Unit 5, Silveroaks Farm, Hawkhurst Lane, Waldron, Heathfield, E.Sussex, UK, TN21 0RS
Tel (0)1435- 813393, International +44 1435 813393
VAT Reg. No. GB 241 7291 72
 

ERIC CLAPTON'S AMPS/EFFECTS SYSTEM

We were contacted by Lee Dixon, Eric's technician, with the request that we build a complete stereo touring system using the amps recently built for Eric by Mike Soldano (SLO 100s) and the effects which had been used in the recording studio. We first noted that all the effects, with the exception of the Wah pedal were stereo rack devices requiring line level (+4dBm) input and output signals; all at low impedance.

As it is not advisable to connect guitars to low impedance effects we made the decision to route all the rack effects to a point between the output of the SLO 100's "pre-amp" section and the "power amp" input. This proved not practical as the measured input sensitivity of the power amp was found to be 11V RMS (+23dBm), a signal level too high to be provided by the effects. We therefore designed an extra gain stage for the power amps to make the input sensitivity +4dBm, and as the amps had been returned to the USA for recording we requested that Mike Soldano add the gain stage. We also asked Mike to make some other modifications to the master volume controls of the SLO 100 pre-amp stages so that the output of the pre-amp stages were also +4dBm.

The next problem was how to link together many effects units that all have a mono input and stereo outputs. The only option was to feed all the effects inputs with the same isolated, balanced signal from the SLO 100 pre-amp sections and as this was to be a stereo system there were two pre-amps, eachwith a footswitch controlled "Clean/Crunch" or "Lead" output. As our system was to be as flexible as possible we decided to used both pre-amps, with their channel switching, and allow either one pre-amp to drive the effects or both pre-amps to be combined and then drive the effects. This produced some very interesting sounds, as it was now possible to use one pre-amp "Clean", the other in "Lead" and combine them together. The other "Clean" and "Lead" pre-amps could now be adjusted to provide different amounts of overdrive and tone and used either separately or combined.

Having split and isolated the selected pre-amp signal to provide low impedance balanced feeds to all the effects the next job was to combine all the effects outputs into one stereo pair suitable for driving the two power amp sections of the Soldano SLO 100s. A problem immediately arose: if each of the effects passed the "Dry" signal to the power amps then they would add together and alter the signal level each time an effect was added. It was also a requirement that when no effects were in use the SLO 100s should sound EXACTLY as they did before we built the routing system. We therefore removed all the "Dry" signals from each effect unit and routed the selected pre-amp signal directly to the power amps at unity gain. As the effects were, in general, all various types of delay and would usually not be louder than the "dry" signal we could ensure that the overall volume would remain constant no matter which effects were included in the power amp input signals. The effects outputs were now to be muted rather than bypassed as we already had the "Dry" signal and did not require another one!

For some special type of effect it might be essential to have only the "Effect" signal without the "Dry" so we arranged for the two pre-amp signals to be muted at certain times but still sending them to the effects. This system was used when the Leslie Effect (Dynachord CLS222) was switched on.

As the effects mutes, the pre-amp selectors and the dry mutes may be required to be switched simultaneously we decided add a matrix controller so that the various combinations could be pre-programmed and selected with just one footswitch. The matrix fitted was 10 X 20; the 10 vertical rows represented the 10 footswitches at stage front for operation by Eric and the 20 horizontal rows the various effects and routings that could be programmed by placing a pin in the crossing point by Lee. Any number of pins, up to the maximum of 20 could be inserted in each horizontal row and those functions or effects would then be assigned to the footswitch associated with that row.

Further refinements were added:
1) all effects inputs and outputs were converted to male and female locking XLR connectors to ensure that vibration in transit would not cause any loss of signal due to jack plugs working loose;
2) An automatic voltage stabilizer, providing an exact 117Vac was built to power the system.

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