Replacing the rheostat with a single fixed resistor
Hello to everyone!
I would like to replace the existing rheostat (8 Ohm - 30 W) inside my loudspeakers with a single fixed resistor just as if the rheostat was in the middle position.
Please note that only 2 pins of the rheostat are used and furthermore a resistor 4 Ohm - 25 W is placed in series to it.
Frankly I do not know where to start because I lack the technical knowledge to be able to make that mod...
Any help or idea will be really much appreciated.
So many thanks in advance! 🙂
Hello to everyone!
I would like to replace the existing rheostat (8 Ohm - 30 W) inside my loudspeakers with a single fixed resistor just as if the rheostat was in the middle position.
Please note that only 2 pins of the rheostat are used and furthermore a resistor 4 Ohm - 25 W is placed in series to it.
Frankly I do not know where to start because I lack the technical knowledge to be able to make that mod...
Any help or idea will be really much appreciated.
So many thanks in advance! 🙂
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A two terminal pot becomes a variable resistor as you have noticed.
Set it to your desired sound output.
Disconnect and measure the resistance at that same desired setting.
Remember to take account of your meter's probe and cable and internal resistances when working with such low values. I need to subtract 0r1 to 0r2 from my hand-held DMM and subtract 0r13 or 0r14 from my bench DMM.
Go and buy a fixed power resistor of the same value or slightly higher than your measured variable resistor.
You can pad down the slightly higher fixed value by paralleling extra low power resistors.
eg
measured value = 4r2
available fixed = 4r7. This measures 4r8 due to it's 10% tolerance.
add a 33r // 4r8 gives a net 4r19
Much closer than the tolerance you can measure to.
Check the power rating to match the 7W 4r8.
Voltage across the 4r8 when at maximum power is sqrt(4.8*7) = 5.8Vac
Power through the 33r is V^2/R = 5.8*5.8/33 = 1W
or you could use two off 500mW/half watt 68r.
Set it to your desired sound output.
Disconnect and measure the resistance at that same desired setting.
Remember to take account of your meter's probe and cable and internal resistances when working with such low values. I need to subtract 0r1 to 0r2 from my hand-held DMM and subtract 0r13 or 0r14 from my bench DMM.
Go and buy a fixed power resistor of the same value or slightly higher than your measured variable resistor.
You can pad down the slightly higher fixed value by paralleling extra low power resistors.
eg
measured value = 4r2
available fixed = 4r7. This measures 4r8 due to it's 10% tolerance.
add a 33r // 4r8 gives a net 4r19
Much closer than the tolerance you can measure to.
Check the power rating to match the 7W 4r8.
Voltage across the 4r8 when at maximum power is sqrt(4.8*7) = 5.8Vac
Power through the 33r is V^2/R = 5.8*5.8/33 = 1W
or you could use two off 500mW/half watt 68r.
Half of 8 is 4, so use a 4 Ohm Wire Wound 20Watt resistor glued to the aluminium chassis for stability.
Green 50 Watt 4 Ohm 5% Aluminum Shell Wire Wound Resistor | eBay
Will be ideal.
Green 50 Watt 4 Ohm 5% Aluminum Shell Wire Wound Resistor | eBay
Will be ideal.
First of all thank you very much for your two very appreciated replies!
I think I begin to understand the issue...
Also I think I need some further hints, please:
1. I have to measure the resistence of the rheostat WITHOUT the series resistor connected, right?
2. The power resistor value in Watts I think cannot be 7 W if rheostat is 30 W and the series resistor is 25 W... but I do not have enough knowledge, so sorry if I can be wrong. Please note that the single fixed resistor has to be in series with the front tweeter and that the whole loudspeaker is suitable for use with amplifiers rated up to 200 W RMS.
3. Once reaching the resistance and the value in Watts of the single fixed resistor then I have to remove also the series resistor 4 Ohm 25 Watts, right?
Sorry if I seem pedantic, but it is due to this technical ignorance... 🙄
Thank you so very much again!
I think I begin to understand the issue...
