Two questions out of ignorance

[Daddy Dee]

O.K. I've got two questions unrelated, except that they are connected by my ignorance.

I'm considering taking on a modest newbie DIY project to recase the tripath amplifier in the Sonic Impact amp. It's level of difficulty appears to be not too far ahead of my skills, which are next to nothing. Am trying to figure out which potentiometer to use for volume control. My understanding is that this will work by attenuating in input signal. One poster I was reading used a 100K attenuator. In this usage, is the term "attenuator" interchangable with "potentiometer". Also, what would be the expected difference in results if a 25K pot was used instead. Another project on the horizon (sure, Dee) is the PWK minibox. It calls for 25K pots to build it.

Another related question...What is the difference between a potentiometer and a rheostat?

Some DIY threads I read talk about how different pots affect the sound. I tend toward the wire is wire philosophy, assuming the wire is decent. What about pots?

Have been doing a little reading on the internet about the use of tripath amplifiers. What is the difference between a "class T" amplifier and a "class D" amplifier. I notice that the upcoming Klipsch 2.1 iFi uses a "class D" amp.

Any help is appreciated. If speaking technically, please type slow.

[richinlr]

Can't really help with the differences with what a pot, attenuator and rheostat are.

But I can tell you that my Electrocompaniet pre-amp has just about the sweetest volume control I have ever encountered. I can open it up and maybe find the brand on it.

The main reason I like it is because of the way it controls volume.

With the exception of the new 'digital' controls, volume controls are generally not linear. The ones I have seen generally don't give much control at the low volume end of the control - they just seem to get real loud real quick.

On the other hand, you have to turn the Electro control to about 2 o'clock before that starts to happen. It gives you a great deal of control over the volume range you in which you primarily listen.

Don't have a clue how find out how a pot responds in a given circuit and it might have to do with the load on it.

This probably didn't help you at all...

[3dzapper]

"One poster I was reading used a 100K attenuator. In this usage, is the term "attenuator" interchangable with "potentiometer". Also, what would be the expected difference in results if a 25K pot was used instead. Another project on the horizon (sure, Dee) is the PWK minibox. It calls for 25K pots to build it.

Using a 100K Ohm pot will allow greater control over the output by requiring a larger adjustment for each increase in gain. A 25K pot would make the output too loud too fast and may give close to full output at the minimum position, not a good idea.

"Another related question...What is the difference between a potentiometer and a rheostat?"

A potentiometer is a resistor or series individual of resistors usually made of carbon. A potentiometer designed for volume control use has a logirithmic taper, that is the further you "open" it the faster the resistance falls off. A rheostat is made from wire wrapped around a form to dissipate large amounts of power and is not designed for audio use.

"Some DIY threads I read talk about how different pots affect the sound. I tend toward the wire is wire philosophy, assuming the wire is decent. What about pots?"

As in everything, you get what you pay for up to a point. A cheap $1.00 pot will likely have more impurities in the carbon, a lower quality wiper contact, terminals and quality control. A higher quality such as an ALPS use purer materials manufactured with better quality parts and more testing. I use a lot of ALPS "Blue Velvet" pots in my projects. They are small "stepped attenuators" that is individual resistors placed around a shaft with a wiper. I posted a picture of the "guts" once, I'll try to find it again and put it up. The ALPS go for from 10-20 dollars.

There are pots that cost much more. But IMO the ALPS or Noble are adequate for 99% of our needs.

Rick

[3dzapper]

Here is the pic of the workings of an ALPs stepped potentiometer:

Rick

[JJkizak]

You can also use 5 turn and 10 turn precision wire wound pots. (no noise)and very reliable. My choice.

JJK

[Guest Anonymous]

i would for sure get a good pot, because they do make a difference, that is why on some high end amps there is no bass or treble control because some audiophile said that it will ruin the sound having another 2 pots on there, personnally i do not buy that, but i do believe that good pots make a night and day difference, but at the same time the law of diminishing does return at some point!

[Deang]

James -- what do "5 turn and 10 turn precision wire wound pots" do, and look like?

I realized about a week ago that I couldn't explain the difference between an L-pad and a potentiometer. Anyone feel taking a shot at this for me?

I wonder why PK used the 25K value for the minibox?

[3dzapper]

An L-pad has two opposing calibrated pots joined by a common shaft, with the better ones wire wound. So as you turn the knob to decrease the sensitivity of the driver, the value of the series resistance goes up while the parrallel resistance goes down and shunts more signal to ground. and visa-versa. All the time maintaining the nominal impedance to the source.

Rick

[JJkizak]

Dean:

Five turn and ten turn wirewound pots are drum shaped and the resistance lets say is 100k. They just wind the wire around a core then wind the core inside the drum and then the shoe follows the core. There are different scenarios but what it does is split the control adjustment into a very precise control (ten turns to travel across the windings of 100k instead of one turn. There are a lot of cheap ones on electronic boards (miniture) that are not very good in quality. The good ones will usually be about 2" in diameter and about

3" or 4 " deep. And they are extremely reliable and expensive. We used them a lot in some high falooten Parametric amplifiers (1.0db noise figure, 40 db gain, 10 meg bandwidth, 400-960 megs). ($50,000.00). They also don't generate any noise and because they are wirewound they are extremely stable. Also for every small movement of the adjustment the change is constant, not like carbon where it will not change for a bit then jump up or down in resistance.

