\\\ viva Analog 360+ Bass Preamps ///

Modernized class-A Performance

** 2008 Pro Edition **

 

Photos and Text (c) JC Maillet 1996 - 2009

Welcome to my Acoustic360 repro page !!

I build a high-quality compact pedal version of the Acoustic 360 Bass Preamp. The unit is roughly 1" in height so the pedal will fit in most gig bags. The guts use robust amplifier-grade construction (Alpha 24mm Pots, Switchcraft JACKS, Lorlin, C&K, Carling Switches, ...) and the Variamp filter circuit is built using a great sounding audio-grade inductor custom-wound for me by Cinemag USA. The preamp costs $500US - this includes the AC adapter, a custom finish, and air shipping to US/Canada.

The Fuzz BLEND control is new and original in design, allowing fully independent mixing of fuzz and straight signals, the FUZZ bypass switching is also re-designed to minimize switch pop/noises. The fuzz circuit uses selected 2n1306 Germanium devices providing a fairly faithful reproduction of the 360 distortion qualities - and with the BLEND control any amount (up to x3 over-unity) of clean signal can be mixed in for a heavier and fuller sound.

On the inside there are two user adjustable trim-pots. The one near the center of the board sets the gain of the output amplifier.  This needs to be set carefully since the output amplifier circuit can put out around 22volts (pk-pk) signal. The 360+ comes with an instruction sheet that describes how to best adjust this trimmer.

The pop-less MUTE function is achieved through an original constant-current optical switching design. Since class-A circuits are sensitive to sudden variations in rail current and typically passes on the variation as a click noise to the output it was necessary to devise a switching circuit that leaves the rail currents constant. The MUTE circuit has no effect on sound quality when disengaged.

The power supply circuit features an over-kill design that rivals battery operation in terms of noise/ripple output. The new PSU circuit combines passive and active filtering where the 60Hz and 120Hz components find themselves lying below the -96dbV threshold of my 24-bit soundcard (with gain full up); and that's when using an ordinary unregulated AC adapter.

Using the 360+

The 360+ is a complete preamp system which includes an internal fuzz circuit. The Acoustic 360 preamp was originally meant to drive Acoustic 361 speaker bins which contain a power amplifier. The 360+ is designed to drive a more general class of power amplifiers and does so with better fidelity than the original design on account of an added output line driver stage - what I've been doing from the start. This gives a clearer overall response - the bottom is always strong and clear, the top-end sharp and accurate thus preserving the clarity and punch of the original signal path.

Some players may want to try plugging the 360+ in the front of an integrated amplifier (meaning into another preamp first) but the combination sometimes produces extra hiss just like when cascading any two "full" preamp circuits. There's no harm in trying but the 360+ is mostly intended to drive a power amplifier or a magnetic tape recorder, A/D converters (soundcards and digital recorders) and PA channels/inserts are usually fine as well.

Some integrated amplifiers come equipped with pre-out/power-in jacks that break open the signal path between the output of the preamp and the input of the power amp. This would be the preferred entry point for the 360+. Vacuum Tube amplifiers can be easily modified to receive a SLAVE input, some already come equipped with them. The purpose of having a variable gain output amplifier on the 360+ is to provide the signal swing required to drive some of these high powered vacuum-tube power stages. Otherwise, the 360+ can handle pretty much any type of load.

Background on the Acoustic 360 Design

From a spectral point of view two interesting things are going on in the 360 circuit. On the one side, there's the Variamp circuit; a single class-A stage providing a 5-band selectable filter function. Unlike multi-band eq circuits the relatively restricted 360 circuit exhibits considerably less phase distortion.  From a dynamic point of view all the gain stages are devoid of capacitive bypassing in the emitter circuits.  Together they account for the type of transient response the 360 is known for. 

The lower graph shows the five frequency bands of the VARIAMP filter, they are centered around: 50Hz, 100Hz, 200Hz, 400Hz and 800Hz. The EFFECT control sets the amount of cut or boost produced by the stage. When the EFFECT control is turned full clockwise the circuit produces maximum boost, when full counter-clockwise the circuit produces maximum cut.  When the EFFECT control is set at 12 o'clock the circuit response is flat. It's also worth noticing how the bands become relatively narrower as they go higher in frequency - thus affecting a lesser frequency span.  In combination with the VARIAMP 5-position filter we find the passive RCA tone stack (load) typically seen in valve type circuits from the late 50's, notably in guitar and bass amplifiers. It's worth noting that the sudden BASS control jump characteristic of the RCA circuit still applies in the 360. Since the RCA tone circuit naturally produces a mid-scoop, this combined with the VARIAMP filter response yields interesting tone shaping combinations.

 

The five VARIAMP frequencies are depicted as RED dots at the bottom of the tone circuit response graph to show the relative placement of frequencies between the two filter circuits. Notice how the five VARIAMP frequencies all lie somewhat within the mid-scoop area of the BASS/TREBLE tone stack filter response. It's also worth noticing that the two upper-most Variamp frequencies (at 400Hz and 800Hz) get swamped by the treble shelf when the TREBLE control is set high enough. This means that getting more use of those two upper VARIAMP positions may require turning the TREBLE control down. You can tell in the VIDEO clip (below) these two positions aren't doing too much at one point because I've got the TREBLE control set too high. The other thing to note is that the FUZZ circuit comes before all that eq'ing circuitry, so the eq. sections have a dramatic effect on the resulting FUZZ sound as well.

