need help with the dirty sam... weird bias problem...

marrstians · January 29, 2005, 07:24:59 PM

ineed some help with the dirty sam... there's a weird bias problem and i'm not sure where it's coming from... when i first plug it in it sounds find, it records like it's supose to and plays back fine... after a little while the guitar input will sound like it misbiases itself... the recorded signal will still sound good but if you play your guitar it will sound like a misbiased transistor... if you unplug the cord and plug it back in it will sound fine again for a couple of minutes... weird...

the only other thing i noticed is that the "safe" switch doesn't make it safe... if it's off it will record like normal but if you turn it on it will not record the guitar signal but wont save the old sample either...

the second thing i'm not really worried about but the first problem makes it unplayable after a couple of seconds.... any help would be appreciated...

ethrbunny · January 29, 2005, 10:11:36 PM

<wild ass guess>
capacitor in backwards? Sounds like it gets charged up by the signal.
</wag>

Im glad to hear you got this working. Mine has a fault somewhere thats making it v noisy.

marrstians · January 29, 2005, 10:14:49 PM

i'll poke around and see if that's it... it is a noisy pedal so maybe you're is working ok... :wink:

ethrbunny · January 29, 2005, 10:19:27 PM

Noisy how? mine has a loud hum with the playback cycle. It goes away during recording. I tried recording 'nothing' by grounding the input pin but the humming sound was still there.

marrstians · January 29, 2005, 10:24:03 PM

yup, sounds like it's working... you might have a little more hum than others but that's what these type of circuits do... that's why it's kindof a lo fi thing, as zvex suggests... i uped the pots a little so i got a little more headroom but it's still pretty noisy... did you have any problems with the original guitar signal?

marrstians · January 30, 2005, 02:21:13 AM

i checked the polarity of the caps but they seem to all check out... it sounds like the TL074 is misbiased but if that were the case then the playback signal would sound misbiased as well 'cause the last stage of the loop goes back through the TL074, correct? what should be my next step?

ethrbunny · January 30, 2005, 08:08:46 AM

Well I'd say its more than just a hum - its more of a buzzing - its as loud as the playback signal and it coming from both of the SP+ and SP- pins. It makes the unit unusable.

marrstians · January 31, 2005, 12:45:36 AM

hmmm... i'd clasify the sound of mine more like a hiss than a buzz... i'm not sure about yours... any more ideas on my problem?

ethrbunny · January 31, 2005, 10:12:19 PM

There are few things that have to happen to make the 'safe switch' work. Read the datasheet on the Winbond chip and try to simulate the effect by tying the appropriate pins to ground. If that works ok then backtrack up the line to the safe switch itself to see where its mis-wired. That might help to solve the other problem as well.

News:

need help with the dirty sam... weird bias problem...