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Old 4th June 2010, 04:16 AM   #11
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If you can access the volume pot, you can take the signal off that. Will avoid signal degradation from the internal power amp.
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Old 4th June 2010, 04:52 AM   #12
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Well, yes and no. The volume pot is a good place as long as the load is a high impedance. And you can't run it very far before it degrades from cable and stray capacitance.

The ideal situation is to modify the amplifier by adding a buffer between the volume control and the line output.
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Old 4th June 2010, 02:15 PM   #13
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@ashok good suggestion, that sounds interesting...

@bob do you have any details or schematics for such an approach? i am very interested in learning / implementing as many options as possible - especially what you are talking about - as it sounds like your suggested approach would bypass the volume control and just output an independent and consistent signal.

and what distances are we talking about here? i need about 7' of cable run before i reach the mixer...

thanks guys
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Old 4th June 2010, 06:08 PM   #14
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Well seven feet of cable, I assume shielded, would be a couple hundred picofarads. A bit much; you will lose some highs.

A buffer can be made simply - for tube, a cathode follower and for solid state a cheap op amp or even emitter follower. Maybe four or five small parts and you are good to go.

What power supply is available? Of course another option you may like would be to build a buffer in its own box with a wall wart and run a short line to the volume control. That would be minimally invasive regarding the amplifier.
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Old 5th June 2010, 03:02 PM   #15
diblet is offline diblet  United States
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it would be great to have these internal for each unit if i could figure it out - however i have 4 reel to reels i need to mod - three solid state and one tube (and possibly two solid state portable record players).

i don't have any schematics for the solid state players yet, only the tube - all plug into the wall and have transformers (or in the case of the tube, will have an iso transformer soon, 115/230 VAC, 0.7/0.35 A, 80 VA). the solid state players can run off of c or d batteries, but i can't ever think of a time that i would use the line-out buffer only running off of batteries...

also, any places you would suggest looking for a design for an external box? if i can't figure out how to modify them internally - this seems like a good way to go.
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Old 5th June 2010, 04:05 PM   #16
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You can make a unit that runs off an internal 9V battery or, as I mentioned, a wall wart. Any unity gain system should work fine. If you have an op amp, especially one that will work off a single supply, you are good to go. Else you can split the supply. That's what I did with my guitar amplifier. It had about 35V single supply so I made a resistor divider to run a quad op amp I had laying around.
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Old 7th June 2010, 03:06 PM   #17
diblet is offline diblet  United States
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excellent - thanks for the suggestions

i'll try and do some research on op amps - any suggestions on good places to learn?
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Old 7th June 2010, 04:50 PM   #18
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My best source of theory has been data sheets and application notes from IC manufacturers. Check National Semiconductor, Analog Devices, Linear Technology, Texas Instruments. Of course they are biased toward their own products but after reading a few of their notes you may gain some perspective.

However, since you are probably going to build a unity gain buffer, nearly any op amp IC will do a good job. The bandwidth and noise considerations are not a problem with nearly any of them. Your main concern is what supply voltage they require. You can find single supply units, capacitor couple the input and output, bypass the power, and you are good to go.
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Old 7th June 2010, 06:00 PM   #19
diblet is offline diblet  United States
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great, i'll have a look - thanks for all the advice!
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