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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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It is currently 18 Sep 2014, 12:42

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PostPosted: 31 Mar 2014, 10:42 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Everyone, I wanted to share some information on using these regulators in high gain / low noise tube circuits. It should apply to any circuit though. I have found that the basic schematics for the LR8XX do not always show the use of a capacitor between the adjust terminal and ground. I have found that this component is extremely important to keep the noise level down in the output circuit. Using a 1.0uf poly will reduce the noise in something like a preamp by several db. In one case about 8! This capacitor needs to be rated for the output voltage. It is in addition to the one on the output terminal that is there to prevent oscillation. I normally use a 1 uf there as well. If the distance to the point of use of the regulated voltage is under say 3 inches you can put the cap at the point of use (often a tube socket pin). Bigger capacitors on the output are fine but anything over 1uf on the adjust is unnecessary.

Good listening
Bruce

More info: The Care and Feeding of LM317 and LR8 Integrated Circuit Regulators, Particularly in Valve Circuits.

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PostPosted: 13 Sep 2014, 03:16 
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Joined: 09 Oct 2012, 19:43
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Location: Vancouver Canada
If you do add the filtering at the adj terminal should the bypass diodes be added as well? Or do you think they might add more noise back in again?


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PostPosted: 13 Sep 2014, 08:34 
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Hi, The reverse diode (rectifier) is there to protect the LR8 from voltages on the output side that are higher than those on the input side. This can occur quite easily and often particularly in tube circuits. If you turn the equipment on and then off quickly the filters will charge up and it is usually the case that the one furthest from the transformer will have the least drain (slowest to self discharge). Then the voltage there will be higher than on the ones closer to the trannie. If the potential exceeds the rating of the LR8 for reverse voltage it will zap immediately. The protection rectifier does not seem to have any effect on noise.

Good listening
Bruce

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