Moondog muts without a preamp

[Erik Mandaville]

In an effort to get used to not having the AES preamplifier in the near future, I rearranged the placement of the Moondogs so they could be connected straight to my CD player with a 2 foot interconnect. The difference in sound between active and passive pre-amping is noticeable, but bypassing the extra gain stage is something I can get used to -- at least for awhile. There is unquestionably enough gain with the two or so volts from my CD player, and the Moondogs are biased to swing to full output with that level of input. The short interconnect factor is ultimately most important with this arrangement. What I find missing is perhaps some of the 'body' that was there with the tube preamp. My reassurance in this comes from the fact that I have a couple of very good preamp schematics in my file, which also use fantastic 6SN7s. The original Ultrapath preamp is one I'm also considering for a future project.

Volume on the Moondogs is controlled by way of 250K stereo pots, with both sections paralleled for an overall input impedance of just around 125K ohms. Turning the volume up a little approximates the 100k ohm input impedance specified on the schematic. Sensitivity is still very high, and sounds as good as a fixed resistor from grid to ground on the input tube. I also connected a 100 picofarad capacitor between the input on the volume control and center wiper, which works beautifully as a high pass filter, allowing otherwise lost details in the music to come through as the volume control is turned down -- a trick sometimes used on vintage tube guitar amps, as well.

Also, for those of you all with Moondogs -- Kelly, Leo, Edster, and others...I completely redid the grounding scheme on the amps. All signal and power supply grounds are now separate, and use a two point grounding on the chassis. For some odd reason, this DOES NOT induce ground loop hum. The star signal ground is connected to the chassis right at the area of lowest impedance (input side) and the PS is on the opposite end grounded to the PS transformer mounting bolt. I also wrote Ron years ago about the importance of removing all the clear coating in the area of the chassis ground...by scraping it off. I was getting hum problems after they were first built, which was remedied by making sure the ground connection was hitting bare metal.

I also connected the ground tab on the input Jack to ground by way of a .01MFD cap, which effectively shunts stray RFI junk from getting into the signal. It really works!

Just to share with you all,

Erik

...I think Leo is doing some pretty fascinating work with the Moondogs in terms of parafeed output! Cool!

[mike stehr]

You'll probably build that Ultrapath pre-amp before I will.

So post your progress if you do go about it.

I'm pretty much dead in the water as far as audio related income goes for a couple months or so.

The first thing on the list is to get my Cornwalls home.

Kudos on the Moondog mods, Eric.