Also I think I need some further hints, please:
1. I have to measure the resistence of the rheostat WITHOUT the series resistor connected, right?
2. The power resistor value in Watts I think cannot be 7 W if rheostat is 30 W and the series resistor is 25 W... but I do not have enough knowledge, so sorry if I can be wrong. Please note that the single fixed resistor has to be in series with the front tweeter and that the whole loudspeaker is suitable for use with amplifiers rated up to 200 W RMS.
3. Once reaching the resistance and the value in Watts of the single fixed resistor then I have to remove also the series resistor 4 Ohm 25 Watts, right?
Sorry if I seem pedantic, but it is due to this technical ignorance... 🙄
Thank you so very much again!
Thank you for your very appreciated reply!
Please tell me if "Half of 8 is 4" means that if the rheostat is 8 Ohm so its middle position should be 4 Ohm... because I think so too, but I have not yet made any measurement... so I may say that later. 😉
Please, also kindly tell me if in your opinion once that the rheostat is marked as 30 W then also the single fixed resistor should be just as 30 W, or not?
Finally, are "Aluminum Shell Wire Wound" 30W resistors existing in "audiophile" taste? Any suggestion? 🙄
Thank you very much again!
Yeah! 🙂
I was going to think exactly the same thing: thank you so much!
25 or 30 Watts?
Please note that the fixed series resistor is 25 W while the rheostat is 30 W...
I was going to think exactly the same thing: thank you so much!
25 or 30 Watts?
Please note that the fixed series resistor is 25 W while the rheostat is 30 W...
The rheostat is rated for maximum current through the wiper.
That maximum current is then used to determine the "full resistance" power rating.
The current rating can be found by working backwards from the power rating and the resistance, 30W 8ohms gives 1.94Arms. Maybe about 4A peak for very short term transients.
Will your tweeter see all of that 1.94Arms? Or does some get shunted to ground?
What is the continuous rating of the tweeter?
The ceramic cased resistor has CW25E marked on it.
Are you sure that means 25W?
How big is that ceramic casing?
That maximum current is then used to determine the "full resistance" power rating.
The current rating can be found by working backwards from the power rating and the resistance, 30W 8ohms gives 1.94Arms. Maybe about 4A peak for very short term transients.
Will your tweeter see all of that 1.94Arms? Or does some get shunted to ground?
What is the continuous rating of the tweeter?
The ceramic cased resistor has CW25E marked on it.
Are you sure that means 25W?
How big is that ceramic casing?
Many thanks for your time.
I'm fairly sure though...
Also, I've just discovered that rheostat "upright position" does not match with its "middle" one...
Anyway its "upright position" is OK for me and then I've just measured the related resistence of the rheostat and it is 2.8 Ohm, possibly it is really 2.4 Ohm if I subtract just 0.4 Ohm due to my meter's probe and cable and internal resistances...
Needless to say any further addressing will be most welcomed.
I have no idea about that, and no info at all currently available...
Nope.
I'm fairly sure though...
60mm x 15mm x 15mm.
Also, I've just discovered that rheostat "upright position" does not match with its "middle" one...
Anyway its "upright position" is OK for me and then I've just measured the related resistence of the rheostat and it is 2.8 Ohm, possibly it is really 2.4 Ohm if I subtract just 0.4 Ohm due to my meter's probe and cable and internal resistances...
Needless to say any further addressing will be most welcomed.
To match the 2r4 you can buy a 2r7.
Try that with the existing fixed resistor.
Is that close enough for what you want to hear?
If it needs to be lowered slightly then start adding parallel highish value resistors as suggested in my "trimming" comment.
Try that with the existing fixed resistor.
Is that close enough for what you want to hear?
If it needs to be lowered slightly then start adding parallel highish value resistors as suggested in my "trimming" comment.
Thanks again for your valued instructions: now I can begin to see the whole thing.
OK for 2R7 value, but 25W, 30W or 50W? (< Please note that your answer to that question is quite important for my final understanding.)
