[rumatt]

I am thinking of building a very simple passive preamp. I want:

- A high-quality potentiometer or attenuator
- A high-quality toggle switch to choose between two inputs
- An input that bypasses the toggle in case I want to plug in direclty

That's it, so it should be very simple, but I'm pretty confused about the whole idea of a shunt resistor, and how to wire it.

1) If I buy a high quality attenuator (DACT maybe?) do I still need to use a shunt resistor? Chu Moy's writeup says you do not, but I don't know what they mean by "high quality". The Luminous Audio Axiom uses a Noble pot and a shunt resistor.

2) I don't understand the wiring diagram in Chu Moy's writeup. He has three wires connecting to the pot? I thougth the pot and resister are in series with each other, and together connect the lead and the ground in parallel with the preamp output.

3) Any ideas where to buy a high-quality toggle switch? I'd love to find a single site that sells all the stuff I need (Switch, attenuator/pot, some good wire, and a good resistor).

Thanks!

EDIT: Another option would be to just build a switch box and use two of these inline attenuators on the inputs to the amp.

 

[PeterR]

Quote:

Originally Posted by rumatt 1) If I buy a high quality attenuator (DACT maybe?) do I still need to use a shunt resistor? Chu Moy's writeup says you do not, but I don't know what they mean by "high quality".

Chu wrote "a high quality audio taper pot is the best solution", with the emphasis on "audio taper". The shunt resistor was used in this case to modify the attenuation characteristic of a linear pot to be more suitable for audio use. If you use a pot with a logarithmic curve, this is not necessary.
Quote:

The Luminous Audio Axiom uses a Noble pot and a shunt resistor.

Different thing. This one uses one fixed resistor as series element and the pot as variable resistor to ground.

Quote:

2) I don't understand the wiring diagram in Chu Moy's writeup. He has three wires connecting to the pot? I thougth the pot and resister are in series with each other, and together connect the lead and the ground in parallel with the preamp output.

Not quite sure what you're talking about, but anyway, yes, there are three connections with a pot - input, output (th wiper) and ground. A shunt resistor goes from output to ground.

 

[rumatt]

Thanks for the reply Peter. I decided that I officially don't know what the hell I'm doing and instead of DIY I ordered a pair of the inline attenuators mentioned at the end of the first post.

From what I've read, the cable after the attenuator should be as short as possible, so sticking these attenuators on the back of the amp could in theory be better anyway. At $80 total, I figure it's worth a shot.

 

[randytsuch]

Check out the schematic here,
http://www.audioasylum.com/audio/twe...ges/57604.html

I made a passive like this, with a alpha pot and a 10K resistor. Sounded pretty good, it replaced a tube preamp, but was later replaced by a TVC.

The nice part is the signal does not have to go through the pot, just through one resistor, so you can get a couple of high quality resistors. Since you only need two, doesn't cost too much.

percyaudio is a good place to find diy parts, btw. Also partconnexion, who is a sponser.

Randy

 

[Tomo]

Hi,

I also built a similar Shunt-Type Stepped Attenuator. I found it fine, but impedance across the attenuator changes with pot position. So you have to be careful which amp you should use.

Eventually, I went back to Series-Type Stepped Attenuator which has fixed input impedance.

Tomo

P.S. Now I use polymer potentiometers from ALPS and Noble. (Zippo difference if you ask me.) Too bad I still have 4 more 2-by-24 position rotary switches in my box. ... I am keeping it till I am SUPER-RICH.

 

[sunfowerggs]

Many thanks to ur post. I love it.

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