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Old 15th January 2010, 02:08 PM   #1
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Default Tube buffer/preamp. Where to place voltage divider?

Purchased one of these low cost tube buffer/preamp 'kits' on ebay. I really just wanted to get involved with tubes on the cheap ; and delve into the unknown, at least for me.
There is little info about them , other than the 6N11 tubes in SRPP config. I asked the seller about what the circuit 'gain' was , but was told there is no gain , it's a preamp . I'm guessing something was lost in translation.

My intent is for use in one of my many cd players; as a tube output buffer. Pretty straight forward. I have a Rotel 855 , which has plenty of room for the board to fit. Hook up was easy. The 'problem' is that my real preamp clips , as I expected . My guess is this circuit has about 15db gain , given that a 12db fixed attenuator I have(RCA type in series) gives me close to normal levels with no clipping.

Now to my question. I want to add a fixed voltage divider inside the player. Should I place this at the input of the tube circuit, or on the output , before the rca out? Does this affect the output impedence? Which values (high or low resistence) will prevent overloading the stage before it. Or is there a way to reduce the gain of the tube circuit itself?

Thanks
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Old 15th January 2010, 02:22 PM   #2
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Why on earth would you want to add an extra stage of gain to an already too high output CD player? An SRPP stage will have gain near the mu of the tube used, so x15 is completely reasonable.

Then you are wanting to add a voltage divider? I operate on the principle of KISS, and doing what you are doing is far from KISS.

If you can turn the circuit into a cathode follower, this will give x1 gain, but could be a bit complex to work out.

The other solution is to keep the SRPP circuit, but add negative feedback to bring the gain down. Do it with a (say) 100k R from the input terminal to the grid of the lower triode, and another 100k R from the junction of the 100k and the grid, to the output - after the output cap.

Neither will improve the sound you already have though, but at least will be usage.

Regards, Allen
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Old 15th January 2010, 02:41 PM   #3
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Allen

I'm honored by your reply(I think)

Yes , this very likely does not make sense. Like I said , just looking for a cheap way to get involved with tube circuits. My ultimate goal is for changing one of my Philips based players(1541) to resistor I/V , and using the gain the circuit provides. But I am still interested in knowing the best place to reduce total output ; at the input or output. I did insert a quick voltage divider on the output of the tube board and it appears to work adequately.

Just getting acclimated

Thank-you
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Old 15th January 2010, 02:49 PM   #4
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Then what you have in that kit is perfect for what you finally want to do. SRPP will do just fine in amplifing a resistor I/V.There has been a thread here about exactlythis - well worth a read. Some silly name I can't remenber. "Lumpomator" or similar.

If you must add an attenuator, then before the gain stage will result in lower distortion, but more output noise, after will have more distortion but less noise. Chose your poison.

Regards, Allen
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Old 15th January 2010, 03:07 PM   #5
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Allen

You are correct; Lampizator. I must admit , his site is where I got the idea. He promotes it for use with resistor i/v , as mentioned.

Without getting to specific , is this a low distortion design? Is the tube used(6N11) the weak link? Is there a lower gain tube substitute? Or one for higher gain , should I need additonal gain when resistor i/v is implemented.

Thanks
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