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Old 13th June 2018, 12:10 PM   #21
mashaffer is offline mashaffer  United States
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I was so intrigued by the Mu Follower idea that I stayed up way too late last night playing with Spice. Probably the most exciting aspect to me is that I would no longer need to attempt to cram another tube socket in the already crowded chassis.

I started using the 12AY7 as I wanted to take advantage of the low microphonics but the bandwidth was not good. I suspect Herr Miller was getting in the way. Using the lowly 12AU7 provided much better results extending f3 from 15k out to over 35k. PSRR was dramatically better than my first circuit and even better when I used a 6N1P as the top tube.

I think this is well worth pursuing.
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Old 13th June 2018, 04:13 PM   #22
mashaffer is offline mashaffer  United States
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Quote:
Originally Posted by Eli Duttman View Post
JMO, put JFET source followers as close to the arm pillar as is possible. HF performance of piezoelectric carts. is poor enough, without cable capacitance exacerbating things.
Once again you are right on the money Eli. My issues with getting a Mu Follower gain stage to work well were entirely due to the large source impedance created by the balance and volume control networks which were tailored for the Ceramic Cart. Much better to toss the original pots in favor of smaller values and use a separate follower at the tonearm to buffer the cart. Good frequency response and gain for the 6N1P gain stage and less concern for cabling issues. All for just the cost of a few jfets, a handful of pots and a low voltage power supply. It also makes it quite easy to add a phono preamp and semi-modern TT at a later date if desired.
Attached Images
File Type: png 6N1P_MuFollowerSchem.png (113.6 KB, 52 views)
File Type: png 6N1P_MuFollowerOutput.png (57.2 KB, 50 views)
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Old 13th June 2018, 08:54 PM   #23
mashaffer is offline mashaffer  United States
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With semiconductors going obsolete in what seems like hours it is quite frustrating to find currently available suitable devices. I search mouser or digikey with every suggested part number only to get "0 results found". I probably have suitable devices on various boards in my junk box but unlike our fire bottles they are nigh on impossible to identify visualy.
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Old 14th June 2018, 12:01 AM   #24
Eli Duttman is offline Eli Duttman  United States
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Don't forget NTE's replacement catalog.

FWIW, I went to Mouser and came up with this somewhat pricey part. The turned on internal resistance is low (8 Ω), which is (IMO) a positive indicator.
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Old 14th June 2018, 01:45 AM   #25
mashaffer is offline mashaffer  United States
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Thank you Eli. I have seen other types over $10 so not as bad as it could be. Will probably get a second pair to drive the subwoofer amp too. Seems like jfets are going out of style.
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Old 14th June 2018, 01:17 PM   #26
mashaffer is offline mashaffer  United States
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This could work out well as I have an unused 5V PT winding. A doubler and sufficient filtering should provide a convenient supply. For a few lowish current followers I don't think a regulator will be necessary.
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Old 14th June 2018, 03:58 PM   #27
Eli Duttman is offline Eli Duttman  United States
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Use Schottky diodes in the doubler, along with a low dropout adjustable regulator, and 11 VDC should be easy. Given the small net draw, transformer regulation effects could easily let you squeeze a reliable 12 VDC out.
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Old 18th June 2018, 02:57 AM   #28
mashaffer is offline mashaffer  United States
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Getting closer to a final design and doing some real physical work. Cleaned out remnants of old circuitry and began chassis prep adding a tag strip.

Clean.jpg

I changed it around to use the octal socket after all so that both the initial gain stage and the tone control use a mu follower. This gives plenty of gain to make up for the balance control and provides enough drive to handle the power amp and subwoofer amp connections without a separate buffer. I also tweaked the caps in the tone controls to move them a touch farther out from the midrange.

FULL_MU.png
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Old 20th June 2018, 04:48 PM   #29
stocktrader200 is offline stocktrader200  Canada
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I drove the passive tone stack in SX780 with 6922 triode and 2N5401 CCS. The volume pot is driven by the other half gain of 12. The 100k Volume / loudness/ balance stack sees a signal of 20v p-p with a max overload above 100v p-p. The SS amp has a gain of 15 to keep s/n high. sound quality is outstanding while frequency response is flat at center positions to .2db from 20 to 20khz. I also moved the boost / cut curves further from the 1 khz point. lower bass - higher treble
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Old 20th June 2018, 08:48 PM   #30
mashaffer is offline mashaffer  United States
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That sounds like an interesting set up Stocktrader.

I wonder if it would be wise to use grid stoppers on the upper tubes too.
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