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Sokrates used to be around the marketplace talking to other people. This is exactly what this pre-amp (actually it's a buffer) is doing. The marketplace is where all the source devices and speakers come together. But it is more than a buffer. It is a source switch that is operated via a remote control and more importantly it transforms the signals from unbalanced (cinch/RCA) to active balanced (XLR). This is important because in an installation with active speakers it is likely that the source devices and each speaker are all powered from different wall outlets. Connecting devices with PE (e.g. a PC) to the pre-amp typically worsens the hum situation.
Primary Design Goals:
- remote source selection for 5 or more balanced or unbalanced devices and remote volume control
- sustainable elimination of hum in the system
The input stage features bootstrapping of the common mode signal. The input as well as the output stage are built with OpAmps. Within reasonable limits, it is possible to connect source devices with balanced and unbalanced outputs. The ASP of the speakers are hosting the corresponding active balanced input stages, which are virtually identical to those of the pre-amp.
Why not simply using audio transformers and be done with this ? Because the good ones seem to be unobtanium here. But I will keep looking. However, there is no urgency as the current solution is working well.
Sokrates contains a simple but effective phono pre-amp that can be replaced by a different one if desired. It drives a Denon DL-110 high output MC cartridge of a Denon DP-37F turntable.
The device also features a headphone amp made up from two OpAmps per channel to drive a Sennheiser HD650 with 300 Ohm impedance.
The volume is regulated by a high quality motor pot. This can be easily replaced by even better gear such as a switched resistive ladder if I feel like doing so. The input selection is implemented using high quality relays. The remote control and the associated controller board is an off-the-shelf solution.
As it stands now, no caps are required to block DC in the signal path of the pre-amp, except to protect the headphones, the pot and the MC cartridge.
All the caps are lowest distortion PP (Polypropylene) or PPS (Polyphenylene Sulfide) types with appropriate tolerances. The small capacitances in the feedback paths of the OPamps are C0G/NP0 multilayer ceramics also with lowest distortion.
Rev. 0.1 was built with OPA134 / OPA2134 but rev. 0.2 will be based upon LM4562 and the corresponding single and quad OPamps (LME49710, LME49740). The phono amp will feature one of the best in voltage noise class: BJT OPamp AD797.
Status of the project:
- 13-Mar-2012: Version 0.2 is up and running and everything works as desired.
First things first: Hum is no issue as was with rev. 0.1.
Did the LM(E) OPamps change the sound ? That cannot be answered clearly because too many other variables changed at the same time, e.g. the output stage, the layout of the grounding scheme, and the entire power supply. In total, version 0.2 sounds a pinch warmer and more relaxed, especially the highs, and it has a notch more clarity and details.
The completely re-designed phono stage also works well. The gain had to be increased from 100 to 110 so that the level difference is minimized when switching sources. Interestingly, the MC cartridge produces different output offsets left and right just after the AD797 and a 22 Ohm or 27 Ohm resistor between the ground wire of the turn table and earth brings the offset down in one channel and up in the other one. This is when electronics really become interesting and make a fun time in the basement...! Anyway, the offsets are now within reasonable levels and the following filter stage takes care of the small remainder of DC right away.
The noise contribution of the phono stage is really minimal. You can only hear the hiss when you hold your ear right against the speaker membrane. On the other hand I am very happy with the high output (1.6mV) of the Denon cartridge, which makes it possible to go with only one gain stage.
I do not own many LPs anymore (or not yet ?). But one of the few I have is also present as a CD so I can directly compare them. With this album (FGTH - Welcome to the Pleasure Dome), the CD has more clarity in the mids and highs. If I don't switch back and forth I don't miss a thing, though when listening to vinyl only.
The air core coils of the output stage have become much bigger than calculated. Even a safety margin was eaten up soon so that the boards look a bit like space ships (not Klingon this time).
As a bottom line, the work has paid off. All the changes are positive, especially the improvement in clarity and detail resolution.
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