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Old 15th August 2012, 10:30 PM   #1
hopat is offline hopat  United States
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Location: Los Angeles
Default Passive Pre Chassis and Budget Allocation

Hello everyone,
I need some advice on how to distribute parts cost on a midlevel passive preamp. A few months back I built a voltage divider using a 100k linear pot and some 10k resistors from Radio Shack. I connected this to some gold plated Teflon RCAís that I had laying around and it beat out the sound of my Hovland HP100! Since then Iíve been thinking about building a nicer version with a source selector and some higher quality parts. Iím pretty new to all of this so I could really use some advice as to where I should spend my money, and where I should look for a small pre-drilled chassis for my project. Iíd like to keep the project under $200 if possible, but could go as high as $300 if it would make a huge difference. If I could add the source selector to the pre and have a slight increase in sound quality at the same time I would be ecstatic! I need to have 3 unbalanced inputs and one set of outputs. Hereís the parts list that Iím looking at, and some questions regarding those parts.

1 small pre-drilled chassis, with 2 knobs ($65-100)

Does anyone have a good recommendation here? I would like to keep the signal path as short as possible, and back mount everything if I could, but I have no ability to alter a chassis that I buy. If possible I would like to get a chassis that was narrower rather than wider.

2 x 10k Resistors ($1)

Should I go expensive ala Vishay or Audio Note, or keep it simple ala Holco or PRP?

1 x 100k linear Stereo Volume Pot ($25-50)

Maybe a Noble, Alps blue, or Audio Note log taper? Are there high quality linear taper pots that I could look at? The voltage divider takes a linear taper pot and makes it approximate a log taper pot, so is a log taper pot a viable alternative in this circuit?

1 x Source selector switch 4 poles 3 selections ($10-20)

Right now Iím looking at a Connex switch from parts connection. Iím having trouble finding any alternatives at the 10-20 dollar price point. (Would it make more sense to shell out for a more expensive pot like a TDK or a high end selector switch?)

5 ft. Hookup wire ($8)

Possibly Cardas or Kimber. Has anyone had success using the 23.5 gauge Cardas hookup wire at parts connexion without using a soldering pot?

? x RCA Female chassis mount jacks ($0)

I have more of the jacks that I used in the previous amp (I think that they are Connex jacks from the parts connexion)
Would it make sense to invest in any high end alternatives for the output at my price point or should I just keep it simple and work with what I have?

If I were to splurge by adding up to $50 to the budget where should that money go? Thanks so much for all of the advice! I
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Old 15th August 2012, 11:48 PM   #2
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Join Date: Sep 2011
Location: Netherlands & Australia
Hi Hopat,

If you want to go semi-passive (simple JFet buffer but no voltage gain) you might consider the Nelson Pass B1. I own an original FirstWatt B1, and it is very good.

If you go for the B1, $200 should be plenty. There is a pcb available from Nelson Pass himself (with or without matched JFets):
DIY Order Form
Another option is the modified B1 pcb from the diyaudio store:
P-MEZB1-1V10 - Mezmerize B1 Buffer Preamp (Makes 2 channels; Rev 1.0) - Boards

Other people have seen very good results by eliminating traditional resistors and pots by using LDR's. You might want to check the following topics:
Lightspeed Attenuator a new passive preamp
Lighternote attenuator
Light Dependant Resistor Current Control

Enclosure:
Check out the modushop Galaxy enclosures. Shipping is a bit high to USA, but the diyaudio store is going to offer some of the enclosures (search the forum, I am not sure where it was listed which ones they were going to import) from their US location, so no import or high shipping costs.
modu

Resistors
Pass uses mainly Dale. Seems to work fine. At less than 20cts each, you can always try something else later on.

Pot
Consider using to a stepped attenuator. Either with relays or with a 24 (or more) position switch and resistors. Check ebay for stepped attenuator using a switch.

Source selector
If you go for mezmerize B1, relays would make sense. You can find switches (break before make) on ebay.

Wire
I have been using ptfe silver plated stranded copper wire from Apexjr. Steve has good prices on other stuff too.
Wire

RCA
I have just finished building a DAC and have used Neutrik NF2D-B. Great quality. Good price. Details from a Dutch webshop:
Neutrik RCA zwart chassideel wit NF2D-B-9 kopen? | Goedkope | Promo | RCA / Tulp

Good luck!

Albert
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Old 16th August 2012, 01:07 AM   #3
hopat is offline hopat  United States
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Join Date: Feb 2012
Location: Los Angeles
Thanks for the info Albert. I'm really wedded to this particular design at the moment, but I appreciate the suggestions. As to the resistors, do I need to worry about which series of Dale that I'm using (CMF or RN) or are they all good for audio applications?

I like the idea about picking a stepped attenuator up from ebay. Does anyone have an opinion about this option?

Dale 23 Step Ladder-type Potentiometer Volume Control Log 100k Ohm Stereo | eBay

Finally, I can't see the pictures on the modushop website, but I'll try and find any info that I can on this site.
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