Audio Research SP-3 Preamp


Rebuilt and modified 4-2007.

 

Note that an extensive list of links and references is included at the bottom of this page.

History and Marketing Materials

The SP-3 preamp was introducted by Audio Research in 1971, and was in production for at least 6 years in various incarnations, and several thousand were made.  It has received numerous rave reviews and accolades while in production and even as recently as 12/2003 when The Absolute Sound magazine listed it as one of its 10 "Components to Remember", along with the ARC D-150 amplifier.  It was listed as an Absolute Sound Editor's Choice as "State of the Art" in 1975, as "Class II" in 1976, and back up to "Class I" in 1977. 

This preamplifier has been sold in at least 5 incarnations:  the original SP-3, an SP-3A update, and SP-3A-1 update (which appears to have been the bulk of the production), an SP-3B update in 1978 (upgrading to the SP-6 circuit and an improved power supply, see newsletter below), and an SP-3C update in the late 1990s that is still available today (more on the SP-3C in the Modifications section below).   The fact that over 30 years after it was introduced, ARC is still upgrading this unit is testimony to its popularity and to ARC's committment to its products.

A November 1975 ARC pricelist shows tha list price of the SP-3A-1 as $795 (thats $3,140 in 2006 dollars adjusted for inflation).  That pricelist also shows modifications from the SP-3 to SP-3A-1 for $125, and from the SP-3A to SP-3A-1 for $25.  A June 1977 pricelist shows the SP-3 to SP-3A-1 upgrade as $200, and the SP-3A to 3A-1 still as $25.  The 1978 Newsletter announcing the SP-3B (shown below) indicates an upgrade price of $470.
 
ARC Newsletters related to the SP-3:
 
test
 
 

Above: July 1976 Newletter announcing mods to SP-3A-1.  Click to enlarge.

Above: Page 1 of June 1978 Newsletter announcing SP-3B.  Click to enlarge.

 

Above: Page 2 of June 1978 Newsletter announcing SP-3B.  Click to enlarge

 

Above: SP-3 ad from Audio magazine September 1973.  Click to enlarge.

 

The original SP-3A-1 sales flyers:

Above: front, click photo to enlarge.

 

Avove: Back side, click photo to enlarge. 

Another SP-3A-1 sales flyer:

Above: Front, click to enlarge

Above: Back side, click to enlarge.

Modifications  

 My Audio Research SP-3A-1 preamp was built and purchased by its original owner in 1975, making it over 30 years old when I bought it in 2007.  The unit had been extensively modified previously (probably in 1985 based on receipts with it) with all new caps (both coupling and power supply), phono EQ changes, and more recently with the addition of a Curcio HV power supply regulator (Thanks to Samir Shah, the previous owner).  Unfortunately, most of the changes (except the Curcio regulator), had been done 20+ years ago, and the electrolytics were leaking and way off nominal values.   In addition, the heater caps had been increased in value almost 20 times the original value, resulting in large turn-on surges which fried one of the heater rectifiers. 

The photo below shows the preamp before I started repairing and modifying it, and before Samir added the Curcio regulator.  Note the Siderealcap couplers and extra heater capacitors (Blue cans plus the smaller black cans) which were leaky.  The two silver caps on the right rear are the only original stock caps in the entire unit, and they are unused with the Curcio regulator.  The RCA connectors were stock and some were very loose, and all controls are stock. 

Above: My SP-3A-1 as received.  Click photo to enlarge.

 

Audio Research (links below) is currently offering an upgrade to the SP-3, called the SP-3C (the  SP-3B was a 1978 upgrade to the SP-6 audio circuit, using a new power supply PCB).

The SP-3C upgrade was originally created because the electrolytic caps in the power supply are no longer available, so a new power supply board was developed.  Newer HV regulators, a revised heater circuit (also regulated), and a muting circuit are also included on the new power supply board.  In addition, the SP-3C upgrade replaces every cap in the audio circuit (with Relcaps, Wimas, and polystyrenes), replaces the volume control with an Alps unit, and replaces all 24 of the RCA jacks with modern gold plated ones.

The photo below shows an ARC modified SP-3C, not my unit.  Notice the yellow Relcap couplers and red Wima caps on the audio board, and the entirely new power supply PCB (at the top in the photo). 

Above: An ARC SP-3C.  Click photo to enlarge. 

 

Since my unit had already been so extensively modified, returning it to its stock original state wasn't really an option, and I didn't want to pay the $1,000 SP-3C upgrade price, so I decided to make the audio section as close to the Audio Research SP-3C as possible, and use the Curcio regulator for the HV power supply, leaving the tube heater circuit stock (unregulated), but with moderately increased capacitance. 

