Clipper Ship - a 1590A 'charge pumped' dual boost w/ Ge clipping

[midwayfair]

This is partly for the lols. If anyone sees any opportunities for refinement, lemme know. I'll see if I can get one of these going and film a demo this weekend. Early tests are promising.

Here's version 1 of the Clipper Ship, a warmified boosty overdrive with germanium diode clipping and a "charge pumped" (JFET voltage gain) volume recovery/boost following. It was inspired by a certain legendary IC-based overdrive with two side-by-side circuits and obscure germanium diodes that was, well, too large a circuit to fit in a 1590A ...

We should moderate our expectations of how close you can get to the sound of a true charge pumped IC design with just two discrete transistor circuits and JFET voltage gain, but here it is:


(Sorry for using values instead of names, but this was the easiest way for me to go from schematic to layout.)

UNVERIFIED Layout, 20x11 perfboard:

The layout is unverified, but I've been over it several times.

Power Section
The power section on Q2 (after a standard power filtering section) comes from RG's SRPP, which was a modification of Jack Orman's famous mini booster. It's a pretty simple design that uses a FET to increase the DC gain, which is then used as a DC source by the transistor that amplifies the guitar signal. Note that RG's version of this circuit includes noiseless biasing. The circuit is very quiet even on a breadboard. It also includes the 1K resistor trick to essentially double the gain. The 1uF cap between the gate and source has an effect on the tone. It must be unpolarized. I used the largest unpolarized cap I had, but you can  get a larger value (and thus darker tone) by putting two tantalum capacitors with their negative sides soldered together, and the positive sides connect to the gate and 1k resistor as shown. I suggest tantalums because they're smaller and sound a little better. Why bother? Because this is one of the few places in the circuit where it seems you can shape the tone without it affecting the overall volume or harmonics. The size of this cap also has a notable effect on noise.

A 2.2K resistor limits the current to Q1, which helps keep the volume in check so that it doesn't overdrive the input of the Q2 section.

OD section
The Q1 circuitry is a very simple booster design with a diode clipping section. I used a low-gain Silicon transistor (4JX11B1634B, gain range ~30-75) because it gave a warmer overdrive tone -- and it's the gain range that matters here, because most of the overall sound characteristics are going to come from Q2. The minimum resistor of 2.7K was chosen because without it, the distortion is actually unpleasant and harsh, it's not good fuzz at all (ever actually cranked a SHO all the way up? am I the only one who thinks that sounds bad?). The small output capacitor was chosen so as not to drive the second half of the circuit too hard.

The 1N914 can be any diode you like (there is an extra hole above it for if you want to use a longer diode). It does not have a dramatic effect on the sound, but I could hear some subtle differences between a Zener and the 1N914. I went with the 1N914 because it seemed to offer more compression.

Diode clipping section
My layout shows symmetric clipping for clarity. I highly recommend sticking with Germanium or BATs here, as other diodes may clip too subtly. I'm actually using a single pair of Ge diodes in mine, one of which has a Fv of a whopping .85v (I was testing diodes and found a "bad" one ). OA1160 sounded spectacular in a symmetric pair, and BA41s also sounded very good here. Feel free to experiment or put some options on a switch.

Boost section
Q2 is designed as a gain recovery stage, using the same transistor gain value as Q1. The gain is hardwired to be as much clean volume as I could get. Technically, then, the 2.7K resistor could be on a trim pot. (For anyone curios why the first section distorts but the second one doesn't despite the resistor being the same value, the answer is ... I don't really know, but I suspect it has something to do with the DC gain circuit providing more voltage and thus clean headroom to this transistor overall.)

A 330pf cap tames some noise and keeps the treble in check.

Output section
The output section has Mark Hammer's Stupidly Wonderful Tone Control with a generic .022uF cap for treble cut. This is follow followed by a simple voltage divider volume knob. The minimum series resistance of 57K means that the output impedance is fairly high for a boost or overdrive even when the controls are wide open. Total output is somewhere around 20dB.

Optional Second Diode Clipping Section
The diode clipping section is in the first circuit, which is a little unusual. You could get FAR more distortion by running the diodes before the tone stack (more on this in a minute), so why do it this way? Several reasons: the diode clipping itself is MUCH softer at that point in the circuit and I am not a user of heavy distortion. Aside from my own personal preferences, the gain is actually so high on the end of the circuit that there's simply no way to clean up the pedal without adding a saturation control, or a limiting resistor, plus you also will likely need a blocking diode to keep things from getting fizzy, and there's a big volume drop, which negates all the hard work done to make the second section a high-headroom boost and increases the size of a circuit that's otherwise very simple. And although you could flip the circuit so that the DC gain is applied to the first half of the pedal, there's still the problem of lost dynamics from overly dramatic clipping and few ways to handle getting the dynamics back without overdriving the input impedance following section, so you'd lose the ability to use the pedal as a clean boost.

It's a little weird, in the end, that I didn't include the second diode clipping section, given that I built this while messing around with circuits to test diodes for their sound qualities. (This is the origin of the name ... I was actually calling the version without the switch the "Gold Nugget.")

The DIYLC files are available by request. The art is creative commons, so you need to make your own layout if you think you can make a profit off Yet Another Overdrive.

[Mark Hammer]

There've been several requests lately for instructional materials. So let me compliment you on your detailed writeup.  That's how the newbies learn. Good on ya!

