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PostPosted: 11 Apr 2017, 19:22 
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Joined: 02 Oct 2014, 20:36
Posts: 28
here in the south of hell i just can't find the lr8's regulator...can i use a irf 840?
thanks.


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PostPosted: 12 Apr 2017, 07:20 
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Joined: 24 Oct 2010, 07:05
Posts: 296
If you are going to change power supply I'd made a good shunt regulated PS like SSHV2. At this currents it will allow to eliminate most of the RC strings and allow cleaner supply with very low Zo.


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PostPosted: 12 Apr 2017, 09:34 
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Joined: 03 Jan 2017, 15:42
Posts: 6
I made a six output adjustable LR8 power supply with that low impedance thought in mind. Mouser has the LR8 for $0.64 so why not try it? I also used 2oz copper pcb with twice the normal copper.


Last edited by RiverPhil on 12 Apr 2017, 12:05, edited 1 time in total.

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PostPosted: 12 Apr 2017, 10:49 
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Joined: 24 Oct 2010, 07:05
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LR8 itself has the output impedance in the range of 10Ом (мах. 30Ом) @120Hz and the impedance changes (rises) rapidly towards the higher frequencies in the audio band. You can of course use C, CRC...-like strings after the regulator to lower the impedance and make it smoother across the frequencies, but the capacitors should be good and - for best results - filtering should be good too for that.
Active circuitry like shunt regulators uses only active regulation for the same thing, and stable low Zo value across the whole audio band. Mentioned regulator for example has around 100mOhm and saves the impedance almost in the whole 20-20k Hz band.


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PostPosted: 12 Apr 2017, 16:26 
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Joined: 04 Jun 2008, 20:59
Posts: 4052
Location: Arizona, USA
Hi, I think a response I made went into limbo land. I actually made one of the preamps with 6 regulators. One for each tube. It was no quieter and didn't sound any different than ones with only two regulators. If figure resistors are cheap and less prone to issues than regulators. You can fry a whole channel's worth of LR8s if you are not careful all at once. One is easier and cheaper to replace than 3. I like to put the TO92 types in sockets and mount the flat pack ones on small "surf boards". Then I plug the boards into IC sockets. It makes life a lot easier if there is any sort of failure. Now the answer to a question you didn't ask. I use the regulators as active filters and only secondarily as regulators. They are really good at that and will filter noise quite well. Some folks have noticed that I put relatively large capacitors on the adjust and output terminals. They do indeed make the regulator less "fast". But since that was not paramount to me it is OK. The filtering effect is magnified. Yes you could use other pass devices, but this is simple.

Good listening
Bruce

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PostPosted: 23 Apr 2017, 11:28 
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Joined: 10 Nov 2013, 05:03
Posts: 11
Hi, I have also built this great phono amp. I use balanced input to LL1678 and balanced output but I just wondering is it possible to change phono stages to balanced mode. Is it difficult to make?
Turntable connected this way http://www.waynekirkwood.com/images/jpg/Balanced_Arm_Wiring_small.jpg
LL1678 balanced this way http://www.audionote.co.jp/images/products/sfz_img5e.png

BR/Alahti


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PostPosted: 24 Apr 2017, 03:05 
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Joined: 24 Oct 2010, 07:05
Posts: 296
While it is definitely possible to make it balanced I'd go another way. There are a lot of balanced phono amps schematics around, you can just choose anything you like. In most cases the balanced counterparts have as much as twice elements, so you should be going to prepare for that.


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PostPosted: 24 Apr 2017, 15:04 
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Joined: 04 Jun 2008, 20:59
Posts: 4052
Location: Arizona, USA
Hi, My experience with balanced circuits is that they can be useful in EMI noisy environments. In typical applications I feel they add more complexity than is justified. In vinyl reproduction the limiting factor is the record itself. The max S/N is in the -75db range and that is seldom achieved. A more typical value is -65db. The preamp is already well quieter than that. A balanced input type preamp can be constructed but the extra complexity of the circuit is likely to increase the noise level rather than decrease it. (more parts = more noise most of the time) What I would do with respect to your system is keep the balanced input to the SUT and make the cables short to the preamp and use unbalanced there. Some things I have found useful are put the SUTs in a steel box (not aluminum or cast pot metal). It makes them more immune to hum. You can put the box inside the preamp but be aware of the orientation of the coils and the power transformer. You may have to rotate one or the other to get it really quiet. Use good quality interconnects. Use a chassis ground from the turntable to the SUT and on to the preamp. My experience with Lundahl SUTs is the case of the SUT should go to the signal ground and not the chassis ground. It seems to be less sensitive to hum and noise that way. Seems contra intuitive.. but works.

Good listening
Bruce

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PostPosted: 26 Apr 2017, 11:38 
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Joined: 10 Nov 2013, 05:03
Posts: 11
Hi, thank you for the answers. I have already separated LL1678 to the metal chassis and seems to work much better in this way.
Question is about signal ground, have I connected it right? Cause I still hear some little hum.

Another question is how or is it possible to attach my Grados 80i - 32 ohm to the output. Now is the volume not enough.

BR/Alahti
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PostPosted: 26 Apr 2017, 13:47 
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Joined: 11 Nov 2015, 13:19
Posts: 10
Hi,

I using this phone preamp with MM type head more then a year and I really very satisfied by his sound. Now I decided to make some upgrade and bought MC head which parameters are:
Input load impedance 100 ohms
Output impedance 10 ohms
Nominal output voltage 350μV

Could some one tell me if this head will work with this riaa phono preamp and step up transformer? If yes, what nominal resistor R1-R4 I should use?


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