Top Layer and Silkscreen

Bottom Layer with Top Silkscreen

Build instructions


Overview:

First read both the BOM (DIY F1 BOM pdf) and the First Watt F1 Service Manual. The BOM inventories all the recommended parts needed and the service manual explains the theory and actual operation of the circuit.


Preparation:

This amplifier requires matched parts to achieve a similar level of performance as the commercial version. Purchase multiple copies of all resistors in the signal path and for biasing of the mosfets Q1-Q4. Part of the magic in Nelson Pass’s designs is his willingness to invest in a higher labor cost for assembly, such as parts matching, and rely less on multiple gain stages, current mirrors, and other compensation or correction circuitry. His methodology of reducing the number of parts in the signal path makes for very clean and clear sounding amplifiers. Parts matching is important to achieve high bandwidth and a low noise floor.


Match the critical components in the signal path and carefully layout the passive parts on a sheet of paper or foam block, with each labeled according to their placement on the board. For example, label a blank paper as right or left channel and poke one leg of the passive part through the paper and label that part of the paper by the board designation such as R1, D1, C3, etc. Measure each copy of R1 and R2 from your batch of 221 ohm resistors and use the two that match the closest and poke them through the build component paper and label the paper accordingly. It is not necessary to match Q5 & Q6, nor D2, D2, C3, C4, R19, R16, Q7, R20, or LED1.


Warning: The transistors are static sensitive and should never be placed on paper or in foam. Label these parts by using a permanent marker or a small sticker with the part’s match value. ESD sensitive parts must be kept in non-static/ESD safe containers.


This board includes 2 extra 3 watt resistor spaces to help the DIYer compensate for slight matching issues for DC offset. Nelson covers this in this excellent F1 service manual. The extra parts are R27b and R28b on this F1 amp board. R27b & R28b parallel the bias resistors R27 and R28 respectively in the schematic.


Pin pads for C1 &C2 can accommodate various poly 1uf capacitors, and the BOM recommends either panasonic or WIMA.


These boards are not ROHS compliant as they have been tinned with Lead. This mean the boards can be stored for much longer periods of time before the solder pads oxidize. Make sure the pads are clear and shinny. Use a PCB cleaner as needed to prepare the pads prior to construction.


Assembly:

After the necessary parts have been matched and labeled, assemble can begin. It’s a good idea to place the resistors slightly above the surface of the board to allow for better thermal cooling of these parts. This practice is essential for the 3 watt resistors as they tend to get very warm. A good practice to use a spacer between the resistor and the PCB for uniform construction. The 1/8 watt RN55 Dales can be placed directly on the board, but slightly spacing them slightly above the board is a good practice. RN55’s consider 1/16” or 1.5mm spacing. The Panasonic 3 watt resistor spacing should be 3/16” or 4.5mm above the board. It’s especially important to elevate the 3 watt resistors above the board to allow them to cool by convection. A small wood shim repeatedly used for each resistor works great for consistent spacing.


While planning placement of components within the amplifier, make sure the mosfets are

Start with the components lowest to the surface to the board and proceed to the next taller components.


  1. 1. Test fit the PCB. Mount the empty board to the heat sink with the appropriate amount of stand-off above the heat sink. The mosfets should be placed at the vertical mid-point or slightly lower from the vertical midpoint on the heat sink. The power mosfets are mounted and soldered last, but make room for them when drilling and tapping the heat sink for the PCB mounting stand-offs. A good practice is to use a disposable to-247 mosfet for estimating the mounting hole for the power mosfets Q1-Q4 & Q7.

    Construction option: A common method for assembling the power mosfet’s is to mount them with the mount holes at the mid-way vertical point on the heat sink with a large washer, hex head screw, and Bergquist thermal pad. The tips of the pins are then bent upwards 90 degrees to fit through the bottom of the PCB. Mark-off the power mosfet mounting holes at the desired distance from the edge of the PCB and centered with each power mosfet’s Drain pin pads. Remember these mounting instructions are for layout and planning purposes and the actual mounting and soldering of the power mosfets are last in the construction process.

  2. 2. Assemble and mount the power switch, fuse, PSU stand-offs, etc.

  3. 3. Blow and/or vacuum out the chassis to ensure no metal filing are floating around. At this point ALL chassis drilling, tapping, and maching is finished.

  4. 4. Mount and test the PSU and verify it is working properly.

  5. 5. Verify all components are ready and matched, and boards prepared for assembly

  6. 6. Solder in all parts in the following order:

  7. A) All 1/8 watt RN55D’s

  8. B) D1 & D2 protection diodes

  9. C) P1 & P2 pots

  10. D) All 3 watt resistors

  11. E) Q5 and Q6

  12. F) LED 1, if mounted on the PCB

  13. G) C4

  14. H) C3

  15. 3. Solder all power, ground, and input and output wires to the board before mounting the PCB to ensure clean joints.

  16. 4. It’s oftentimes a good idea to use a flux remover to clean the board.

  17. 5. Mount the power mosfets Q1-Q4 &Q7 to the heatsink. Use a hex head machine screw through a wide washer with a small hole, mosfet, then Bergquist pad. Be careful not to over tighten the mounting screw. Over tightening will either crack the mosfet or cause a small “volcano” raising the heat sink aluminum around the mounting hole, thus causing very poor heat transfer.

  18. 6. Up turn the power mosfet pins if not done so already, and mount the assembled PCB to the heat sink. Take care to adjust and align the power mosfet pins to the solder pads. With the board mounted, solder the power mosfets.

  19. 7.Last, make the final connections to the PSU and audio input and output connectors.


Power-up the amp with either a variac or take a “plug and pray” approach for no curls of smoke. Most builders will smell an oder of dust and coatings burning off new parts.


Hopefully the build was successful and you are now listening to your favorite music.


Enjoy!


-David

DIY First Watt F1 Clone Build


Introduction

The F1 is a transconductance “current” amplifier with low noise, balanced inputs, 10 watts of power, and dissipates 100 watts.


The group buy is based on the boards as imaged below. A Bill Of Materials (BOM) lists the parts needed for this particular board. This amp requires large heat sinks and the Conrad Heatsinks  part #MF35-151.5, one per channel is recommended.


Thank you to Nelson Pass for authorizing the Group Buy for non-commercial purposes ONLY.


Bill Of Materials: DIY F1 BOM pdf

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