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Audio Research SP9 – fault, tubes and matching - Long

On the weekend I took the opportunity to buy an Audio Research SP9 hybrid preamp which looks great but apparently has an infrequent intermittent fault and was hence available at a very good price. It is the original model built in late 87 and does not appear to have been modified.

Objectives
My first objective is to upgrade from the phono preamp in my Yamaha C2 preamp which is currently connected to my Plinius 9100 integrated. My second objective is to find out how good my Yamaha B-2 VFET power amp can sound with a different preamp. I don’t expect it to beat the Plinius but if it does, I will have to decide how much I value remote control.

The Fault
The fault apparently causes sounds of a speaker threatening nature but I have not heard it myself or had the chance to speak to someone who has. I have a tech looking over it and he has not yet heard the problem or found anything amiss other than the original tubes which test as marginal.
Suggestions on the cause of the problem from various sources have included:
- Failure of the B+ 285V regulator
- Intermittent tube failure
- Intermittent connection on one of the tube pins
- Intermittent grounding fault at the volume control
Does anyone have any similar experience or have ant thoughts on this?

The Tubes
Replacing the old tubes seems like a no-brainer whether it fixes the fault or not. My tech has a pair of the original tubes (can’t recall the model) but recommends going instead to the milspec 6922 which has more voltage headroom. ARC went to 6922’s in the Mark III version of the amp.
I can get a pair of Philips US 6922’s locally. Would they be a good choice?

Matching
The SP9’s output specs are 250 ohms main out, 850 ohm recorder out, recommended load, 60K ohm and 100pF. (20k or 30k minimum)
The Yamaha B-2 input resistance is 22k and the Plinius line inputs are 47k. The main out would go to the Yamaha and the recorder out would go to the Plinius.
Is the SP9 likely to sound near its best if I just connect with low capacitance cables or should I be modifying something?

David



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Topic - Audio Research SP9 – fault, tubes and matching - Long - energyandair 12:15:20 04/10/06 (8)


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