FET Driver

[soggybag]

I have been working on Tim's FET driver. It sounds pretty good, but squeels at high gain. I have been biasing Q1 and Q2. I have adjusted the 47K resitor above Q1 and reading the voltage where the 10K and 1uf meet the lower leg of Q1. It seems that I had to go all the way down to about 11K before I could read 1/2V+, about 4.7V, my battery read 9.4V.

I found this post: http://www.diystompboxes.com/smfforum/index.php?action=post;topic=19626.0;num_replies=2
Where the author talks about adding sort of a double high pass filter on the output. Looking at the schematic here: http://www.lynx.bc.ca/~jc/meteorModded.gif
Would that be R20 and C13 and R21 and C14?

[brett]

Hi
Yep, those 4 components make 2 single-pole filters.
Tp calculate the cutoff (-3dB) frequency, use this formula:
f = 1/(2.pi.R.C) where
pi=3.14
R = resistance in ohms (ie kohms x 1000)
C = capacitance in farads (ie uF/ 1000 000)

cutoff frequencies of 6 to 12 kHz are often useful for taming high-pitched noise
cheers

[soggybag]

Thanks for the reply Brett.

After figuring out a few mistakes in biasing, I had been mesuring wrong as I had the source and Drains confused, I have Q1 and Q2 biased at about 4.6V.

Everything is working and the sound is pretty good. There is a problem at high gain, I hear a high pitched whine. It goes away if turn the tone control down on my guitar or back off the gain on the FET Driver. I think I will try and tack the filters mentioned above on the output of the FET Driver and see what happens.

[soggybag]

I added the Meteor filter on the end of the FET Driver, this cleaned up the sound by removing some high end hiss, this is an improvement.

I'm still getting the high pitched whine when I turn the gain all the way up. I think reducing the overall gain would not hurt as there is gain to spare. Can anyone give me some ideas on how to reduce the gain?

I'll take a guess that I might add a resistor the .1 cap and the drive pot. Or maybe between the drive pot and Q2.

On second thoughts, maybe it would be best to change the gain of Q1. I would guess this would be something to do with the 10K resistor coming off the Source of Q1.

Or, and this might be the best idea yet, I could swap out some FETs and see what happens with some lower gain FETs if various positions.

If anyone has any thoughts or recommendations I'm open to suggestions.

[brett]

Hi.
Are you using the 51pF CBRITE cap?  That cap potentially lets a lot of ultrasound through when the gain is turned up.  I'd be tempted to remove it.
Lower gain FET like the MPF102 would probably work well here, given the number of gain stages.  Some people don't like MPF102s, but I've never had a problem with them (except that they have lower gain than J201s).
Alternatively, keeping the J201s but removing the first source bypass cap (C2) will reduce gain a lot (several dB) and might solve the oscillation issue.
cheers

[Cliff Schecht]

I added the Meteor filter on the end of the FET Driver, this cleaned up the sound by removing some high end hiss, this is an improvement.

I'm still getting the high pitched whine when I turn the gain all the way up. I think reducing the overall gain would not hurt as there is gain to spare. Can anyone give me some ideas on how to reduce the gain?

I'll take a guess that I might add a resistor the .1 cap and the drive pot. Or maybe between the drive pot and Q2.

On second thoughts, maybe it would be best to change the gain of Q1. I would guess this would be something to do with the 10K resistor coming off the Source of Q1.

Or, and this might be the best idea yet, I could swap out some FETs and see what happens with some lower gain FETs if various positions.

If anyone has any thoughts or recommendations I'm open to suggestions.

Sounds like it could be your guitars pickups. Are they properly potted and/or do you know that the coils are tight? Any vibrations in the coils will cause microphonic feedback. How loud are you playing? I've had the same problems with effects, but most of my noise problems were alleviated after potting the SD Hot Rails that I had to alter to get to fit in my Charvel.

[soggybag]

Thanks for the reply Brett, I have not used the 51p cap. I have built the FET Driver exactly as the schem on Tim's site, with the addition of the filter on the output from the Meteor.

The one change I did make was to confuse the Source and Drains of the FETs, they are switched for all of the FETs. It seems to be working as is. I tested the first FET in the correct configuration, by bending S and D legs around, and it sounded the same, so I left it alone.

I going to definitely try out the MPF102, I think I should have a few in the parts bin.

Thanks for the reply Cliff. I think there should not be a problem with the pickups, I have never had a problem in th past.

[soggybag]

I tested a few FETs this morning.

MPF102 did not have enough gain to get any overdrive sounds. I tried a few others, but the j201s sounded best.

[aron]

There is a problem at high gain, I hear a high pitched whine.

Is this only happening when your amp is really loud or does it happen at low volumes but when the FET driver is cranked but the amp is soft?

If it only squeals at loud volumes, it could be your pickups. Otherwise I would think it might be oscillation in the layout and/or wires.

[soggybag]

Thanks for the reply Aron. The sound is not pickup feedback. I can hear it with headphones. It seems to appear as I turn the gain up past about 75%. At this point there is a lot of gain. Maybe too much, I was thinking it would sound OK if I lowered the gain, which might alos clear up the problem.

It could be the wiring. But, everythng is pretty tight and arranged with the three FETs in a line. There is not a lot of extra wiring. I'll post a picture later this evening when I get off work.

The sound is very similar to the BIASB 2.

Speaking of controlling the gain. I think it must be possible to control the gain by adding a resistor on one of the lugs of the 300K gain pot. I would guess that adding a resitance in series between this pot and Q2 would reduce the gain?

I used a 500K pot, if I understand this correctly at low resistance I'm getting higher gain. So the larger pot is only giving more range on low gain end. Changing the pot would have no effect on max gain.

[soggybag]

I think I am on to the problem. It seems like the positioning of the wires causes the whine. The wiring I have is pretty short, about 2".

There is so much gain using J201s, I think it may be too much, it might be better with several lower gain FETs. I tested a few and the sound was bad, but I realized the bias was way off when I changed FETs. I did not use trimmers so setting the bias is not very easy.

I was also thinking it might help to put a filter between Q1 and Q2. It might clean up some harsh high end before goes into the second gain stage.

[aron]

Did you try using shielded in and out wires?

[soggybag]

A shielded wire is a good idea. At this point everything is still outside of the box. I think this issue may need some planning when comes time to box it all up.

[lovekraft0]

A couple of other ideas that might work:

• 1. Install a small cap in parallel with Q1's drain resistor- something between, say, 110pF and 500pF. This is an old Fender trick for correcting stubborn HF oscillation (as seen in too many CBS-era amps).
• 2. Add a small (100R or so) resistor between V+ and Q1's drain resistor, and bypass the drain resistor to ground with a good size electrolytic cap to isolate the power supply as a potential feedback path.

These are both tube amp workaround techniques ("bandaids", if you will), but should work here provided the right values are used. They can also make things very muddy very quickly, and potentially cause other problems as well,  so apply sparingly - it's probably better to rework the layout if possible.