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Passive Pre-amp

E

earl of sodbury

Guest
Slapped together a little volume-control-on-a-stick: 50k Alps pot, 2 switchable inputs, 1 output, earthing terminal; star-earthed; silver in teflon (i/c leftovers) point-to-point wiring.

Up to a point it works - sounds lovely, switches fine, polarity AOK, but volume is WAY too high - starts Bloody Loud gets a bit Bloody Louder: it's all volume and no control if you catch my drift.

Do I get a higher-value pot (
sad.png
) - if so can anyone suggest a value?

Or can I cheat with resistors in the line-in- again, any suggested values?

I'm using it with an MF X-P200 power amp: Input sensitivity for 125W, 8 Ohms: 1 Volt; Input impedance: 27k Ohms.

I know I should be able to calculate this stuff but I'm effectively "dysnumeric", AKA "a spacker with numbers"
icon_redface.gif.eb51a1f92a0314cdfc14dd3cb4473da1.gif


churz, eofs

 

meninblack

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My Rotel passive pre has resistors on each input. I'll take the lid off tonight and see what value. I'll see if there's anything written on the volume pots while I'm in there.

 

kennyk

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thecrossovernetwork,
try a pseudo shunt:

shuntpotvol.gif


and use a larger value for R , 68 or 81 k perhaps?

(don't forgetto wire the pot up backwards,or the volume control willwork anti-clockwise, IYSWIM)

edit: this effectively brings the pot out of the music signal path. I've done this using 47k VISHAYs with the PreII I'll be bringing to the bakeoff.

 

MartinC

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goodpost.gif.00822561e807cf35c79b3503f2e609d0.gif


I was about to try to descibe that, but the picture makes it far clearer than I ever would have managed.

 
E

earl of sodbury

Guest
Thanks gents - if I can figure out what it means in practice I'll give it a go...

churz, eofs

 

meninblack

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OK, what's in my pre-amp?

Each input phono has a resistor wired between the phono outer ring (signal earth) and the chassis earth. These resistors are coded YELLOW YELLOW BLACK BROWN BROWN RED which I think means 4.4 kohm - I don't think the code makes sense the other way round.

The left and right volume pots are 10 kohm, wired as regular in Kenny's sketch.

The output phonos have resistors wired between signal hot and the phono centre, coded BROWN BLACK BLACK BLACK BROWN RED. Again, I think this code only works one way round and is 100 ohms.

Hopefully someone more knowledgable can tell us if this makes sense....
pray.gif.84955f3b1709939cd9186fd2e3d88a8e.gif


 

kennyk

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thecrossovernetwork,
meninblack wrote:

OK, what's in my pre-amp?Each input phono has a resistor wired between the phono outer ring (signal earth) and the chassis earth. These resistors are coded YELLOW YELLOW BLACK BROWN BROWN RED which I think means 4.4 kohm - I don't think the code makes sense the other way round.

The left and right volume pots are 10 kohm, wired as regular in Kenny's sketch.

The output phonos have resistors wired between signal hot and the phono centre, coded BROWN BLACK BLACK BLACK BROWN RED. Again, I think this code only works one way round and is 100 ohms.

Hopefully someone more knowledgable can tell us if this makes sense....
pray.gif.84955f3b1709939cd9186fd2e3d88a8e.gif
best stick it on a meter, to check but I would have expected a small resistance between signal ground and chassis. other way around is 211 ohms with 5% and 25ppm temp (if one of the yellows is actually gold)

 

meninblack

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Good idea - except some bugger at work has "borrowed" my multimeter!
angry_smile.gif.8c0f636dd68eb7da5c2577e161ee9e27.gif


 

kennyk

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thecrossovernetwork,
talent my arse. my only talent is plagerising somebody elses work and knowing how to upload it to my own photobucket account :dude:

I can't draw a straight line with a ruler. true story: Engineering Drawing at college. the lecturer stopped. looked at my 'effort', held it up to the rest of the lads and said "That is an abortion".

 

mosfet

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Well thank fuck for that.

Anyone with competence in any engineering discipline should not be able to draw anything free hand. Crap drawings only. Them’s the rules.
thumbs_up.gif.3c8ee62eda0e86146178ab30b9facd86.gif


 
E

earl of sodbury

Guest
Now working thank-you verymuch gentlemen - albeit after a fashion: if anyone wants any advice on cobbling together a system that majors on extreme shrillness: I'm yer man
38786-6.gif.132cf51c49b8cd26f08985f56fa67647.gif


At least now I can listen to the news and rip the guts out of the X-150 and replace 'em with something respectable...

churz, eofs

 
E

earl of sodbury

Guest
kennyk wrote:

what did you use for resistors?
I was hoping no-one would ask that...

I tried, variously,acrusty sock, some baby hamsters and a damp paper bag, but none of them worked terribly well, so I just wired it up properly (yes, that's right
icon_redface.gif.eb51a1f92a0314cdfc14dd3cb4473da1.gif
:oops:
icon_redface.gif.eb51a1f92a0314cdfc14dd3cb4473da1.gif
),until such time as I get some resistors of the correct value...

:minikiev:

 

kennyk

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Aug 8, 2005
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thecrossovernetwork,
ok. in that case some 47ks or 39 ks should be about right, if the pot is working ok. try some maplin cheapies first to determine the best value, is my advice.

 
E

earl of sodbury

Guest
kennyk wrote:

ok. in that case some 47ks or 39 ks should be about right, if the pot is working ok. try some maplin cheapies first to determine the best value, is my advice.
Thanks!

 

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