Fuzzy EA Trem

[gez]

Did you replace the MOSFET?  If it's fried, no amount of tweaking will get it to work properly.

[smashinator]

Yep! I forgot to mention that.  The voltage change was nominal.

[gez]

Just doesn't make sense.  If the thing is measuring 4.5V at drain and you've got the right resistors in there, then you should be measuring more than .6V at the source.

If you had a wrong value drain resistor (too large) that would explain your readings (and clipping problem).

Do you still have the problem of the falling gate voltage?  If so, what's going on at the drain as it falls?  (disconnect LFO/set depth to min whilst taking measurements)

When handling the MOSFETs you are taking static precautions aren't you? (all over scuba diving outfit plus pouch containg wolfbane strung round your neck)

[smashinator]

Static?  Shoot, I'd been throwing them in the dryer with all my clothes so I could get big, wooly distortion out of them.  

Just kidding, but my static precautions consist of touching the metal base of the table I work on immediately before grabbing a mosfet.  :wink:  

I'm not having the falling gate voltage problem any more.  My voltages do fluctuate a tiny bit, but it's just going back and forth in a range of less than a volt.  I'll try setting the depth to min. / disconnecting when I measure it again.

I'll fool around with the drain resistor tonight.  I'm pretty sure it's right (measured it with the DMM), but I might have goofed.

[gez]

My voltages do fluctuate a tiny bit, but it's just going back and forth in a range of less than a volt

The fluctuation is a result of the LFO which is why I said to disconnect it (pull 22u cap) until your measurements are done.  Either that or set the rate high and depth to min.

If the voltages are about right then all it boils down to is your pickups being too hot.  If that's the case then here's an alternative way of doing the buffer.



Haven't tried it with this circuit, but should be ok.  Sometimes doing things like this introduces noise, we'll see...

[smashinator]

Thanks.  I'll let you know what happens.

[smashinator]

Ok, I put a 3.9k resistor on the collector (collector voltage is 4.6) and now my source is 1.3 volts.  Is that about right?

It's WAY better than it had been and the trem isn't so wussy, but I still get some distortion when I play hard.

I didn't think my pickups were so hot.   :shock:

Anyhow, is that voltage above about right?  If so, I guess I'll be trying the alternative buffer.

[gez]

Ok, I put a 3.9k resistor on the collector (collector voltage is 4.6) and now my source is 1.3 volts.  Is that about right?

Close enough.  

It's WAY better than it had been and the trem isn't so wussy, but I still get some distortion when I play hard.

I didn't think my pickups were so hot.   :shock:

What the hell are they?!    The original EA had problems with clipping (it was set up for even higher gain), which is why Rob Strand suggested these values.

Anyhow, is that voltage above about right?  If so, I guess I'll be trying the alternative buffer

Best to try the buffer I posted, this will cure your clipping problems and allow you to use a 4k7 for the drain resistor (the lower value you subbed will give slightly less depth).  

Personally, I prefer this type of arrangement, just that sometimes it can be a little noisier (shouldn't be a problem here though)

[smashinator]

My pickups are the stock pickups that came in my Epiphone Explorer.  

I hope this isn't a dumb question, but if I use the other buffer, and put the 4.7k resistor back in, aren't I going to be back to having the wrong voltages again?

[gez]

I hope this isn't a dumb question, but if I use the other buffer, and put the 4.7k resistor back in, aren't I going to be back to having the wrong voltages again?

You didn't have wrong voltages but, by the look of things, wrong readings.  The trimpot is there to ensure that you always end up with the correct voltage at the drain, the size of drain resistor determines the voltage gain of the amplifier (along with the composite resistance at the source) and, because we're always setting the trim to give a drain reading of 4.5V or so, it determines drain current.  Restoring the drain resistor to 4k7 won't upset bias, it just gives you a slight increase in gain....again.

This is why you had a problem with this design, there's sizable voltage gain and then the signal is attenuated.  So, if you have a hot signal there's no way of attenuating it before it recieves all that gain and the end result is clipping...unless you try something along the lines of the alternative buffer I posted (it doesn't matter what the gain is, you can reduce the signal going into the amp so that what comes out is at the same level as the original signal - plenty of headroom, no clipping)

[smashinator]

Cool.  Thanks for explaining that!

I'll try out the other buffer and let you know how it works!

[smashinator]

Sweet!  The alt. buffer does the trick!

Now for my next question: How can I quiet down the buffer?  It hums a bit (if I'm not playing, I get a nice Hum-Trem effect...).  It's on a breadboard right now, with lots of nice, long wires crossing each other at non-90-degree angles, so perhaps onces it's in a box with tidy wiring that problem will go away.  Any other things I can do to quiet it down?

Thanks for all the help!

[gez]

First of all try tidying up your layout to see if you can get rid of the hum (there's no point boxing it in the hope that it'll go away - though that's always a possibility).  If this doesn't do much then try screened cable for the input.  

Failing that, try a smaller value pot.  470k down to 100k (though no lower).  Bit of loading if you go as low the 100k, but I don't think it's going to be a problem with those damn pickups of yours!  

[smashinator]

Awesome.  I figured the messy wires were probably the problem, but if not I'll try out a different pot.

Thanks!

[gez]

Afterthought.  Did you have the hum with the old buffer too?  If so, then how are you powering the circuit?  If it's from a wall wart then this may be the problem (try a simple RC filter).  If it's from a battery, then it's probably just layout.

If the problem is unique to the new buffer, then follow the steps I outlined in my last post.

Wow, this thread is getting looooooong!  

[smashinator]

No hum before.  I really do think it's my wiring, though.  It's just on a breadboard, with lots of long jumpers all over the place.

Right now it's powered by a 9volt battery.

I've actually learned a lot from this thread.  I've got all sorts of mods I'm going to make to my tremolo planned out based on things I've learned from trying to get it to work right!