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My passive preamp project – DIY vs Deep pocket

It has been a while since I last took out my soldering iron.

In the last couple of years I have been listening to a Marantz PM8003 integrated Amp. But recently I was attracted to the idea of a passive preamp. (Just a volume control)

From a purist’s perspective, a passive preamp is better because less is more. Removing the active pre stage allows the signal to go directly to the power amp, which means less distortion and colouration.

Before investing more money on a DACT stepped attenuator, TVC and other passive solutions, I decided to test the above hypothesis on my system. So, $50 and 3 hours of my weekend later, I created a simple ALPS pot housing in an aluminium case. (See photos) Nothing much to write home about, simply an RCA socket with decent cable, and one set of input with one set of output. The size of the enclosure is slightly larger, which leaves some room to upgrade to aTVC or DACT attenuator ☺

Evaluation:

The most striking improvement from this passive pre is the improved sound stage and transparency. The sound stage was no longer confined to my two speakers and the back wall, it was much wider and deeper; well exceeding my expectations.

The separation also improved noticeably. What used to be a group of musicians, can now easily be defined as an individual player, or rows of players from back to front. It made the whole experience more life like, with the same effect being attained from a well recorded musical. I could tell that each of the performers were moving on the stage as if they were right there.

Compared to the active pre amp it was very transparent There was better coherency which was probably due to less phase shift distortion. This quality made the playback of large scale music so much more enjoyable.

Not all good news

This passive pre is not without short comings. Lower frequency extension is not as good as the active, which is expected.

Not having a remote can be a pain for some people. My DIY pre certainly has no remote control, but it doesn’t concern me.

My passive pre only has one set of inputs. The additional input could be included by adding an input selection knob switch and a larger enclosure.

Due to the lack of active pre stage, I needed to turn up the volume by 20% to obtain the same SPL.

Is it worth it?

You bet! 3 hours and $50 is all it took for a result that would surely have cost me much more if I was to have bought a branded passive.

I may also look into a TVC passive which is suppose to perform better than my cheap ALPS pot.

Please feel free to share your passive pre experience and any suggestions you may have to improve my project ☺

Photos:
http://i37.tinypic.com/250nfp4.jpg
http://i33.tinypic.com/11r7tw0.jpg
[http://i34.tinypic.com/10cnuaf.jpg
[http://i35.tinypic.com/eiqed1.jpg
http://i36.tinypic.com/2j0xoht.jpg
[http://i37.tinypic.com/m8lfyg.jpg
http://i33.tinypic.com/16hr9md.jpg
chatta
chatta OP
314 posts
 
Hi Ngjockey,

Thank you for all the great info! I am aware of the MF buffer years ago but never read into the subject then.

Based on your suggestion it seems like my pot is connected in series. Is that right?

Someone suggested a shunt type but it has that very annoying sound leakage at lower levels if you know what I mean.

I understand that a pure passive is hard to match to a lot of systems. I read that speaker sensitivity also plays an important role in the whole issue. And that is where the buffer stage comes in. According to my googling, pro-audio use buffers extensively to match different stages and resolve long cable run issues. An audio buffer stage is supposed to provide little to no gain and hence minimal colouration.

My thinking at the moment is to add a unity gain buffer stage behind my pot or stepped attenuator. Which basically turns my project into a low gain pre amp In particular I am looking at this Australian made audio buffer from Burson Audio.

http://www.bursonaudio.com/burson_buffer_160.htm

What do you think of that idea. Also what is the sonic different between a stepped attenuator to a pot?
ngjockey
1,042 posts
 
http://diyaudio.co.kr/wwwboard1/data/board1/compare.pdf

The "series" I was referring to is not the same as the link but not worth getting into for your purpose.

Usually, buffers are placed before passive, as demonstrated on the Burson website. Although, with a 50k pot, you might be right.

Best way to answer last question: Yes. Once you've tried an attenuator, you'll never want a pot again.
ngjockey
1,042 posts
 
Should also point out that many sources double Vout when using XLR outputs and Zin can double as well with balanced inputs. Can be impractical for additional parts required, except with TVC.
vetnik
23 posts
 
pots or stepped are fine but passive with transformers are much better.
no problems with impedance matching, more body, better soundstage.
tvc is your next step.
then music first audio.
and finnaly ypsilon electronics passive/active preamp.

Nikos
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