A Simple Quiet Preamp. Active crossovers require a preamp. So I needed one. Basicaly, these are just a source selector and volume control. What could be simpler? Well, I have built a few different versions now, and learned a bunch, and have settled on my favorite design. I offer this to you to cut down on the expense of using active XO's (you'd otherwise need to buy a preamp if you dont already own one). I went with a minimalist design, feeling that the preamp should really do as little as possible. The main signal path uses only 2 op amps. Here is the flow of operations:
There are 3 inputs: 2 are passive, and 1 is active with gain (the Aux input). Gain Stage#1 was put at the Aux input in case you want to amplify one piece of gear relative to the rest (say cassette player or phono). More active or passive inputs can easily be added, up to 6.
These inputs go to a passive source selector switch. This is then fed to a source-out / pre-in loop, so you can connect an external processor (such as a graphic EQ). This loop is optional, but putting this loop here allows the EQ to see a nice high voltage under any volume setting since it is before the volume attenuator. This is an unusual function, and nice to have since it will reduce noise where it matters most: at low volume settings.
Finally, we have the gain stages and volume control. I have some very nice pots for the volume control (about 15 individually springed tines contact the carbon trace). Gain is set to prevent clipping at all stages prior to the volume attenuator from any source 2V RMS or less. The final gain stage is programmable by you to meet the needs of your amp.
Other functions: The real trick in designing a preamp centers around knowing what the other equipment it interfaces with (CD players, tuners, cassette players, amps) are expecting to see in terms of input impedances, voltages, and DC offsets. Input impedances can be determined by you, following my reccommendations. The docs discuss additional (optional) buffers that can be added depending on your gear. Parts are included for optional RF filters. There is a suggestion on interfacing with tube gear. Output DC blocking is an extra option needed only if you are connecting directly to an amp instead of feeding an active XO. I have some massive polypro's for this.
I have not settled on pricing yet. It will be very inexpensive if you dont need a chassis, maybe $35 with power supply. Gold-plated RCA jacks will be about $1.50 each. $15 for the chassis. $6 flat rate shipping/handling.
You can E-mail me at: johnpomann@yahoo.com
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