Fender Princeton 112 Plus, Distortion Channel

[Triffid]

I use a Fender Princeton 112 Plus, solid state amp for my practice amp and (call me crazy) I actually enjoy the distortion channel on this amp.  It is pretty simple (gain, contour, teble, bass, volume) but It is also very veristile.  My question is if anyone knows what kind of distortion it uses, is it something that I can reproduce in a DIY project?  Is there a standard place that I can find this kind of stuff?

Thanks in advance

[Mike Burgundy]

http://www.mrgearhead.com/faq/schematics/fender/Princeton%20112%20Plus%20Sche3BB.pdf

It's a feedback-loop clipper, but with an interesting twist. Looks like there's two clipping points with different response - see the LED, diode and resitor network on the feedback loop of U2B. Anyone played around with different-clippers-with-resistor-networks before?
Contour control looks interesting, too.
Should be easy to DIY (just 072's, and working from a 9V battery shouldn't  have too much an effect on sound, if any. If it inadvertedly does, use a charge pump to up the voltage and solve that problem)

The filternetwork before U2A is important, so go for 4 single opamps (two TL072's) and have an input- and ouput buffer, cut the circuit before R18 and Rc7.
Now, experiment the crap out of this circuit untill you've got something great, and report back
hih

[Triffid]

Ok, a couple of things

1) I am completly impressed that you could figure out what was going on so fast.

2) I am completly confused looking at that schematic.  I see the distortion channel (middle top).  I don't know what you mean when you sa U2B and U2A.  I see R18, but I don't see Rc7, can you point me in the right direction there?

Thanks a lot, I am looking forward to building this

[Triffid]

Ok, U2A and U2B are is one of the TL072 Op Amps right?

Thanks again

[Mike Burgundy]

Um. Okay, let's follow the circuit from the input. I'll run through this quick, before I get some needed sleep.
Zoom in page 1, top left, at something like 200% or whatever you need to be able to reliably read the schem.
From the input, there's an opamp that's shared by both channels, marked U1A. Shouldn't be too interesting for capturing that distortion bit, although there's some shaping going on I think. After that, the signal gets split into the clean channel (tone control stack R8,9,10) and the gain channel, which is what were after.
R18 is there to slightly isolate the gain "input" from the clean "input" so skip that. the 22u (Wow, biggie) C10 is a coupling cap (DC-blocker) large enough not to muck with your sound. Can be omitted as long as you take correct DC-isolation measures in the final version. I think. There might be a slight interaction with the following network, though.
Next is a tone-shaping network (2nd order lo-pass plus something hi-pass) and a gain opamp U2A. There's a regular hi-freq filter (C14) and some shaping through manipulating the feedback loop with R22/C15.
Gain pot, coupling cap, next stage.
This is the clipper.
Regular opamp (U2B) with feedback clipper stage (compare this to a tubescreamer schem if you take out the LEDs LD1&2) with a twist as stated.
Next an interesting adjustable filter-stage (R27 to "drive", the "contour" circuit), drive volume and a tone control network.
from this the output of the "distortion-module" is taken from the junction between R34 and R37.
U7C (which is what I meant when saying Rc7) is the switch selecting channels - one of them anyway.
So: U2 means opamp package number two. This is an arbitrary designation by the designer, so we can all talk to each other about the same single opamp without having to describe the entiore circuit to everyone. Have a look at the circuit-the input opamp is designated U1A - the first half of the first TL072 dual opamp (a dual contains *two* opamps, hence A and B). This goes on through the schem.
You've already found R18,  "Rc7 should be explained as well.
Don't forget hese opamps want a "dual" supply ( +/ground/-) so you should at least incorporate a resistive devider to get Vref, at half the supply voltage. Yada, yada... we're here if you have more questions, although for me it's bedtime for the next hours.
hih

[Triffid]

Ok, I hope this is ok (legally), but here is the section of the schematic that I would like to recreate.  First off, this will be the first pedal I've attempted where the plans weren't completly laid out form, so please forgive my newbie q's (thanks).  I have a couple of issues...