And, please try to follow my related thoughts to kindly find if they were correct...
To mostly reduce soldering points I would like to replace the existing series resistor (4 Ohm - 25 W) too, so I would buy a single power resistor 6R8 - 50W in order to replace both existing ones.
Resistors fine-tuning apart: do you think that both that practice and the related wattage (50W) are appropriate?
OK for 2R7 value, but 25W, 30W or 50W? (< Please note that your answer to that question is quite important for my final understanding.)
And, please try to follow my related thoughts to kindly find if they were correct...
To mostly reduce soldering points I would like to replace the existing series resistor (4 Ohm - 25 W) too, so I would buy a single power resistor 6R8 - 50W in order to replace both existing ones.
Resistors fine-tuning apart: do you think that both that practice and the related wattage (50W) are appropriate?
Are you absolutely sure that only two of the connections are used? I'd expect something like that to be used as a variable L-Pad.
To replace trace out a circuit diagram, measure the resistances and replace with a single high quality resistor. Personally I use the mundorf 25W models, which are fine (presuming this is for a tweeter) and cost pretty much nothing. plus postage.
To replace trace out a circuit diagram, measure the resistances and replace with a single high quality resistor. Personally I use the mundorf 25W models, which are fine (presuming this is for a tweeter) and cost pretty much nothing. plus postage.
nannoo, thank you for your appreciated reply!
To be honest, I'm sure of quite nothing... and I do not want to disassemble the loudspeakers.
I'm reasonably sute that both the fixed resistor (4 Ohm/25W) and the rheostat (8 Ohm/30W) are part of the whole crossover (this is a 3-way system): please note that both fixed resistor and rheostat are placed just close the binding posts while the crossover is placed (I guess) elsewhere inside the cabinet.
And yes, both fixed resistor and rheostat are in series to the tweeter (I cannot see and I do not know if also other components there are).
I was thinking to Mundorf resistors too, but in my not yet proven opinion 25W is a too low value considering both fixed resistor (4 Ohm/25W) and rheostat (8 Ohm/30W) existing in series to the tweeter.
Finally, if I understand what you say I should replace only the rheostat (and not both ones) and then a 25W fixed resistor should be enough, right?
In your opinion is there no way in order to replace both components with a single one?
Thank you!
To be honest, I'm sure of quite nothing... and I do not want to disassemble the loudspeakers.
I'm reasonably sute that both the fixed resistor (4 Ohm/25W) and the rheostat (8 Ohm/30W) are part of the whole crossover (this is a 3-way system): please note that both fixed resistor and rheostat are placed just close the binding posts while the crossover is placed (I guess) elsewhere inside the cabinet.
And yes, both fixed resistor and rheostat are in series to the tweeter (I cannot see and I do not know if also other components there are).
I was thinking to Mundorf resistors too, but in my not yet proven opinion 25W is a too low value considering both fixed resistor (4 Ohm/25W) and rheostat (8 Ohm/30W) existing in series to the tweeter.
Finally, if I understand what you say I should replace only the rheostat (and not both ones) and then a 25W fixed resistor should be enough, right?
In your opinion is there no way in order to replace both components with a single one?
Thank you!
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I wouldn't get hung up on the resistor power rating. I haven't seen a 1" dome that would handle more than 10 Watts for a minute or two. In practice you will find that your resistor is well protected by your fusible tweeter.
Any resistor bigger than 10 Watts will likely be fine. You also have the option of keeping the fixed resistor and adding a 2nd of a value equivalent to the pot, or replacing both with a resistor of the combined total resistance.
David
Any resistor bigger than 10 Watts will likely be fine. You also have the option of keeping the fixed resistor and adding a 2nd of a value equivalent to the pot, or replacing both with a resistor of the combined total resistance.
David
So many thanks: practical, authoritative, simple!
I was thinking about the following (high-cost apart) in order to replace both the existing components...
(MRES20-110) - 6.8R 20W M-Resist Supreme Resistor (6R8) Hifi Collective
Thank you again, really very appreciated. 🙂
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