help with preamp

Harry Muff · September 15, 2018, 09:32:00 AM

hello, I am building a preamp but its not turning out. everything works fine , nice boost that makes the amp sound nice and the switch works too but no matter how I slice and dice it with a million different parts the effect pauses/hesitates/delays when i turn it off. I dont think its supposed to do that?

the green ringer I been working on is still ticking but I got it to only tick when my amp gain is above 5 ..maybe its my amp. I am using a orange crush 20 129.99$$ special ..seems like a good amp but maybe not for pedals? as others have stated, nothing in a GR circuit to be ticking/motorboating ..i dont care about the ringer but the pre amp has me baffled ..I got about 30 hours into this thing which looks like a simple hour long project ..i literally switched out hundreds of parts and different breadboards and power supplies; and I went ahead and tried a few on the vero with solder , got same bad results..im confused here, this is weird. could it be the fets im using? I thought it would be a faulty electro cap doing this ..I had a pedal one time that paused clicking it on and I think it was a faulty cap in the end..as you can see this one only has three and one is the boost on the switch

GibsonGM · September 15, 2018, 09:56:46 AM

Sounds like you need to look at your switching, and turn this into a True Bypass switching scheme! Extremely easy to do - will take it out of the loop immediately when you step on the stomp switch.

As wired, the effect remains 'in the circuit' when it is off - it is 'hanging on it', giving you that drag you are noticing. Search for "true bypass 3PDT", put this baby in a stomp box and wire it up, it will make a good booster

Hope that helps.

Harry Muff · September 15, 2018, 10:16:10 AM

thanks gibson! I do have it on a switch. got a few 3pdt stomps here .was googling some stuff and seen beavisaudio schematic for a 2n5457 stratoblaster. but this one sounds pretty good. beavis has a 47uF instead of the 10uF and says its for more gain. im going to hook up that true bypass switch ! thanks for a quick response too

GibsonGM · September 15, 2018, 11:19:58 AM

YW: About every option you'd ever want is right here, Harry!
http://www.generalguitargadgets.com/how-to-build-it/technical-help/switching-and-wiring/bypass-sw-options/

Using a 3PDT (1 pole is for an indicator LED; you can use a DPDT if you have no LED) will let you totally remove the thing instantly, no wait for caps to drain and so on.

There are so many forum members, usually you can get an answer in no time! Welcome aboard, enjoy your builds and learning all this new stuff!

PRR · September 15, 2018, 04:15:57 PM

> the effect pauses/hesitates/delays when i turn it off. I dont think its supposed to do that?

Your switch *appears* to just disconnect power. Power up/down on an audio amplifier is normally chaotic. We normally do not turn-off power while on stage. To un-boost, we leave the power to the booster but switch the *signal* to bypass the booster.

Harry Muff · September 15, 2018, 04:28:56 PM

Quote from: PRR on September 15, 2018, 04:15:57 PM
> the effect pauses/hesitates/delays when i turn it off. I dont think its supposed to do that?

Your switch *appears* to just disconnect power. Power up/down on an audio amplifier is normally chaotic. We normally do not turn-off power while on stage. To un-boost, we leave the power to the booster but switch the *signal* to bypass the booster.

I redid my testing jig with the true bypass as suggested and I like it like much better now for the testing of the circuits. after working with the voltages, I finally got it sounding right with a drain voltage at approx 4 volts; this seemed to stop all the problems. the 2k2 gives me around 7-8 volts. I have a 47k drain resistor in place of the 2k2. I am thinking this is a byproduct characteristic of an unbiased fet lol . this thing is pretty cool. its all I need for the little amp I have.

GibsonGM · September 15, 2018, 05:29:35 PM

Glad to hear it! I've also had to go that high with drain resistances if substituting a FET in a circuit...it is common.

Harry Muff · September 16, 2018, 07:51:58 PM

I meant the 6.8 resistor!! this thing has a nice punch to it. it may be clobbering my little orange 10" speaker. I could see this opening up on a 30 watt tube amp easily. I dont think it changes the frequency , sounds like my amp but way louder!

I didnt think to mess with the supply resistor. havent had too much experience biasing a fet but from some googling I thought you biased through the drain output so thats what I did until I saw roughly 4 volts.

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help with preamp