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Ted Kendall · 2nd Jan 2011, 7:58 pm

The problem with the 5532 was that the asymmetrical collapse of power rails on switch-off sent a large current pulse through the head, magnetising it. This does not occur with the 931, presumably because the bias currents are much smaller.

The A67, developed in parallel with the A700, has a similar rep amp design. The machines were slightly in advance of the available technology but, given careful selection of 931, could be made to work perfectly well unless you needed to apply large amounts of top lift to retrieve an ancient tape.

The B67 substituted a three transistor discrete first stage, feeding an IC EQ stage not unlike that in the A700. Placing one of these in front of the existing circuit and reducing the gain around the 931 would serve two purposes -the first gain stage would be quieter, to the benefit of S/N ratio; and if the resulting improvement were not sufficient, the removal of the IC from the head circuit would permit substitution of a quieter IC.

Masochists could consider the use of the A810 rep preamp built into the block - this would bring similar benefits and probably reduce hum pickup, another A700 bugbear, for which the deployment of B67 shielding plates was recommended.

2nd Jan 2011, 7:58 pm	#13
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,670	Re: Revox A700 repro IC The problem with the 5532 was that the asymmetrical collapse of power rails on switch-off sent a large current pulse through the head, magnetising it. This does not occur with the 931, presumably because the bias currents are much smaller. The A67, developed in parallel with the A700, has a similar rep amp design. The machines were slightly in advance of the available technology but, given careful selection of 931, could be made to work perfectly well unless you needed to apply large amounts of top lift to retrieve an ancient tape. The B67 substituted a three transistor discrete first stage, feeding an IC EQ stage not unlike that in the A700. Placing one of these in front of the existing circuit and reducing the gain around the 931 would serve two purposes -the first gain stage would be quieter, to the benefit of S/N ratio; and if the resulting improvement were not sufficient, the removal of the IC from the head circuit would permit substitution of a quieter IC. Masochists could consider the use of the A810 rep preamp built into the block - this would bring similar benefits and probably reduce hum pickup, another A700 bugbear, for which the deployment of B67 shielding plates was recommended.