The photo below shows my unit after modification.  Note that the audio board is nearly identical to the SP-3C, with only two changes: the tone controls (including V7 & V8) are not in circuit (although all audio circuit parts are in place should I ever decide to use it), and second, the phono EQ is changed to that published by Brian Clark in Audio Amateur magazine in the 4/1980 issue (link below), based on his extensive modeling of this circuit.  Some of the caps were sourced from ARC, including the 1.0ufd/450v Relcap couplers and some of the small value polystyrene caps.  The Curcio high voltage regulator board is standing vertically at the right rear.  The power supply board contains the raw DC supply (2 large black caps) and the heater supplies (the 4 green caps).  The rectifiers were also replaced, after this photo was taken - details below.  All photos from this point down are my modified unit.

Above: My SP-3 after modifications.  Click photo to enlarge.

Audio Circuit 

Below is a closeup of the audio board and controls.  Note the new Alps 100K volume pot and a new Alps 100K balance control (which I believe are no longer made) which I salvaged from another project.  Note the unused sockets for the tone control tubes V7 and V8 at the center of the board.

 

Click photo to enlarge.

Phono Section 

Below is a look at the revised phono section.  Brian Clark did an extensive analysis of the RIAA phono EQ for this and other tube preamps published in The Audio Amateur 4/1980 issue, entitled Active RIAA Equalization Network for Vacuum Tube Preamplifiers.  Based on his research and analysis, the polystyrene caps (silver and blue) which I used are his values for this circuit: 3000pf and 893pf, plus his recommended resistor values of 86.6K, 1.44Meg.  These were all selected from larger batches to be well under 1% tolerance.  When I measured the new phono frequency response, I got +/- 0.25dB of flat, which is exactly the spec of my Hagerman inverse RIAA filter (link below).

 Click photo to enlarge.

Power Supply Regulator

The photo below is a closeup of the Curcio regulator board (link below), which provides four 400VDC low impedance regulated supplies, feeding the phono and line sections.  This regulator has enough current capacity to supply the tone control stage if I should later decide to power it up, and each section draws only a few mA of current.

 Click photo to enlarge.

 

Below is the raw DC power supply.  For the HV supply, I used Nichicon GU 390ufd/420V caps in series from Digikey (link below), similar to the setup ARC uses in the SP-3C.  For the heaters I used Nichicon Muse 3300/50V.  I also changed the rectifiers, using SF4007 ultrafast soft recovery diodes for the HV supply (from Partsconnexion.com) and GI856 high speed fast recovery diodes (from Mouser) for the heaters.

 Click photo to enlarge.

Input/Output Jacks  

Below is an inside view of the new RCA jacks, from Partsconnexion.com (their part #Connex 53451 - gold over brass, teflon insulation), link below.  I expected replacing these 24 RCA jacks to be a bigger job than it was, but it was just a couple hours of methodical work.  Note the Curcio regulator is temporarily on the other side of the preamp, allowing easy access to the jacks.

Click photo to enlarge.

Below is a rear view of the new RCA connectors.

Click photo to enlarge.

 

Below is the facplate view.  I replaced two of the knobs which were scratched with new stock from ARC (which amazingly to me, look identical to the originals after all these years).  Otherwise the face was in very good condition, just needing cleaning.  I will note that the cover for this unit is very open, and 30+ years of dust and grime on everything internally was a pain to clean, using a ton of Q-tips.   Now off to trying various tubes in the unit...

Click photo to enlarge.

 

Reference materials

ARCDB info on the SP-3 - This is an excellent site for ARC fans, with lots of info and photos on every Audio Research model, many with schematics (including the SP-3).

ARC SP-3A-1 schematic - from ARCDB.WS

ARC SP-3C schematic - from ARCDB.WS

Audio Research's SP-3C info

Curcio Audio's information of their SP-3 mods and HV regulator

Joe Curcio's SP-3 mods and regulator installation instructions

Absolute Sounds "Components to Remember" on the SP-3 and D-150. - Sorry, this link is no longer on the web, but is in their December 2003 issue.

An Audio Asylum post on the SP-3, with suggested mods.

Stereophile magazine's reviews of the SP-3 from 1973 - 1975

A review of the SP-3 from AudioShopper

The following article is available in back issues from AudioXpress:

   http://www.audioxpress.com/index.html

   Active RIAA Equalization Network for Vacuum Tube Preamplifiers, article by Brian Clark, The Audio Amateur 4/1980, page 45+. 

 

Tube rolling in the SP-3:

     Audio Asylum post on tube rolling the SP-3

     Another Audio Asylum post on tube rolling by Samir Shah (whom I bought my unit from)

     Yet another Audio Asylum thread on SP-3 tube rolling.

 

Parts Sources:

http://www.partsconnexion.com/ - source for the new RCAs

http://Mouser.com - source for Wima caps, rectifiers, etc.

Digikey.com - source for HV power supply caps

Michael Percy Audio - source for heater caps, etc.

Hagerman RIAA filter

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