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Building passive preamp - impedance matching

post #1 of 11
Thread Starter 


Boy am I confused :D

I thought building a passive preamp would be a walk in the park, but I am having some trouble grasping the concepts of impedance matching.

Here is the situation:
I wan't to run my Bottlehead Crack with the 100k attenuator at max, because I think it sounds better that way. It just seems much more dynamic? I think all tube amps sounds better at max???
Currently my Nano Patch+ passive preamp is attenuating -40db to -50db to control the volume.

I know I have to pick the right attenuator for my preamp to get the best possible match with my system => best SQ.

My DAC outputs 75ohm at 2,5RMS
The Bottlehead Crack's input impedance is 100kohm (http://bottlehead.com/crack-otl-headphone-amplifier-kit-specs/)

I need aproximately 45db attenuation. What ohm rating should the attenuator I pick for my preamp have? 
Can anyone wrap their heads around this? I am at loss :(

Thanks in advance!

Edited by Rune3400 - 5/18/14 at 4:49pm
post #2 of 11

You have a typo:


The approximate specs are input impedance 100K ohms, output impedance 120 ohms, gain about 15dB into a 300 ohm load like a Sennheiser HD600, max output about 10V into a 300 ohm load.

post #3 of 11
Thread Starter 

True I added a "k" now :)

So far I found this helpful page: http://www.dact.com/html/technical_info.html

Some things seems to suggest that 50k would be good be good, but honestly I think I am going with 10k unless someone can tell me why not to do that. It seems like the most suggest thing for a passive preamp and I have very short RCA cables so the load won't be too much.

post #4 of 11
2.5vrms/75ohms output will drive many phones without any amplifier. It will certainly drive virtually any amplifiers input impedance without issue. Not only that, it will drive many phones to unbearable volumes. The stock Crack has 120ohm output impedance, so it is a distinct dis-improvement on your DAC run stand-alone.

I guess all this is complicating the task of composing a suitable reply.

I'd just put a large value series resistor in each DAC channel's input to the Crack, start by trying a 220k and adjust from there. The Crack has about 15dB of gain, the resistor should cost a bit more than 12dB.

You seem to be labouring under the misapprehension that you are somehow running the amplifier harder if the volume control is turned up but the input signal is smaller. This is simply not the case. Controlling the volume at the input is controlling the volume at the input is controlling the volume at the input.

post #5 of 11
Thread Starter 

Thanks for the reply!

I can see how it makes sense that I am not driving the amp harder by turning up the volume, but something is definitely happing when I set the amp at max and add a passive volume controller? Maybe somehow the impedance matches better or I don't know what? I apologize for my ignorance....

Can you come up with any explanation as to why I am hearing an improvement with max signal + preamp -40db vs the Crack alone? It just sounds more crisp, or perhaps it's more accurate to say that without preamp sounds more "dull".

What would I gain by adding a resistor in your opinion? Being able to use a larger portion of the crack's volume pot?

post #6 of 11
You say you've got 40~50dB of attenuation in there, that sounds like a lot to me, so I ignored it, and offered a guesstimate of what I thought would be a good starting point, bearing in mind that I expect you to experiment.

You obviously want to be operating closer to 10 on the output scale rather than 1, so you can achieve this by losing a fair chunk of signal voltage across a series resistor. The DAC O/P impedance is negligible, so a 220k resistor will reduce the voltage seen by the pot to slightly < 1/3 its previous value, leaving the amplifier with an output swing of 5~6dB over 2.5v, say 4.5v. You can obviously adjust to taste. If you're still way down near the bottom of the pot, try a Megohm or more.

post #7 of 11

I can believe that you need 40db if you don't listen loud.


The best way to get 40db of attenuation in your context is with a 10K/100r resistor divider right at the amp rca inputs. To have a bit of margin, I'd use 10K/300r.



You'll then be able to use the pot of the crack nearer to the top of its range.

post #8 of 11

Perhaps a simpler solution: how about just replace the stock pot with a higher quality one?


or how about Bottlehead's own passive preamp... http://bottlehead.com/?product=submissive-volumesource-control-kit

post #9 of 11
Doesn't the output impedance seen by the amp matter?
post #10 of 11

Yes, but assuming you don't have some odd capacitance shenanigans going on at the input, as long as the source output impedance is roughly a tenth (or less) of the amp input impedance it should be fine.

post #11 of 11
Originally Posted by davidsh View Post

Doesn't the output impedance seen by the amp matter?

Yes, but not in this case, when it's only 75ohms. Amplifiers' input impedances are typically in the 1000s of ohms. When the output impedance starts to approach these values, then the output volume will fall. Since the amplifiers input impedance typically does not vary much with frequency, unlike phones, there should be no effect on the overall output quality except for the SNR, and no noticeable effect on that in this case.

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