|Hamingredient inspired chassis 😉|
In the early 2000s, I tried to unload a half dozen or so of my used/test good Telefunken 12AX7s on eBay. The buyer promptly requested for a return claiming that they didn't test well in his TV-7. Was that a sign...? 😉
So instead of re-listing, I stashed them away. I had forgotten about their existence until they showed up during a 2020 lockdown clean-up spree.
I met my buddy Lar through Audiomart in the late '80s. He's a trained sound engineer and audio hobbyist who shares my passion for the classic Austin Mini.👍 We hang out and talk shop at ham radio festivals, audio and radio shows. He's firmly committed to tube phono circuits employing active RIAA EQ and posts his projects at Audiokarma and Lenco Heaven.
|Shure M65 schematic|
The classic Shure M65 "chrome cutie" (as Lar fondly calls them) is one of his favorite platforms for modification. Keen eyes will notice the similarity of this topology to the phono section of the classic Dynaco PAM-1 and PAS-2/3 tube preamps, albeit operated at a lower B+ voltage and with AC heated filaments.
|One channel shown|
The 0.47uf ceramic disc input coupling caps, C1 and C2 + the 1M shunt resistors, R3 and R4 are discarded. Likewise, the 100k positive feedback resistors (R11 and R12) are eliminated, which I also used to do to my Dyna PAS preamp hacks. IME, this procedure improves transparency. The most significant performance upgrade is Lar's recomputed RIAA EQ capacitor values (see the above schematic) tweaked for flatter response.
I didn't have a Shure M65 but had enough parts to clone one so I could put my rediscovered Tele 12AX7s into good use in the midst of Covid-19 lockdown.
This one chassis configuration sounded fine on first turn on but had an annoying low level hum I couldn't eradicate.
After trying all possible grounding schemes, I yanked the power transformer out and built an outboard power supply. Problem solved!
My power supply is slightly more sophisticated than the stock M65. The full wave rectified B+ is filtered by a 20H/20mA choke followed by a 47uf/450V cap, which splits to separate left and right channel rails with additional decoupling and filtering. The tube filaments are DC heated.
|1 kHz |
stock RIAA EQ
Using the stock RIAA EQ capacitor values, lower frequencies sound bloated with a noticeable treble boost as noted in the Shure M65 literature, which is also visible in the above 1 kHz square wave.
LAR modified RIAA EQ
The 1 kHz square wave is squarer and the phono preamp sounds so much better with Lar's tweaked cap values!
I don't think I was ever able to derive this level of musical satisfaction from any of my past 12AX7 phono efforts using active RIAA EQ. I believe the sonic purity stems from the simplicity of the circuit itself. The Dynaco PAM-1 and PAS-2/3 circuits have an additional cap after the negative feedback RIAA EQ tap + elaborate switching and a balance control in the PAS, all of which can cause veiling.
In spite of the Shure 5000 ohm output impedance spec, this is still a relatively high output Z device. When connected directly to the 10k ohm input of my Nobsound NS-02g amp, the loss of extension at both frequency extremes was discernible. It sounded best driving a 100k ohm load using low capacitance cables, 3 ft. long, max!
This phono preamp synergized very well with my favorite magnetic cartridges - Nagaoka MP10, Pickering XV15, Shure M3D and M7D, amongst others. So after a couple of months of enjoying the Lar mod M65, I decided to build it on a nicer (if a bit cramped 😁) cast aluminum box with two pairs of inputs.
Instead of boring you with audiophile adjectives, I'll just say that my RCA passive RIAA EQ octal sounds effortless and airy, while the EAR 834P derived loctal is detailed and dynamic and the Lar retro-resto Shure M65 is sweet and alluring!
This phono preamp is a most welcome addition to my tools for enjoying LP reproduction. I highly recommend following Lar's phono preamp adventures at Audiokarma!