[pinkfloyd4ever]

What about the optivol? Same principle as the lightspeed for about $30, and I've seen nothing but great reviews about it

 

[luvdunhill]

Quote:

Originally Posted by amb /img/forum/go_quote.gif Yeah, 750 ohms is not too high, but is it 750 ohms in any attenuator position? As far as I know the JT is completely passive (relay switches in/out a bunch of resistors to make a variable voltage divider), in order to have an output Z of 750 ohms or less in all positions, the resistors will have to have very low values, which means it could be a difficult load to drive for the preceding stage. I couldn't find a schematic on twistedpearaudio.com website's JT page, so I couldn't verify.

yes, pretty close to a constant 750 ohm impedance. Input impedance ranges from approximately 2.2K to 10K, but this should be fine in Al's case

 

[00940]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif oh, and for the person that recommended the DS1666, look at the DS1802.. it's used in some commercial gear that I think highly of, so it should be worth investigating more.

The DS1802 has some problems compared to the ds1666:
- single supply, which means you have to be creative not to have coupling caps ( Pedja Rogic Audio Pages - DS1802 Silicon Potentiometer )

- 5V supply means that it will not properly accept the standard 2Vrms of a cd player output.

But it is easier to implement (no need for a pic)

 

[amb]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif yes, pretty close to a constant 750 ohm impedance. Input impedance ranges from approximately 2.2K to 10K, but this should be fine in Al's case

OK. 2.2K is quite low, users of the Joshua Tree should be mindful of source compatibility. A usual rule-of-thumb is that the input impedance of the amp (which is usually dominated by the volume pot/attenuator) should be at least 10x higher than the output impedance of the source. This means that the source should have an output Z of around 200 ohms or less. Many sources do not meet this criterion.

 

[error401]

Regarding PGA23xx, I have 3 spare boards around if you're looking for them. The design is here.

Beware that these are designed to be driven from a low impedance output, so you may need to add a buffer for good performance. I'm decidedly mid-fi, in it for the fun of it, but I'm quite satisfied with the performance. They sound very good for the money, and are more linear and have better channel matching than any pot you'll ever find. Downside is you're adding an opamp stage, complexity and possibly a power supply if you can't poach power from the amp.

 

[luvdunhill]

Quote:

Originally Posted by 00940 /img/forum/go_quote.gif The DS1802 has some problems compared to the ds1666: - single supply, which means you have to be creative not to have coupling caps ( Pedja Rogic Audio Pages - DS1802 Silicon Potentiometer ) - 5V supply means that it will not properly accept the standard 2Vrms of a cd player output. But it is easier to implement (no need for a pic)

well, I have no doubts that the designer that used it understood and took adequate measures to address the above.

 

[j4cbo]

On a related note, what's a (preferably reasonably-priced) nice feeling single-channel pot to use for controlling some other attenuator?