I built a DIY Elliott Sound Project 88 Rev C - audio preamp 12 months ago and have found the sound quality to be comparable to anything else I have heard in my audio club rounds.
However, as a constant tweaker I decided to convert it to a switchable active/passive unit.
After the usual research I decided on a shunt design with quality resistors. The only additional parts were 2 high tolerance 47k resistors and a 4 pole 2 way switch which was used to isolate and bypass all electronic circuitry. Cost $15.
If you want to experiment, the circuit for both channels is just a matched pair of 47k resistors running from the input to the output and one end of a 50k pot connected to the output with the pot centre tap earthed. (I had to reverse my pot connections because it was already wired but it works fine)
There seem to be a million more complex ways to skin a cat but this is a cheap way to give it a go yourself.
The original ALPS Blue pot, rotary switch, inputs and outputs were untouched except for input from the switch.
The active/passive switch can be used for instant A/B testing with power still supplied to the amp.
The results were surprising, as I had expected a change in level when the electronics were shut down.
The first surprise was that levels were identical. Gain can altered by switches on the powered preamp but at good listening levels are obtained at 3 o'clock (from the top)
The second was that active and passive sound quality was almost identical with the with or without the electronics. The passive section has a slighty more detailed bass extension and voices are more textured, but not much. Without an A/B test there is no noticeable difference.
I have come to a couple of conclusions.
1. The ESP preamp seems to add or subtracts nothing from the signal and is very close to being a 'straight wire' unchanged by electronic components, which should surely be the goal of all equipment in the signal path.
This seems to be remarkable performance from a preamp that cost less than $200 to build with power supply, ALPS pot and heavy duty gold contact rotary switch.
2. Do I really need a powered preamp at all?
Now I must make a nice waxed Scandinavian birch case with a black perspex panel.
DIY is such fun and costs next to nothing!
Below: The wires on top and 2 x 47k high tolerance resistors were added. The electronics were isolated and picked up with the 4 pole 2 way switch on the right.
