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PostPosted: 19 Jan 2012, 18:09 
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I am a retired audio engineer of 45 years in Hollywood, LA, California. During my career I designed pro-equipment used in many studios, control rooms and a couple of disc/vinyl mastering houses (as they are called). In reading over this thread and looking into the usefulness of the K301 design ... my comments are purely academic, not having actually built the K301.

Having said that, my suggestion concerning the power supply would be to break-off the power supply part of the circuit board and discard it. Then buy five, 6-volt heavy-duty lantern batteries from batterymart.com at $3.00 each. Hook them in series for the 30 volts required. So for $15 you get pure DC, enough to last many, many hours (or years), and forget all the other hum and buzz problems you're trying to solve.

My current preamp (an older Bolder design) used a complex power supply that I dumped and replaced with batteries. The sound became rock-solid, with black-velvet quietness, which I would say is due to the absolute removel (or never was there in the first place) of all fluxuations, noises, and grung from the AC line.

Just go pure DC for this straight-forward preamp and let it sing!


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PostPosted: 19 Jan 2012, 21:12 
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Further thinking on using a battery power source . . . rather than discarding the AC powered power supply, break it from the board and mount at a distance. Using a DPDT switch, connect one leg to the AC powered supply (30VDC) and the other leg to the five, 6-Volt batteries (30VDC), with the position of the switch supplying either the synthesized DC from the power supply or pure battery DC to the active tube circuitry.

When listening, flip the switch to battery power. When not listening, flip the switch to "stand-by" from the AC powered supply. This will assure that the tube filaments remain heated, thus insure stability of characteristics and consistancy of sonic signature. Removing thermal stress from the tube will also extend its useful life.

Try this to reduce microphonics. Place the preamp board, with the tubes as flat to the board as possible, into a pan of fine white sand (dry), face down, with as much of the tube in the sand as possible. The more sand around the glass bottle, the better. Otherwise, fashion several layers of led foil around the glass bottle. Led is the best damping material you can use. Hold the led tightly to the glass using several twist ties. It would also help clearify the amplification by isolating the preamp from stucture borne and air borne sources, namely the stereo speakers propagating into the same room inwhich the tubes are located. Place the preamp board inside a heavy wood box or the hollow of a concrete block with the ends closed and sealed. Of course it's always best to place the turntable and preamp in another room.

I know this sounds flippant, but in all seriousness, for maximum reduction of microphonics put the preamp board (and tubes) into a bucket and cover with fine white sand. Use cable connectors. Place near turntable.


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PostPosted: 24 Jan 2012, 11:20 
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Joined: 22 Sep 2011, 11:22
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I was wondering if anyone knows what is the stage resistive load for this preamp. I'm somewhat of a newby when it comes to this, but from what a read and understood from the schematics of this amp, the resistive load is specified by the R1 (or R101) resistor. So that would make a 100k stage resitive load for this preamp. Is that right?


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PostPosted: 24 Jan 2012, 16:03 
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beaubbe wrote:
I was wondering if anyone knows what is the stage resistive load for this preamp. I'm somewhat of a newby when it comes to this, but from what a read and understood from the schematics of this amp, the resistive load is specified by the R1 (or R101) resistor. So that would make a 100k stage resitive load for this preamp. Is that right?

If you are talking about the load on the cartridge it is decided by R1 (R101). On the original schematic that is 100K which I would say is too high for most MM cartridges. My suggestion would be to make R1 (R101) 47K. Look to your cartridge specs. for the suggested loading. It can vary quite a bit.

If you have a MC cartridge I would leave R1 (R101) at 100K. The MC preamp will provide loading to the cartridge. If you don't have a MC preamp take a look on this forum for my Le Pacific MC preamps or Hiraga MC preamp.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 24 Jan 2012, 21:56 
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Thanks for the info! I'm gonna leave as it is but I'm gonna use some rca male plug what a resistor and a rca y-splitter to adjust the loading of the cartridge.


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PostPosted: 24 Jan 2012, 23:49 
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You could you a 500ohm trim pot. and just dial-up the impedance required. Not only that you could adjust it in real time and stop when it sounds the best.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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PostPosted: 13 Mar 2012, 16:59 
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Hi all.
I'm just about finished building this phono stage. This is my first diy electronics project since helping my brother construct frankenstein stereo systems when I was 10. It is amazing we didn't electrocute ourselves back then. Anyway, I have a very simple question. Do the 4 LEDs require directional placing? If so, how do I know which side takes (+) and which (-)?
Thanks for any info! Bill


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PostPosted: 13 Mar 2012, 17:53 
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jetorama wrote:
I'm just about finished building this phono stage. This is my first diy electronics project since helping my brother construct frankenstein stereo systems when I was 10. It is amazing we didn't electrocute ourselves back then. Anyway, I have a very simple question. Do the 4 LEDs require directional placing? If so, how do I know which side takes (+) and which (-)?

Yes leds are directional. The longer leg is positive. But I'd leave them out. They are only there for looks and leds are noisy.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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PostPosted: 13 Mar 2012, 18:03 
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Thanks. I'll do as you suggest and remove them. Will I then need to bridge the gap with a wire?


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PostPosted: 14 Mar 2012, 05:56 
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jetorama wrote:
Thanks. I'll do as you suggest and remove them. Will I then need to bridge the gap with a wire?

No, don't bridge the gap.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


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