

  • MOSFET_matching.png
    MOSFET_matching.png
    69.2 KB · Views: 1,410
If you build the amplifier with regulators it is desirable to match the power MOSFETs used there as well minimizing output voltage differences. Simple, single point matching of the MOSFETs at 4A is sufficient for the regulators. Bear in mind that it is entirely possible you will not get a perfect match between P and N devices.

Happy sorting…
Dave