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Old 14th June 2018, 09:51 PM
MrKettle MrKettle is offline
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Location: Warwickshire
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Default "LCR3" phono stage

Having heard a bit about LCR phono stages, a thought went through my head about what one could be like if it were to be part of the WD series. Somehow that thought has actually turned into a working project.

The first thing to do on the phono stage was, of course, the power supply. I've put it in a bigger modushop case, replaced the choke with a Hammond 157G, added another 157G in place of the phono output resistor, added another 100uF to each of the lines where there is space on the main board and then put in a dropper resistor between the output pin for the phono stage and the socket. And then put in another heater supply rated at about 3A (from the LM338) referenced to ground rather than c. 40V, which I have also routed to the Pre3 rather than the raised heater supply to reduce stress on the 82s.

The circuit I have ended up with looks pretty much like the attached file. A nominal 6.2k impedance seemed to work pretty well with standard component values with only one combination of resistors and one combination of caps required to get the right values. Using a 1k2 and a 30R together means that the 30R can be adjusted up or down to get the right response. The inductors were built to spec by Sowter, the circuit takes into account their stated series resistance in the equalisation. I can let anyone have the Sowter type references if you are interested.

Just to note, the eq network is impedance matched with the output impedance of the cascode rather than the resistor after the stage.

In terms of the choice of gain stages, the aim was to use standard valve types. Someone else mentioned that a cascode would work well for the first stage, so given there is a raised heater supply available an ECC88 seems to be a fairly obvious choice. The final stage could have been an ECC83 stage like the first stage of the Phono3, but I have been pondering CCS loading so thought I would try that.

The actual circuit actually used the DiyHifiSupply CCS using the 10M45S devices. With an LM338, there is a need to get the supply voltage right to work in the middle of the envelope, which was a bit of a problem as even with a balance matched 81 the anode voltage was still about 8 or 9V different between the two sections. 10M45S can cope with hundreds of volts drop (assuming the right amount of heatsinking) so I have instead run that off the the full HT supply dropping about 70V or so.

The coupling caps around the final supply and the cathode follower were causing some problems with higher values with an apparent resonance around 0.25-0.5Hz. 0.1uF doesn't lead to any appreciable drop in bass response so I have ended up there.

I've used Kiwame resistors, ClarityCaps for the cascode cap and the eq sections and Mundorf for the coupling caps. End result? Really quite good, definite step up from the Phono3 (but then the retail cost of the Phono3 is the same as the cost of the inductors). Very clean and detailed, gets out of the way very quickly but still with oomph when required. The eq was also very accurate straight out of the box with no fiddling required, which says something about Sowter and about the accuracy of the other components.
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File Type: jpg Final LCR circuit.jpg (76.0 KB, 23 views)
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Old 15th June 2018, 01:11 PM
Mark Mark is offline
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Join Date: Feb 2007
Location: London
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Default Cases

Apologies for the 'off topic' response, but thanks for signposting modushop cases - just the sort of thing I am looking for

Mark
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