[Wayner]

It's been a tough spring on this old boy. I f'd up my Achilles tenon awhile ago so I hobble around here like Yoda. Tough to drive the truck downtown for coffee in the morning, too. I also have several projects going on around the house, so lots of activity.

I have been thinking about this project for awhile. I was reading some of Frank Van Alstine's Audio Basics and decided to go down this path. Basically the box has 3 zones to it. The top has left and right RCA inputs, the center section features DIP switches and a couple of adjustable resistors, and each channel has probe pin sockets for a volt/ohm meter.

Frank's formula for determining proper cartridge loading works great, but for some cartridges, the values are not known (usually for coil capacitance) so there is a bit of a roadblock for some cartridge types. I decided to build this box and provide about any kind of resistive load a guy (or gal) would ever need.

After a bunch of research, I selected some standard (matched pairs) of resistors for the DIP switch banks, and left switch bay 1 to go thru a 100k ohm variable resistor. The combination of resistors should give the user about any value they are looking for.

I have fooled with the Green Longhorn, and as Frank has suggested, benefits from a 20K ohm load across the hot and cold of the coil. The first thing noticeable was symbol crash, and it was very refined and well executed. I then fooled around with my AT440MLa and at 20K it was muffled, so I went up the ladder, dialing in the variable at 42K and having a nice response to that, tho I need to fool more with it.

I am letting my ears tell me if I have hit the right values, right or wrong, but seems to work so far.

 

 

When I have more time, I want to record some sweeps and see where I'm at with some of these settings. It is fun.

Wayner 

[Delta Wave]

Pretty sweet! 

Now, just put in a fancy-schmancy box and you could charge a few grand. 

[neobop]

Very nice, Wayner. Did you consider using a high quality stereo 100K pot? A couple of guys on VE found some pretty good ones apparently, and tested them for channel matching. I guess you could get mono ones and mark the settings on the dial (case).

I was a little surprised at the number of people that use alternate loading for their HO carts. OM40 - 42K, M97 - 62K, etc. With a box like that, it's easy to hear the difference, right away. 

[Wayner]

Because my only source for this project was rat shack, I kept the design simple, opting for the separate adjustable resistors.

It's very interesting to see how the tank circuit reacts to each cartridge model and type as you go up and down the frequency. You could almost call it a "tone control", but it is more of a cross-over type of behavior, with a slope that seems to be rather aggressive. As an example, setting my AT440MLa at 20K, pretty much completely muffles the sound, while the Longhorn Green sprung to life. I now have the AT around 45.5K and may be a nice value for those that think the cartridge is a tad bright. Of course, that exact value won't work for everyone with that cartridge as the rest of the circuit (interconnects, tonearm wire, connectors....etc) is not factored in.

My intent is to use this box as a tool, rather then a permanent fixture, and after finding a comfortable loading value, use the simple resistor soldered on an RCA plug (hot to ground) and install that into a nice gold plated Y connector (one RCA male, 2 RCA female).

Wayner