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Update 6-21-2025: Chassis & faceplate notes
Update 6-21-2025: Chassis & faceplate notes
For anyone who wants to replicate my chassis and faceplate, the following may be helpful. The chassis was custom made for me by Pi-Metal Products. Contact Jin@pi-metals.com and specify the following (I believe he assigned it #12-08102PM):
- Dimensions: 8” wide by 10” deep by 1.75” tall
- No “Front” (the dress panel sometimes used for rack mounting)
- No Handles
- Two “Rear” panels (no holes in the "2nd Front", which makes it the same as the “Rear”)
- No ventilation holes in the sides (or any other panel)
- Aluminum, with press in threaded inserts, Gold Alodine, Painted black.
Although Pi-Metal supplies black #6 screws, I usually replace them with stainless button head screws. You'll need to drill the rear panel for your chosen RCAs, DC jack, ground lug as well as the MC/MM button on the PCB. The latter is the only position-critical hole, so you'll need to mark where to drill once the PCB sits on your chosen standoffs (I used 5/16"). The front panel also needs drilling, but you'll need the faceplate first.
Note that for this low profile chassis, I used the alternate C110 and C111 power supply caps from Mouser, #661-EKYB350E332MM25S (25mm tall).
The faceplate is from Frontpanelexpress.com, and if you'd like to order the same or similar panel, send me an email at mpbarney@gmail.com and I'll send the FPD file. You can obviously edit anything about the panel (lettering, hole position and sizes, etc) before you place your order with them. I didn't want the power switch nut to show, so the switch is threaded in from the rear and the panel is tapped M6 x 0.75 for the Tocos switches I use (now discontinued). The center LED hole is the standard 3mm, and the mode switch hole is 3/8". I added 4 recessed holes around the perimeter to match the look of my Benchmark DAC, so you'll need to visit your local hardware store for four 1/2" long #8 black socket head cap screws.
Since the panel from Frontpanelexpress is machined around the outside edges, I usually sand those edges down with 220 sandpaper so they are not so shiny.
With the panel in hand, you can use it to mark where to drill the Pi-metal front panel (which will be the sub-panel in your finished preamp). I use blue painters tape on all 4 sides to hold the two together while I do the marking and drilling. For the power switch, LED, and mode switch, I simply mark their locations on the sub-panel, then use a Greenlee punch to make a 1/2" hole for each. The four perimeter holes on the sub-panel for the cap screws can either be drilled and tapped 8-32 as I do, or more simply just drilled with nuts used inside on the caps screws. Those screws actually hold the front panel to the sub-panel.
This project is my build of Wyn Palmer's latest and greatest phono preamp, V4. Its a follow-on to my Wyn Palmer V3 Moving Coil phono preamp build, here. That V3 preamp can be built either for moving magnet (MM) or moving coil (MC) cartridges, and is not easily interchangable between the two. This new V4 design is switchable between MM and MC with the push of a button. You typically need to switch cartridge loading components (as you likely know): MM cartridges almost always use a 47Kohm resistive load, but the capacitance load my need to be changed; for MC cartridges, the capacitance matters little but the resistive loading must be optimized. This preamp allows for that with two banks of switches on the main PCB, and the MM or MC settings are switched with the MM/MC gain switch. Although this preamp has equally accurate RIAA accuracy to the already great V3, its several dB quieter in the audio band and has adjustable gain from 35 to 70dB. Also instead of a V3's SMPS (switching power supply), the V4 uses a linear power supply.
This project started in a thread at Audiokarma, here. Those interested in the design process should read this thread. It evolves over a year+ and about 1,000 posts, so you'll need to block out a considerable amount of time!
There is also a build thread at Audiokarma here. For anyone building the V4 preamp, I strongly recommend reading that thread, its just shy of 170 posts as I write this, so quite manageable. I also strongly suggest reading the draft build notes post in the previous thread, here. I did find one typo there: the J25 terminal connector is Mouser # 651-544-2895.
A big thank you to Wyn Palmer for creating and publishing this design and to everyone involved in designing the PCB, testing, etc.
Above: Schematic of the phono preamp. I know you can't read it here, but this gives an idea of the circuit complexity. If you want to see the details of the schematic, you can download it from Wyn Palmer's Dropbox here. Just be prepared to zoom in to read it.
The blank PCB. Note the power supply section on the left can be detached for mounting, as I've done in my implementation below.
There are a couple dozen surface mount components (including the JFETs) , but using a fine tipped soldering iron, flux, and a bit of patience (maybe a magnifying light) it's easily done without special tools. Here's a good video showing techniques for mounting SMD parts.
A big thanks to pfarrell at DIYAudio who had a small run of the PCBs made and very graciously sent me one.
IMPORTANT NOTE: C50 is shown on older PCBs with the incorrect polarity. The Gerber files in the build thread have since been corrected, but if you are working with an older PCB, or just want to check, you should confirm that the negative cap mounting hole connects to ground (e.g. at the audio input). If not, simply reverse the polarity of this cap when installing it.
