Audio Research SP 3

[kcin]

I had a ARC SP8 MkII for over 30 years that I bought new. I sold that unit because I had moved on and felt it just was not getting any use.
I probably should of kept it for nostalgia- I really enjoyed that pre.

Perhaps 20 years ago or more, I pulled the trigger on an SP3 that was advertised as non working and refurbished it. I found a black and gold face for it that was absolutely perfect and kept it for along time before I again sold it due to lack of use. I should of kept it too. It was a beautiful example.

Just a couple of weeks ago I found another SP3 marked with the factory paper "A-1" sticker that was botched by someone and modified with pseudo Curcio mods and with 12Au7 s (?) in the line section. Any way I have reversed those kludges and so far rebuilt the power supply.

I was going to re- stuff but I managed to just use snap ins for the heaters and Kemet makes a very nice 550v cap for the first stage- then Mundorf 200+200uf for the decoupling.

Some holes needed to be drilled under the original cans to makes these fit but it turned out very neat.

SP3's have many variants on the PS boards - this one had an SP3 A amp board not SP3 or SP3-A1 -so a little different than my previous one. It was upgraded to A1 status by the factory and noted by the sticker.

The amp board in all models are virtually the same with just some parts deletions or value changes to get from SP3 to SP3 A to SP3 A1

This one is not as beautiful as my old black and gold- but I will make it electrically sound.


Next is tube sockets and various resistor and cap changes - as I test. Lots more work.

 

[Jalousie]

Looks cleaner inside than the one I have.

 

[Blue Shadow]

Worth all the work. They sound great.

Maybe the new ARC will have something like them in the future.

 

[stanko]

I've been using an SP3 since 1978. Had an SP6, SP8, and SP10 but always went back to my SP3.

 

[arts]

Still using mine

 

[kcin]

There is something to be said about the SP3

I was taking voltage readings last night after the PS upgrade. Some on V1 and V4 were well off on the plates.
Everything else lined up. This evening I began taking resistance measurements. the plate resistors on these tubes were SP3 values at 150K and not 301K like the SP 3A (A1) and the voltages were high.

This will bring things in line as there will be a greater voltage drop across the larger resistors ( more gain too).
as it has the SP3 A 1 sticker on the back and inside by the transformer. Weird.


Here is a pic of a shoot out when I had my old SP3-- hoping to do the same soon as I work through this one.

 

[kcin]

Worth all the work. They sound great.

Maybe the new ARC will have something like them in the future.

Wouldn't be cool if ARC took a page from Mac and re-issued a modern SP3 with a regulated PS, modern controls that were accurate and tracked properly , added a mute switch, ditched the convenience outlets and maybe added an IEC inlet used better boards and parts.

I bet they would sell very well with the last wave of boomers still with us.

This way we could let the vintage SP3s be and not do a 3C mod for those that want the pain and suffering of refurbishment of vintage

 

[Blue Shadow]

And go for original sonics with tubes evaluated for that to avoid the more modern sound of ARC.
Those with or seeking ARC are split between the new sound, what the solid state sound of tubes with clean, clear sound vs those enjoying the vintage sound of music with more meat on the bones.

 

[Blue Shadow]

Here is a pic of a shoot out when I had my old SP3-- hoping to do the same soon as I work through this one.

What no results from this listening session?

 

[kcin]

That preamp stack comparison was some time ago after I got my ESLs redone. The short story is:

The ARC had better low end than the Marantz 7, the Marantz 7 had more detail and airiness on through the treble, the Consolettes stole the show.

Each was refurbished with new PS caps , the Marantz 7 was stock in the pre circuit, the Consolettes I had to change the bumble bees as they were bad .

I still have the 7 and Consolettes. I'll have an up to date comparison - by end of summer- I will add the Counterpoint I re-did to the line up- it may very well steal the show?

 

[arts]

One of the few modern (relatively speaking..) ARC preamps that holds to the more traditional ARC design philosophy is the SP16.

Six 12AX7's (3 phono, 3 line) and a single stage of regulation for each stereo stage. Not a lot going on, which is probably why it sounds so darned good. There's something to be said for not a lot of crap getting in the way. I bought mine just for the remote, which I promptly misplaced, so I wouldn't have to walk across the shop to mute or adjust the volume.

 

[kcin]

Here's why my voltages were off if anyone runs into the same issue. Plate resistor on v1 and v4 150k vs 301k on A-1 and cathode resistor on v301k instead of 100k

Also the feed back network is definitely A1 and this I believe with the PS was the major mod at the factory

My PS board is SP 3A1, my feed back circuit is 3A-1 and my phono and first line stage section SP 3 ---sheesh.

The punched SN is A the sticker is -A1 My guess is that it was an early A1 factory mod --- there were lots of changes

 

[gadget73]

so it sounds like they increased the open loop gain by increasing the plate resistors, but compensated that with more feedback so the end result is the same as original ? That should reduce distortion if so.

 

[ceylon]

very cool. i eagerly await the next update.

 

[kcin]

so it sounds like they increased the open loop gain by increasing the plate resistors, but compensated that with more feedback so the end result is the same as original ? That should reduce distortion if so.

A strange set of changes.

The situation is that on mine the feedback network is like the newest A1...this may of been part of the update as the resistors are definitely factory.

I am waiting for a few parts and will chime back in soon.

 

[gadget73]

Depending on actual needs, maybe thats not a bad mix. No idea what the gain of the line stage is on these things, but if the source is hot and the amp has reasonably high input sensitivity, having lower gain in the preamp may have an advantage. More feedback but less open loop gain makes for less gain end to end.

 

[kirk57]

I've been using an SP3 since 1978. Had an SP6, SP8, and SP10 but always went back to my SP3.

You prefer the SP3 to the SP10? That’s high praise…

 

[BillWojo]

Never had a SP3 but went from a LS15 to a SP8 ver5 and never looked back. Very musical compared to the sterile to me sounding LS15.
I'm thinking that younger folks that grew up on SS would actually enjoy what the LS15 has to offer. Its a very good preamp with both balanced and unbalanced inputs/outputs along with a nice metal remote.

BillWojo

 

[tubelectron]

Good job @kcin !



I nearly did the same restoration work with my SP3A; except for the cosmetics that were still premium - unlike on yours, it seems.

In fact, last year I made my first venture in Vintage-Legendary Audio Research Corporation gear, by purchasing a SP3A1 (champagne and black frontface, like yours) and a D76 in pristine condition :

About the SP3A1 :

PSU restoration :

Preamp restoration :

Additional details :

Signal output, square waves, after modifications :

The gain reduction I performed brought the sensitivity and background noise level of the device back to current standards. So yes, it's a fine sounding and great performance preamplifier...

T

 

[Salectric]

Good job @kcin !

View attachment 2937875

I nearly did the same restoration work with my SP3A; except for the cosmetics that were still premium - unlike on yours, it seems.

In fact, last year I made my first venture in Vintage-Legendary Audio Research Corporation gear, by purchasing a SP3A1 (champagne and black frontface, like yours) and a D76 in pristine condition :

About the SP3A1 :

PSU restoration :

Preamp restoration :

Additional details :

Signal output, square waves, after modifications :

The gain reduction I performed brought the sensitivity and background noise level of the device back to current standards. So yes, it's a fine sounding and great performance preamplifier...

T

Am I reading your notes correctly that you have a 47K in series with the output? If so, that is a very high output impedance on the linestage.