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Old 21st December 2010, 03:53 PM   #1
ixe13 is offline ixe13  Canada
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Default input resistor on the JE LAB 300B

Hi,

I dont wanna use the volume pot on the input of this amp, do I have to replace it with a resistor of the same value?
I've looked around on different schema on many sites, and the value are quite different...

If not the same value, What would be the best value ?

Thanks for your help...

Yves

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Old 21st December 2010, 04:21 PM   #2
cjkpkg is offline cjkpkg  United States
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I am using a 300ohm resistor direct from the input to the grid of the 6sn7, then a 100K resistor from input to ground. No pot on the inputs - signal comes from a preamp.

This combination helps to prevent oscillations and gives the grid a reference to ground.
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Old 21st December 2010, 04:26 PM   #3
k9swc is offline k9swc  United States
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You'll need to replace it with TWO resistors in series, the sum of which equals the original pot, connecting the point the two fixed resistors are connected to the point in the circuit where the wiper arm of the pot is connected (V1A grid). The problem will be determining just what values to use; I'd try a 68k + 33k or 82k + 18k, with the higher value connected to the input (lower volume) or reverse them for higher volume.
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Old 21st December 2010, 04:33 PM   #4
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If Yves does not want the input signal attenuated just the 100k resistor will do. No voltage divider needed then.
Cjkpkg, the 300 ohm resistor is grid stopper, you must put the 100k resistor before the grid stopper resistor.
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Old 21st December 2010, 04:45 PM   #5
DF96 is offline DF96  England
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Best to add an input capacitor too, in case whatever comes before puts out some DC. Also keeps subsonics away so less intermodulation with signals we can't hear anyway.
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Old 21st December 2010, 04:49 PM   #6
k9swc is offline k9swc  United States
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True, Pieter, but using just the 100k resistor would be the same as running the amp with the volume control all the way up. I haven't found many amps that are run that way - I run most of mine around 3-4. Wide open (10) would probably blow my eardrums if not the speakers...Not to mention making the neighbors very unhappy!
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Old 21st December 2010, 05:06 PM   #7
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k9swc,
Of course, but Yves might have some other volume control already.
I don't like to attenuate input signals with plain voltage dividers; then look at the schematic and see where you can lower the gain of the driver stage, for instance by removing the 47 uF cathode capacitor.
This amp has un unpractical input sensitivity of 250 mV RMS to begin with.
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Old 21st December 2010, 05:16 PM   #8
ixe13 is offline ixe13  Canada
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Hey guys,

Thanx for the infos,

I forgot to mention that I'll be using a passive preamp with this amp.

So the idea would be to have the amp input wide open and the control the volume with the preamp.

But then, use that way, am I gonna get end up with noise picked up on the way to the amplifier?

Also, I dont mind adding a cap as suggested by DF96. What would be the value (although I could look around on different schema on the internet)

I guess the one would go in parallel with the input resistor?

I'm not that good to make the calculations and make the right decision.
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Old 21st December 2010, 05:22 PM   #9
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The input cap direct at the input in series, before the 100k resistor which goes to ground.
220 nF should do.
However I would check first if the cap is really necessary; most sources have output caps already.
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Old 21st December 2010, 05:28 PM   #10
ixe13 is offline ixe13  Canada
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Thanks pieter t

I'm using a Cambridge audio CD player and (Heresy) a IPOD through the output
of a Ihome radio.

Surprisingly, with 320 KPBS MP3 the sound is still pretty good...
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