SPACEMAN EFFECTS: Redstone Germanium Preamp

Yet to build this myself, I quite like the idea behind this - I am partial to anything with bass / mid/high controls for guitar and a germanium transistor.  Bonus points for also including a discrete op-amp, which I've been playing with lately.

Before I looked at the schematic for this, I was playing around with modding a Boss Blues Driver to make the tone stack a real tone stack with external pots and removing the last stage, just leaving the two discrete op-amps.  This is a refined version of where I was heading, so I think I'll park that project for now. 

BOOST STAGE

The boost stage uses a LND150 MOSFET which is common on several Spaceman Effects designs.  It could just as easily have been a JFET-based boost - this is a clean boost to make up for the gain loss at the EQ stage.  A little bit of high-end is also filtered off at this stage, but nothing substantial.  You may not even hear it.

EQ 

It's a fairly standard bass / mid/high tone stack with some tweaked values.  This could be modded to typical values for Marshall or Fender, but there's a decent chance Spaceman went this way for a reason - it looks pretty balanced to me.

DISCRETE OP-AMP

It's a Boss-style discreet non-inverting op amp using two JFETS and a PNP transistor (as found in the Blues driver).  There are a couple of resistor value changes, and the PNP transistor is, of course, germanium instead of silicon - hence the germanium preamp name.  The GE transistor will add some extra flavour, but maybe not much more. 

According to the Dead End FX build docs, a 1T308B transistor is used - it makes sense to use Soviets for a pedal with a large production run.  If you don't have a 308 - try something around 70-80 hfe and low leakage (most Soviet-era transistors have low leakage)  Japanese transistors might also be a good choice.

Potential mod:  As this is just another type of op-amp - diodes can be added across the output of the op-amp and inverting input for some soft clipping.  It works on LTspice, so it should work in practice. I'm definitely going to try this.    

SPACEMAN REDSTONE GERMANIUM PREAMP - VERO LAYOUT

The layout below is missing the soft switching to save on size.  If you don't have ferrite beads, jumper it on the input, or use a low-value resistor in the power section (100ohm).  You could quite happily not use the MOSFET in the power supply section, but I had the room, so may as well keep it in.

The same layout as above, but this one is for a Soviet 1T308 transistor, which uses the less common BCE pinout, as many Soviet-era transistors do - you could always use the one above and bend the legs. Still, I find that annoying, so I made another layout to suit.

SCHEMATIC

I redrew part of the Deadend FZ schematic, as I found it difficult to picture the discreet op-amp.  I have not included the soft switching or fancy power supply in the drawing below.   

VIDEO DEMO

MARSHALL: Blues Driver / Boss Discrete Opamp

If you've read any recent posts, you may have noticed that I'm experimenting with discrete opamps - specifically replacing standard opamps with simplified discrete opamps - I've only played with the Joe Davisson diode compression opamp so far (with some success).

While I was checking out DIY Stompboxes, I saw Rob Strand was experimenting with the Boss discrete opamp that's used in the Blues Driver.  I've occasionally looked at it but have never gotten around to playing with it.  

Anyway...  it appears to work as a drop-in, and I thought a Blues Driver / Blues Breaker hybrid might be fun.  They are totally different circuits by the way - similar in name only.

More to follow on the Boss discrete opamp.  Feel like I need to spend some time playing with this and the Blues Driver.  I have an interesting idea in mind for the Blues Driver; I'll post it if it's successful.  

MARSHALL BLUES BREAKER X BOSS BLUES DRIVER DISCRETE OPAMP

It looks like a Blues Breaker on LTspice.  But I'm always a little suspicious of anything using JFETS on spice as they're notoriously variable beasts in real life.

BOSS BLUES DRIVER SCHEMATIC - OPAMP MARKED

Here's the Blues Driver schematic, with the discrete opamp marked. A good analysis can be found here if you are interested in further reading.  

MI AUDIO: Discrete V2 Crunch Box

I'd already done the Blues Breaker in this format, and it sounded pretty good, so I thought why not try the MI Audio Crunch Box, as it's a high-gain derivative of the Blues Breaker.  

They have a similar topology - the Crunch Box has the soft clipping diodes removed and hard clipping LEDs added after the op-amps.  The LEDs probably just round things out a little, as the second op-amp is clipping hard. 

It looks pretty similar to the opamp version on LTspice, but with a bit less gain (which for me might be preferred).   I hope to get a slightly more dynamic distortion out of this compared to the stock MI Audio Crunch Box.  The stock Crunch Box is not really my kind of thing.

Untested at present - this is a future build file for me.  Try it if you want, but who knows what it will be like at this stage.

