passive pre-amp questions?

[alanbeeb]

Hi - if I was thinking of using a passive pre-amp between source and power amp, what things should I look out for?

Should I be be looking at things like the input impedence & input sensitivity of the power amp, and the output impedence of the source? What effect do these things have? What sort of range of values will be Ok with a passive preamp if they are important?

thanks, Alan

 

[michaelab]

With a passive pre that uses a resistors to control volume then you do need to look into all those impedance questions. DACT (who make some of the best regarded resistive attenuators for passive pres) have a page explaining it. The also have a tool you can use to help you calculate what will work well. The most critical thing is actually the cables between the pre and power. They should be as short as possible and of the lowest capacitance possible.

OTOH, if you were to go for a transformer based passive (like mine) these problems don't exist. Music First make a nice looking passive pre that uses the same S&B TX102 transformers as mine.

Michael.

 

[LiloLee]

Generally the higher the impedance of the attenuator the better, so most people go for 100k. Having said that I haven't had any problems with my 10k one.

As for the sensitivity issue. Provided that the source generates enough output (usually 2V) then the input sensitivity of the amp shouldn't be an issue.

 

[Pete]

I have an Audio Synthesis Passive.

Its the version with all Vishay resistors in it.

I've used it with the following sources, amps and cable and have never had a problem...

Sources:
Meridian 506.24/MF X-Ray/Sony PS2/Creative Labs cheapo 5.1/Yamaha DSPA5/MF E50 Tuner/Pace Ondigital radio box.

Cables into amp:
Tandy 5m/1st ultimate 0.6/blue heaven 1.0/quattro fil 0.5/valhalla digital 1.0/valhalla IC 0.3/Nobrand cables 2m and 0.5m from 3.5mm jack to phonos.

Cables out of amp:
1st ultimes 1.0m/BHeaven 1.0/quattro fil 0.6m/valhalla 0.5m

Amps:
MF E11 integrated[used as power]/Chord SPM1200C/Mark Levinson No.334

Speakers[not that it matters]:
Celestion A1/B&W Nautilus 804/Wilson Benesch Disco's

Every combination has worked brilliantly. It really cured a big system problem on my mates 804's that nothing else could fix.

They reccommend short lengths and it did make a difference to the signal level on the cheaper cables, but you just turn it up more to compensate, and as its a passive, the SQL gets better as it gets louder anyway cos your using less resistors in the signal path!