Passive Preamp
- Thread starter HarryHaller
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UrSv said:
hahahahaaha... sorry, that was a good one though. damn near spit out my lunch all over my keyboard.
shunt volume
Wow this is the most right on thread! I finally understand how a shunt volume works. I'm doing a passive preamp w/ remote volume control and I bought an Alps motorized pot - It's 10k linear. Will this work and what resistor value should I use for the shunt setup? Is the 'Fake Law' arrangement mandatory for a linear pot? I know an earlier post dealt w/ this but I didn't quite get it 🙄
My source (CD) line out is 2v, I don't know it's output impedance (or even what this means! other than it comes into play...) My amps (2 monos) are 500mv input sensitive and 100k input impedance.
Thanks Harry Haller and all. Man do I feel like I'm in the right place😎
Peace and Music
Sanaka
Wow this is the most right on thread! I finally understand how a shunt volume works. I'm doing a passive preamp w/ remote volume control and I bought an Alps motorized pot - It's 10k linear. Will this work and what resistor value should I use for the shunt setup? Is the 'Fake Law' arrangement mandatory for a linear pot? I know an earlier post dealt w/ this but I didn't quite get it 🙄
My source (CD) line out is 2v, I don't know it's output impedance (or even what this means! other than it comes into play...) My amps (2 monos) are 500mv input sensitive and 100k input impedance.
Thanks Harry Haller and all. Man do I feel like I'm in the right place😎
Peace and Music
Sanaka
I plan too to build an unbalance and balanced passive preamps with ALPS motorized pots with IR remote control. I will do as dorkus, use high quality relays for switching the inputs and a wall wart to supply power. But I won't use a microcontroller and all the IR receiving components will be housed in a separate box.
WOW!
ok so i haven't ordered parts yet to make my new passive pre, so i got itchy and decided to hack up my old ghetto volume control box. i rewired the 10k alps pot in shunt mode, and placed 15k Holco resistors i had lying around between input and output. total signal path is literally an inch long, consisting only of the holco resistor soldered directly to the jacks, and some 30 gauge Cardas litz connects the shunt pot. incidentally, i tied the wiper and right leg of the pot both to ground, and the signal to the left leg (rheostat configuration).
i fired it up, and WOW! it sounds fantastic! much better than the pot by itself in standard hookup. treble is a little brighter but more transparent, and there is a density to the mid-bass that i didn't hear before. imaging is also much improved, and there is less fuzz to the sound. very very nice. it should sound even better when i get a better pot and play with the resistors (Caddock or Mills maybe).
marc
ok so i haven't ordered parts yet to make my new passive pre, so i got itchy and decided to hack up my old ghetto volume control box. i rewired the 10k alps pot in shunt mode, and placed 15k Holco resistors i had lying around between input and output. total signal path is literally an inch long, consisting only of the holco resistor soldered directly to the jacks, and some 30 gauge Cardas litz connects the shunt pot. incidentally, i tied the wiper and right leg of the pot both to ground, and the signal to the left leg (rheostat configuration).
i fired it up, and WOW! it sounds fantastic! much better than the pot by itself in standard hookup. treble is a little brighter but more transparent, and there is a density to the mid-bass that i didn't hear before. imaging is also much improved, and there is less fuzz to the sound. very very nice. it should sound even better when i get a better pot and play with the resistors (Caddock or Mills maybe).
marc
it is fairly arbitrary, but in general you want it at least 10k and below, oh, 25k or so. the series resistor will determine your minimum load impedance, as well as affect the scale of output impedance as you adjust the control. i prefer to keep things on the low side to keep impedances as low as possible (but not too low), but HPotter said 22k worked best in his system. i used 15k since i had it lying around. the value of the pot is not too important as you are going to be using it as a simple variable resistor anyway, but it will determine the minimum attenuation of the circuit. e.g. if i have a 10k pot and 10k series resistor, even with the volume cranked all the way up i will have 6db attenuation, whereas with a 100k pot i would have less than 1dB attenuation.
Thank You Dorkus
Now I feel comfortable enough to play around with stuff. How do you calculate min. attenuation like that? Impedances still make this newbie's head spin 😱 Would you mind recommending values for my setup? Source is CD player w/ 2v line out, output impedance unknown. Amps are .5v sensitive and 100K input impedance.
FBJ - Are you doing your IR remote from a kit of some kind? I'd be interested in which one. I'm not at a point of comfrot building one from scratch yet. Also are you housing this separately because it may inject noise into the sound? How significant is this factor? I was planning to put my IR stuff in the same chassis with everything else.