MI AUDIO CRUNCH BOX V2 - DISCRETE OPAMP VERO LAYOUT

MI AUDIO CRUNCH BOX V2 - DISCRETE OPAMP - SCHEMATIC

MARSHALL: Blues Breaker, Discrete Opamp Mod

I've had some success with the Joe Davisson diode compression discrete opamp on some other circuits (namely the Cornish SS-2), so I thought I might try a Blues Breaker - it works...  sounds good.  

Apart from the lack of IC, there are a couple of other minor mods to a stock Blues Breaker, mainly a larger input cap (Morning Glory) and some larger resistors on the inverted input of the first opamp (King of Tone).  I'd also recommend a C250k pot for gain, if you want more dirt (which is what I'm using).

I've increased the bass response on mine - all you have to do is change the 10n cap on the bottom row to something bigger, and you'll get some low-end back.  I like 220n, as it seems to keep all of the bass that I started with, but some might find this too much.  Could be cool to have this on a switch.  

For anyone with a keen set of eyes, you might notice the RC networks on the inverting input of the first stage going to ground instead of vbias - it was convenient to do on the layout, and it makes zero difference to the sound.   

DISCRETE OPAMP - BLUES BREAKER - VERO LAYOUT

DISCRETE OPAMP - BLUES BREAKER - SCHEMATIC

And here's it looking a mess, but 100% working, attached to the breadboard - I made a couple of errors initially, so this is not exactly the same layout as above, as I fixed it on the fly.   Note to self: don't attempt creating layouts followed by soldering them when really tired.

 

 

VISUAL ANALYSER

440hz sine wave input

Scope taken on the output, with the volume turned down a bit.   Starts with gain on 100%, sweeps tone control up and down a couple of times, and then reduces gain.

 

The output of the first boost stage, gain 100% (note the input signal sits at -25)

 

The output of the clipping stage (gain 100%), before any filtering from tone controls and caps to vbias - it has a lot of harmonics / high-frequency content, and it's bordering on being a square wave.

JOE DAVISSON: Diode Compression Opamp

Given that I've been playing around with a few circuits using this as a replacement for the opamp, I thought I should make a little vero layout, which can be dropped into an existing circuit to test it.

If you are unfamiliar with this, it's a simplified discrete opamp - basically, build it, and place the appropriate leads into the matching locations in the socket for any standard opamp (TL071 as an example).  Of course, if it's a dual, you will need two...

The schematic and more information can be found via this link. 

But why?  There are heaps of perfectly good opamps out there.  That's a very good point - it sounds different in most cases, to my ears a little smoother; perhaps something to do with the lower gain?  Apart from that, it's just another fun thing to play with.

JOE DAVISSON DIODE COMPRESSION OPAMP - VERO LAYOUT

CORNISH: SS-3 Modded Diode-Compression Opamp

Details of this project can be found here - as I started with a modded Pete Cornish SS-2 using the Joe Davvisson diode-compression op-amp, and then moved on to this.  Essentially the same as the SS-2, but with the addition of a lo-cut.  

After a bit of testing, I’d suggest a C50k for the sustain pot, followed by B50k, and A50k if you have none of the previous.

CORNISH SS-3 WITH DIODE-COMPRESSION OP-AMP - VERO LAYOUT

The layouts works - I added a buffer on top of this build.  

BOSS: Modded DS-1, Diode Compression Opamp

A somewhat polarising classic - the Boss DS-1, with a slight twist, as this has the Joe Davisson discrete diode-compression opamp onboard instead of an IC.

BOSS DS-1 DIODE COMP OPAMP - VERO LAYOUT

BOSS DS-1 DIODE COMP OPAMP - WAMPLER STYLE JCM MODS

This is what I ended up being happy with on the breadboard - it's basically the Wampler JCM mods, but I used a germanium and silicon diode in series for hard clipping rather than two silicon.  The combined forward voltage brings it up around the same level as the original diodes used in the DS-1.   

I also increased the size of the resistor on the emitter from 22 to 470 ohms to reduce gain in the first stage.  You could happily go a bit smaller than this, as the discrete opamp has less gain than a real opamp.  I may have taken a tad too much away - so maybe have a play with that yourself if you plan on trying this.

The most important change in terms of sound is the replacement of a resistor in the tone section, which really changes the character of the sound (mids return).  

So with all of these changes - it sounds like a nice distortion with some character, but not much like a DS-1.  

BOSS DS-1 DIODE COMP OPAMP SCHEMATIC

Some common mods included - minus diode clipping mods, as I'm sure we all know what this involves.  I've kept mostly stock values on the schematic while I test mods.  The only exception is the clipping diodes, where I have two on each side in series, as I just discovered the original diodes had quite a high forward voltage (0.92v typical, 1.2v max), which is close to 2 x 1N4148 in series.  

