RCA: BA-31C Preamp, Vero Layout (The Balmoral Broadcast)

The RCA BA-31C would have been found in broadcast consoles of the day (late sixties), as a microphone preamp module.  

I think it may have been the starting point for the Hudson Broadcast, as they look quite similar and I've heard some references made to RCA.  If it is what they used, I can see why they did - it's very tweakable, with some nice options for setting gain as well as high and low-end roll-off.   

The original runs at +/-30V.   I was aiming for a 9v single rail supply to keep it simple, and of course without expensive transformers.

RCA BA-31C MICROPHONE PREAMP - ORIGINAL SCHEMATIC

RCA BA-31C - MODDED FOR GUITAR

I tried to keep things simple, while still maintaining the structure of the circuit.  

Note:   This is still a work in progress, still some fine tuning and changes in progress.

GAIN

Like a Hudson Broadcast, it runs from clean boost to an overdriven fuzz.  It's not super fuzzy like a fuzz face, but you can hear it.

After experimenting on the breadboard, I decided to use an A1K pot for R6, and set R10 to 22k - 68k (anything a bit bigger than the original 6.8k resistor is fine, so long as it still has some feedback).  Decreasing R5 also provides a bit more dirt.  

FREQUENCY RESPONSE

With a few capacitor value changes the frequency response is easily adjusted.  

Increase C5 to roll-off high frequencies, although this is only needed at the highest gain settings, on lower gain settings, the tops are already reduced.

Decrease C3 to roll-off lows after Q1.  1u is pretty flat, can go down to relatively small values (22n as an example).  

MODDED RCA BA-31C ON THE BREADBOARD / SCOPE

So it works....  this is using a B5k pot for R6, and I think a 100n on the output as it was sitting right in front of me at the time.  Will change over to 1u.

PNP is a germanium 2SB457, NPN is a BC108.   It really isn't at all fussy about what germanium is used for Q2. 

 

caution: this starts at low volume and increases - watch your ears / speakers.

VERO LAYOUT - THE BALMORAL BROADCAST

Initially I thought I would end up with a few switches for bass and treble - but for now I decided to keep it simple.  Think I will most likely land on gain, volume, and a fat switch (which will change the coupling cap value between Q1 and 2).  For me it doesn't get bright enough to need control on the treble - if anything it's a bit low on treble on cleaner settings.

At low gain settings, it has a slightly rolled off high-end, with is nice and bluesy, maybe a bit of a jazz tone.  I did try it with a boost in front, which works pretty well, although it can use a bright cap across the boost level control.

This layout is a little different to the schematic above, as I decided to change the output cap.  I've called it the Balmoral Broadcast - due to some similarities to the Hudson, and I live in Balmoral.

 

Note:  not final versions

REVOX: A77 Input Amplifier Clean Boost, Overdrive / Light Fuzz

This is the input amplifier from a Revox A77 reel to reel (circa ~ 1967) - which also happens to be almost identical to the mic preamp in Trident A-Range mixing consoles from the early seventies.

So far out of the preamps that I've been working on, which is the Neve, RCA and this, I'm happiest with this one.  Apart from sounding cool, it also retains a pretty flat response, and could be used as an "always on" sweetener.  I was probably just hearing very light distortion, but I did prefer the tone of my guitar with this on running clean, rather than off.  It felt a bit more forward and bigger sounding.

It’s very very loud - tons of output level available.  This could happily be described as a clean boost as well as an overdrive / light fuzz.  

Due to the insanely high output, I think this is a good candidate for some form of High or low pass filter for some tone shaping.  If you’re using it to smash another pedal or front end of an amp, some bass cut might be handy.   Note that on the highest gain setting (which is fuzzy), it does roll-off high and low frequencies.

The original uses silicon transistors, I've used Japanese 2SD128 germanium transistors, which are usually around 40hfe.    

REVOX A77 INPUT AMPLIFIER SCHEMATIC

Note the switching used to set impedance and recording (gain) levels.  As I was aiming for boost and fuzz, the resistor switch on the input has been removed on my layout.  The fixed resistors controlling gain have been replaced with a pot.

TRIDENT MIC PREAMP SECTION FOR COMPARISION

REVOX A77 INPUT AMPLIFIER FREQUENCY RESPONSE

On the highest gain setting, high and low frequencies roll-off.  It's very flat elsewhere.

The output is very high, and the overdrive / fuzz does not occur until the last couple of hundred ohms when resistance to ground is heading towards zero (hence the reverse log pot)

my notes from spice re the schematic above:   

• input cap can be 1u without effect response
• 1u output cap only has slight low frequency roll-off, even with a 5k volume pot
• decrease C4 to 10u to roll-off low frequencies
• increase C2 to increase high frequencies
• decrease R8 to reduce fuzz
• C6 can be smaller (down to 10u)
• C5 can be removed, or increased to boost treble from 8k / 10k
• R5 can be removed for a very slight gain boost

There are so many variables, I haven't show the LTspice results here, or it would be a very lengthy post.  