Now I feel comfortable enough to play around with stuff. How do you calculate min. attenuation like that? Impedances still make this newbie's head spin 😱 Would you mind recommending values for my setup? Source is CD player w/ 2v line out, output impedance unknown. Amps are .5v sensitive and 100K input impedance.
FBJ - Are you doing your IR remote from a kit of some kind? I'd be interested in which one. I'm not at a point of comfrot building one from scratch yet. Also are you housing this separately because it may inject noise into the sound? How significant is this factor? I was planning to put my IR stuff in the same chassis with everything else.
you should be familiar with how to calculate parallel resistances as well as the voltage divider equation, both of which are quite simple. try looking them up if you are unsure, you can also find them in any basic circuits or physics book i think.
what type of CD player do you have? i would go with a setup like mine, maybe 15k series resistance with a 10k - 50k pot. the exact value of the pot is not too important, i find 10k works fine.
what type of CD player do you have? i would go with a setup like mine, maybe 15k series resistance with a 10k - 50k pot. the exact value of the pot is not too important, i find 10k works fine.
yup I'll try it
I can calculate resistance, and I will study up on voltage divider. Not sure how this translates to dB of attenuation, though.
My CD player is a Sony consumer model for now - CDP CE375. It has variable out but I'm doing the pre as part of the long range plan which includes a more audiophile player in future (I need source selection anyway). Plus i hear digital volume control can mess with the sound so I'd like to leave the player's line out wide open and control vol at the pre.
Thanks again!
peaceandmusic
Sanaka
I can calculate resistance, and I will study up on voltage divider. Not sure how this translates to dB of attenuation, though.
My CD player is a Sony consumer model for now - CDP CE375. It has variable out but I'm doing the pre as part of the long range plan which includes a more audiophile player in future (I need source selection anyway). Plus i hear digital volume control can mess with the sound so I'd like to leave the player's line out wide open and control vol at the pre.
Thanks again!
peaceandmusic
Sanaka
dB = 20 * log(x)
where "x" is the voltage ratio. (note VOLTAGE, not power)
so let's say you have an voltage divider that reduces the signal by half (.5):
x = .5
log(x) = -0.301
dB = 20 * -0.301 = -6.02dB
which is what we were expecting, since 6dB implies a magnitude of 2 (or half in the case of -6dB).
now all you need is the voltage ratio of your divider. the equation for that is:
R1 / (R1 + R2)
where R1 is the shunt resistor (pot in this case), and R2 is the series resistor (fixed).
so with a 10k series resistor, and a 10k pot turned all the way up to its full value, you get:
10k / (10k + 10k)
which is obviously .5 - hence my previous statement saying with these values, there will be a minimum attenuation of 6dB, since R2 is fixed and there is no way to increase R1 beyond 10k with a 10k pot. with a 100k pot and the same shunt resistor, the ratio x goes up to .91, which is obviously much less attenuation - you can do the math to see how that is equivalent to less then -1dB.
marc
where "x" is the voltage ratio. (note VOLTAGE, not power)
so let's say you have an voltage divider that reduces the signal by half (.5):
x = .5
log(x) = -0.301
dB = 20 * -0.301 = -6.02dB
which is what we were expecting, since 6dB implies a magnitude of 2 (or half in the case of -6dB).
now all you need is the voltage ratio of your divider. the equation for that is:
R1 / (R1 + R2)
where R1 is the shunt resistor (pot in this case), and R2 is the series resistor (fixed).
so with a 10k series resistor, and a 10k pot turned all the way up to its full value, you get:
10k / (10k + 10k)
which is obviously .5 - hence my previous statement saying with these values, there will be a minimum attenuation of 6dB, since R2 is fixed and there is no way to increase R1 beyond 10k with a 10k pot. with a 100k pot and the same shunt resistor, the ratio x goes up to .91, which is obviously much less attenuation - you can do the math to see how that is equivalent to less then -1dB.
marc
you can do the math
You forgot the effect of the amp input impedance in parallel with the 100Kpot.....
You forgot the effect of the amp input impedance in parallel with the 100Kpot.....