ORIGINAL BOSS DS-1 SCHEMATIC

 

ON THE BREADBOARD

Looks like a mess... and no input or output buffers at this stage.  Speaking of buffers, I may even try one between the tone and volume control to stop the interaction between the two.

 

ON THE SCOPE / FFT

Straight from the boost section - this is actually pulled back a bit, using a smaller resistor than the stock one, as it's normally distorted before it even hits the opamp (this is a common mod)

The next shot is the op-amp output and then at the clipping diodes. I need to change the diodes, as the original diodes have a forward voltage of about 0.9v, which is a decent step up from a standard 0.6v.  This certainly explains why the output was a bit lower than expected.

Caution - start with low levels.  Adjust the gain control, then tone - trace taken at the output of the tone section.   440hz sine wave

CORNISH: Modded Cornish SS-2 using Diode Compression Op-amp

This is a Pete Cornish SS-2 using Joe Davisson's diode-compression op amp.  It sounds pretty good - a lot like the regular SS-2, but without any distortion artifacts on sustained notes as it fades outs.  It's also quieter in terms of noise.  This one is getting boxed up, and now I want to try a SS-3 version.

For my ears it sounds best at 18v, and with a Cornish buffer in front.  You can notice the buffer on cleaner sounds in particular, as the high frequencies are a little brighter. 18v sounds more open with better string clarity.  9v still sounds good, but it's more compressed and less dynamic in my opinion - may be a tad thicker sounding as a result of this, which some people may like.

Being based on the MXR Distortion +, higher gain settings roll off bass and treble to provide more of a mid-hump.  It also sounds pretty good with the tone control wound all the way back.  Low gain settings sound pretty nice too, as the frequency response flattens out.  

The tone control is definitely needed as the treble can need a bit of smoothing out on some settings.  It works well for a simple tone control.

I tested it on a transistor and valve amp - both had their own charms.  Could not say which one I preferred.  The valve amp liked it more when it was just breaking up, and then pushed with the SS-2 - but then again when doesn't a valve amp not like being pushed on breakup?

Things to tweak if you feel like playing around with the tone;

• C2 - definitely increase to 22n (input cap).  10n forms a 66hz HPF with R8, 22n forms 30hz HPF

• C8 - 10n capacitor paired with diodes to ground (forms a 1.5khz LPF with R11)

• C5 - 10n capacitor from clipping section into tone 3, rolls off some bass (I'm using a 22n here)

• C9 - 10n capacitor from tone 1 to ground

• Germanium Diodes:  different combinations, types - usual diode mods...

• Reverse log gain pot like the SS-3

CORNISH SS-2 USING DIODE COMPRESSION OP-AMP - VERO LAYOUT

SCHEMATIC

FREQUENCY RESPONSE - LOW TO HIGH GAIN

ON THE BENCH TESTING SOME CAP VALUES

And a buffer to match

ON THE SCOPE

caution - start with low volume levels 

 

First adjustments to gain, then tone.   440hz sine wave input source

 

This is at the diodes, after the first opamp, with adjustments to gain

Gain off the first opamp is pretty high, it's really squashed by the germanium diodes.   Like hard clippers, diodes are fun to play with.  With silicon diodes, it looks like there's some additional clipping happening in the second stage.

VOLTAGES

Strangely at 18v it behaves a little differently compared to 9, 12 & 24.  Here's 18v - note the mix of harmonics compared to other voltages.  

 

 

All other voltages to follow in ascending order - no changes to any settings, just the voltage.

ON THE BREADBOARD

And here’s the mess that was the breadboard 

After the previous post for the Dan Armstrong Red Ranger, I thought I might do layouts for the rest of the series, starting with the Blue Clipper - I then got sidetracked, and now I'm looking at a Blue Clipper using Joe Davisson's diode-compression opamp.  

I had to change the bias from the original values of the Blue Clipper on the positive input of the opamp to get it to work (update: as it turns out the values on the online schem were wrong).  

The 33n cap at the end of the signal chain is rolling off a lot more high frequencies than I'm normally happy with as well - that will change at some stage.  There's a fair chance this will evolve into something resembling a Distortion + / Pete Cornish SS-2 with mods.  I have the SS-2 version running on LTspice and a layout ready to go.

note:  I did end up turning this into a SS-2, so the Blue Clipper using a diode-compression opamp project will not progress any further, as I'm happy with the SS-2.   

DAN ARMSTRONG BLUE CLIPPER WITH DIODE COMPRESSION OPAMP