1) I assume I would also need some sort of output capacitor, what size should I use?

2) From Mike's advice I will replace the 16Volt supply with 9V and see how that sounds first.  He also recommended that I use a voltage divider (I think) to supply Vref of the TL072 with half the supply (4.5V?).  I understand what a voltage divider is, but I don't know what Vref is, or how to supply it with half the supply, or why that is significant... any help on this?

3) Mike also suggested setting up input and output buffers.  What is the reason for this?  Any suggestions as to which ones to use?

Thanks in advance for any help you can provide

*edit* external link because of how annoyingly large it is
http://www.castagno.org/112plus_dist.png

[RDV]

I'd try it like it is with a .1uF outcap to start with, but coming out of a tone stack you probably won't need one unless you do use the 2nd half of a 2nd TL072 for an output buffer. For the buffer wiring check out the'Basic Buffers' article at AMZ. I might try this one myself.

Have Fun.

RDV

[Triffid]

Another thing... it doesn't say what kind of LED's or Diodes to use here, does it matter?

Thanks again

[al3151]

If I was a bettingman i'd say the diodes are 1n4148 and the LEDs are your simple small red ones.

[petemoore]

Socket those Clipping diodes...
 These Si Amp 'Fuzz competitions' would seema good source for Si amp Fuzz schematics...many shoppers choose the Si amp of purchase by the sound of these Opamp clippers.
 The Engimeers probably do extra extensive testing on these ckts. as how well they do here will directly affect the sales quotas of that amp.
  My experience is that most of the Fuzz Boxes that come with [in] Si amps are pretty good...I've known a couple Ppl that drag a practice amp around for the Fuzz that's in it and 'direct line' that to another amp.

[Mark Hammer]

My money is on 1N4148's and red LEDs as well.

There are some distant ties between this clipping circuit and the "dynamic limiter" found in Fred Nachbaur's "Dogzilla" amp.  Both use a combination of cascaded diode pairs in combination with "bypass" resistors.

My guess is that you like your Princeton because the "distortion" channel is lightly compressed and only moderately dirty, with kind of a singing quality, right?  That would be the outcome of a circuit that tries to be both soft clipper and limiter at once.

[aron]

Is there a way to predict what the clipping network will do?

Is this just a pair of Si diodes with the addition of LEDs with series resistance or?

I've done this sort of thing before, but I never understand how it worked. I believe there are a couple (or more) schematics with similar clipping networks.

[Triffid]

So, does anyone know if it will be ok if I connect pin 4 of the TL072 straight to ground, or do I have to come up with a dual supply?  What is the best thing to do in this case?

Thanks in advance

[Mike Burgundy]

You either come up with a dual supply (Check GEO for a voltage inverter) or make the opamp *believe* it has a dual supply.
This is done by hooking the opamp up to V+ and ground, and referencing the +input and output to Vref - which is HALF the supply voltage.
The opamp *thinks* Vref is "ground" and sees a supply of +1/2V and -1/2V.
This requires a resistive devider to make Vref (check *any* battery-fed  dual opamp circuit for this technique, a tubescreamer f'rinstance) and changes to the circuit to reference the opamps to Vref and isolate DC levels from bits of circuit that are hooked up to real ground.
This is a rather big one to take on if you're inexperienced, though. Just a word of warning

[Triffid]

Thanks for the reply Mike,  and your ongoing help... I am rather new to this stuff, but I've created the beginner project here, a couple of FF's and a RangeMaster (all simple I know).  I figured why not give this a try.

I am looking at a few dual op amp curcuits like you mentioned, tube screamer is one of them.  Here are a couple of questions if you don't mind.

1.  I assume that when you say "isolate DC levels from bits of circuit that are hooked up to real ground"... this can be done by placing capacitors in certain spots.  What significance would the value of those capactiors have?

