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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

Showing 4 responses by ngjockey

ngjockey
1,042 posts
 
Built a TVC using S&B 102's, when they were still available, and before the Promitheus made DIY seem cost ineffective. Was a lot of soldering but not nearly as bad as fitting 46 resistors on a 23 step attenuator. Didn't compare the attenuator/TVC to attenuator/resistors because the latter was for an active preamp upgrade much earlier. Considered a pair of IAG Audio's pre-assembled attenuator/Sowter TVC as an easier alternative. TVC's are indifferent, whether you use balanced (XLR) or single-ended (RCA), which is not the case for AVC's or RVC's.

Still have the TVC but the whole system is optimized for that, with high Vout/ low Zout source with balanced IC's and high gain amps.

The DACT is series, as opposed to ladder, and has more impedance variation than a TVC, but is always additive.

Might want to look at Goldpoint and Marchand attenuators.
ngjockey
1,042 posts
 
>>and impedance variation seems to be an important issue. What is that? I use a 50K pot in my project. and that Pass DIY B1 buffer what is that? <<

Pots and ladder attenuators present a constant input impedance (Zin), varying by frequency (duh). Series attenuators will never present less than device following it. I've preferred series attenuators for specific "tuning" applications but haven't used one as a master volume control, yet. TVC's and AVC's... I'll just direct you to http://www.stevens-billington.co.uk/page102.htm, which was well documented, for an example.

Lowest possible Zout and highest possible Zin would be the theoretical ideal but reality is never ideal. Many other factors. Tubes tend to have a higher Zout but not usually an issue. When there is an impedance mismatch, both low and high frequency gets rolled off. As I type this, it occurs that capacitance and inductance becomes the real issue. If you've looked at a passive xover, you'll follow my logic.

Choosing a Zin value for a pre-assembled attenuator, which is a relatively recent option, is system dependant.

A buffer is an active stage that has little or no gain. Primarily used for impedance benefits. Placette Active is another example. Musical Fidelity and Space Tech make tube buffers.

One difference between RVC's an TVC's is that the former attenuates to heat and the latter converts to amperage. What that means, I have no idea. When I built my TVC, knowing (or thinking I knew) the difficulties with xformers, I didn't expect it to work well. The chief designer for a respected audio brand (my brother) sat in my living room and told me they wouldn't work and why until I pointed out that he was praising one minutes earlier. Even John Chapman of Bent Audio told me he had to be convinced at first. 'Chatta', as you have learned, any passive might not work in every system.
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.
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.
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