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Old 3rd April 2005, 06:12 AM   #1
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Default passive preamp (autoformer)

Hi all,

where to put this subject ? Just finished my preamp......but it is out of most catagories.

So i put it between the amplifiers (although it amplifies nothing, it only makes the signal weaker). And why between tubes: because the rest of the system is all tubes. Passive's do seem to have a higher attraction to tube-addicted people anyway.....

The result is a minimalistic:
- 4 x balanced in;
- 2 x RCA in;
- switchable balanced (to power amplifier) or RCA (to headphone amplifier) out;
- ground lift.

Very open and detailed. Before this i had a active preamp which was good.....but not so "natural" as this passive one.

Regards,
Reinout
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Old 3rd April 2005, 06:27 AM   #2
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Default inductive switches

Hi again,

in the following post i'll give some explanation + pictures of the passive preamp.

Here's a picture of the switch itself:
- this one is formally stereo. I use 2 of them for i wanted to attenuate the balanced signals properly;
- main ingredienst are permally cores, an ELMA 24-step switch and lots of time winding the coils !;
- nicely potted. It's definitely high quality. Nothing wobbly on the items;

Remember, these are coils and not transformers.
Sowter uses the transformer style (so with primary and secondary coil). These Tribute's are "only" coils with only the primary.

Brand: Tribute. This brand is into anything what has to do with transformers/coils/tubes/audio. Normally he makes only stuff for private persons, but once in a while you see Tribute-items in other brands as well. For instance in those weird and out-of-this-world Electronluv stuff.
I think i must be lucky to live in such a small country where you can buy these items. The owner/winder of these goodies even brought them to my house in person !

I saw complaints from fellow DIY-builders with delivery-speed of Tribute. It's a one-person business, and even not his primary job ! For him it's a hobby and the goal is high quality. Give the man a break and let him do his stuff. Rest assured: your order will come through and will be of the highest standard.
Sorry....not affiliated with this guy.....but i'm simply afraid the audio community will lose some of it's treasures.

Reinout
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Old 3rd April 2005, 06:30 AM   #3
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Default lay out preamp

The lay out of the preamp was simple indeed:
- volume knobs at the front;
- at the topplate the selector switch;
- the switches XLR/RCA out at the end;
- fill the rest with the connectors;

Topplate and bottomplate are 2 mm stainless steel. The housing itself is 15 mm aluminium. I use this style throughout my whole set.

The eye is listeling as well !

Reinout
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Old 3rd April 2005, 06:33 AM   #4
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Default mechanically ready

all bolts and nuts tightened....let's start soldering !
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Old 3rd April 2005, 06:36 AM   #5
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Default simple soldering

The inner world of the preamp is simple enough: no powerlines or other items that influence each other......

So from the connectors...to the selector switch....to the inductive switches...to the output switch...out.

Reinout
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Old 3rd April 2005, 06:38 AM   #6
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Default We have lift of !

Hi,

the last picture of this serie: the end result working.
Very pleased soundwise and optically. A clean result.

The heart of this project are of course the inductive switches....and they work !

And now some music,

Reinout
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Old 3rd April 2005, 06:56 AM   #7
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Hi Reinout


Everything you make seems of the highest construction quality. Very impressive indeed.

As i need another attenuator (already have tx102) could you let us know how to order Tribute and what is the price. Any technical info? Inductance?
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Old 3rd April 2005, 07:36 AM   #8
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Default tribute adres

Good morning analog_sa,

thanks for the compliment.....and how about your DIY-projects ?

Tribute: website (a bit old...but there is a contact email-adres):
http://www.tribute-audio.nl

My poweramps have a 10K input transformer...and working woth tapped transformers as a preamp is not very nice doing that with those impendances. You're bound to lose high notes.
That's why i'm using autoformers....and it works very well without the rol-off.

Another issue with my set up: i can squeeze 5V from my CD-player....and the switches must be able to withstand that.
Again: no problem.

Sorry...never measured the inductance. But i'm sure Pieter (of Tribute) does have that figure available.

Another builder with these goodies measured the frequency throughput:
- full open (no attenuation): 2 Hz - 2 MHz
- half open (halfway): 5 Hz - 105 KHz

And no popping noises during switching !

Pricewise: i bought them last year while still almost prototype. Don't know the current productionprice. I believe it spend 275 for 2 switches....but please don't hold me to that number.
Positive: you do not need to buy a quality switch when ordering these attenuators....the ELMA-switch is part of the complete package.

Reinout
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Old 3rd April 2005, 08:34 AM   #9
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Default more info

Hi analog_sa,

the other builder (at www.dhtrob.com but in dutch) also measured resistance and inductance on the switches:
- 35 ohm resistance;
- 180 He inductance (at customer specification).

As you already use the T102 you know the pro's of an autoformer:
- at high attenuation ( = low volume) still enough inductance to have correct low throughput;
- source sees constant inductance AND low resistance.....easy load.

Hope this answers your questions ?

Reinout
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Old 3rd April 2005, 09:19 AM   #10
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Quote:
Hope this answers your questions ?
Yes, thank you very much.
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