Proof of concept: An $8.01 passive preamp... in a cigar box!

Redboy

a few good watts
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I've built a few copies of Harry Haller's great little passive preamp already, but I’ve only used better quality components to date. It’s pretty easy to justify good quality stuff when the total parts count for a stereo pre is – get this – seven!

Each time I’ve wired one together though, I’ve had this nagging notion that the ultra-low budget option might not sound that much worse. Well, the proof is in the pudding, as they say, so...

here’s your pudding!

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How much did it cost?

Cost breakdown:

I picked up the empty cigar box from a thrift shop for $0.40
Wire is a few short strands of CAT5 that I got for free.
Knob is optional so I’m not including it in the cost breakdown.

All other parts are from Radio Shack:

• $3.49 100K Dual-Ganged Stereo Volume Control
• $3.99 Panel-Mount RCA Phono Jack (4-Pack)
• $0.13 10K Carbon film resistors (qty 2)

TOTAL COST: $8.01

(Full disclosure: the cost of the resistors is based on a 100-pack sold at the shack for $6.49, so your actual costs for this project may run higher…)
 
Damn fine looking contraption IMO.

Does it still smell like a good cigar? That would be even better!
 
I just got the 5 pack of resistors at rat shack and they were 99 cents, And these little units do sound sweet! I included a mono switch in mine too total cost was less than 15 dollors including case.
 
Hi Redboy: I'm gonna predict something — if you can hear a difference, it is going to be very slight. And, if anything, it may come from using the resistors you are using in terms of balance. IMHO, a metallic film 0.25 % costing 1.50 as opposed to carbon comp at 23 cents might just make a small difference everything else being equal. But that's just splitting hairs. I'm fascinated to know what you hear.

I love this little component. A great teaching tool, and it really works! I remember Haller writing that he thought it might become a classic. It makes you think about those other more involved designs mentioned by Haller and written up on Elliot Sound Products web pages entitled something like '... the better volume knob'.
 
Hmmm ... on second thought, I shoulda thought more about what pots you may have been using before. With cheap pots, I'd be in the mood for using one per channel — like what I did. But that would be pushing to price up towards $12.00.:D
 
Great job!

Ya know, that could have some great uses in the integrated world, maybe even tubed. You could put a small power amp in that box with that passive pre, and well, you got yourself a small, great sounding integrated. could even be battery powered.

I know that if I ever build one, I shall look into putting an amp inside the box, and using it as a portable stereo system with a pair of small speakers like the minimus 7....
 
This may sound stupid, but are you guys into seperates, and what would you use this for? I do like it though.. Thanks for the pudding!

I could be having a brain fart...
 
Hi Redboy: I'm gonna predict something — if you can hear a difference, it is going to be very slight. And, if anything, it may come from using the resistors you are using in terms of balance. IMHO, a metallic film 0.25 % costing 1.50 as opposed to carbon comp at 23 cents might just make a small difference everything else being equal. But that's just splitting hairs. I'm fascinated to know what you hear.
My thoughts exactly. We'll see.

I've used nice Noble pots (about $25 ea), Shinkoh resistors (about $3.50 ea), and gold plated RCA inputs in the others that I've built, but I struggle with understanding how the pot can affect the sound quality as long as it tracks well. Correct me if I'm wrong, but the pot is simply shunting more signal to ground as you decrease volume. Doesn't this mean that the pot isn't even in the signal path? The resistors are key in this design:
First, let us start out with the schematic of this passive device, which I copied from the diyaudio.com thread. The picture is courtesy of planet-10 (Dave Dlugos).

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Well...if you're not into vinyl....this is a cheap ticket into separates.
Pick up a used, good quality power amp on CL or e-bay....and you're there!

Steve
 
This may sound stupid, but are you guys into seperates, and what would you use this for? I do like it though.. Thanks for the pudding!

I could be having a brain fart...
saltwater, this is just a really simple preamp - no tone controls, no switches or balance controls, just a volume pot. So I would run Source > pre > amp > speakers.

EDIT: Yeah... what they said!
 
Why not just get a power amp with a volume control? That unit will degrade the sound of the source as you push the output toward ground. An active unit will do better.
 
Redboy:

I'm STILL trying to figure out your "resistors are the key" question myself?
It seems almost that it does NOT matter what value your pot is....10K, 25K, 50K, 100K?
Sooo confusing.................??

A quick question for yuz resistor experts:

If a resistor is a MF 1/4 watt 1% of ANY value.......what difference does it make WHO makes it?
I know Holco & Dale are considered desirable.....but at a PRICE!?
I mean, isn't a 1% tolerance a 1% tolerance.....even if I made the resistor in my kitchen sink!?

Steve
 
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