Building a passive preamp/attenuator

[brianpowers27]

I would like to build a passive preamp/attenuator.

-- Input
-----RCA
-- Output
-----XLR (Balanced)
-----RCA

--would a simple unbalanced design be as simple as connecting 2 rca jacks to a pot and then to the outputs in series?

--I looked through the PE selection of Pots and didn't find anything that jumped out at me. Should I be looking for a rating of 2 watts?

[Æ]

Re: Building a passive preamp/attenuator

I would like to build a passive preamp/attenuator.

-- Input
-----RCA
-- Output
-----XLR (Balanced)
-----RCA

--would a simple unbalanced design be as simple as connecting 2 rca jacks to a pot and then to the outputs in series?

--I looked through the PE selection of Pots and didn't find anything that jumped out at me. Should I be looking for a rating of 2 watts?

This isn't what you are looking for but your inquiry made me think of it.
I've actually wanted to build a passive preamp myself. I've collected some different articles and some parts/circuitry in that direction.

It's a good read, even though it's a bit corny and rather long.
Schematics and technical info is included.

http://www.stereophile.com/solidpreamps/54/

[philiparcario]

Re: Building a passive preamp/attenuator

I would like to build a passive preamp/attenuator.

-- Input
-----RCA
-- Output
-----XLR (Balanced)
-----RCA

--would a simple unbalanced design be as simple as connecting 2 rca jacks to a pot and then to the outputs in series?

--I looked through the PE selection of Pots and didn't find anything that jumped out at me. Should I be looking for a rating of 2 watts?

I recommend these pots from apex. these are quite good for the money.

http://www.apexjr.com/Bourns.htm

the 100k will work they are mono 1 for each channel

[philiparcario]

Re: Building a passive preamp/attenuator

read this link

http://sound.westhost.com/pots.htm

any of these rcas will do depends on what you want to spend

http://www.parts-express.com/webpage.cfm?webpage_id=3&CAT_ID=39&ObjectGroup_ID=124

I truly will say if you are going diy make them mono. you will get the right tracking. most stereo pots have tracking issues in volume. see this link for a high end diy 5 channel mono preamp that I am selling on ebay
If you built this diy it has about 150 in parts

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=160289769396&ssPageName=STRK:MESE:IT&ih=006

[brianpowers27]

Re: Building a passive preamp/attenuator

AE -- Thanks for the article about buffered "passive" preamps. How much does the resistance change from the pot really affect the sound?

Can anyone remark on the quality of a purely passive device without buffers?

[philiparcario]

Re: Building a passive preamp/attenuator

yes I have used a few from this builder

http://www.richardleeaudio.com/

very nice clean sound and I built a few using. alps blue velvet mono pots. nice sound I kept short connections. I have used low cost stereo ones from ebay not bad for 10 or 15 dollars.

http://cgi.ebay.com/SCOSCHE-RLC-EFX-CAR-AMP-REMOTE-LEVEL-CONTROL-BASS-KNOB_W0QQitemZ290275229621QQcmdZViewItemQQptZCar_Electronics_Parts_Accessories?hash=item290275229621&_trksid=p3286.c0.m14&_trkparms=66%3A4%7C65%3A3%7C39%3A1%7C240%3A1318

the ebay ones are good for fullrange they track pretty well.

If you want to build a good one I have some nice mono pots from bourns they cost 10 plus shipping I would sell them for 6 with free shipping

http://www.partsconnexion.com/catalog/Controlsswitches.html

click on potentiometer at the bottom and scroll to the bourns monos 91k series I can sell you a pair for 12 with shipping included sealed. I have more. pm me it you want some.

[Fred T]

Re: Building a passive preamp/attenuator

I built this passive using an Alps pot and RCA's purchased from Partsconnexion.I used 10k as I think it gives the best comprimise between input and output impedance.

[philiparcario]

Re: Building a passive preamp/attenuator

i Built This Passive Using An Alps Pot And Rca's Purchased From Partsconnexion.i Used 10k As I Think It Gives The Best Comprimise Between Input And Output Impedance.

Nice Look!

[Fred T]

Re: Building a passive preamp/attenuator

Nice Look!

Thanks.
The little Hammond cases do look good.

[rgrickerson]

Re: Building a passive preamp/attenuator

AE -- Thanks for the article about buffered "passive" preamps. How much does the resistance change from the pot really affect the sound?

Can anyone remark on the quality of a purely passive device without buffers?

If you use reasonably good components the effect on the audio will be that there is no effect.
Just good clean transfer of whatever is at the input. Most of the perceived differences we hear in cables, resistors, tube amps etc. is mor emotional than real.

[Æ]

Re: Building a passive preamp/attenuator

If you use reasonably good components the effect on the audio will be that there is no effect.
Just good clean transfer of whatever is at the input. Most of the perceived differences we hear in cables, resistors, tube amps etc. is mor emotional than real.

More to it than just that. You have to watch the input and output impedances, the way it transfers signal from one component to the other. Maybe I can dig up an article or two so you'll understand it better. Here is a bit of reading you might find interesting.

http://www.stereophile.com/content/aunt-coreys-homemade-buffered-passive-preamplifier-page-2

[AJ]

Re: Building a passive preamp/attenuator

I would like to build a passive preamp/attenuator.

-- Input
-----RCA
-- Output
-----XLR (Balanced)
-----RCA

--would a simple unbalanced design be as simple as connecting 2 rca jacks to a pot and then to the outputs in series?

--I looked through the PE selection of Pots and didn't find anything that jumped out at me. Should I be looking for a rating of 2 watts?

It is basically that simple, but you have to watch out for a few pitfalls. Your amp must have an input impedance a magnitude higher than the value of the pot, but the pot value can't be so low as to load down the source. 10K is usually a good compromise. What amp and source are you using?

A better solution is a buffered volume control which would have an opamp one either side of the pot, giving the source a steady, easy, load and the output would have a low impedance to drive whatever amp.

To get a balanced XLR output you need something to create the balanced signal. This can be a couple opamps or a transformer.

[AJ]

Re: Building a passive preamp/attenuator

...just realized how old this thread is...