Re: you can do the math
oh right... yeah, that's true. i was assumming the theorertical infinte load. 😛
if you want your calculations to be precise, you will want to factor this in as harry says. just parallel the input impedance of the amp with the impedance of the pot to get the net shunt impedance. in other words, "R1" in my previous post is not just the value of the pot, but the value of the pot in parallel with the amp's input impedance.
so in the case of the 100k pot and a 100k input amp, you get a net shunt impedance (R1) of 50k when the volume is turned all the way up. with a 10k series resistor, this means a min. attenuation of about 1.6dB.
marc
HarryHaller said:
oh right... yeah, that's true. i was assumming the theorertical infinte load. 😛
if you want your calculations to be precise, you will want to factor this in as harry says. just parallel the input impedance of the amp with the impedance of the pot to get the net shunt impedance. in other words, "R1" in my previous post is not just the value of the pot, but the value of the pot in parallel with the amp's input impedance.
so in the case of the 100k pot and a 100k input amp, you get a net shunt impedance (R1) of 50k when the volume is turned all the way up. with a 10k series resistor, this means a min. attenuation of about 1.6dB.
marc
I know some high end companies that use much lower transmit frequencies for IR remote controls if the IR ciucuits are housed in the same chassis as the preamp circuit than normally used.
But I will use a separate box because I plan to buid 3 or 4 prototypes (preamps) using the same receiving circuit so why build more than I have too. I am using the Holtek ICs for transmiter and receiver.
Also I have a complete PIC programer, software, and PIC ICs setup but I just don't have time to learn how to use it yet.
😎
But I will use a separate box because I plan to buid 3 or 4 prototypes (preamps) using the same receiving circuit so why build more than I have too. I am using the Holtek ICs for transmiter and receiver.
Also I have a complete PIC programer, software, and PIC ICs setup but I just don't have time to learn how to use it yet.
😎
hmm, i was about to jump on the PIC bandwagon too, but after some consultation with my EE friend (i was an EE too, but only in school 😛 ) and looking around at what people on the 'net were doing, i decided to go with the Atmel AVR parts, which are very easy to use and have a nice developer community. not that the PIC developer community is limited to a couple geeks living in a garage or anything... anyway i've just got my AVR developer kit working and am pretty excited, can't wait to play around with some programs.
I am mainly going to use (in the near future) PICs cause I have found many very well written how to books. And since I have been reading these books off and on for about an year, I have came of with all the concepts I plan to use regarding microcontrollers.
I have been told and read that microcontrollers can be put in some type of sleep mode when not in use. This sounds like a good way to avoid the timing and switching problems that might interfear with sensitive circuits.
I have been told and read that microcontrollers can be put in some type of sleep mode when not in use. This sounds like a good way to avoid the timing and switching problems that might interfear with sensitive circuits.
yup, most any modern microprocessor will have a sleep mode so you can minimize noise transmission. i'm not totally sure i will be able to do that for my multichannel preamp project, as i'll be driving an alphanumeric LCD or VFD that may or may not have a buffered controller... my control unit will be in a totally separate chassis anyway, with optocoupled control lines to the audio circuit boxes, so i'll attempt to bypass the noise problem altogether. if all goes according to plan, my control unit will look like a standard rack unit, while each of my audio channels (6 total for multichannel) will be housed in small enclosures. there will be a big power supply to power all the little audio channels, bringing the total number of chassis to 8. 😛
OK, so....
I built a shunted voume control using my 10K linear pot and a 15K resistor. This worked pretty well and there was no "9 o'clock jump" in the volume response. More like fairly smooth vol. increase plateauing at about 180 deg. - the last 90 deg. of rotation did not increase volume furthur.
This was a better outcome than I expected so since things were going well I decided to do the Law Fake. I put a 1.5K (10K/6.67) resistor from signal out to ground. Now the plateau happens at about 90 deg rotation! - the last 180 deg does not increase volume, and max level is very attenuated.
I realize looking at it now that this was pretty stupid. The Elliot law fake design [ http://sound.westhost.com/project01.htm ] is for a tradtional series wired pot.
Can anyone help me do a shunted, law faked pot?
Help much appreciated 😀
peaceandmusic
Sanaka
I built a shunted voume control using my 10K linear pot and a 15K resistor. This worked pretty well and there was no "9 o'clock jump" in the volume response. More like fairly smooth vol. increase plateauing at about 180 deg. - the last 90 deg. of rotation did not increase volume furthur.
This was a better outcome than I expected so since things were going well I decided to do the Law Fake. I put a 1.5K (10K/6.67) resistor from signal out to ground. Now the plateau happens at about 90 deg rotation! - the last 180 deg does not increase volume, and max level is very attenuated.
I realize looking at it now that this was pretty stupid. The Elliot law fake design [ http://sound.westhost.com/project01.htm ] is for a tradtional series wired pot.
Can anyone help me do a shunted, law faked pot?
Help much appreciated 😀
peaceandmusic
Sanaka
for shunt attenuator we have shunt resistor R1 and Potentiometer P1....... in your case P1 is made by R2 and linear pot P1 .... you can try simply reversing the connections on the pot such that the plateau will then occur @ 180 deg. as per before or switching the circuit position of P1 and R2 relative to each other.
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