Just checking: Lpad and Hpad attenuator values

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi, I just registered. I have been reading up on attenuation, and I'd like to check with you guys before I order my resistors and start with the soldering iron.

My system has got to much gain: Roon volume attenuation is regularly at -6 to -9 db, sometimes at -12db. I like the sound better at "fixed volume" setting so I want to attenuate the signal.

I have two rca sets of rca outputs on my Schiit Yggdrasil DAC. One of them will be prone to my experiment. I want to put the attenuation inside the chassis at the rca output. This way I can compare and I can switch If I want to.

Output resistance of the DAC: is 75ohm according to Schiit. About 200ohm according to a review.
Input impedance of my preamp is 100k ohm according to the maker.

Is the following correct and a good idea?

If I use Lpad with 75k ohm r1 and 36kohm r2 I arrive at 12-ish db attenuation at 101kohm. Is it a problem that the 101kohm is probably higher than the real life input impedance of my preamp?

When this works I later plan to do the same with the XLR-connection (but I have only one output here, so I want to start with the rca first to compare the sound quality and because this is easier). With the same values of source and load, is H-pad 5 times 36k ohm a good idea for about 15db attenuation?

Thanks!!

Azh
 

PRR

Member
Joined 2003
Paid Member
I agree that "75r" should be penciled as 1k. We don't really want the small-signal impedance, but what it can *drive*. Any serious modern audio output should drive 1k.

10:1 rule. Can the pad be 10X more than 1k yet 10X less than 100k? Yes, 10k.

Quick figure: 12dB is 4:1 which implies 3:1 resistors. So 7.5k and 2.5k. Or anything close: you want both channels =equal= but you don't know if you want 12.0412..dB exactly. 6.8k:2.2k, 10k:3.3k, etc.
 
[ 25k and 2.5k aren't common values, 24k and 2k4 are E24 ]



If the output impedance is 200 phms, then using about 500 ohms for the attenuator input impedance will keep the Johnson noise down. 75k:25k is likely to give a distinct increase in the noise floor.


So 390 : 120 ohms perhaps? Or 750 : 240 ohms for lighter loading on the DAC.
 
Thank you for your replies! It was good to ask because I really are a bit of a noob in this.. I just discovered that 12 db might be to much because this would of course limit me toward the upper end.. I am now thinking at 6-8 db.. Your input is really valuable, I keep on going back to the calculators with these numbers. Should I use the 100k as "load", or 10000 as the load, so that the attenuator is nicely 10x from the spurce And 10x from the load (earlier I thought that I should mimic the load in its resistance, good that I did not go that way!)
 
Mark, I see that your figures will make the resistance of about 1000, I think this is on purpose? I am a bit confused about the 10X rule of thumb between source and attenuator and 10x between attenuator and load.

Do I understand correctly that if the resistance of the attenuator is lowered from 10000 to 1000 the load is easier for the DAC and gives less noise on the signal?
 

PRR

Member
Joined 2003
Paid Member
.... keep the Johnson noise down. 75k:25k is likely to give a distinct increase in the noise floor.

A 2V signal with 96dB dynamic range, 30uV noise, we'd have to get over 2Meg for real Johnson noise. We want lots of margin, especially if it costs nothing, but 20k should really be plenty low.

I'd go still lower because "20k" loads and interconnects are often ill-behaved. Like falling toward 5K at the top of the audio band.
 
Ok, thanks PPR!

Right now I am thinking: Rser 6200 ohm, Rpar 3900 ohm gives me 8.5ish db and 9953 ohm. Between my dac with 75 ohm or 200 ohm output and my preamp with 100k ohm input I think this fits nicely? I see the resistors which I want in 1 watt available in these values.

Or I could go more in line with what Mark suggests: Rser 680ohm, Rpar 390 ohm which gives me 8.79db and 1068 ohm. The resistors I think about buying in these values are only available in these values for the 2 watt versions, a bit pricier than the above option.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.