Audio Research LS7 Preamp Restoration

I bought 2 Audio Research LS1s as the start of a new system. The LS1 is a hybrid FET/Tube/MOSFET design, using a 6922 in the tube role. It is fully regulated on the 400V+ high-voltage B+ as well as the heater supply. The LS7 is very similar, the power supplies are very close, but only uses 1 semiconductor as a current source and four 6922 vacuum tubes in the gain and output stages.

The LS7 is a simpler preamp, despite having more tubes. Its cable routing is shorter, the PC board is less complex and it lacks switching and balance that the LS1 has. The LS7 being a little newer and being in spectacular condition, didn't require a great deal of work. The main change was to replace all the old electrolytic capacitors and upgrade the power supply film caps to Solen Metalized Polypropylene and the audio path capacitors to USA made Auricaps.

Photos

Taping up the PC board to drill IEC power cord hole, 2 RCAs for a second output and mounting holes for the new electrolytic can capacitors.

The targets to remove. The REL-CAPs and PC board mounted electrolytics. On the LS1s I cutout the tube sockets and replaced with Belton Micalex ones. I haven't done that to the LS7 yet, but I plan to.

Electrolytics updated from 470uf + 100uf to 800uf and 350uf. Outputs updated from 2uf to 6.8uf Auricaps.

The RCAs cleaned up nicely with some Simichrome polish. Note the additional last set of RCAs I added to have a second output from the preamp. I need two; one for the power amp, one for the subwoofer amp. They are the exact same type as originally used by Audio Research.

That's a nice picture. Four NOS (new old stock) Sylvania 6922s added. Note I added silicon dampening rings as well in the final assembly.

Setup and playing with a Rega P3-24, EICO HF-87 and B&W 703 tower speakers. The sound is very, very good.

So How is the LS7 vs the LS1? Pros and Cons of Each Design

Both preamps sound fantastic and are worthy of a place in any audiophile system. They are both excellent performers and the differences in the 2 are very subtle. I would imagine the LS1 is the best bet for most users, but the LS7 with some serious time spent matching high quality tubes can attain a slightly higher fidelity.

LS1

One tube is much cheaper; the solid-state devices make having a premium tube not necessary. The circuit is very forgiving of the typical variations in vacuum tubes
20db gain vs 13db gain for LS7
Solid state devices have lower output impedance and can drive solid state amps and longer cables better
6 inputs vs 5 on LS7
Balance control
Tighter, better low bass and extension
Two output RCA jack sets

LS7

Slightly better and more open soundstage; better depth and placement
The internet rumor of 'lean bass' likely comes from the 2uf output capacitor driving solid state amps; increasing this I hear no lack of bass at all
13db of gain is fine for most systems; however, may not be enough with some equipment
DC coupling from input to output makes the tube selection very, very, very tricky as each section difference is multiplied in the DC levels through the preamp
Tube selection and matching is critical because of above
The trade off for increased hassle of setup and lack of controls is simply a better sound
Much, much more expensive to re-tube
Only has one RCA output set

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Audio and Electrical Engineering

Audio and Electrical Engineering