Microamp not unity gain??

[nordine]

Is the microamp NOT supposed to give unity gain at minimum volume setting?

At minimum it gives me a huge volume boost.. maybe not that huge, but way noticeable....

is this normal?
similar experiences?

[nordine]

btw, i've checked everything, used exact values
IC=TL072

used tonepad schematic: http://www.tonepad.com/getFileInfo.asp?id=6

increasing R3 evens out the gain discrepances, but it's a tricky way... and i don't see anyone doing it, also

[aron]

I haven't looked at the schematic for ages, but mine doesn't seem like it boosts near minimum. I have to turn it up a ways to get it to really start boosting a lot.

[nordine]

having tried everything, i'm starting to think maybe the equipment on which i'm testing is the cause of this apparent misbehavior.... i'm amplifying the signal in a home stereo (not the best one)... could it be that the impedance load put in the signal can be that severe to cut its volume... so i get it back amplified to what it should be, due the microamp buffering characteristics?

[Mark Hammer]

If the pot and series resistor and feedback resistor are *exactly* the value they say they are, the minimum gain will be around 1.1 or so.  If the pot is a little less than its stated value, and the feedback resistor is a little more than its stated value (both well within the tolerance for their value), then the minimum pot setting will produce gains greater than 1, though not likely more than 1.4 or 1.5.

[bwanasonic]

i'm amplifying the signal in a home stereo (not the best one)...

Good way to blow your speakers, if you care. The transients from a guitar can fry them pretty easily, I can attest from personal experience. Cooked a few stereo amps that way as well, back in the day (some nice McIntosh tube ones included).

Kerry M

[nordine]

If the pot and series resistor and feedback resistor are *exactly* the value they say they are, the minimum gain will be around 1.1 or so.  If the pot is a little less than its stated value, and the feedback resistor is a little more than its stated value (both well within the tolerance for their value), then the minimum pot setting will produce gains greater than 1, though not likely more than 1.4 or 1.5.

This is pretty weird

considering this is a pretty simple project (built the phase90 last night w/out problems) i can't explain me what is happening

tried exact values in everything, and get a damn huge boost.. even tried with the 741, swaped several tl072's (just to be sure), and nothing...

seeing this almos hasn't explanation, which resistor should i increase/decrease to match levels?... a 4k7 on 470ohm place matches the outputs... this shouldn't generate any problem right?

ps: i've been running my instrument in the stereo for ages, i think it's tough enough for it 

[Mark Hammer]

There are a few things going on.  First off, a guitar plugged directly into your stereo is such a hopeless impedance mismatch that the loading will cut the apparent signal level considerably.  Keep in mind the stereo is expecting to see 600ohm loads from tape decks, CD players, VCRs, tuners, etc.  Your guitar is WELL above that (around 10x if not more).

Additionally, the volume you hear is the sum of the entire spectrum.  Simply restoring a degraded treble will create the appearance of increased volume.

So, between what the loading does to the overall level, and what it does to the treble, sticking a suitable buffer between guitar and stereo input can be expected to increase apparent level considerably.  It has nothing to do with chip choice, and everything to do with feeding your stereo something it was never planned to amplify.

Feel free to offset this by replacing the 10k output resistor with a 10k log pot.

[nordine]

THAT WAS IT

a mixture between the stated impedance affair and the treble loss and apparent volume rise

so i tried bypass and non bypass modes pre buffered by a fetzer valve

thanks a lot Mark and others

cheers