DIY - Passive Preamp - Help Please

Surely it couldnt be too hard to design and build a passive pre amp. Im thinking input and out put fly lead, attenuater or perhaps three inputs, with sockets, switches and an attenuator - All housed in a RF sheild, well aluminium box. Anyone offer any Advice to a happy solderer?

 

heres a nice pre in kit form, so you can do all the soldering you want!

http://www.hificollective.co.uk/kits/glasshousehome.html

 

I've recently bought this:- http://www.hificollective.co.uk/kits/passive_pictures.html

Sounds fine to me and you say, seems easy enough to build, although I bought it ready built due to lack of time.

Edit: Chris, type slower!!!

 

Ive checked it out but seems a bit expensive for what it is

 

seems pretty damn cheap to me..

but they also sell the stepped attenuator seperately (and plugs and silver wire etc) if you want to buy the parts.

 

components like alps pot £10 Glasshouse selector £18 - seems that there is scope for a cheaper alternative or even
Dal Vishy Stepped Attenuator £45 and Elma Selector £40 add vat and a little ove £100 - add a metal box a few sockets etc and it leaves you to experiment with wires, personally Id use silve or copper solid core.

 

Price looks pretty good or you could try this off fleabay...I've got one and its pretty good ....not great but a easy solution!

Item number: 5869651612

 

I'm sort of thinking a small aluminium box Dal Vichy Stepped attenuator input and out put sockets or fly leads and use it with my CD or output from monitor audio 1010/computer source.

I looked at other designs and considerd an aluminium box sectioned down the centre, front to back with a piece of aluminium. Input from left side to selector then output through small holes in centre partition to attenuator then output led down right hand side to sockets on the rear. Internal wiring solid core copper.

 

Hi Dreftar

There is a BIG difference between the stepped attenuator they sell and an Alps blue pot.

I know, I swapped between the two!

If you economise on the other components (like cheaper RCA sockets, not using silver wire or an aluminium case etc) I would try to keep the stepped attenuator in the loop - it loses so little of the signal which is vital in a passive. Carbon tracks are pants in comparison, trust me!

 

Dev, any chance you could expand on that at all (or have you even posted a review somewhere I haven't spotted?)? I'm slightly tempted to investigate one of these...

 

Chris has already done a review somewhere. I think he compared it to a TX pre and his own Basie. I don't do reviews, er I mean I can't because I changed my power amp at the same time so don't have a reference.

I am hoping to organise a bake off soon and ask participants to bring along pres (passive and/or active) so that we can have a proper comparison.

 

Hi Martin

I checked out your equipment list, are you intending to use it with an 8000P, or have you got more kit changes in mind?

I think it sounds fantastic, and for a couple of hundred don't think you can do much better..

.. having said that, not every power amp is happy with a passive pre, depending on the level of input they are expecting to see.

..could you get the opportunity to borrow a passive from somebody to try?

 

Hi Chris,

Fairly idle musing from me really, but I have always thought passive pres make a lot of sense and that Glasshouse kit looks reasonable value for something that by the looks of it finishes up looking pretty good as well as having decent components? I'm warming to the idea of making one as I type this, which is silly as I really have no idea if a passive is a good idea in my system! If one of these did though work for me I could sell my Densen and free up some cash . A better CD player is what I'd really like...

Right now I am only actually using one source, with everything going through my DAC, so dreftar's idea of pot with one input and one output is an option, but may be restrictive in the long run.

One other thing is that my 8000P is fairly high gain so I'm using the very bottom end of my current pre most of the time, whereas I guess by picking an appropriate pot for the kit I could get a better match.

Martin

 

for Martin C....
You would be better adding attenuation before the pre ...theres a very good technical reason that escapes me at present ! this can be done with a couple of res in each ic they need to be the very best you can source, a minimum of dale vishay's.

As to the 8000p I don't think a passive will really work well with this......
I've owned one along with its matching Q pre, Its not very forgiving and is ruthlessly revealing to the point of mugging you [me] each time I listened ! I think it will sound horribly flat and shreeky. I have nothing to back this up ...but it was certainly an amp that I could not live with. Hope this doesn't put you off but just give you something to think about.

 

dreftar - before I forget, Lilolee on this forum has a passive pre that is essentially a potentiometer in a tin can, with with either some sockets or possibly flying lead connections. Might be worth a pm as he doesn't seem to have spotted this thread yet.

Fair point; I was sort of thinking of this as a possible secondary gain, but I'll take your word for it it's best to attenuate the input. I guess it follows that it is better to have the attenuation before the pre, rather than between the pre and power then? I might give this a shot with my headphone amp where there is a real issue due to my sensitive 32 Ohm 'phones. Any chance you could say a little more about 'good' resistors makes to use? I've seen lots of threads talking about caps but not resistors...

Thanks for the comments, much appreciated. I have a vague recollection that you might have heard an 8000P that was not up to it's best? Although the Audiolab pre's are very bright (the 8000C is awful, the Q better) the 8000P really doesn't share that trait in my experience. Well, compared to say something from Quad it will sound thin, but not compared to more typical solid state fare. All that said, when I switched to my Densen part of the reason was the smoother top end than the 8000Q, so you could well be onto something... I won't be gambling on one of these kits for now. I will try to get hold of a passive pre to try at some point, or might out of curiosity make a rough and ready one with a cheaper pot and a single pair of inputs and outputs. I'll see...

 

It was the outside that was in poor condition ...actually covered in grease and dirt and came up well wih a little effort.

That sounds an excellent route......just an alps pot will give you a feel .....if you like where its taking you you can then try the stepped atten. route.

 

How about this to boldly go into the passive zone?
Maplin Rotary Switch = £1.49
Maplin phone sockets 8 @ £2.13 = £17.04
Maplin Aluminium CAse £4.99
Alps Blue pot log 50k £10
coulple of knobs, hook up wire of choice etc £10

Total £43.52

or with Dale Vishat stepped attenuator + £35 (45) plus vat etc etc

should be able to have a try for under £100 and make better for another £50 or so.

Regarding manufactures profits - they do make on components as well as kits - usually the cost of a complete item is less than the sum of its parts

This proposal could be done in steps ie the stepped/ attenuator or pot only with fly leads - If it sounds ok etc then add on the rest bit by bit.

Anyone wanting to have a go?