DIY six channel passive preamp

[dvenardos]

I am going to build a six channel passive preamp based on the gold point MV-6 stepped attenuator.

http://www.goldpt.com/dims.html

Initially I will probably wire this up just as a volume control with six in six out. However, in order to not keep this a simple project I would like to add a selector switch. Ideally the selector switch would allow me to switch three stereo inputs and two six channel inputs. So, position 1-3 would switch three stereo inputs to the l/r out and position 4-5 would switch two 5.1 inputs to the same l/r out and the four additional channels. That is overkill as I really only need to switch the stereo inputs as I will have only one 5.1 input.

Wiring the stepped attenuator looks pretty simple, but when adding a selector switch I am totally clueless as to which switch I would need and how to wire it. :dizzy:

http://www.goldpt.com/selector.html

It seems like I would need some kind of i/o board to wire the switch to. I can't find any information on how to wire this or what kind of a board I would need to hook it up to. Anyone know how to do this and where to point me for instructions?

Thanks,
Don

edit: added a drawing of what I am doing. Decided to add an additional input to the front.

Parts List:

[B]Qty	Part #	 Description	        Ext Price[/B]
1		 case	
1	MV-6-20K attenuator	        $349.00
1	2P-6T-3D switch	                $137.00
7		 red rca	
7		 white rca	
3		 black rca	
3		 yellow rca	
3		 green rca	
3		 blue rca	
[B]26		 total rca	        $42.00[/B]
8		 red wire	
8		 white wire	
4		 black wire	
4		 yellow wire	
4		 green wire	
4		 blue wire	
[B]32		 total hookup wire	$35.00[/B]
28		 brown wire (ground)	$10.00
2		 knobs	                $30.00

[B]Total			               $603.00[/B]

[dweekie]

I am going to build a six channel passive preamp based on the gold point MV-6 stepped attenuator.

http://www.goldpt.com/dims.html

Initially I will probably wire this up just as a volume control with six in six out. However, in order to not keep this a simple project I would like to add a selector switch. Ideally the selector switch would allow me to switch three stereo inputs and two six channel inputs. So, position 1-3 would switch three stereo inputs to the l/r out and position 4-5 would switch two 5.1 inputs to the same l/r out and the four additional channels. That is overkill as I really only need to switch the stereo inputs as I will have only one 5.1 input.

Wiring the stepped attenuator looks pretty simple, but when adding a selector switch I am totally clueless as to which switch I would need and how to wire it. :dizzy:

http://www.goldpt.com/selector.html

It seems like I would need some kind of i/o board to wire the switch to. I can't find any information on how to wire this or what kind of a board I would need to hook it up to. Anyone know how to do this and where to point me for instructions?

Thanks,
Don

It's fairly simple, although a bit crowded. You'll need to go very slow with the wiring because you really don't want to backtrack.

2P-5T-3D = 6 Pole, 5 Positions, 3 Decks
(2 Poles per deck)

That's what you want. 6 pole = six channels, 5 positions = 5 inputs, 3 decks = 3 of these back to back ()

You'll want to go RCA in -> selector switch -> volume pot -> RCA out. I don't think you'd need any external pcb.

For your stereo inputs, you'll just have to mentally think of them as 6 channel inputs with 4 channels unconnected to make it easier to picture in your mind.

On the pot, you'll see a "c" along with a set of numbers - 1 through 5. The "c" is a common output, which you can connect to the volume pot. There will be 6 total sets of c-5, 2 sets on each pcb as the pic shows. Each number pertains to an input.

So, for example, lets say you want to connect your first set of inputs into the selector switch. Each channel will go to all the "1"s on the selector switch. There are exactly six "1"s on the switch, one for each channel (last 4 not connected for stereo). For the second input, all the channels will be connected to the "2"s, and so on through 5 inputs. Then all the "c"s are connected to the 6 inputs on the volume control.

Hopefully I got that right through the diagrams. Does that make any sense at all? :toilet:

[dvenardos]

Thanks dweekie, that makes perfect sense. :idea:

[dvenardos]

These look good for RCAs copper with gold plating and six color boots:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=350152072064

What should I do with the ground for the RCAs?
edit: found the answer here:

http://www.users.bigpond.com/aagreen/passive03.html

[dvenardos]

added parts list.

[dweekie]

I'm unsure of the quality of those particular RCAs, but with the cheap connectors from China, I usually test them out individually with a spare cable before installing them. I basically connect and reconnect the cable to a socket a few times in my hands to see how tight or lose it is, and, on occasion, a few will fall apart or not work properly. I also match the resistance of the sockets for stereo pairs since they can vary, but that'll be a bit more difficult in your case. The more expensive connectors tend to avoid these issues, but that should be expected of course.

[dvenardos]

I'm unsure of the quality of those particular RCAs, but with the cheap connectors from China, I usually test them out individually with a spare cable before installing them. I basically connect and reconnect the cable to a socket a few times in my hands to see how tight or lose it is, and, on occasion, a few will fall apart or not work properly. I also match the resistance of the sockets for stereo pairs since they can vary, but that'll be a bit more difficult in your case. The more expensive connectors tend to avoid these issues, but that should be expected of course.

Thanks for the tip. It might be worth considering better rcas. Of course, that really jacks (:fryingpan:) up the price, but I could also add them a few at a time and make it an ongoing project. :crazy:

[dweekie]

Thanks for the tip. It might be worth considering better rcas. Of course, that really jacks (:fryingpan:) up the price, but I could also add them a few at a time and make it an ongoing project. :crazy:

I think you'll be fine doing a bit of QC testing on your own :stirthepot:

[dvenardos]

I just can't seem to leave well enough alone. :dizzy:

I have decided that I want to add home theater bypass to this preamp so that you can use it either as complete avr replacement or use it in conjunction with an avr. The only way I can think of implement HT bypass is to put another switch after the selector switch and before the attenuator similar to this:

http://www.goldpt.com/schm_pls.html

In the above schematic it looks like they used a selector switch in reverse so that the output of the first switch would go to the common of the HT bypass switch which would have one output (normally input) going to the attenuator and one output going directly to the line out. This would effectively make the preamp either a preamp or a six channel switch. I could use a 3P3T2D goldpoint switch and have on, bypass, and mute positions or I could just use a 6PDT toggle switch which would toggle from on to bypass.

Problem is the goldpoint switch is kind of expensive to just implement HT bypass and 6PDT toggle switches are kind of hard to find and I definitely don't want to degrade the signal quality because the whole purpose of this preamp is to get the absolute best sound on a budget.

Any thoughts or recommendations? :idea:

Thanks,
Don

[TomS]

Mike Galusha (mgalusha) on Audiocircle has one of those 6 deck MiniV Goldpoints for sale at a very good price. Brand new, never used. It's worth a shot. Tom

[dvenardos]

Mike Galusha (mgalusha) on Audiocircle has one of those 6 deck MiniV Goldpoints for sale at a very good price. Brand new, never used. It's worth a shot. Tom

Hey thanks. His is the 10K model and I need the 20K model.