Talking minute changes here where carbon really is unusable. The brush in the wirewound is copper or ?

JJK

[Jim E]

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On 1/22/2005 10:03:21 AM Daddy Dee wrote:

O.K. I've got two questions unrelated, except that they are connected by my ignorance.

I'm considering taking on a modest newbie DIY project to recase the tripath amplifier in the Sonic Impact amp. It's level of difficulty appears to be not too far ahead of my skills, which are next to nothing. Am trying to figure out which potentiometer to use for volume control. My understanding is that this will work by attenuating in input signal. One poster I was reading used a 100K attenuator. In this usage, is the term "attenuator" interchangable with "potentiometer". Also, what would be the expected difference in results if a 25K pot was used instead. Another project on the horizon (sure, Dee) is the PWK minibox. It calls for 25K pots to build it.

Another related question...What is the difference between a potentiometer and a rheostat?

Some DIY threads I read talk about how different pots affect the sound. I tend toward the wire is wire philosophy, assuming the wire is decent. What about pots?

Have been doing a little reading on the internet about the use of tripath amplifiers. What is the difference between a "class T" amplifier and a "class D" amplifier. I notice that the upcoming Klipsch 2.1 iFi uses a "class D" amp.

Any help is appreciated. If speaking technically, please type slow.

----------------

Whether you use a stepped attenuator or a potentiometer, audio controls are usually logarithmic taper types when used directly with an analog signal. Linear taper pots are more common where the signal is used for voltage or current control on an electronic circuit, digital chip or VCA (voltage controlled amplifier). Generally it is better to stay away from wire wound controls. Cermet and stepped (with fixed resistors) components seem to be the most quiet and have better precision.

[mike stehr]

Dee,

http://www.penguinlovers.net/audio/Attenuator.html

Here's the stepped attenuator I use, it's 100K. I think you can get other values. It's a shunt resistor ladder type I guess? It uses 23 steps and has a resistor value for attenuation level at that step, from silent to wide open. I think the link explains it a bit.

eBay seller is kyc111. They come up for auction assembled sometimes. The kit would be tedious to say the least.

It's better than all the carbon pots I have, I can hear the difference. I have a 100K Alps pot in a broken SS integrated amp, I could compare the two. But I think the attenuator will still win.

[Daddy Dee]

Thanks all. This is helpful for understanding what I need to do for this project.

Mike, that would be quite tedious soldering, indeed. Saw that he's got some on Ebay now. I wonder how many of those one guy could turn out in a day?

[WMcD]

I'll give a shot at reostat (sp?) and potentiometer.

They are basically the same type of device but of different size and application, and different things connected to the input and output.

Hard to do without diagrams. If you have a schematic of an amp, you'll see how the "pot" is set up.

In both you have a arc of resistance material and a slider attached to a shaft. The contact point moves along the resistance as a wiper. You also have a connection to either end of the resistance. So all together there are three connection points possible.

Generally a reostat is a high current device where the resistance material is a set of windings, probably nichrome wire. Only one end of the resistance and the wiper is used, i.e. just two of the possible connections. This was used in stage lighting and crude motor control.

As you turn the wiper, resistance between the two connections is increased. Therefore you can dim down the stage lights. The rest of the electrical energy is disapated as heat in the windings. Very cludgy by any standards. They were replaced by variacs and SCR dimmers in such an application.

So we see that a reostat was used as a variable resistance (two connections only) to vary current by being a variable series resistance. And typically there was a linear relation of resistance along the physical length of the resistance. They were used even before vacuum tubes were invented. A handy gadget. They were adopted in a small version to audio in the early days and the terminology held over.

A potentiometer is basically the same except that we are using the three connections. An important part is what is connected.

A pot is typically driven by an amp (pre amp) which has relatively high current capacity. Relative is an important concept. The driving amp output (usually a preamp) is connected across the resistance. Call one end the signal input and the other end ground. Thus the voltage varies across the physical lenght of the resistance.

As the wiper is turned, we can pick off any voltage we need. Of course the voltage picked off is always less than the input. So it is called an attenuator, or variable attenuator of voltage.

Voltage is electric potential, and hence the name, poteniometer. "Meter" does not make too much sense if we think of meters as something which are indicators. But the device does "measure" out the signal as we twist the control shaft.

In order to make this work properly, the output of the pot (wiper to ground) must be attached to a following amp which has a relatively high input impedance. This means it does not load down the attenuator and does not alter the overall resistance "seen" by the driving amp. Here the set up is quite distinct from the application of the L-Pad where the speaker load is significant.

Up above with the reostat I snuck in the thought that the resistance along the physical lenght of the resistor is linear and therefore every degree of twist on the knob give a linear (staight line) increase or decrease in voltage drop. It works that way with lightbulbs or motors.

But in audio we have this issue with our ear-brain combination. We perceive changes of audio level logarithically. That basically means that perceived steps in loudness require doubling of actual signals (1, 2, 4, 8, 16, 32), at each little steps. Or if we want big steps, it goes up as 1, 10, 100, 1000. The math works out the same. Powers of two or powers of ten. We could use any base.

This is honored in audio or log type "pots" where the resisance (voltage drop in the attenuator) does not vary linearly with each degree of wiper knob twist, but rather in that log' type of progression.

Best,

Gil

[Daddy Dee]

Gil.

Thanks for lining this out.