DRBILL Posted March 13, 2005 Share Posted March 13, 2005 Of course, there is no such thing as a "passive" preamplifier. But what are you going to call "a channel-selector with an antennuator for line signals?" I knocked this one out in a couple of hours so forgive the aluminum project box for this prototype. How does it work? Very well, indeed. It has a sort of clean urgency about it --like washing your eye-glasses after mowing. Will it replace a standard preamp? No. In an ideal world with pristine, excellently-recorded CD's (which is less than 5% of my collection) it would serve nicely. But most of my CD's need some degree of tonal "adjustment". But for those few stunning CD's, it is a wow. It is good with CD's, Tuners, Tape, etc. No Phono. Am I going to build some? Nope. But I'll be glad to send parts lists and schematics to those interested. DR BILL Quote Link to comment Share on other sites More sharing options...
Daddy Dee Posted March 13, 2005 Share Posted March 13, 2005 Very nice. I'd like to be on the list. Quote Link to comment Share on other sites More sharing options...
greg928gts Posted March 13, 2005 Share Posted March 13, 2005 Creek OBH-12 I have a couple of these and a few OBH-10's and they work great. Of course it's because they work so well that Creek decided not to make them anymore. Isn't that just the way? They're out there in the used marketplace. Greg Quote Link to comment Share on other sites More sharing options...
DizRotus Posted March 13, 2005 Share Posted March 13, 2005 I'd like a parts list and schematic please. Quote Link to comment Share on other sites More sharing options...
Klipschfoot Posted March 13, 2005 Share Posted March 13, 2005 My PS Audio IV has a passive mode option. Are you saying that it is not a really a passive feature? I had this upgraded at the factory several years ago and it is one of my favorite pieces. Quote Link to comment Share on other sites More sharing options...
leok Posted March 14, 2005 Share Posted March 14, 2005 I've always preferred simple attenuaton over active "preamplification." I'm currently building a 4 channel version to handle front rlc and sub from muli-channel SACDs. Bill, nice job on the stereo version you have. I recommend anyone interested give it a try. Leo Quote Link to comment Share on other sites More sharing options...
speed3 Posted March 14, 2005 Share Posted March 14, 2005 I've been searching the DIY forums on the net for a few months for something exactly like this. Please put me on the list for parts list and schematic. Thanks. Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted March 14, 2005 Share Posted March 14, 2005 Great work! a small passive can work surprisingly well, and I've used different ones I've made over the years. You can make one with these easily found parts: 1) 10K potentiometer (audio taper) stereo control/two mono for balance ability 8) RCA jacks (6 total for inputs, 2 for R/L outputs) 1) make before break 2 pole 6 position selector switch Knobs as needed for the volume and selector switch controls. 1) chassis (aluminum or steel would be best for RF/EMI immunity. And: Run a bare ground bus wire (18AWG solid copper would be fine)through the ground tabs on the RCA jacks (if using insulated Jacks). Ground to chassis at one point. Wire inputs to channel-specific inputs on the selector switch. Connect selector switch from each pole to inputs on volume controls Connect center wiper of volume controls to channel appropriate L/R RCA out Connect a ground lead from the ground lug on the volume control to the circuit ground or chassis ground connection. If stereo control, jumper the grounds together on the volume control. For some channel separation, the gound connections on the output RCAs can sometimes be effectively made with a small value resistor, although I've never found the need to do this. Keep cables short, and it should be good! I've made these in bread baking tins before. Radio shack carries everything needed to make a decent passive volume control/soure selector. You could even use a higher value potentiometer, such as 50K ohms, but it's best to find a happy medium between the control's input sensitivity and output impedance into the following stage. An ideal way to do this, which can be extremely good I think, is to use a transformer coupled input. This would present to the signal a very good high input impedance -- say 100K ohms -- and much lower output impedance (good thing!) than can a conventional volume control. This can be very expensive, though, depending on what transformers are used. Have fun! Erik Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted March 14, 2005 Share Posted March 14, 2005 Dr Bill's passive has uninsulatetd RCAs, so uses a ground plane approach rather than an SPG -- single-point-ground. Thus, there is not the need for a ground bus on the RCA ground tabs. The barrell of the jack makes the ground connection on the chassis, itself, which is the common ground. Grounding is strange, and what works in one case, might not in another. Erik Quote Link to comment Share on other sites More sharing options...