2.  Would the voltage divider displayed here work for my needs...http://www.runoffgroove.com/tubereamer.html

3.  What are the practical differences between  using the fake out method and using a voltage inverter to get +-9volts.   Would the battery run out faster with the voltage inverter method?  I couldn't find an inverter @geofex, but would this one work:  
http://www.aaroncake.net/circuits/vinvertr.htm

Anyway... I have got a ways to go before I completly understand this, but I'm working on it... Thanks again...

[Mike Burgundy]

1. I assume that when you say "isolate DC levels from bits of circuit that are hooked up to real ground"... this can be done by placing capacitors in certain spots. What significance would the value of those capactiors have?

Correct. if you look at the tube reamer schem, the input cap isolates the input from DC, the 1M to Vref puts the + input firmly at half voltage DC-wise but allows a voltage swing (if you simply hook the input up to Vref directly, you end up with exactly the same as when you hook it up to ground. It's essentially the same).
The value of the cap would be large enough to be full-range for guitar. It forms an RC-filter with the impedance of the next stage. 1u should get you waay in the clear.

2. Would the voltage divider displayed here work for my needs...http://www.runoffgroove.com/tubereamer.html

Yes

What are the practical differences between using the fake out method and using a voltage inverter to get +-9volts. Would the battery run out faster with the voltage inverter method?

Well, yes. Plus the devider has less parts - so it's cheaper and easier.

would this one work:
http://www.aaroncake.net/circuits/vinvertr.htm

Yes. So would this:
http://www.geofex.com/circuits/+9_to_-9.htm
The 555 circuit might switch at audible frequencies, and cause problems. The GEO-circuit has the benifit of working above audible frequencies, BUT does involve a more expensive, hard to get chip.

[Triffid]

Thanks again man... this is fun stuff..

So is it this simple then (changes in red)...

*edit*, external link because of how annoyingly large it is
http://www.castagno.org/112plus_mod.png

[Triffid]

Wait, this doesn't make sense does it....

The point is to get 4.5V DC flowing throug +input to output right... Which means I need a ground connection on the output side and +4.5V on the +input side right?   I'll have to work on that... argh

*edit* man, now I'm all confused again

[spongebob]

It helps not to think in absolute numbers when looking at the power supply, don't think of ground as a zero volt point but as the point that's in the "centre" of your suplpy voltages.

You can use the resistor divider method to create a "virtual" bipolar power supply, which will give you V+, V- and GND. V+ is 4.5 volt relative to GND, V- is -4.5 volt relative to GND.

Code Select Expand


from battery +  -------------------o V+
                        |
                       10k
                        |
                        |---------o GND
                        |
                       10k
                        |
from battery -  -------------------o V-


I don't think you have to change the schematic in order to get it working from a battery, you just have to change the power supply connections. Wherever you have ground connections in the schematic connect them to your "virtual" ground at the resistor divider.

But before you pull out your soldering iron let someone with more expertise comment on this  :lol:

[Mike Burgundy]

no, wait!
You don't need a current flow, you need to *reference* the input at 4.5V. The opamp will then force the output at that level all by itself.
Think of it this way:
For an opamp to work correctly the in- and output pins need to reside (DC-wise) right smack bam in the exact middle between the positive and negative supply lines. Got it?
With a dual supply, the exact middle between -9V and +9V is: zero. Which is ground level.
With a single 9V supply, the opamp wants it's in- and outputs to sit at 4.5V which is exactly between 9V and 0.
BUT there's a catch - the AC signal swings around 0. You have to isolate the 0=ground bits from the "0=4.5V" bits of the circuit, and that's what the caps are for.
See if this little bit of reading material sheds light
http://focus.ti.com/lit/an/sboa092a/sboa092a.pdf

Spongebob is almost right, so was your edited schem.
Here's what to do:
-Keep in mind voltage supply is the same as ground as far as AC signals are concerned.
-hook up R21 to Vref instead of ground.
-flip C10's polarity (+ to the opamp)
-that's all you need to change on the input of U2A.The opamp forces its output to the correct DC level, and C10 blocks the DC level from the input.

-pin 5 (U2B) goes to Vref.
-on the output (before R27) you do need a cap so the volume knob isn't loaded with DC - it will be very noisy otherwise.
That's it.