The pile of parts from Mouser. Note that the published Bill of Materials (BOM) in the build thread can simply be imported in Mouser, to save searching for parts or typing complex part numbers and possibly making an error. Note that some parts may occassionally be out of stock at Mouser, but the build thread mentioned above is the place to post asking for acceptable replacements - Wyn is very helpful.
The BOM for the parts is in Dropbox, here.
The following info on parts is pretty much all available in the build thread, but I've consolidated what I learned here. Parts that were out of stock for my build (March 2025), along with recommended replacements:
R25 & 30 (33K, 1%): 594-MBB02070C3302FCT
R82,83,84,85 (6.81K, 0.1%): 71-CMF556K8100BEEK
C71,74 (0.018uf, 1%): 80-C0805C183F5G
C81 & 85 (4700pf, 1%): 81-GCM21B5C2E472FX1L
Note that the BOM incorrectly lists the quantity of R66,71,184,185 as 2, must be changed to 4
If you'd like lower profile main power supply caps, C110 & 111, shorter versions are Mouser #661-EKYB350E332MM25S, however, when checking all these Mouser part numbers, I just noticed this is out of stock until July 2025.
I used the following header pins:
538-22-23-2031 (3 pin) - for the mono/stereo/warp filter switch connections
538-22-23-2021 (2 pin) - for the 6 and 9 dB gain connections
The corresponding connector housings/shells are:
538-22-01-3037 (3 pin)
538-22-01-3027 (2 pin)
And the female pins for all are: 538-50-29-1638
The 3 power supply terminal blocks I used are: 538-39890-0302
I ordered a custom chassis from Par-metal.com, using the same dimensions as the chassis for my Wyn Palmer V3 phono preamp: 8" wide by 10" deep by 1.75" tall.
These are all aluminum with pressed-in steel threaded inserts. The chassis is alodine coated (to resist corrosion and provide an electrically conductive coating) and painted on the outside.
Here's the (mostly) assembled power supply section. 18VAC input (from a wall-wart) on the right, power switch connections on the left, and outputs to the audio board on the bottom in this photo.
This is the assembled power supply with the audio board. The DIP switches at the top right are for cartridge loading and the red pushbutton switch on the rear selects moving magnet or moving coil.
I installed the DIP switches facing the same direction for ease of use, and added a label on top to make settings clearer.
The temporary DC connector at the rear of the power supply allows testing the supply subassembly before connecting it to the audio board.
Another view of the audio board, from the rear.
Test fitting the PCBs in the chassis.
Drilling done, PCBs mounted in the chassis, along with the rear panel RCAs for input and output. Color coded wiring for easy signal path identification - red is right, blue is left. I used Vampire gold/teflon RCAs.
You may also note the row of white connectors on the front of the audio PCB for mode switching and gain switching.....
The switching connection points. The design provides for a base gain of 35dB in MM mode and 55dB in MC mode. It also allows for an additional switchable 6dB gain in the input section as well as an additional 9dB gain in the output section, providing a wide variety of gain options.
It also includes the Warp feature like the V3, and a mono mode, both switchable from these connectors.
Wiried up for initial system testing. The yellow (+15VDC), green (-15VDC), and black (ground) supply power to the audio board.
The white twisted pair to the left of the power supply PCB is the 18VAC input from the wall-wart, and the white and black twisted pairs to the right go to the power switch.
First operational test: 1KHz signal, bottom trace is from my signal generator which also feeds an inverse RIAA filter then the phono preamp, top trace is the output from the phono preamp.
Most builders of this prreamp use toggle switches for what I'm calling the mode switch: Mono, stereo, or the Warp filter. I decided to use a rotary switch (like the mode switch on a traditional preamp) to accomplish the same thing.
This shows the 3 position switch from the rear, an adjustable Grayhill from my parts box.
See below for the front panel labelling for that switch.
For the 6dB and 9dB gain options, I believe most builders again use toggle switches. Since I change cartridges infrequently, I expect to remove the cover to set the internal dip switches for cartridge loading. So I've simply used jumpers (also called shunts) to connect the pins to engage either the 6 and/or 9dB gain options. This will keep my front panel less cluttered and also prevent accidental gain changes.
Here you can see the black jumpers connecting the pins for the 9dB output gain, but the 6 dB input gain jumpers are simply stored on one of their pins, not making a connection between the 2 pins.
The Mouser part number for the jumpers I used are 737-MSBH-G.
All wired up. At the rear (top in this photo): power input on the left, RCAs, and ground lug barely visible on the right. At the front: power toggle switch on the left, power LED in middle, and the mono/stereo/warp switch on the right.
I used a 3mm blue LED, and after experimenting, used a 100K resistor for R25 to get the brightness I wanted.
I also added a printed sheet for input load settings on the right.
The front panel layout as ordered from Frontpanelexpress.com.
I designed the faceplate to mimic the look of some Benchmark gear it will sit near.
Front panel installed.
All buttoned up.
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