Audio Research SP8 Preamplifier
Written by Bryce Ringwood   

First - I apologise for not taking pictures of this subject.

I am often asked if I can replace a turntable having a piezo cartridge with a modern stereo turntable using a magnetic phono cartridge. I usually reply with a qualified "Yes but you need a phono box". At this point everyone agrees to replacing the piezo cartridge and stylus and that's the end of the story.

The Audio Research SP8 is a "phono box" par excellence. Magnetic cartridges respond to changes in velocity of the needle in the record groove, whereas piezo cartridges respond to displacement. In addition magnetic cartridges deliver a much smaller output voltage around 5mV compared to as much as 0.4 V for a piezo cartridge.

I prefer to keep the original turntable for reasons of authenticity, but in my mind, the piezo cartridge and turntable is heavy and potentially damaging to vinyl records. If listening pleasure is the sole criterion, then, the modern turntable is the answer.

Inexpensive phono boxes start for around R160-00 and are kind of OK, but a really good audio preamp is a much better bet. The Audio Research SP8 was made in 1982 and cost in the region of (wait for it) around ... R20 000-00. Little wonder I was confronted by a very nervous owner.

The price reflects the care taken in the design. The power supply circuit is partly solid-state and consists of two HT supplies independently regulated using a 12BH7 / 12AT7 combination in a series regulator for the 409 volt supply, and a TIP41 pass tranistor for the 390 volt supply. Zener diodes are used for reference. A 555 timer is used to  delay application of the HT until the audio valve filaments are hot.

The audio chain consists of 12AX7 double triodes with unbypassed cathodes feeding a 6DJ8 cathode follower. The gain and balance controls follow, then follows an almost identical chain of 12AX7 / 6DJ8 providing the output. A filter circuit follows, with the final output passing through optocouplers to the output phono sockets. The gain of each channel is therefore carefully controlled, and with the balance control in the central position, each channel performed absolutely identically. The coupling between stages consisted of up to three capacitors in parallel - one for the high frequencies, one for the mid-range and a large value for the low frequencies. Does this make more sense than a single capacitor ? - I don't know, but a lot of people think its a good idea. The second 12AX7 in each chain has an FET wired as a diode between cathode and grid. This probably protects the valve from a positive grid excursion when switching from stand-by. The anode of the previous stage is directly connected to the grid.

The optocouplers act like small value resistors in series with the output under normal use. When the amplifier is muted, they act as very high value resistors, removing the sound.

While testing the amp, I couldn't resist giving  the input vale a light tap to see if there was any microphony - there wasn't. My guess is that this amp is about as close to perfection as you're going to get.

And the problem ? A naughty cabon resistor randomly turning the amp on and off via the optocouplers.

 

 

 

 
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