What to measure in passive pre-amp

[amirm]

I have a Schiit Sys passive pre-amp on loan: http://www.schiit.com/products/sys

What are suggestions for testing it?

I can think of channel separation, frequency response and channel matching as levels are reduced. Any other ideas?

 

[Blumlein 88]

Input and output impedance. You also might need to measure this at a few volume settings as it will change different ways depending upon the design of the attenuator circuit. Check full volume, -6 -12 and -18 db at least I would think on the impedance.

Low level channel matching is a biggie with these, but you already have that listed.

 

[Wombat]

How does a passive pre-amplifier amplify? Aren't they just impedance matching attenuators unless they use a step up transformer?

 

[Sylafari]

Can confirm low level channel matching issues when used with bookshelf speakers. If I remember correctly it was only the left channel worked and the right channel was inaudible.

 

[stunta]

How does a passive pre-amplifier amplify? Aren't they just impedance matching attenuators unless they use a step up transformer?

I think it's just a misnomer. Generally it's a passive volume control with one or more inputs. No gain.

 

[amirm]

There is nothing but a switch and potentiometer on this:

 

[RayDunzl]

I used nothing more than a pair of 10k pots mounted on some curtain hanger hardware screwed to a wood "coaster" between the CD player and the power amp for many years.

And a pair of Radio Shack knobs.

Not because it was better, but because it worked and it was all I needed.

No measurements, but nothing bad noted, nobody complained.

California Audio Labs DX-2 player, Acurus A250 amp, and the Martin Logans.

Two separate pots so balance tracking wasn't an issue (if you thought it wrong, go adjust one a hair) and sit back down.

 

[DonH56]

Some passive preamps use transformer attenuators (obviously this one does not). I suppose you should perform distortion measurements to check PIM (see e.g. https://www.keysight.com/main/application.jspx?cc=US&lc=eng&ckey=2127060&nid=-33157.0.00&id=2127060 -- many other references on the web) but this is another of those things I just can't see being a major concern at audio frequencies and power levels. BUt I've been wrong before.

Just how long before you get tired of this Schiit?

 

[Wombat]

Some passive preamps use transformer attenuators (obviously this one does not). I suppose you should perform distortion measurements to check PIM (see e.g. https://www.keysight.com/main/application.jspx?cc=US&lc=eng&ckey=2127060&nid=-33157.0.00&id=2127060 -- many other references on the web) but this is another of those things I just can't see being a major concern at audio frequencies and power levels. BUt I've been wrong before.

Just how long before you get tired of this Schiit?

Searching for the Holy Gruel?

 

[Sal1950]

I ran the Mod Squad Line Drive for over 20 years, great choice if you match the source and end components well.
Atkinson did some extended measurements on it back in 1987
https://www.stereophile.com/content/mod-squad-line-drive-passive-preamplifier-measurements

 

[Soniclife]

Is there any possibility of phase shifts in devices like this?

 

[Speedskater]

The John Windt 'Hummer Test' would be would be a good check for noise and interference.

 

[DonH56]

Is there any possibility of phase shifts in devices like this?

For a purely resistive passive device phase shift in the audio band should be negligible. If it is AC coupled, there will be shift at LF. A transformer-based attenuator will have more phase shift across the band.