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Old 15th June 2003, 10:06 AM   #1
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Red face preamp for GC?

i have a 5687 tube transformer coupled preamp.

1.May i know if i can use it to drive the GAINCLONE?

2. if i can, may i just use a fixed value resistor at the input to replace the 12 step attenuator?

Pardon my silly question.......hope someone can advise.

Thanks in advance.
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Old 16th June 2003, 07:53 AM   #2
Nuuk is offline Nuuk  United Kingdom
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Hi commstech, I'm not sure about the compatibility of using your preamp but as regards the amp part, just leave out the pot and replace it with a resistor to ground. Anything from about 56K to 100K should be OK.

I'm currently building a 5687 preamp although not transformer coupled.
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Old 17th June 2003, 01:59 PM   #3
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Default Can't you just leave out the pot

I am a bit confused here. I am thinking of building my own GC, to work with my existing pre-amp. Can I just leave out the POT? Why do I need to have a resistor to replace the pot?

thanks!
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Old 17th June 2003, 06:42 PM   #4
mikelm is offline mikelm  England
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Default Re: Can't you just leave out the pot

Quote:
Originally posted by alvinlim
I am a bit confused here. I am thinking of building my own GC, to work with my existing pre-amp. Can I just leave out the POT? Why do I need to have a resistor to replace the pot?

thanks!
Hi,

The answer to this depends on wether the GC is inverting or not.

either way you could do some damage to something.

Either way it would work fine without the resistor UNTILL you unplug the lead between the pre-amp & amp. With the lead unplugged the GC input would not be tied to earth.

For inverting mode this would mean the gain of the amp would become one and it would almost certainly oscillate. Even with a 100K resistor to earth the gain would still only be about 3 and may still oscillate so you may need a small cap across the resistor. perhaps 100pF. otherwise something could get VERY HOT

For non-inverting mode the i/p would no be referenced to ground so this and therfore the o/p will probably float up to quite a high DC voltage - enough to destroy a loudspeaker.

Hope that helps...

cheers

mike
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Old 18th June 2003, 12:25 AM   #5
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Default Will 1M R at input be sufficient?

HI!

Thanks for the reply - I just checked the spec sheets for the LM3875. They specifiy a 1MR in place of the POT. Will that be a good value?

Thanks!
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Old 18th June 2003, 11:50 AM   #6
mikelm is offline mikelm  England
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Default Re: Will 1M R at input be sufficient?

Quote:
Originally posted by alvinlim
HI!

Thanks for the reply - I just checked the spec sheets for the LM3875. They specifiy a 1MR in place of the POT. Will that be a good value?

Thanks!
should be fine for non-inverting

for inverting I'm not sure, you would have to try it carefully & see what happens but the cap will be needed

cheers

mike
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Old 19th June 2003, 01:09 AM   #7
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Default rin - rf

Just read somewhere that Rin should be equal to Rf to minimise DC offset -

So, if my Rf is 22kR, my Rin should be the same (noninverting)
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Old 19th June 2003, 01:20 PM   #8
Nuuk is offline Nuuk  United Kingdom
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Quote:
Just read somewhere that Rin should be equal to Rf to minimise DC offset -
alvinlim, you're getting confused! If Rf = Ri then you will get no gain!

If you are talking about the inverted Gainclone, then the resistor from non-inverting input to signal ground should be the same as the feedback resistor to minimise the DC offset.
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Old 19th June 2003, 03:44 PM   #9
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Default You are right - I am confused!!

YOu are right - I am confused now. I think we are using slightly different terminology here.

If you see the attached schematics for a non-inverting GC (courtesy of RJM), R2 is tying the non-inverting input to ground. If I drop the Pot, do I need to have another resistor in its place?

thanks!
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File Type: gif diy_gc1.gif (13.8 KB, 543 views)
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Old 19th June 2003, 06:33 PM   #10
mikelm is offline mikelm  England
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Quote:
Originally posted by Nuuk
the resistor from non-inverting input to signal ground should be the same as the feedback resistor to minimise the DC offset.
Hi Nuuk,

sorry to be perdantic.... but I think 'should' here, should be could, unless the should is qualified...

i.e.

If low DC offset is the aim then non inverting resistor should = Rf
( if the there is a blocking cap somewhere in the input cct )

However if the ultimate sound is the the aim, it might be that the non-inverting resistor should just be non !

whilst playing with designs in spice I noticed that any tendency towards ringing could be damped effectively with quite a small value resistor at the NI i/p, perhaps 100 ohms. It's on my list of things to try. It may be just enough to smooth out the sound if that is needed.

But, all other things being equal, I do think that for any resistor to sound as good as a zero ohm link it will have to be quite good quality.

cheers

mike
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