LPD Push Push Overdrive Booster

[Groovenut]

Hey all,

I designed this circuit early on in the new millennium for a friend of mine. He plays several styles, mostly edge of breakup stuff, through tube amps. He wanted something that would just push things over a bit.

It's a fairly basic design. JFet input stage, BJT buffer stage, gain control, clipper stage, tone control, level control.

Now before anyone says "Hey those mosfets are wired wrong!", they are meant to be wired that way. Keep in mind, I designed this when my knowledge of circuits was just beginning and most of what I came up with was by bread board experimentation.

At low gain settings the signal is clipped at the top of the waveform and as the gain is turned up the lower half of the waveform is clipped progressively until at full gain both sides of the wave form are clipped approximately equally.

I can post scope shots of what the clipping is doing if anyone's interested.

The trick to the circuit is how the bias on the Jfet is set. I use a drain trimmer to set the voltage at ~5 to 5.5VDC. I know there are some who don't like trimmers on the board, so you can also use a pot wired pin 1 to drain, pin 2 to V+ and dial in the bias voltage. Remove the pot, measure the resistance between pins 1 & 2, then choose a resistor that's close to that amount.

I personally use a scope do dial in the waveform while adjusting the bias but wanted other options for those without a scope.

I used a J201 but since those have gotten scarce as hens teeth, a 2N5457 should work just fine



As always, thoughts and comments are welcome!

[Kipper4]

Intresting Lawrance.
More mosfet clippers!!!!!
This time to ground rather than nfb loop.
Quit now or I'm never gonna get done with bread boarding. 😀Thanks for sharing

Why such a small input cap?

[Groovenut]

Intresting Lawrance.
More mosfet clippers!!!!!
This time to ground rather than nfb loop.
Quit now or I'm never gonna get done with bread boarding. 😀Thanks for sharing

Why such a small input cap?

Normally I would use a source resistor bypass cap as HPF to shape the low freqs prior to clipping, but as Im not using one (I wanted the local feedback of the source resistor unbypassed), I chose to use the input cap. The HPF knee of the 1nF/1M RC is 159Hz. Gets rid of the subsonic through low freqs but still leaves plenty of bottom on the guitar tone. Maybe Im fooling myself, but I feel like I can hear a difference in the amount of second order harmonics from the Jfet amplifier stage with the HPF at 159Hz as apposed to 450Hz (I experimented a bit trying to find the knee I settled on).

Thanks!

[Groovenut]

Happy Friday everyone!!!!

Here is a single sided pcb for etching and component layout. Hopefully I didn't make it too compact 

Board outline measures 1.2" X 1.1"

Curvy traces for extra mojo 

I allowed for .035" on the component holes, .04" on the wire holes and trimmer.




This is the first time I've posted one of these, so if there's anything different I need to do please let me know.

Thanks!!!

edit: updated layout and vero

OK anyone who does vero please give this a look to see if it's ok. I usually do pcb so this is out of my comfort zone

[midwayfair]

Your MOSFETs are clipping as just the silicon body diodes. It's fine if you intended that, but they're pretty expensive for silicon diodes.

This thread goes into detail:

http://www.diystompboxes.com/smfforum/index.php?topic=90474.0

You can use a single MOSFET as two diodes to good effect.

[Groovenut]

Your MOSFETs are clipping as just the silicon body diodes. It's fine if you intended that, but they're pretty expensive for silicon diodes.

This thread goes into detail:

http://www.diystompboxes.com/smfforum/index.php?topic=90474.0

You can use a single MOSFET as two diodes to good effect.

Jon's post started me thinking about the diode clipping arrangement on this. I designed this circuit very early in my understanding of electronics and as such it was almost all the product of breadboard experimentation.

Fortunately, I discovered Jack Ormans wonderful article on the many methods of clipping some years back and started using single mosfets as asymmetric clippers. A good example is my latest design http://www.diystompboxes.com/smfforum/index.php?topic=112265.0

Come to find out through some scope probing, that the way I had wired the mosfets wasn't doing quite what Jon said in his post. The body diode was only clipping one half of the wave form at half a volt. The other half was being clipped by the gate-source diode at about 2.5 volts. Pretty standard for a single mosfet clipping stage. At the time I was working on this circuit, I thought it was Q3 that was clipping at 2.5 volts. Turns out that Q3 is completely superfluous. It does absolutely nothing (well not really nothing, it does clip the waveform on one side at half a volt, but Q4 does exactly that as well so Q3 is not needed).



Here are some sim screen shots, that are exactly what I found on my actual scope with the circuit on bread board

If I eliminate the second mosfet, the lower half of the waveform will not clip




See, superfluous...


Thanks Jon for providing the impetus for understanding.

I will redo the schematic and layout accordingly

[Groovenut]

Updated Schematic! The PCB and Vero layouts above have been updated to this schematic.

[Groovenut]

I had some time to throw together a couple of audio demos

This is the Push Push into a Marshall 100W Plexi amp

https://soundcloud.com/lawrence-petross/lpd-push-push-marshall-amp-demo

This is the Push Push into a clean Fender Vibroverb amp

https://soundcloud.com/lawrence-petross/lpd-push-push-fender-amp-demo

[Kipper4]

Thanks Lawrance.
Great demo. Great chops.
It just loves that Marshall don't it?
Good work.

Edit
Is this the single MOSFETs version2 in the demo?

[Groovenut]

Thanks Lawrance.
Great demo. Great chops.
It just loves that Marshall don't it?
Good work.

Edit
Is this the single MOSFETs version2 in the demo?

Thanks Rich!

I agree, it likes the Marshall thing

This is the single mosfet version like the revised schematic.

[Kipper4]

Thanks mate. its nice, really nice.

[Groovenut]

Here's my shot at a perf layout




Here are corrected pcb layout and trace map

[duck_arse]

(I think you're missing the top pad for R3.)

[Groovenut]

(I think you're missing the top pad for R3.)

you are correct sir. Thanks again for the heads up. Fixed!

edit:not only did I fix the missing pad but I redid the layout after I found another error from the pcb layout. Everything should match the schematic now.

[Renegadrian]

Nice, I thought that the 1nF at the input would cut too much bass, but it sounds quite balanced...

[Groovenut]

Nice, I thought that the 1nF at the input would cut too much bass, but it sounds quite balanced...

Thanks!

If you have a look at the HPF F3 of 1nF/1M it's ~159Hz. Since there is no source bypass cap and the circuit is supposed to clip, I chose to filter the low frequencies at the input of the jfet. If the circuit was going to be clipping much more I would have done additional HP filtering at C2/R4.