2. amps-preamps
  3. 1268737214

Who is using passive preamps and why?

Seldom has there been any discussions on passive preamps in the forums and although my experience with them has been limited I have found them so far to be very enjoyable and refreshingly different. They seem to fall into their own category, somewhere between solid state and tube. Finding a preamp that is satisfing has been difficult. Some active solid state preamps can be very good but they seem to inject grain to some degree in the upper registers and some tube preamps are not too far behind. So far I think they should at least be matched up with an amp that has sufficient gain which is often overlooked. Which passives are you using and with what amp? Why do you like them?
phd
herman
2,242 posts
 
Pubul, the output voltage isn't the issue, just about any CD player has enough voltage to drive any amplifier to full volume. The problem occurs when the CD player output impedance (Zout) is too high compared to the load it sees which is a combination of the input impedance (Zin) that it sees from the passive and the amp.

Many passives have a Zin of 10K or less and this is further reduce by the Zin of the amp. At full volume they are in parallel so a 10K passive and a 10K amp would result in a 5K load for the pre. That is problematic for many CD players and phono stages no matter how much voltage they can generate with no load applied.

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pubul57
3,712 posts
 
Herman, does the 50kohm of my attenuator address that issue, or does it cause problems on the other end into my 100kohm amp load?
herman
2,242 posts
 
Even with the volume turned all of the way up the CD player is loaded with 33K ohms and the Zout of your player is only 50 ohms, a ratio of about 700:1. Conventional wisdom says at least 10:1 with higher better and no benefit beyond 100:1. I think 10:1 is a bit low but you are in a very good place. I would be tempted to try a Placette or a transformer control with those ratios.

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clio09
3,353 posts
 
Herman,

What formula are you using for the calculations.

From your original example, I would have never thought an amp with !0k in put impedance to be a good match for a passive. More like 100k ohm in my mind.
herman
2,242 posts
 
hmmmm, can't find where I said 10K was a good match. Which original example are you referring to?

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