REVOX A77 INPUT AMPLIFIER - CLEAN BOOST / OVERDRIVE / LIGHT FUZZ

Work in progress on the bench - as you can see, it's the mojo layout (also know as the unnecessarily large version).   Approx 65mm x 40mm on 0.15" vero with 1w carbon film resistors

And it’s boxing day - using my ‘spare no expense’ cardboard box wiring rig to get the spacing right before finishing it off in the enclosure.  

It’s not the greatest looking, but I do like the sound.   Really really loud by the way.

ON THE SCOPE / FFT

caution re audio levels - start low.  This is bench test of my build.

 

 

   

NEVE: 1073 Channel Amplifier

I went down the Neve rabbit hole with this post, looking at how the channel amplifier works and trying to use one of the preamp stages as a fuzz/overdrive.

Long story short - I couldn't come up with something that I was happy with using the mic preamp stage, so I have walked away from this one for the moment.   Came close, but just not worth spending any more time on.

I will try the other amplifier sections and see if I have better luck.

On the upside, it got me interested in what other preamps were out there, and I had much better luck with the Revox A77, the same as a Trident A-Range mic preamp.   I ended up making one of these and boxing it up, and it's pretty cool.  

- - - - - - - - - 

So what am I trying to do?

• Modify/abuse a Neve preamp so it can be a fuzz or overdrive

• Preferably running at 9v, but I can live with something larger using a charge pump (the Neve runs at 24V)

• Maybe include some EQ from the Neve, but simplify to keep it pedal friendly

• Avoid transformers.  Yes, I know, everyone says the transformers are a big part of the sound, but I don't care what everyone says cause proper Neve-style transformers are costly and large, and I'm not making an overdrive/fuzz pedal with a really expensive studio grade transformers known for their low distortion

• Avoid the insanely complicated Sensitivity switch (it's the input gain control, which sweeps all the way from mic to line level)

So what am I not trying to do?

• Recreate a Neve channel amplifier on vero (madness)

• Use unobtainable and/or expensive parts

• Reverse engineer a JHS Colourbox

NEVE 1073 CHANNEL AMPLIFIER SCHEMATIC

You can find a full manual, including schematics, here    This may be better than what you have seen previously, as it is the complete manual with tech specs.

SIGNAL PATH / BLOCK DIAGRAM FOR CHANNEL AMPLIFIERS 1066, 1073, 1078

A few partial schematics floating around on the web do not contain this diagram, which is very easy to understand compared to the next one, which is more commonly found on the net.

BLOCK DIAGRAM

This diagram is confusing, but it makes more sense with the reference above and the other items below, where I've covered the EQ sections individually.

NEVE MIC PREAMP SCHEMATIC

Three different preamps are used in the channel amplifier, which are used as gain stages in various places.  The multiple boards are made up of two or three of these preamps combined.  Some are used as gain stages for the active EQ, Mic and Line inputs and, finally, an output amplifier.

As it happens, this was used as the clean preamp on the Way Huge Pork Loin.  

MODDED MIC PREAMP SCHEMATIC

Just gain and level at the moment.  The frequency response is very flat for a pedal, so for my taste, it doesn't need a tone control.  Even though it has an element of fuzz, it doesn't have excessive high frequencies.

LTspice

It doesn't look exactly like this on the scope; the wave isn't as squared off as this.  It was more triangular with flat tops.  Hopefully, after I work out the gain sweep properly, I'll add some actual traces at a later date.

OUTPUT AMPLIFIER

LTspice

Frequency response varies quite a bit, depending on where I take the trace from (note letters on trace vs schematic).  I have done a bit of a dodgy workaround where the transformer should be, so it might be a result of this.

MISC

Now I know you can't see any details here, and there's still a little more to add - but this is why I will not be attempting to recreate a Neve channel amp on vero - it would be gigantic.

THE NEVE 1073 EQ SECTION

NEVE BA205 BOARD (LOW & HIGH SHELF EQ)

THE ASSOCIATED PREAMP

WHICH TRANSLATES INTO THIS

It's quite similar to an active James EQ, with the low shelf frequency being selected by a rotary switch (switching between different values capacitors), with a pot to control cut and boost.  The high shelf just has a cut and boost pot.

Hopefully, this is easier to understand, and it gives me a starting point if I ever decide to use some of it in a pedal or adapt it in some way.  The next section in the signal chain is the mid-range control (Presence), which looks quite challenging.   

NEVE BA211 BOARD (SWEPT LO-MIDS TO HIGHS)

I'm pretty sure this has an error - instead of C & E (which are preamp in/outs), it should read K & M.  K & M are the in/outs of the other preamp on the same board.  C & E have already been used for the low-high EQ section. 

There also seems to be some differences between the schematic, some including a 10k option, others not.  There's also an alternate schem for the board above, with an extra resistor. 

