Attenuate output of your preamp/input of amp the right way: voltage divide with input impedance.

hey everyone it's Jason from odd audio I got a simple one here today but it's one that I don't see a ton of information on and I had my own problem to solve here and this is as far as I'm concerned the best solution and so I wanted to cover it in a quick video so what it is is you have a preamp that has too much game for your amp and so that can be either you know in most cases you just can barely turn the volume control up and you're cranking the music um more annoying is on like an older amp uh preamp design or especially tube preamps there's always a noise floor to deal with and sometimes it can be kind of high if there's no attenuation so you know you kind of want to just get that noise level down down further so you're not hearing it through your speakers so what you'll often see is people talking about attenuators and this is what this is but it's it's using the attenuation with our existing circuit and the first thing you'd think is if you want to attenuate and you know you can't easily modify the preamp to have less gain in the first place is oh I'll put a volume control on the output or I'll put like a passive uh attenuator passive volume control between my preamp and my amplifier and that's a good idea in theory but the problem is that you're messing with the impedance and so what that means is you're putting a voltage divider in line and the voltage divider which here's the circuit because this is what we're working with is you have a series resistance that is coming this way and then you have the parallel resistance which is coming down to your ground and what happens is if you have an something between the preamp and the amp you're now paralleling another resistance with the input impedance of your amp and when you parallel resistors you can think of it the easiest way to think of it is you're having so if you put two 10K resistors in parallel they're 5K now it's it gets more complicated than that if you're using you know that's an easy example because they're both the same value uh but if you have two different values it's a little more complicated that use an online calculator it's not hard to figure out but anyway parallel resistance bad because then you're lowering your out your input impedance on the amp that the preamp is seeing you don't want that because you want as high an input impedance as possible that's an easier load for the preamp to drive think of it the same way as speakers a higher impedance speaker like eight or 16 ohms is easier for an amp to drive than a four or two ohm load so how do you get around that uh it's actually really easy all you need to know is what's your input impedance of the amp and you're going to use that as this half of your voltage divider so all you need to do is add one resistor in series with the signal and then it creates a voltage divider with your input impedance of the amp and you're not adding any extra resistors in parallel that are going to mess it up so you need to know first of all the input impedance of your amp so in my case my amp has a 100K input impedance and I wanted Min -12 DB just kind of as that was the number I was looking for and it was the number that I knew from testing would bring my noise floor down to an acceptable level and so- 12 DB when you convert that into voltage is uh you know if you had 10 volts in after the divider it'd be 2.5 volts out so it's one quarter of the voltage in and that you can again use an online calculator to say oh I want min-2 DB I want minus 10 I want minus 20 whatever it may be and you can convert that into a volt voltage number in the gain or loss and so then it's just a matter of saying okay so I want it to be one quarter of the voltage in I have 100K here going to ground so that's my R2 of the circuit so what's my R1 well use the calculator if you go backwards you say I've got 10 volts in I want 2.5 volts out I know the value of this resistor is 100k that gives me 300K as R1 series resistor in this case I just used a 330k easy enough just something out of the the part spin uh but if you wanted to be really precise well you could use the 300K and all you're going to do is build this and all this is is two rcx the female the male as you can see and inside of the body here is a 330k resistor that is just connected along the positive or the center pin and then the outer shell is grounded from end to end as well so make sure that's you in the case of this type of connector it's actually not grounded directly to the body so you have to add a wire through to make sure that the ground is connected and that's it so then when you plug this in to your amp and you plug in your usual cable on this side boom you've created a voltage divider and you haven't messed up the input impedance so as far as I'm concerned this is this is the answer um you'll see attenuators like this sold off the shelf but unless they're asking you hey what's your input impedance well how does that work well it's be all it is is they're putting a voltage divider inside of this so there would be two resistors inside and then we have the same problem the parallel resistance issue screwing up the impedance so not those are not a great solution this is simpler you can easily build it yourself I mean however you want I made these just kind of like a little Standalone and some spare uh fancy audio file RCA connectors I had but really like it's literally just a piece of wire with a resistor in the middle so build it however you like uh but I like this that it's reversible it's not a permanent modification in any way I'm not soldering anything into the circuits of either the amp or the preamp I just plug that in and use my existing cable good to go so this is this is the way to do it uh this is way simpler obviously you kind of need to know what attenuation you want and you need to know that variable of what is your input impedance on the amp so that you can then calculate out the divider but that should be fairly straightforward you know look on the the spec sheet or uh sometimes in the owner's manual usually in the service manual they will give you that number and then you can make your calculations based on that that's it it's pretty simple and it doesn't screw anything up except for adding a single resistor into the signal path which eh that's not really a big deal let's be real like yeah it's nice to get everything out of the signal path but a single resist is about the least damaging thing you could do so that's it

Channel: OddAudio

Views: 29

Keywords: oddaudio, oddaudiocanada, odd audio, odd audio canada, audio, hifi, preamplifier, preamp, amplifier, attenuator, voltage divider, impedance, audiophile

Id: IwaNX_dNWd0

Length: 8min 11sec (491 seconds)

Published: Sat Jun 15 2024