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

[Redboy]

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!

http://www.audiokarma.org/forums/attachment.php?attachmentid=84128&stc=1&d=1206367970

http://www.audiokarma.org/forums/attachment.php?attachmentid=84129&stc=1&d=1206367970

http://www.audiokarma.org/forums/attachment.php?attachmentid=84130&stc=1&d=1206367970

http://www.audiokarma.org/forums/attachment.php?attachmentid=84131&stc=1&d=1206367970

[Redboy]

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…)

[Redboy]

Truthfully, I don’t know yet. Stay tuned…

[gmilitano]

Nice work. I hate paying for enclosures.

[240sx4u]

Damn fine looking contraption IMO.

Does it still smell like a good cigar? That would be even better!

[Redboy]

Nice work. I hate paying for enclosures.Me too, though I'm probably better off buying the real deal, based on the number of odd little boxes and whatsits around here that didn't work out!

[Twenty20Man]

nice silk screening ( in....out)

[Endspec]

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.

[lorne]

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'.

[Face]

Very cool!

[lorne]

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

[vinyldavid]

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....

[ejfud]

Real nice Redboy!

[saltwater]

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...

[Redboy]

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).

http://homepage.mac.com/tlinespeakers/hhaller-passivepre.gif

[gearhound]

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

[Endspec]

It can be used inplace of a tube or SS pre amp.
like CDP>passive line stage> power amp.

[Redboy]

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!

[grateful]

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.

[gearhound]

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

[Redboy]

Steve, resistors are key because they are directly in the signal path. Your signal travels through them at all times. As to the quality of the resistor, I have heard differences in different brands/ types of resistors. I can't explain what makes the difference, but if I can hear it, it's there (I don't have a "golden ear" - I often cannot hear a change).

[bricktop]

Can you still put cigars in it? I bet it sounds better with a little tobaccy in there.:D

Nice work!

[ashok]

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:

I think that both the series resistor and the shunt pot are important.

Here is the standard pot:

http://home.att.net/~agopalak/wsb/media/70320/site1046.jpg

and the series-shunt pot:

http://home.att.net/~agopalak/wsb/media/70320/site1047.jpg

In both cases, the amplifier or other device that is connected to the output of the volume control responds to the voltage that is developed across the variable resistor R2. It is just that R1 is fixed in one design (series-shunt), and variable in the other (standard).

So, if I believe that the series resistor can affect the quality of the sound, then I should also believe that the quality of the conductive element in the shunt pot will also affect the quality of the sound.

As an example, I point to the Luminous Audio Axiom series-shunt volume control I used to own. The series element was a Caddock resistor. The shunt pot was an Alps blue RK27. The device worked great - nice detailed sound etc.

But the point is that the shunt pot is no slouch by any means, and is used often as a standard volume pot.

Whether a Caddock sounds better than a Shinkoh, or vice-versa, or does a Radio Shack resistor beat both of them hollow - who knows? :D

[lorne]

First I should say that both of the editions I have built have used the same 10K pots that cost the equivalent of under $10. I use one/channnel, so matching is not an issue. Many people have written to say that the heart any pre section in is the quality of the attenuator — the pot being the central issue. Being the natural sceptic, I refuse to be herded into buying more expensive pots until I understand why. And I am beginning to understand. OK, so I may not rush out and buy the Rolls Royce of taper pots, but I may order some Alps Blues, Blacks ... whatever.

So here is a couple of comments I read in a recent post in our DIY forum — maybe you saw them:

http://www.audiokarma.org/forums/showthread.php?t=155659

Dansk wrote: Yesterday in our search for parts we came across an old Alps 50Ω pot, a BX2 model. I'm wondering if anyone knows anything about the Alps pots here. The BX2 seems to be an old, out of production model, that (I assume) has been replaced by the RK27. ....
It seems to be in great shape, we hooked it up to a power supply and oscilliscope and the channels track together perfectly, with less than 0.1 volts difference between them at any point. The scale is definitely logarithmic and climbs smoothly. .... (In contrast, we compared this to a Radioshack pot that jumped all over the place and had big dead spots. There's something to be said for quality manufacturing.)

Interesting, huh? Then —

PakProtector/Douglas wrote: I found the inexpensive Radio Shack 100k Alps pots to be a little bit variable on tracking. I also discovered that they could be tweaked by twisting the plates and then applying a bit of super glue to lock them down. With $10 to buy three I could get a very nice one. I do prefer to get the $15 50k ones as sold by Partsconnexion. They match/track quite well, and the 50k gives an octave better response based on the grid capacitance creating an RC filter with the pot resistance.

Redboy's audtion will be interesting. — Lorne

[lorne]

grateful wrote: 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.

To paraphrase Harry Haller in his original DIY post (see schematic in Redboy's post above): we could argue this all day, but this design is so simple and inexpensive to build that you should just go and build one and judge for yourself). And I'm not trying to be facetious.

It may or may not work for you. Go over the posts here and over at DIY Audio and become aware of the primary issues. In some cases — that is in some systems — an active amp may turn out to to be the most expedient solution and give the best results. Impedance matching is the central issue with this design. YMMV.

As for you suggestion: I am thinking about putting one of these passive circuits direct into my ST-70 when I rebuild it.

[lorne]

Carbon, carbon compound, metallic film, wire wound — 5%, 1%, 0.25%. Grain, noise, matching — and some people claim to hear tonal variation while others are doubt or outrightly reject the notion that we can hear a difference.

I feel as gearhound does: I am not going to gnash my teeth over the fact that I've not got Dale or Caddocks cuz I don't understand what the difference is supposed to be. I am using inexpensive pots, but there are generic 1/4 Watt film R's rated at 0.25% used for R1.

If a body wants to build three or so nearly identical units but with alternate bits, there will be some cool subjective evaluations as a result. As I said somewhere before, these little suckers a great teaching tool. And Redboy is doing a cool thing here. Hurry up and do some listening Redboy:yes::banana:

[Redboy]

If a body wants to build three or so nearly identical units but with alternate bits, there will be some cool subjective evaluations as a result. Alternately, you could easily cobble together one of these with a selector switch to A/B different resistor pairs...

I just need more time in my day! :sigh:

[ejfud]

I sure love mine that Redboy wired up for me.

It features 2 inputs and 2 outputs and sounds great with the Maggie el84 clone.

http://i29.photobucket.com/albums/c254/ejfud/DSCF0623-1.jpg

[apurcell22]

This pre combined with this attenuator http://www.altronics.com.au/index.asp?area=item&id=K5026
posted by avionic sounds like just the ticket for a maggotbox in my bedroom system!

[apurcell22]

No remote volume control is a deal breaker for the old lady in the bedroom:no: