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PostPosted: 14 Dec 2020, 15:50 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Mark, correct. I have attached a schematic of the preamp with the modification. The preamp is the same as in earlier posts with some possible component variations that essentially do not alter the performance. The 30 VDC power source can be nearly anything. I used a wall wart. It need not be clean and really any value from about 28 VDC to 35 VDC should be fine. The design does not permit charging while in use. This is intentional as you would need to filter it well. Since I estimate battery life at over 200 hours continuously this is really not an issue. I used 18500 batteries, but the 18600 are more common and would extend the playing time by about 30%. Folks building this mod should be aware that when fully charged the batteries can deliver a lot of current. I smoked a number 20 wire when building this unit. They can deliver several amperes for a short while. The setting of the regulator is somewhat unusual as the terminal voltage on the battery string desired is about 23.4 volts. Running them to 4.0 volts is not recommended by the battery companies. You would expect that setting the regulator for that value would get them there eventually. I observed that it would not. A no load regulator output of nearly 27 volts was needed to trickle charge the string at 25 ma. BTW for those of you that would say check your meter ....The one I use is accurate to 5 places when compared to a known voltage precision reference. Now the important part. Does it sound differently? In short no. The S/N of the original AC powered design was in the -100db range, essentially at the limits of what I have confidence in measuring. This one is the same. The biggest advantage is that it is portable.

Please excuse the hodge podge in the schematic. I just pasted the mods on an earlier AC powered version.

Good listening
Bruce
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IC Phono Preamp With Batteries.jpg


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PostPosted: 14 Dec 2020, 16:38 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
Good work Bruce hopefully this will entice some one to at least make the preamp. I’m making mine on 10 x 20 holes of Vera brd. or strip board I think it is called in USA. This allows for the diff. PS on the same brd and some mounting holes. Compact but workable. I’m sure you can’t make a cheaper or better sounding preamp without more complexity and more money.

One a similar thread I notice yesterday on a Billy Holiday 180gm remastered re-issued on one track for one second there was some distortion in a track with the tube preamp. I replaced the new JJ 12AX7s with some Psvane 12AU7s top of the range T series MKIIs. Played the track again, distortion gone. Back to the JJs distortion back. Tried some new Mullard re-issue 12AX7, distortion back with or without tube rings.

Back to the lightly used Psvane, with tube rings, no distortion. There appears there maybe some track distortion but the X7s are (I think) oscillating to it or just emphasising it. I might add the U7s are 6db quieter but manageable. The U7s are staying in at this point. Not quite as lively as the JJs but still enjoyable.

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Projects: | “La Ruse” - tube SRPP ECC83 preamp | "Noir” - Psvane tube preamp | ”fiih-300” - 300W Holton based amp | "Sortilege" - 180W Tripath Class D Mundorf upgrade amp |


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PostPosted: 14 Dec 2020, 17:54 
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Hi Mark, It might get some folks inspired. I use similar boards. I like the dual row types that Radio Shack used to carry. I have seen some eleswhere recenty but don't remember where. Possibly an Asian site. I am using JJs in most of the preamps without any issues but have one using EH 7025s. Supposed to be the lowest noise. I measured only about 1 db though. They sound pretty much the same. Try some of the long plate JJ ECC803S if you are already not using them. The small plate ones seem to be more sensitive to issues IMO. Pass on the gold pin version as they seem no better...just cost more.

Good listening
Bruce

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PostPosted: 16 Dec 2020, 21:37 
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An update on my last post. The U7 didn’t have enough gain so back to the JJ A7s with no other issues after about 6 hours straight of vinyl listening. JJs are staying.

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Projects: | “La Ruse” - tube SRPP ECC83 preamp | "Noir” - Psvane tube preamp | ”fiih-300” - 300W Holton based amp | "Sortilege" - 180W Tripath Class D Mundorf upgrade amp |


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PostPosted: 23 Jan 2021, 19:33 
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Joined: 03 Jan 2020, 16:10
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Hi Bruce, a year after i read your detailed answer, LOL. Thanks a lot, in fact, the project is finally realising, i was just locating components in perf board, so all the suggestions and aknowledge is really welcomed. I even found a nice vintage metal box for peanuts. Hope it turns well. Greetings and good 2021!


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