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Old 28th December 2010, 06:11 PM   #441
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Quote:
Originally Posted by Variac View Post
It certainly deserves to be built by more people. Now we need a power supply schematic...
Click the image to open in full size.

You should, however, "Support Your Local Sheriff"

(every time I think of Joan Hackett my heart cringes, a life lost too early!)
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Old 24th January 2011, 05:22 AM   #442
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Quote:
It certainly deserves to be built by more people.
I can agree with that statement. I finished my preamp enough to have a listen, then just a few cosmetic items to take care of to wrap it up. A very nice sounding preamp.

There is zero 60 Hz hum, but just a slight hiss. My midranges are Audax PR170, about 96dB. It is independent of the volume control. At listening position you can't here the hiss. Is this normal, if it's not what can be done?

Since there has been some questions on tuning or loading for the Cinemag input transformer, here is what I found to work. With a 100K pot. a 10K ohm resistor by itself worked well enough. With the volume at maximum there was a slight peaking on the leading edge of a 10KHz squarewave. A 500pf capacitor would get rid of the peaking but in the end decided to leave the 10K resistor.

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Old 24th January 2011, 07:02 PM   #443
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What sort of power amp are you using? It's possible that the gain is just too high for your setup. We can proceed from there.

(FWIW, mine was dead quiet when used with a tube amp having a gain of 6 (15 dB) and speakers having about 93dB/2.83V sensitivity)
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Old 24th January 2011, 11:29 PM   #444
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I have a Bi-Amped setup with F3's through a MOX active X-over. Not much gain.

I didn't mount the heater ceramic disc capacitors C106, 107 at the socket pins. Could this be my problem? It will be the first change I make.

Thanks for the reply

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Old 24th January 2011, 11:32 PM   #445
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It can't hurt. I'd also double check the the FETs are all OK (they can get damaged and go noisy but still work after a fashion) and that you don't have a noisy (relatively) 6SN7.
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Old 24th January 2011, 11:39 PM   #446
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The hiss is the same in both channels. You can start to hear it at about a meter away. The FETs are both right at 8ma. The boards are from Jackinnj, which came with the FETs already setup.

So it sounds like I should be able to reduce the hiss?
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Old 24th January 2011, 11:43 PM   #447
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I'm sure you can. One other thing to check is the regulator- do you have a scope? If so, connect it to the reg output with a capacitor (0u1 will do just fine- value not critical) and see what the output noise is. The fact that the noise is the same on both channels potentially points in that direction.
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Old 25th January 2011, 03:25 AM   #448
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I don't have a "hiss" issue on mine.

You can often knock down the noise of an LM317 regulator with some significant bypass on the Adjust pin -- and remember that it is significantly elevated above ground. I am just lifting a page from the "Last PAS" power supply of many decades ago in which the authors used 470n polypropylene in parallel with 330uF -- both probably rated 400VDC.
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Old 25th January 2011, 06:18 AM   #449
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No significant noise on the regulated output. A Fluke meter showed 3 to 4 mV and the scope was between 15 and 20 mV peak-peak.
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Old 25th January 2011, 09:44 AM   #450
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Quote:
Originally Posted by BDP View Post
No significant noise on the regulated output. A Fluke meter showed 3 to 4 mV and the scope was between 15 and 20 mV peak-peak.
That's quite a bit larger than I have- we may have found the problem. Is the output bypass the same as shown in the article? If so, then you may have a funky 317.
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