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Old 8th July 2010, 10:12 PM   #1
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Default input impedance of the Tubelab SE?

Does anybody know what it is? The reason I ask is that I'm trying to figure out if I need a preamp for my system. I have enough gain without a preamp, so the only reason I'd need one is for impedance matching. I understand the input impedance of your amp should be 10 times the output impedance of whatever's driving it. If the output impedance of my phono stage is low enough, I don't see any reason to add an active preamp.
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Old 13th July 2010, 03:38 PM   #2
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It depends how you built it. If you used a pot, the input impedance is approximately that of the pot's resistance. If you didn't use a pot, the input impedance is approximately that of R8, which is 121k by the book.
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Old 15th July 2010, 03:03 PM   #3
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Thanks! I clipped those two resistors when I put a volume pot in the Tubelab. I don't think I replaced them when I removed the pot, I guess I probably ought to.

It sounds like I should be fine with a passive preamp from an impedance point of view.
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Old 16th July 2010, 01:11 AM   #4
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I don't think I replaced them when I removed the pot, I guess I probably ought to.
5842's can be unstable in some situations. Having some path from the grid to ground is necessary. If your passive preamp ALWAYS has some resistance across its output terminals you can get by without a resistor. If a switch is involved that is open during switching, add a resistor. It doesn't have to be 121K. Something under 300K is OK unless you are using LED bias. In that case 150K is max. These values are from the 5842 data sheet, and higher values have proven unstable (the plate voltage wanders around too much).
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
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Old 16th July 2010, 09:28 PM   #5
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Good to know, thanks. Currently I have the inputs connected directly to a Firstwatt B1 buffer, so I guess I'm safe without the extra resistors. I think I'll try taking the B1 out and just put a nice stepped attenuator in there to see how it sounds.

Before the B1 I was using a Transcendant Audio Grounded Grid preamp. It seemed to have a livelier sound and better tonal balance than a passive pre, but it did add a little distortion and a buzzing sound I could never get rid of. I built the thing from scratch, so it's probably because of a mistake I made rather than a problem with the design.

I also built a Lightspeed Attenuator, which was crystal clear, but it didn't have the punch of the Grounded Grid or the B1. The tone seemed off, too, and if I listen to it for more than 30 minutes I get a headache even at low volume.
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