Custom Passive Pre Amp

Custom Passive Pre Amp

The Passive pre amp that I chose to build is a very simple 3 input design with 1 output and no tape loops. Each of the left and right channel inputs are connected to a very high quality 2 Way 6 position Elma selector switch (3 are redundant) with gold plated contacts. The selected input is then fed to the Dale Vishay stepped attenuator which is connected to the output phono sockets.

I have used a brushed aluminium case to house the pre-amp which measures 100 x 60 x 130 mm (WxHxD) which was bought from Maplin Electronics for about £3. The custom built pre-amp uses 8 teflon insluated gold plated phono sockets which cost £2.20 each. For the internal wiring I used sharkwire pure silver cable interconnect cable which is available from Maplins as well priced £26 metre which is very expensive but 1 metre should be enough for this design. The Elma switch and the Vale Dishay Stepped Attenuator were bought from Hi-Fi Collective priced £40 and £45 plus VAT respectively.

Silver solder was used to solder the wires which are quite awkward given their size. I could have used another type as Hi Fi collective can also supply some HGC pure silver wire which is a lot easier to work with but I used the Sharkwire because I had some spare and it made sense to use it as my interconnect cable was made from the same cable.

Once everything was soldered in place, a multi meter was then used to check the resistance between the input channel and output channel which for a 10K stepped attenuator like the Dale Vishay I used should measure about 2.6Kohms, and the resistance between the ground and the input should be zero. Once I was happy that the wiring was correct, the lid was put on the amp and the pre amp was connected to my system consisting of a Quad 67 CD player, Quad 11L speakers and Quad 405-2 power amp.

Even though the Vishay Dale resistors need about 100 hours burn to reach their sonic optimum it was evident straight away that this pre-amp had lifted a veil off the sound that the previous Alps "Blue" based pre amp had masked. The sound was so much smoother and the treble was much brighter and detailed without ever been harsh - it was an incredible and very rewarding experience to hear such detail from the Quad equipment and well worth the outlay for a superior and transparent pre-amp. With the new pre amp in place every note in the mix just seemed to last longer with the decay in the studio clearly audible and instruments taking on a more natural coherent soundstage with a top end that I almost thought impossible from a solid state amp.

I could write more but the only way to do a pre amp like this justice is to listen to or build one yourself. More so than ever now I am convinced a passive pre amp is better solution sonically than a well designed active one.

This entry was posted on Saturday January 6th, 2007 at 11:34 AM and is filed under Amplifiers. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response.

16 Responses to Custom Passive Pre Amp

Renze dijkema Says:
January 19th, 2007 at 8:53 PM

Hello,

Nice article about the vishay-stepped attenuator. It is nice, because now I am running a pair of Quad ESL-57 with a 303 with a build-in Alps Blue as a passive pre-amp. I am working now on a Quad Dual Mono 303 with "your" Vishay stepped Attenuator. The sound I am having now with the blue Alps is a bit harsh in the treble region and after reading your article, it could be bettered by the use of the stepped attenuator? And what amplifier and/or Quad ESL do you use? Hope to hear of you,

Regards,

Renze Dijkema

Hi-fi insight Says:
January 22nd, 2007 at 10:35 AM

Renze,

I am using a Net Audio modified Quad 405-2 with all available mods PSU and Burr Brown OPA627AP op amps etc which feed Quad 11L's and Partington Dreadnoughts. Source is a Quad 67 CD.

Sound is very neutral and the top end is incredibly clean. I have modified the Quad 11L as featured in a previous post.

Be warned the stepped attenuators used above need at least 100 hours to burn in properly before the sound fully opens up.

Paul steel Says:
October 17th, 2007 at 1:38 PM

hi the guy from net audio is a ted loon he is paranoid
i wanted to buy the 405 mod in the end i gave up is he crazy

Hi-fi insight Says:
October 17th, 2007 at 1:52 PM

Paul,

I always found David at Net Audio to be very helpful and extremely pleasant to do business with. He used sell the 405 amp board mods in kit form but stopped that and now only offers an upgrade service, however you can still get the DMSU kit directly.

Either way both are well worth every penny to Quad 405 owners

Paul steel Says:
October 17th, 2007 at 2:25 PM

hi thanks for the info
as for david p i speak as i find thanks again
paul

Andy Says:
May 27th, 2008 at 10:38 PM

A possibly stupid question but why did you use the 10K Stepped Attenuator ?

I am looking into making my own passive preamp to go between my cd, phono stage, pc and my rega mono blocks.

What difference would a 50K or a 100K Stepped Attenuator make to the sound?

thanks

Andy

Hi-fi insight Says:
May 28th, 2008 at 2:36 PM

I chose the 10K because that was the value of the Alps pot in the previous passive pre-amplifier I used.

I am not an expert on pre-amp power amp matching but I do know the value of the impedance can effect treble roll off.

