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In Reply to: RE: Those Cute, Sometimes Chrome Phono Preamps... posted by Interstage Tranny on June 21, 2015 at 16:00:03
Thanks for the very interesting and informative post. I just saw one of those chrome phono stages on epay the other day and was surprised at the asking price. I'm sure that more will come along at more resonable prices; rebuilding one looks like a fun project. I didn't know that there were several companies manufacturing them. Is there a "standard" schematic for them?
Follow Ups:
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...
Hello,
I realize this is an old post. Hopefully IT is still active here. I have a GE MF-1 that I'm trying to update. I'd like to update the power supply to a bridge. However, the currently grounded leg of the secondary is internally tied to the center tap of the filament winding. In the half wave configuration this works and balances the filament supply. If i put a bridge across the secondary then the filament will be unbalanced and probably hum like crazy. Do you have a solution for this situation? I thiught about opening the transformer but it's in a crimped metal can. I suopose i could also rectify the filament.. Any thoughts? Thanks in advance..
Thanks for the further information. This looks like a fun project and I generally have plenty of projects in the queue so that I can gather parts for one or two while building another. I'll be checking epay for a couple of mono units that match.
GE developed a 6SC7 based setup that Fisher "borrowed". I've uploaded Fisher's version.
A link to a post showing the schematic for Lafayette's 12AX7 based knockoff of GE's concept is provided below.
Eli D.
So if i wanted to change the eq on this Fisher preamp schematic, what would I'd want to change out and with what values to make it RIAA?
Peace,
Jake
Boy, you'd need some really short, low capacitance interconnects between that and your preamp. I wouldn't be surprised if the commercial offerings had the output cable built into the preamp, and the length kept very short!
-PB
IIRC, some of the little "jobbers" do have captive O/P cabling.
Switching to full wave bridge rectified B+ should allow a few extra mA. of B+ to be squeezed out of the power trafo. Those extra mA. could go to energizing a ZVN0545A source follower, as was used in the tweaked RCA setup. Bye, bye, load driving problems. :> )
Eli D.
Where can I find the as source follower mod to the RCA phono? The recommended 220000 input impedance for the next stage is a bit too high for my set up.
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