Custom "360+ Pro" Gallery

\\\ 360+ FUZZ DEMO ///

360+ Video

\\\ 2009 viva Analog 360+ DIY Bass Preamp Kit ///

** NEW RELEASE 06/09 **

\\\ ARCHIVE MATERIAL ///

** Acoustic Control 360 Bass Preamp and Fuzz Unit (1997) **

This electric bass Fuzz/Preamp system was favoured by the likes of John-Paul Jones, Jaco Pastorius, and Larry Graham ... aside from the great sounding Germanium/Silicon combo fuzz the Acoustic 360 circuit is based around a versatile and punchy sounding class-A preamp circuit ...

These are pics of my first 360 clone project - I used a hi-Q 1.2Hy Pultek filter coil for the Variamp section, it has both battery and DC regulated operation from an AC wall-wart ... the circuit is stock ... the original 360 had an internal ETF tuning circuit, it isn't included in any of my units (or kit) ...

Acoustic 360 Bass Preamp/Fuzz Schematic (126k GIF)

Acoustic 360 Schematic by Joe Piazza

Acoustic 360 and 270 owned by Atsushi Sugenoya - Patron of the first "360 Earthquaker" Clone

Original Mods and Building Ideas (2002)

The "modernized" version of my first 360 clone includes an op-amp based output amplifier add-on to the basic circuit - this is where op-amps can provide improved system performance ... having greater output drive allows for better retaining of transient (dynamic) fidelity over the stock emitter follower ... this also allows the driving of a wider range of power amp (loads) and cable capacity (length of run) : all the way from modern stuff to old low-Z transistor power amps and low sensitivity vacuum tube slave amps ...

My Hand Drawn Schematic

Op-amp based DI outputs (1/4" and XLR)

18v @ 26mA // Battery Version

Charlie Barth's 360+ Wiring/Components-PCB Diagram

Charlie was very kind to draw a computerized layout and wiring diagram with the component placement on my PCB board

SPDT version of 360+ Layout and Wiring

DPDT version of 360+ Layout and Wiring

 

Photo (c) C. Barth 2003

For those of you who are interested in the 360 Fuzz sound but not the whole preamp Charlie tells me he's come up with a gratifying circuit that does just that. Check it out !!

www.moosapotamus.net

\\\ Early Units ///

Lite/fuzz-less 360+ Pedals and full featured Racks

Player Comments

Hello JC,

I have been working on building an entire Acoustic 360/361 copy from scratch. I am building the entire thing making it as close electrically and physically to the original as possible.

One of the things that kind of blew my mind was the use of the 2N2926 NPN transistors as what appears to be 8V DC Zener diodes by reversing the collector and emitter (and leaving he base floating). I wonder why they did not use an actual zener diode unless the reverse 2N2926 has some special property that was needed in the design? Maybe they had a ton of 2N2926 transistors on hand at the time?

I tried using the backwards 2N2926 in the Fuzz circuit and it actually oscillated somewhat which modulated my fuzz signal. I took the 2N2926 out and put in a 7.5 zener and everything is fine now (I think).

I was just wondering what the real deal is with with these transistors (there are also two in the electronic tuner which I know is not included in your hand draw schematic (Acoustic 360+ Mods).

I also tried using a real inductor in the Variamp but I was not happy with the results so I am going to try your inductor simulator circuit instead.

One other question: As best as I can remember, the Acoustic 360 pre-amp did not have a Master Volume control (as shown on your schematic and all other 360 schematics) so I am assuming that this must have been a factory pre-set internal trim-pot (I have not had access to the real deal Acoustic 360 otherwise I would not have these questions). If so, I wonder what it was set at?

Also my electronic tuner output into the pre-amp is at a very low volume with I assume is normal as there is some high attenuation resistor voltage dividers between the Unijunction output and the pre-amp input.

B.T.W. Do you have any idea how this tuner was suppose to be used? It seems like a joke!

Follow up: Jeff's 360+ Frequency Response Graphs

The plots that I send you are with the coil simulator circuit from your hand drawn 360 schematic not using an inductor. I gave up on the inductor after I found that it was not really 1.5H but more like 0.8H. Even though I used two back to back 3.3uF electrolytic caps in the simulator circuitry, the center frequencies came out extremely close to the calculated frequencies so I am just going to leave everything alone and not mess with it any more so now I am concentrating on finishing the powered speaker. I also gave up on using the upside down NPN transistors for voltage regulation and I went with regular old zener diodes.

All I have to do is build the compression chamber (that what I call it anyway), mount the 18" Vega and then install it in the cabinet. I am now trying to track down some Formica that matches the acoustic blue to go on the front of the compression chamber. I will take some pictures when I am done.

Jeffrey Hicks

Associate Engineer

Audio Technica, US., Inc.

Thanks Jeff! the electronic inductor simulator works pretty well in this adaptation of the Variamp circuit, except for the slight increase in hiss at full boost or cut, it's almost as good sounding as a high quality coil - quite close in fact. About the Zener connected transistor, I had the same problem with all the modern devices I tried, maybe the hFE has to be low or something - ended up using a Zener as well. Interestingly, the three VCOs in the EMS Synthi and VCS3 synths are all based around a transistor connected the same way - and obviously designed to oscillate.

I'm heading to Montreal to do the Guitar Show in July, if you are around for the Jazz Festival come see me. I'm bringing an Archtop Bass with me.

Thanks again for a great device-It's all I use to record with, my amp went on vacation after the first time I used my 360+

Vinny

www.vinnycollettiguitars.com

Greg Ridley of Humble Pie - yeah!

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EMAIL: jc AT lynx DOT net