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 potentiometer 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---
IIRC, 50K should work OK.
Hint: If there are three terminals that connect to the innards of the pot, short two adjacent ones together (center and either side).
This prevents a full open circuit when the wiper inside makes a poor contact or hits a bump.
That would only work if he puts the pots in SERIES with the signals and inputs.
I think he's more likely to use them as potentiometers, in which case your offering won't work.
Peace,
Jeff
I've seen a lot of audio wiring diagrams, and that's not the way it's done. IIRC you connect the fixed resistor across the source and the wiper of the pot to the load. The source never has less than 50 ohms across it.
Sorry, for the messup. That was it's never greater than 50K ohms.
You cannot get an effective result using a simple potentiometer. You are attempting to attenuate a signal that involves voltage, current and a frequency range without denigrating it. It requires more that simple series resistance. You can buy audio frequency filters that are designed for that purpose, to correct a mismatched input, they are not expensive, although more than just the cost of a pot. Try one of your local hobby/electronics stores. Failing that, try to obtain a circuit diagram for your FM tuner and try playing with the front-end AGC. If you don't have an adjustable AGC, then that is the part of the circuit that would be more suited to adding a pot, or preferably and tunable RC network.
Regards
Mack
I would have ordered 1K, 5K, 10K, 25K, 50K
A more elegant look is to order an inline attenuator. They cost way more than some pots and are not adjustable, but you won't see dangling wires.
Actually, with today's solid state equipment, it usually means lower impedance. So, IMO, without knowing any details, I would grab a 5K or 10K and because it is audio, one with an audio taper. However, if you have a 50K, use it. Wire the fixed resistor (ends) across the output of the tuner. Then wire the low fixed side (ground) and the wiper to the input of the amp. It should work fine. I've done this at our local access TV studio for a tuner that plays over text. It's only mono, but it's the same idea.
Mike,
Mack's advice is entirely incorrect.
Using pots as simple voltage dividers is an absolutely correct design approach. The size of the pot ultimately is dictated by home much attenuation you need as well as the impedance of the source (your tuner output) and the load (your amplifier input) which are likely to be around the industry standard of 47K ohms.
I would start with the 50K ohm pot which is just about perfect for this impedance. Shunt the top end of the pot from the Line out to ground such that the pot therefore has the total signal voltage appearing across it. Use the wiper (the center contact) as your divided output, and this will be the attenuated signal. As the wiper approaches the top end of the pot, you will see the up to the maximum line out voltage, and conversely, as you turn the pot towards the grounded end you will see further attenuation. Chose the setting to match your other audio inputs.
Obviously you will need to do this the same on both on the left and right channels.
Do not, under any circumstances, play with the tuner's AGC as "Mack the Knife" suggests. This is not in any way whatsoever related to the audio output of the tuner. This AGC gain control is used to adjust the RF amplifiers to compensate for external signal level differences arriving at the antenna, and is not in any way used to control audio output.
Nor is is there any reason to buy "an audio filer". This is not a filtering issue whatsoever, since filters are for modifying frequency response. You do not want to alter frequency response.
I am a graduate electrical engineer with EE degrees and 37 years of experience in hardware design. You were on the correct track to begin with by buying the pots. All you have to do is wire them up.
Smarty
On 5/3/2008 9:46 PM Mack the Knife spake thus:
With all due respect, you're just plain wrong, and this is from empirical evidence (I've done exactly what the OP is asking about, between the line-out output from one receiver into the line-in input to another, and it works just fine).
Besides, just look at the circuit diagram for a receiver, and you'll see exactly what's been suggested--a simple potentiometer--between preamplifier stages.
Smarty, Thanks for your learned and commonsense reply. I have on order in addition to the 50K, 100K, 250K, & 500K pots, a 5K and 10K. I will experiment between the 10K (recommended by some) and the 50K to see what works the best, I have also sent emails to Sangean (the maker of the tuner) asking their opinion. I'm not holding my breath waiting for an answer from them!
---MIKE---
Glad to help Mike. If your amp has a very low input impedance (which it shouldn't) and falls well below the 47K ohm "standard" which is commonly used, then you may find that the other pots will work better in the sense that a larger fraction of their rotational range will be useful. The pot is merely a voltage divider, and, as such, divides a ratio determined in part by the input impedance of your amplifier input. You can experiment and see which one works best, but any of the pots, once set, will do exactly the same voltage division to give you the attenuation your are looking for.
Similarly, a pot with an "audio taper" rather than a "linear taper" will make the adjustment in sound 'volume' more proportional, since the ear perceives sound volume logarithmically and a linear pot seems to have an unnaturally slow response when rotated. Even so, the ultimate voltage division is still all that matters to get you the attenuation you are looking for. Therefore any of them should work, but some will seem much more fussy to set.
Good luck!
Smarty
---MIKE---
Depends on the impedance/s of the output of the sound source and the input to the amplifier. The impedance of a so called 'line input' connection bewteen out put of a tuner and input of amplifier could be as low as 600 ohms! Since the OP doesn not define really define 'too strong' it may be wise to assume a reduction of around 6 to 12 db. And then jigger up a resistor arrangement to reduce the audio voltages to achieve that sort of reduction (one quarter the voltage for example will give power reduction of about 16 times or roughly 12 db.) A 1k pot might be a good starting point. Since it will presumably be rarely adjusted once in place the type of taper etc. may not, unlike a regular volume control that can be varied easily to the desired listening level, be critical at all.
BTW is thi mono or stereo? Or multi audio channel?
Terry asked:
This is two channel stereo. I have ordered dual pots and I am aware of a potential tracking problem. I have had suggestions ranging from 1K to
50K so I am still unsure about the best value. I will probably start with 10K---MIKE---
From: snipped-for-privacy@webtv.net (---MIKE---) Terry asked: BTW is thi mono or stereo? Or multi =A0audio channel?
-------- This is two channel stereo. I have ordered dual pots and I am aware of a potential tracking problem. I have had suggestions ranging from 1K to
50K so I am still unsure about the best value. I will probably start with 10K =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0= =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0---MIKE--- In the White Mountains of New Hampshire (44=B0 15' N - Elevation 1580')----------------------- I had a similar situation a few years back with an office stereo sound system and purchased a stereo attenuator from mcm electronics - it wires in easliy - worked like a charm.
Roy Q.T. ~ US/NCU ~ E.E. Technician [have tools, will travel]
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