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Old 6th August 2008, 11:24 AM   #1
Nuuk is offline Nuuk  United Kingdom
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Default Removing pot from valve pre amp

Quick question (I hope). If I remove the pot from my valve pre to convert it to a buffer do I need to replace it with a resistor to ground, and if so, what value?

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Old 6th August 2008, 12:17 PM   #2
kmaier is offline kmaier  United States
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Yes... could use a 50K resistor unless you have a specific requirement around input impedance to the buffer. Would also recommend a resistor at the output to ground to prevent a DC spike if the output is unplugged/plugged while the stage is active.

Note that you really don't have a buffer but a line stage with amplification and phase inversion. A buffer stage would be better addressed using a cathode follower. Gain would be a bit less than unity and phase would not be inverted.

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Old 6th August 2008, 12:44 PM   #3
ratza is offline ratza  Romania
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I'm using 1M for my ECC83 input.
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Old 6th August 2008, 12:51 PM   #4
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Thanks for the replies guys. I do have a 270K to ground on the output. There is also an output mute delay that prevents anything going to the power amp for about 90 seconds.

I appreciate that this isn't really a buffer, but I would like to try it in my system where volume control is done using the SB3.
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Old 6th August 2008, 01:54 PM   #5
kmaier is offline kmaier  United States
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Fair enough,

I would make one other recommendation however.... regarding the input and output loads. Consider the "ideal" load for whatever source you plan on driving your "buffer" stage with. In most cases where interconnects are used, I split the ideal load equally at both ends of the cable. In other words, if your source would ideally like to see a 100K load, I would terminate each end of the interconnect with a 200K load (in the device). In many cases, a component was specifically designed around a certain load for either damping, feedback, time-constants, etc.

The venerable Dynaco PAS series is one example. The output was specifically designed to have a 50K load. Most will note that their (Dynaco) power amplifiers had a 470K input resistor which isn't even close. However, if you look at the schematics for the PAS, you'll see a 62K resistor on the circuit board at the output and a 510K resistor mounted on the rear RCA panel jack for the output. So, parallel the 62K, 470K and 510K resistors and you get 50K. Anyone driving a different load with the PAS was not operating optimally as designed. Just my $0.02.

Regards, KM
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Old 6th August 2008, 02:56 PM   #6
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Thanks again. That goes at least part way to explaining why so many people get different results with the same piece of gear!
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Old 6th August 2008, 04:30 PM   #7
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You might want to rethink your idea.... using the SB3s volume control isn't the best solution. It operates in the digital realm and will lose data.... better to keep the volume control in your preamp.

Dave
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Old 6th August 2008, 05:06 PM   #8
Nuuk is offline Nuuk  United Kingdom
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Dave, thanks for the advice but I've been through all this.

In practice, adding a pot somewhere in the chain sounds worse than using the volume control in the SB3 (whatever the theory says)!
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Old 6th August 2008, 05:50 PM   #9
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Interesting, I'll keep that in mind... I'd like to get a SB some day.

Dave
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Old 6th August 2008, 05:52 PM   #10
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I'm glad that I did! I won't go back to hunting out CD's again! With a few mods, the sound quality is great too.
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