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

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).
 
Redboy said:
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:
Click to expand...

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

Here is the standard pot:

site1046.jpg


and the series-shunt pot:

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
 
The pots — what's the differnece?

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.)
Click to expand...

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.
Click to expand...

Redboy's audtion will be interesting. — 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.
Click to expand...

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.
 
Resistors — what's the difference

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:
 
lorne said:
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.
Click to expand...
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:
 
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.

DSCF0623-1.jpg
 
I just built one of these and it sounds quite good. Although the volume is way too sensitive. If I increase the input resistors will this let me use more of the volume pot?

Also, dyslexic me attached the input resistors to the output RCA's, after the shunt. I have it plugged in backwards now, but if I have it so the input resistors are last will this cause problems?

Oh, and it cost me $4.49 to build. I had bought the encloser (with stiches and RCA's)for $1, then I had to buy the pot at RS for $3.49.
 
Redboy said:
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:
Click to expand...

Add a multi position switch for different inputs and you got me sold. Heck, I'll give this a try too.
 
I've not build this one but my main passive pre is linear not tapered. I like it much better but it took a bit to get use to not having more volume at the first few notches.
 
I've built both — including a linear one I showed on AK (somewhere) that I built for a couple of bucks using a $1.00 bookstand and a couple of good quality, Bournes type trim pots. It was sort of an experiment. Kinda of a PITA cuz I had to use a screwdriver — once this side and then the other. But once I had the volume to the desired level, and the image trimmed up it worked great. The rig by-passed the tone section of the preamp and the signal went out to the power. Most of my other passives used cheap linear pots, but they faked the law of the pot. If you don't use a circuit that "fakes the law" (see the Rod Elliot Sound Products for an article on faking the law of the pot), you'll need to be very light and sensitive on the volume knob. And I'd recommend using one pot per channel.

BTW, I have to use a digital volume control in a CALL lab where I am doing some language teaching. NO taper at all! No THINKING at all on the part of the engineers! Drives me crazy!
 
First off Redboy, two words: OUT...STANDING!!

My only little comment is about the volume pot. It is in the circuit, acting as a variable resistor so the signal still has to travel through while shunting more or less signal to ground, affecting volume.

A few issues come up with a carbon comp, stereo pot or any other similar type volume pots:

1. Tracking

2. Repeatability

3. Quality and conductivity of the resistive element in the pot

As the pot wears, tracking will eventually suffer, and so repeatability. After a few thousand turns, what was once an even 100k on either side may become 90k and 110k. Minor, but with all things DIY, it's all in the details...and those changes can affect separation and sound stage.

IMVHO, this isn't that large a problem, but there are some who are always looking for or creating solutions to problems which don't exist.

One solution is a variable attenuator--a pot with heavy connection pads and a single resistor for each signal path at every step of increasing volume.

At any point, it acts like a pair of fixed resistors in the signal path, is repeatable and durable in the extreme, good for hundreds of thousands, possibly even a million turns, all while always presenting a perfectly balanced signal input.

Is it necessary? Not when you're getting great sound out of $8 worth of components and a cigar box (LOVE the box!), just pointing out possible differences between volume controls.

Best,
John

P.S. Did I mention I love the box? I have several cigar boxes I use to store tubes, sockets, grommets, etc. and love 'em.
 
Thanks, John! I really enjoy putting these things together, and have experimented a bit with different components and different configurations. I don't think you can do any better at this price point, and like anything audio, the upper limit is waaaaaay out there, far beyond the point of diminishing returns.

Everyone should build one, or at least try their hand at it.
 
lorne said:
I've built both — including a linear one I showed on AK (somewhere) that I built for a couple of bucks using a $1.00 bookstand and a couple of good quality, Bournes type trim pots. It was sort of an experiment. Kinda of a PITA cuz I had to use a screwdriver — once this side and then the other. But once I had the volume to the desired level, and the image trimmed up it worked great. The rig by-passed the tone section of the preamp and the signal went out to the power. Most of my other passives used cheap linear pots, but they faked the law of the pot. If you don't use a circuit that "fakes the law" (see the Rod Elliot Sound Products for an article on faking the law of the pot), you'll need to be very light and sensitive on the volume knob. And I'd recommend using one pot per channel.

BTW, I have to use a digital volume control in a CALL lab where I am doing some language teaching. NO taper at all! No THINKING at all on the part of the engineers! Drives me crazy!
Click to expand...

I agree with Lorne on the use of "faking the law". Linear pots tend to be better at tracking (well, at least the cheaper ones) and "faking" gives a good approximation of an audio taper. The second issue is as Lorne mentioned, namely the use of two separate pots. With this, you no longer have the tracking issue of a stereo pot and it provides the ability to balance the outputs without using a balance control. The simplicity of this approach seems to provide benefits without the negatives.

Best Regards,
TerryO
 
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I agree completely Redboy--once a person leaves behind the monster cable vs. lamp cord, this DAC vs that, and on and on, there's only one place to go:

DIY.

Smell the solder. Feel the pride. Enjoy the result. Ignore those fixated on interconnects.

;)

No offense to those fixated on interconnects. I have my OCD issues too.

Say though...now that you have a herd of those pre-amps...maybe it's time for a toob amp...?

Just sayin.' :banana:

All the best!

John
 
TerryO said:
The second issue is as Lorne mentioned, namely the use of two separate pots. With this, you no longer have the tracking issue of a stereo pot and it provides the ability to balance the outputs without using a balance control. The simplicity of this approach seems to provide benefits without the negatives.

Best Regards,
TerryO
Click to expand...

Except you end up running back and forth tweaking the living daylights out of the balance. I had a receiver with dual volume sliders. The tweaking never ended. :no:
 
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