Net audio 405-3 initial review Says:
May 30th, 2008 at 12:30 PM

[...] Review System Quad 99 CD Player Net Audio 405-3 Custom Built Passive Pre-Amp Quad 11L speakers on Partington Dreadnought stands Interconnects: Custom built solid silver and [...]

Martin Says:
November 23rd, 2008 at 9:06 AM

Can you send any more photographs? What did you use to shield your wires?

Gabor Says:
November 25th, 2008 at 8:03 AM

Hello Guys,

once again,here is a topic that worth some words.
Ladder attenuators.
The real advantage of this design is there are only 2 resistors in the signal path at every position. Hence the superiority to DACT-like series (stepped) ones where the signal passes through all the resistors all the time. (The more components you use, the more distortion you may expect) But, there is a drawback either. Some types of switches break the contact before make the new one so causing a little moment having no control at the input of the amp. Most of amps - thanks God - not too sensitive on this effect but I have seen ones that were. So take care with custom made ladders. ELMA switches give good results - ok, a touch expensive. As every precision equipment.
Dale RN60.
This resistor is a best buy, I also use them within my own designs (recently LZ-127 Mk5). I would hesitate to say they were absolute neutral, they not. There is a slight brightness and a hint of grain but compared to other ones generally used in high-class audio equipments ( such as Roedersteins or Beyschlag) the difference is obvious. Even more so within an amplifier where higher currents go through making the resistor's TC factor the first point to consider.
The overall result of any preamp depends on quite a couple of circumstances ie. interconnect cable lenght and capacity, sonics of the other parts of system and even - active - power supplies.
Luckily, a passive 'preamp' (better to say: controller) is an easy project and has low quantity of components so gives a good chance of fine results. Choosing the right impedance (ie.10K, 47K or 100K) is essential especially if it's been driven by valve phono stage where the coupling capacitor and the following attenuator makes a high-pass filter. If you experience a kind of rolloff in bass that means you use wrong values. 10K is right with CD players (where the output coupling caps are in the 22-100mF range) but 47 or 100K is a must with valve stages that usually uses 1-4,7mF's.
Finally, if you find your RN60's sound too bright or lean partnered with your equipments I would suggest to use Allen-Bradley's for a more warm, easy-going sound with rich harmonics and similar clarity and detail.
Regards,

Gabor, Zeppelin Audio Hungary

Andrew Says:
January 14th, 2009 at 5:38 PM

I too was inspired by the Hi Fi Insight article to give it a try - so glad I did - fun and educational too.
My first attempt worked but was all over the work bench and looked like spaghetti. But....it sounded strangely very good. This was 100 k cheapo carbon pot from maplins. More curious - so I threw some money at it and have in my system full time a unit with a 50 K alps black velvet ( stepped 24 ) pure silver wire with teflon, silver solder and only three inches of signal path from ' in ' to ' out '.
Cast aluminium case with copper sheet lining and bitumen pads to add a bit of weight.
Only 1 metre of interconnect cable ( plaited CAT6 network wire ! ) from CD to Power amp - short !
It's truly fabulous and would advise anyone with no more than £ 40 - 50 spare to have a go.
I have read articles where people say passives are gutless, lack this and that but I've listened and done A / B tests back and forth and I simply don't
recognise what they are saying.
Once all of my interconnects were shortened to half metre lengths things improved a little but as soon as I hooked it up was tremendous in every department. I don't like the Hi Fi language so I won't try - it's just.... RIGHT .
Do it - you'll love it and it's almost free !!

Binu paul Says:
January 19th, 2009 at 11:04 AM

Hello,

Can you send me some details and pictures so that I too can try out something like that ?

Best regards,
Bins

Scott Says:
May 4th, 2009 at 4:29 PM

Hi,
I would like the details to your pre amp as well can you email them to me? My email is scott2@shaw.ca

Jake Says:
September 26th, 2009 at 10:40 AM

Hello

I notice that other posters have asked if there is a detailed build plan available. If there is I'd be really interested and quite happy to cover any cost involved. As an aviad passive preamp fan (rothwell at the mo) I'd love to try and make mt own.

Many thanks, Jake

Hi-fi insight Says:
September 26th, 2009 at 8:36 PM

The are no detailed plans as such as it it only took two hours to build one night.

Basically all the grounds on the input and output all soldered together (no fancy star earthing). The three pairs of signal inputs are soldered to the elna switch inputs and then the L/R outputs are soldered from the elna switch to the stepped attenuator L/R inputs and then the L/R outputs from the attenuator are soldered the output phono sockets and that is it a simple design but wonderful sound.

Poul Says:
January 23rd, 2012 at 11:36 AM

Hi. I want to make a comment to this old thread since it still will be read.

I agree with this being an incredibly good attenuator. However if you have an old Blue Alps just clean it with IE (Teslanon kontakt and tuner spray)

You won't belive how much difference it makes. Even fairly new equipment gets a courtain lifted off the sound.

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