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Those Cute, Sometimes Chromed Phono Preamps...

Posted by Interstage Tranny (A) on June 28, 2015 at 11:37:29

In Reply to: RE: Those Cute, Sometimes Chrome Phono Preamps... posted by Palustris on June 22, 2015 at 13:06:41:

Hi & Thanks ! If you use the "Classic View" as available from the bottom of the AA Home Page, just a few threads below this one is the "Volume Control" thread. There is a posting there for "Lafayette schematic" which shows the "standard" scheme.

Unfortunately, their wiring practices at the few factories make it imperative for us to study each unit carefully. Most important is the switch wiring, as most of the factories used the switch as an intermediate connection point for the two EQ positions, rather than an overall network connection and then to the feedback return from the center terminal. If you reread my post above while studying the actual wiring of the unit you end up with, eventually the strategy will click in place.

While the Lafayette stereo units usually have all their sides cleanly chromed without any nastily glued on labelling other than a decal, I do heartily recommend two mono units, rather than one stereo unit, as the separate power supplies really do "open" the soundstage wide, high and deep. This would require finding two of the same mono units, which could actually take months or years. Besides the benefit of dual mono, the innards are much less cramped, which can allow more capacitance for the power supply and easy fitment of the finest parts inside. If you choose the stereo unit, there is still "some" room to split and separate the HV rails with separate capacitance banks for each channel.

Beefing up the power supply storage capacitance has many benefits, including better bass impact and control, plus notably increased dynamic range. If you can fit a full wave bridge rectifier inside, the slightly extra current available will also translate into slightly higher HV available. One of the mods not mentioned yet, very worthy of trying, is raising the HV supply voltage and increasing the plate load resistance for each tube section's plate.

If you keep the output cables short, you really do not "need" any buffer stage as someone continuously interjects. These units have a rated load impedance of 100K Ohms, which most "Aux In" inputs have. My ss pre which has 40K Ohms listed as the Aux In impedance also works very well with my chrome cuty, even if I use eighteen inch interconnects. The concept of "needing" to drive an EQ network with a buffer stage is also overstated on forums too often; as evidenced by oh so many phono preamps successfully sounding very fine without cathode followers or ss buffers.

One of my standard operating restoration procedures has always been "rebuild as the factory intended; then attempt modifications to improve...". Herein, with these chrome cuties, if we keep it simple first and simply correct the input resistances, install known correct RIAA EQ networks, plus beef up the power supply, we will be well on the way to a very enjoyable tube phono stage...

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