What value pot?

What value pot?

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. 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).


---MIKE---
>>In the White Mountains of New Hampshire
>> (44° 15' N - Elevation 1580')

Re: What value pot?



---MIKE--- 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. 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).

LOL - I'd have advised 10k actually but 50k will do.

Graham

Re: What value pot?


"---MIKE---" <twinmountain@webtv . net > wrote in message
news:8186-481CABAE-426@storefull-3251.bay.webtv . net ...
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. 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).


---MIKE---
>>In the White Mountains of New Hampshire
>> (44° 15' N - Elevation 1580')


Use a fixed "L" pad. Resistors at the shack are still pretty cheap and it
wont ever get scratchy.

Re: What value pot?

In article <8186-481CABAE-426@storefull-3251.bay.webtv . net >,
---MIKE--- <twinmountain@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. 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).

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.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Re: What value pot?

"Scott Dorsey" <kludge@panix . com > wrote in message
news:fviplp$apl$1@panix2.panix . com ...
> In article <8186-481CABAE-426@storefull-3251.bay.webtv . net >,
> ---MIKE--- <twinmountain@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. 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).

> 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.

He can do the same thing with a linear 50K pot.

Re: What value pot?

"William Sommerwerck" <grizzledgeezer@comcast . net > wrote in
message news:ls-dnWsBxY09YoHVnZ2dnUVZ_uGdnZ2d@comcast . com
> "Scott Dorsey" <kludge@panix . com > wrote in message
> news:fviplp$apl$1@panix2.panix . com ...
>> In article
>> <8186-481CABAE-426@storefull-3251.bay.webtv . net >,
>> ---MIKE--- <twinmountain@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. 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).
>
>> 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.

Usually, even 5K can be used.

> He can do the same thing with a linear 50K pot.

Depends on the cable length he is driving.

A 50K pot driven by a low impedance source like a FM tuner has a maximum
output impedance of 25,000. 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.

Re: What value pot?

"Arny Krueger" <arnyk@hotpop . com > wrote in message
news:kJCdnfmoWLGlD4DVnZ2dnUVZ_q-jnZ2d@comcast . com ...
> "William Sommerwerck" <grizzledgeezer@comcast . net > wrote in
> message news:ls-dnWsBxY09YoHVnZ2dnUVZ_uGdnZ2d@comcast . com
> > "Scott Dorsey" <kludge@panix . com > wrote in message
> > news:fviplp$apl$1@panix2.panix . com ...
> >> In article
> >> <8186-481CABAE-426@storefull-3251.bay.webtv . net >,
> >> ---MIKE--- <twinmountain@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. 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).

> >> 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.

> Usually, even 5K can be used.


>> He can do the same thing with a linear 50K pot.

> Depends on the cable length he is driving.

> A 50K pot driven by a low impedance source like a FM tuner has a
> maximum output impedance of 25,000. 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.

Don't take this personally, Arny, because it isn't meant that way. But this
is a classic example of misreading a post. WE ALL DO IT, and I've learned to
reread the original post at least once before sending off an reply.
Sometimes I discover I got it completely backwards.

In this case, the issue was how to determine the FIXED VALUE resistors
needed to pad the output to a usable level:

> 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.

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. (This assumes the output
stage itself has zero source impedance.) 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.

Re: What value pot?

"William Sommerwerck" <grizzledgeezer@comcast . net > wrote in
message news:9oudnYDcQcvHNIDVnZ2dnUVZ_hSdnZ2d@comcast . com
> "Arny Krueger" <arnyk@hotpop . com > wrote in message
> news:kJCdnfmoWLGlD4DVnZ2dnUVZ_q-jnZ2d@comcast . com ...

>> "William Sommerwerck" <grizzledgeezer@comcast . net > wrote
>> in message
>> news:ls-dnWsBxY09YoHVnZ2dnUVZ_uGdnZ2d@comcast . com

>>> "Scott Dorsey" <kludge@panix . com > wrote in message
>>> news:fviplp$apl$1@panix2.panix . com ...

>>>> In article
>>>> <8186-481CABAE-426@storefull-3251.bay.webtv . net >,
>>>> ---MIKE--- <twinmountain@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.

Re: What value pot?

Arny Krueger wrote:

> Quite clearly, the OP was talking about potentiometers.

> Scott was probably recommending replacing the pot with a fixed resistor
> network in the interest of reliability.

