What value pot?
in [Audio Recording]
|
From: Shhhh! I'm Listening to Reason! on 4 May 2008 09:35 On May 3, 10:40 pm, Soundhaspriority <nowh...(a)nowhere.com> wrote: > <twinmount...(a)webtv.net> wrote in message > > 8186-481CABAE-...(a)storefull-3251.bay.webtv.net > > > What is the best value pot to > > use for this. I have ordered 50K, 100K, 250K and 500K > > Man, that's a lot of pot. I rarely go thru more than $200 a week. > > "I don't really have a replacement career, it's a very gnawing thing." Sorry to hear your Baskin-Robbins gig didn't work out. Keep your head up, Bwian. Perhaps a McDonald's will open in your area soon.
From: Clyde Slick on 4 May 2008 10:00 On 4 Mai, 09:35, "Shhhh! I'm Listening to Reason!" <artygu...(a)hotmail.com> wrote: > On May 3, 10:40 pm, Soundhaspriority <nowh...(a)nowhere.com> wrote: > > > <twinmount...(a)webtv.net> wrote in message > > > 8186-481CABAE-...(a)storefull-3251.bay.webtv.net > > > > What is the best value pot to > > > use for this. I have ordered 50K, 100K, 250K and 500K > > > Man, that's a lot of pot. I rarely go thru more than $200 a week. > > > "I don't really have a replacement career, it's a very gnawing thing." > > Sorry to hear your Baskin-Robbins gig didn't work out. > > Keep your head up, Bwian. Perhaps a McDonald's will open in your area > soon. Bwian was caught scooping into the Rocky Road once too often. Maybe managing a hardware store would be a better gig for him, if he can stay away from the nuts.
From: Soundhaspriority on 4 May 2008 10:58 "William Sommerwerck" <grizzledgeezer(a)comcast.net> wrote in message news:qbSdnQuZbaVtNIDVnZ2dnUVZ_rOqnZ2d(a)comcast.com... >> The preferred value of the pot is about 1/10 the output >> impedance of the tuner. Do you have any idea what the value is? > > It would be more like 10x the value. > > If the output stage is a follower, its source impedance -- the impedance > "seen" looking into the driver -- cannot be "matched" by an equivalent > load, > or a loss of signal level and an increase in distortion. > Yes, you are correct. I'm surprised you didn't see the post titled CORRECTION, which I made two minutes after my initial reply. Bob Morein (310) 237-6511
From: William Sommerwerck on 4 May 2008 12:51 "Soundhaspriority" <nowhere(a)nowhere.com> wrote in message news:GM2dnT4Ym7ywUoDVnZ2dnUVZ_hGdnZ2d(a)giganews.com... > "William Sommerwerck" <grizzledgeezer(a)comcast.net> wrote in message > news:qbSdnQuZbaVtNIDVnZ2dnUVZ_rOqnZ2d(a)comcast.com... >>> The preferred value of the pot is about 1/10 the output >>> impedance of the tuner. Do you have any idea what the value is? > > It would be more like 10x the value. >> If the output stage is a follower, its source impedance -- the impedance >> "seen" looking into the driver -- cannot be "matched" by an equivalent >> load, or a loss of signal level and an increase in distortion. > Yes, you are correct. I'm surprised you didn't see the post titled > CORRECTION, which I made two minutes after my initial reply. Because, like most people, I respond to the first post before I've bothered to look at the others.
From: Arny Krueger on 4 May 2008 16:03
"William Sommerwerck" <grizzledgeezer(a)comcast.net> wrote in message news:9oudnYDcQcvHNIDVnZ2dnUVZ_hSdnZ2d(a)comcast.com > "Arny Krueger" <arnyk(a)hotpop.com> wrote in message > news:kJCdnfmoWLGlD4DVnZ2dnUVZ_q-jnZ2d(a)comcast.com... >> "William Sommerwerck" <grizzledgeezer(a)comcast.net> wrote >> in message >> news:ls-dnWsBxY09YoHVnZ2dnUVZ_uGdnZ2d(a)comcast.com >>> "Scott Dorsey" <kludge(a)panix.com> wrote in message >>> news:fviplp$apl$1(a)panix2.panix.com... >>>> In article >>>> <8186-481CABAE-426(a)storefull-3251.bay.webtv.net>, >>>> ---MIKE--- <twinmountain(a)webtv.net> wrote: >>>>> My new digital FM tuner puts out too strong a signal >>>>> compared to my other inputs (TV and CD). I want to >>>>> install a dual pot between the tuner and preamp to >>>>> reduce the volume. Quite clearly, the OP was talking about potentiometers. >>>>>> What is the best value pot to use >>>>> for this. I have ordered 50K, 100K, 250K and 500K >>>>> from Mouser (the pots are cheap - the shipping is >>>>> not). Agreed. >>>> Try a 10K audio taper. 5 or 10K has been my historic choice for this application. Usually 5 K. >>>> The lower the value, the lower >>>> the output impedance that goes into the amp. Indisputable. The effects of a too-high source impedance are primarily high frequency losses due to loading by the output cable. >>>> When you find the correct value, solder fixed resistors and make >>>> a pad. Thus, the discussion of using fixed resistors was started by Scott, not the OP. >> Usually, even 5K can be used. Agreed. >>> He can do the same thing with a linear 50K pot. Yes, and I do this all the time. Nevertheless, audio taper pots are more convenient to use. For small amounts of attenuation (6 dB or less) linear pots work pretty well. For large amounts of attenuations (20 dB or more) linear pots tend to be hypersensitive to small changes in shaft rotation. >> Depends on the cable length he is driving. Again, indisputable. >> A 50K pot driven by a low impedance source like a FM >> tuner has a maximum output impedance of 25,000. As William correctly points out, this is in error. As he says, the correct answer is 12,500 ohms. I guess it was too early in the morning when I posted. Definately before that daily cup of coffee. >> It can >> drive less than 5 feet of standard 35 pF/ft shielded >> cable with a loss of 1 dB or less @ 20 KHz. A 2 meter >> cable would have even more loss. Due to the error above, the lengths should be doubled. This means that typical 1-4 meter audiophile cables are mostly OK for use with the 50K pot. This is especially true if the worst case attenuation (6 dB) is not going to be part of typical use. > In this case, the issue was how to determine the FIXED > VALUE resistors needed to pad the output to a usable > level: Mike the OP asked about potentiometers. The use of fixed resistor attenuators was introduced by Scott. >> Try a 10K audio taper. The lower the value, the lower >> the output impedance that goes into the amp. When you >> find the correct value, solder fixed resistors and make >> a pad. > As the gentleman had already ordered a 50K pot, he could > use it to determine the correct values, without having to > order another one. What I neglected to say was that the > resistor values should be SCALED DOWN to get a lower > source impedance. Whoops. My error was that the mistake you pointed out William, makes the OP's origional order of 50K pots a workable solution. Scott was probably recommending replacing the pot with a fixed resistor network in the interest of reliability. > As for source impedance, it will vary with the pot's > setting. The maximum source impedance for a 50K pot is > 12.5K, not 25K. Right, the 25K number was in error - my bad. > (This assumes the output stage itself has zero source impedance.) With modern SS gear, the tuner's output stage is going to be close to zero, relatively speaking. For example, if you drive that 50K pot with even a 500 ohm source impedance, the source impedance contributes only a few percent to the equations. > But even that's on the high side > unless the amp is near the tuner. A 10K pot would have a > source impedance of no more than 2.5K. Hence all the comments favoring lower resistance pots. However, with your correction in hand, I conclude that the OP's 50K pot would be applicable unless the cable lengths were relatively long. |