Consoletronics Blog: The Valve Phono Stage PH-11

It has been a long time that I've been thinking to make a nice sounding phono stage. The challenge in design and building of it is how to handle the power supply ripple and layout of PCB to minimise output noise level. I auditioned a few brands of SS and tube phono stages and finally I concluded that my ultimate favour for the SQ goes to tube design.

Design like Ear 843P that has 3 stages with RIAA network in the negative feedback loop sounded a little muddy to me. Circuit with no global feedback but consists of passive RIAA network probably is the better way to go. After some searches, I found a promised looking circuit designed by Bruce Heranat at DIY Audio Projects site. The input stage is a symmetrical SRPP which feeds a passive RIAA network. The second stage is almost the same as the first one but it is DC coupled to the cathode follower that is the output stage. The gain is about 45db, this is sufficient for most of the MM cartridges. Using a pair of Lundahl LL1678 step up transformer adds extra 24db gain to suit lower output MC cartridges. The circuit has no global feedback.

Power supply is quite critical for the sound quality as well as the final output noise level of the phono stage. Salas SSHV2 shunt regulator has been my favourite in the past for its clean and natural sound signature. Its low impedance and ripple voltage across a very wide frequency range makes it very suitable for this project. In order to achieve even lower noise level, the power transformer is placed in a separate chassis and the source of filament supply is connected from the transformer chassis to the main chassis. It's then rectified and pre-filtered in the main chassis before feeding to the actual power board. I found this has significantly avoided hum radiation during its journey to filament. It's again filtered by 3 Nichicon caps in parallel with total 10K uF of capacitance, and finally gets to the LT1085 for voltage regulation of filament.

Switching between MM /MC input by relay could cause huge popping sound due to high gain amplification. Output must be automatically muted before actual switching occurs. A new logical control board is therefore designed to achieve that. Also added on the board are delay /mute and Mono/Stereo buttons. Mono button is useful when listening to mono LPs.

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