THE ASSOCIATED PREAMP

WHICH TRANSLATES INTO THIS (HOPEFULLY)

I say hopefully, as the frequency response didn't exactly match the stated values for 2, 3, & 4.  Looked good for the rest.

NEVE BA182 BOARD (HIGH PASS FILTERS)

This is the simplest of all the EQ sections, but it did have me stumped for a while as to why there are two identical sections with the same lettering - it's pretty obvious now; it is because they are connected...    As mentioned above, this is a passive filter, so no preamp is required. 

WHICH TRANSLATES INTO THIS

ELECTRO-HARMONIX: Opamp Big Muff,

 What more can be said about an opamp Big Muff?

ELECTRO-HARMONIX OPAMP BIG MUFF - VERO LAYOUT

VIDEO DEMOS

MID-FI ELECTRONICS: Demo Tape Fuzz

A nice easy one - and it reminds me a little of my Tascam four track cassette recorder days.

MID-FI ELECTRONICS DEMO TAPE FUZZ - VERO LAYOUT

VIDEO DEMO

DINOSAURAL: OPA-101 Overdrive Preamp

The Dinosaural OPA-101 Overdirven preamp is a variation (or improvement) on the Dinosaural Tube Bender.  

This is has the tone control like the the Aion FX version, but I did leave the output stage as a buffer as per the original.  The filter itself is on the original which you can see on the schematic / tracing journal.  Well worth a read as there's a lot of really useful background information on this pedal.

There's also the option to run this true, or buffered bypass.  Personally I'd run buffered, but that's just me.  

DINOSAURAL OPA-101 OVERDRIVEN PREAMP - VERO LAYOUT

LOVETONE: Big Cheese

While I'm doing Lovetone, I have to do a Big Cheese...  this is now the predecessor to the Thorpy Field Marshal Fuzz, now that Dan Coggins is helping out on the design side of things there.  

The schematic can be found at Aion Fx, and this version includes a bit of extra boost at the end.  I'd suggest reading the build docs (schematic) from Aion, as it explains how to set the trimmer and a few other things of interest.

LOVETONE BIG CHEESE - VERO LAYOUT

VIDEO DEMO

LOVETONE: Brown Sauce

Lovetone effects pedals are pretty legendary, and I have meaning to get around to some layouts for a while now.  I thought I might start with the Brown Sauce, which is the least challenging.  

You can find the schematic here on Aion FX - also check out the rest of the Lovetone line that they've traced too, as you will probably never want to attempt something like a Doppelganger on vero.   

LOVETONE BROWN SAUCE - VERO LAYOUT

BROWN SAUCE VIDEO DEMO

VOX: UL Series Preamp

I quite like some of the guitar sounds from The Beatles Revolver album, and after looking at the Vox Supreme, I thought I might have a play with the UL series preamp.

I'm also waiting for the build docs to come out for the Aclam Dr Robert on pedalpcb as a comparison.  It's also a UL730-based pedal - I believe it's the normal channel, and they have also modelled the power amp section.  Keen to see how they have done that.

VIBRATO CHANNEL

Everything below relates to the vibrato channel on the amp, which also happens to be the channel with distortion/fuzz, which is what I'm interested in.

FUZZ / DISTORTION

Strangely there's no control to dial in the fuzz/distortion circuit on the amp itself - it was turned on by a foot switch, and you got what you got.  

The switch increases the gain of T1 & T2 (using T3 and T4 as switches), and at the same time, it ties the OA200 silicon diodes to ground or V+ to create fuzz/distortion.  I'm really not sure which way the diodes connect, as I don't know what's happening with the foot switch.  The schematic is not clear in this regard.  Either way, it all looks about the same regarding wave shapes and frequency response.  

Bypassing the distortion leaves you with a nice clean signal, not unlike the normal channel.

BOOST SWITCH

There's also a boost switch on the front control panel of the amp, but it doesn't appear to do a great deal at the output, maybe a bit of a top boost.

LAYOUT

For some reason, the mid-range control is immediately after the clipping section but before the volume control and gain recovery section (T5 & T6).  The volume control feeds the gain recovery section, with bass and treble controls at the end of the signal chain.  I'm sure they had their reasons at JMI.   This is not a master volume amp either - one volume control per channel, and that's it.

VOX UL SERIES PREAMP SCHEMATIC

VOX UL SERIES PREAMP LTSPICE

This is with a 200mv sine wave at 440Hz, with the distortion/fuzz switched on.  LTspice file is here if that's of any interest.  You may need to change transistors if you don't have the BCs in your library, but the rest should be fine.

Collector T2

Post diode clipping

Output - bass, mid and treble all set to 50%

Output - bass, mid, and treble all set to 50%   Diodes removed from the ground - which I don't believe is part of the normal switching, but I might have to give this a try.  Looks like a rather large square wave.  i.e. a solid fuzz...  

The tone controls are pretty interactive - bass / mid sweep

Mid / treble  sweep