v3spitfire Posted March 14, 2005 Share Posted March 14, 2005 I sometimes use and adcom passive unit for pure two channel listening. I really like it. It allows recording/dubbing, and has video switching. It makes a great unit to tell A/B preamps and to see what they add as a signature to the raw amp. I'de like to have a multi channel one, like the one referred to above. Very nice. Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted March 14, 2005 Share Posted March 14, 2005 It's not more difficult to make one for multi-channel use than for two channels. I think Leo's idea is great, and I haven't seen a passive done that way before. Quote Link to comment Share on other sites More sharing options...
mike stehr Posted March 14, 2005 Share Posted March 14, 2005 I still have my wood box with the 100K stepped attenuator. Selecter? I just go in and out, why make it complicated? Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted March 15, 2005 Share Posted March 15, 2005 I'm not intentionally making it more complicated. But, do you know what happens if the system is powered up and you start yanking out interconnects in order to listen to another source, such as a tuner, for example? I would just rather have the flexibility of a shorting selector switch. I've even made passives with the cable soldered directly to the volume control, and didn't even use RCAs on that end of the cable. It works fine, but for anyone who has another source component, it's probably a little more hassle free to install a switch. A switch isn't complex, I don't think.... Quote Link to comment Share on other sites More sharing options...
DRBILL Posted March 15, 2005 Author Share Posted March 15, 2005 I can't improve on Erik's instructions. Thanks! I found a simple schematic for those who might find it helpful: http://diyaudio.8m.com/cgi-bin/i/Pasive_pa/shematic.jpg Double it, of course, for stereo! This, with Erik's step-by-step and my picture should get you through. Radio Store has everything you need. I often hand the clerk a list while I fiddle with other merchandise. They will be out of one item, but they will call around and send you to another store! Phono Jack 274-346 $3.99 (2 pkg) 100K Stereo Control 271-1732 3.29 (1) Rotary Switch 275-1386 3.29 (1) Control Knobs 274-407 1.79 (1 pkg) 10K 1/4W Resistor 271-1335 .99 (1 pkg)* 4.7K 1/4W Resistor 271-1330 .99 (1 pkg)* Aluminum Project Enclosure 270-238 2.99 (1) *Radio Store doesn't have 15K Resistors (!). Wire a 10K and a 4.7K in series. My unit has three inputs. The above rotary switch can accomodate up to six if you like. PM me if you get stumped. Enjoy. It ain't work! DR BILL Quote Link to comment Share on other sites More sharing options...
DRBILL Posted March 15, 2005 Author Share Posted March 15, 2005 Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted March 15, 2005 Share Posted March 15, 2005 One can also wire two 30K ohm resitors in parallel -- if the other combo isn't available. That will net 15Kohms. Erik Quote Link to comment Share on other sites More sharing options...
DRBILL Posted March 15, 2005 Author Share Posted March 15, 2005 ---------------- On 3/15/2005 6:06:40 PM Erik Mandaville wrote: One can also wire two 30K ohm resitors in parallel -- if the other combo isn't available. That will net 15Kohms. Erik ---------------- ERIK, So true. But I was, for convenience, striving for one stop shopping at Radio Store, access to which most of us have. They don't have 30K's either! DR BILL Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted March 15, 2005 Share Posted March 15, 2005 Ok! Your point is well taken, Dr.! Your way is obviously the one to go with for one stop shopping at one's friendly Radio Shack store! Erik Quote Link to comment Share on other sites More sharing options...
mike stehr Posted March 15, 2005 Share Posted March 15, 2005 "I'm not intentionally making it more complicated. But, do you know what happens if the system is powered up and you start yanking out interconnects in order to listen to another source, such as a tuner, for example? I would just rather have the flexibility of a shorting selector switch" I just turn everything off. I just have the source go into the stepped attenuator, then into the amplifier of choice. I don't have a tuner, or any other types of sources, so I didn't really feel the need for a selector switch. I'm getting a receiver, but it would need the stepped attenuator. No, selector switches are not complicated. I was just joshing....nevermind..... Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted March 15, 2005 Share Posted March 15, 2005 Mike: My situation is exactly the same as yours. The only source I'm using right now is our Denon multi-player. I agree! why use a switch for more sources if it's never going to be used??!! ...I make everything more complicated -- it's just my nature! My effort was to provide info. for folks who might want the ability to switch sources, which is easier on equipment than shutting on and off amps in order to disconnect wires. Erik Quote Link to comment Share on other sites More sharing options...
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