True, but you know that quite often people ask the wrong question rather
than explaining their problem (which the original poster did) and asking
for a recommended solution. Since he already had another place to adjust
the volume, the didn't need a variable attenuator that would, once the
setting to get the level into the right ballpark was determined, would
remain at that setting for the next ten years or so. A fixed attenuator
would be the better solution once he could quantify roughly how much
attenuation he needed . Any pot would do for that. (Or when in doubt, 15
dB is usually a good guess)

--
If you e-mail me and it bounces, use your secret decoder ring and reach
me here:
double-m-eleven-double-zero at yahoo -- I'm really Mike Rivers
(mriv...@d-and-d . com )

Re: What value pot?


<twinmountain@webtv . net > wrote in message
8186-481CABAE-426@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."

Robert Morein
Dresher, PA
(310) 237-6511
(215) 646-4894

Re: What value pot?

On May 3, 10:40 pm, Soundhaspriority <nowh...@nowhere . com > wrote:
> <twinmount...@webtv . net > wrote in message
>
> 8186-481CABAE-...@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.

Re: What value pot?

On 4 Mai, 09:35, "Shhhh! I'm Listening to Reason!"
<artygu...@hotmail . com > wrote:
> On May 3, 10:40 pm, Soundhaspriority <nowh...@nowhere . com > wrote:
>
> > <twinmount...@webtv . net > wrote in message
>
> > 8186-481CABAE-...@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.

Re: What value pot?


"---MIKE---" <twinmountain@webtv . net > wrote in message
news:8186-481CABAE-426@storefull-3251.bay.webtv . net ...
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. 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).


---MIKE---
>>In the White Mountains of New Hampshire
>> (44° 15' N - Elevation 1580')

Mike, 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?

Please regard the forgery by Brian L. McCarty in this thread.

Regards,
Bob Morein
(310) 237-6511

CORRECTION

CORRECTION:

The preferred impedance of the pot is 10X the output impedance of the tuner.

Bob Morein
(310) 237-6511

Re: CORRECTION

On 2008-05-04, Soundhaspriority <nowhere@nowhere . com > wrote:
> CORRECTION:
>
> The preferred impedance of the pot is 10X the output impedance of the tuner.

I've been lucky to deal with inputs that are at least two orders of
magnitude higher impedance than the output I'm connecting the pot
between, and in fact at least an order of magnitude higher than the
rated minimum next stage impedance rating of the output.

There was a rule when chaining passive filters, so a little more complex
than you're doing, to make each one an order of magnitude higher impedance
than the previous to reduce the effects they have on eachother. The
effects for you are probably not a problem as long as you don't go too
big.

If your tuner's output says "Don't connect to anything smaller than 1K"
then don't connect anything smaller than 1K. I know I've used things
like 4K7 because I had it hanging around. Your amplifier's input
impedance may say something like "100K", in which case 10K would be
great.

Googling around I found a domestic amplifier with input impedance 47K.
My 4K7 pot would be great for this if the tuner could drive it. If you
connect your 50K pot across this then at full volume you'll get full
volume and present 25Kish to the tuner. At minimum volume it will be
indeed silent (with caveats, things aren't perfect, extra resistances get
in). When your pot is at 25K you'll have 25K on top and 25 in parallel
with 47 on bottom. That 47 will pull down the 25 so the effect is that
control won't be so linear. Volume will be lower at 50% then it should
be.

If you use a 10M pot into that 47K input then things will be very
non-linear and the control will be pretty unusable. Even with the pot at
90% you'll have 1M on the top and 9M in parallel with 47K on the bottom.
The 47K will make the 9M irrelevant, so in effect your control is at 47K
out of 1M047 or about 5% - but you've only turned it a little way from
maximum. You think you're at -1dB but you're at -26dB so you have 26dB
in that top 1dB of the pot's scale. The difference between mic and line
levels in a control sweep that's meant to be barely audible.

If you know the input impedance of your amplifier, and both channels are
going to be well matched, and you can get well enough matched resistors
(metal film 1% would be fine), you can calculate a fixed pad using a
fixed resistor in series. To cut 6dB use the same size as the input
impedance so dividing the voltage by 2. For 12dB go three times as much,
so 150K in my example.

- Richard

--
_/_/_/ _/_/_/ _/_/_/ Richard dot Corfield at gmail dot com
_/ _/ _/ _/
_/_/ _/ _/ Time is a one way street,
_/ _/ _/_/ _/_/_/ except in the Twilight Zone