[midwayfair]

There've been several requests lately for instructional materials. So let me compliment you on your detailed writeup.  That's how the newbies learn. Good on ya!

Thanks Mark! Since I'm still a newbie myself, knowing that I want to do the detailed write up means I have to really get to know all the bits of the circuit so I don't sound foolish when I'm explaining it.

[midwayfair]

EDIT 7/5/12
The first gain section needs some changes in the layout and schematic. The 2.7K resistor needs to be much smaller -- probably 100-560Ohm, since I still don't want it all the way down to 0 -- once off the breadboard I couldn't get it to overdrive at all, and I'd rather not use the last notch of the OD knob than never have the option to make it distort. I also think the 2.7K in the boost section needs to be smaller to actually produce enough boost, 1.5-2K will still keep it from distorting.

I can't modify my first post. I must have misread some resistor values on my breadboard -- I think they must have been 270Ohm, at least in the overdrive section.

[midwayfair]

Updated schematic and verified layout with the correct values:



I recommend increasing the 2.2K resistor if you're going to use a Si transistor in Q1 so you can get less harsh sounding distortion. The Ge transistor I used in the prototype sounded good and Vox-y with the stock values. Most lower gain silicons sounded fine on clean settings. BS170 and 2N5457 also sounds okay there with the stock value, but hardly ideal.

Edit 7/11/12: Note that I now recommend adding a 150pF cap on Q1 and increased the AC coupling cap in the OD section to 100nF.

[Bill Mountain]

I saw your post in the pics thread.  I'm sorry I missed this thread.  This looks like a cool build.  Thanks for sharing!

[midwayfair]

I decided to revisit this design this week. Overall it was a lesson in having been too clever for my own good. 

Reducing noise
Even though I liked the overall behavior and character of the pedal, the mu amp circuit just introduces too much noise when paired with a BJT, even silicon. I tried everything I could to reduce the noise, but the fact is that the more gain you get out of the circuit, the louder the whine is.

Some testing shows that at no point was Q2 saturating, so the use of a germanium was an unnecessary noise source. And once I was going silicon, the choices were either a Darlington -- which was quiet but also too dark -- or revert to a FET.

I made some compromises on the maximum amount of boost with the least amount of noise and tried to get a more pleasant overall frequency response.

Retailoring the frequency response
Using a FET in Q2 loses some of the brilliance that the circuit gained from the BJT, and grounding the source drove the FET into cutoff when the gain was turned up (not to mention that the volume was simply ridiculous). Grounding the source also meant that I couldn't tailor the overall character with a shunt capacitor. So I used a smallish resistor (560R) and a smallish shunt capacitor (100nF). The resulting circuit boosted highs a little more than it boosted other frequencies, but was never as harsh as treble content with BJT.

The tone control now needed to be retailored, because a 22nF didn't really leave any room (even with the tone maxed) to boost treble. Although cutting it to 10nF made the tone control a little less responsive overall, there was now no perceivable change in treble content of guitar notes at max, and the lower started cut into the highest notes on the guitar. I wasn't completely satisfied with it, because it had a "playing under water" effect when at minimum, which was not a useful sound. A minimum resistance of 10K kept that from happening.

Reducing redundancy and do-nothing parts
Some other simplification was now possible. The 150pF cap on Q1 wasn't really necessary. There was a 1K resistor between the gain stages to cut the overall brightness, but that was no longer necessary, and the 100nF cap after the diodes wasn't necessary either. Gone gone gone. That 330pF? Ze goggles. Deleted!

Final results
The final design now behaves very slightly differently than before. Whereas before the frequency response of the boost and distortion sides was roughly similar, now the boost is a little more transparent than before, while turning up the gain will boost the lower midrange (thanks to the 4.7uF cap on the emiter), expanding the frequency response. The diodes clip a little harder than before (since there is a little more voltage across them), which means that the gain control is also a little more responsive earlier in its travel (even without a reverse log taper). Finally, the tone control is, as described, milder but has no unusable settings -- never harsh or too dark.

Here's the new schematic and layout. It's several rows smaller, drops about 5 parts, AND it's a little easier to put together overall.


https://dl.dropbox.com/u/9878279/Jon%20Patton%27s%20layouts/Clipper%20ship%20schematic%20version%202.pdf

https://dl.dropbox.com/u/9878279/Jon%20Patton%27s%20layouts/Clipper%20ship%20layout%20version%202.pdf

[midwayfair]

Well, I have no idea what was going on with my test unit OR the breadboard last time, but in verifying the PCB, I have confirmed that PNP will not in fact work. It passes signal -- amplified signal even -- but the gain control doesn't work and it just sounds misbiased.

Rats! But I'm glad all is right with the world now.

Here's the link to the build document, which will be where all updates will be posted from here on out.
https://docs.google.com/document/d/1dPlB8Lqn_a1A6Xpb3Lg-2cfTxGCLOkMfNUWALyGE3ZQ/edit

[ch1naski]

I'm currently populating the PCB for this. I've got two 1N60's for D3,D4 (i have a couple of mojo telefunken OA172's but the leads are short and the diodes are longer than the hole spacing on the board).
1N34a in D2. Going with a Russian NPN germanium MP38A (thanks, jon) for Q1, and 2n5457 for Q2-Q3. Hoping to have it boxed and blasting by tomorrow afternoon.

A build report and possible crappy video demo to follow, in the proper sub-forum  .....