help me to understand the gain

You can essentially nullify the gain of the preamp (reducing it to 2db) by placing a pair of 10db Rothwell Attenuators, or similar devices that are available from other manufacturers, at the preamp inputs.

However, to obtain an adequate overall reduction in gain, you would have to place an ADDITIONAL 10 or even 20db of attenuation at the power amp inputs, and there would be a problem doing that.

According to Stereophile's measurements of your preamp, its already very high output impedance of 2.7K at mid and treble frequencies rises to 11.5K at 20Hz. That rise will result in a significant deep bass rolloff when driving most or all readily available passive attenuators. (I've measured the input impedance of my Rothwell's as being about 33K, which is much too low to be driven properly by a preamp having that kind of output impedance).

As I see it, the fundamental problem is that your power amp is much more powerful than what is required by 101db speakers under typical conditions, and although (as was said above) power and gain are two different things, they generally have a significant degree of correlation. I would suggest selling the amp and replacing it with something that is less powerful and has lower gain. Having to pay for fewer amplifier watts may also allow you obtain better sound quality per amplifier dollar.

If you do that, be sure to choose an amp having high input impedance (ideally 100K or more), to be a proper match for the PrimaLuna.

Regards,
-- Al

04-29-11: Davehrab
The PL3's output voltage is 5 volts as measured in a Stereophile review and way to much output for the DNA1's input stage.

With due respect for your well intended comments, I believe you are misinterpreting that spec. That number represents the output voltage at which the onset of clipping occurs, as indicated by a significant rise in distortion. The higher that number the better, everything else being equal. The actual output voltage at any instant of time will equal the input voltage multiplied by the gain that is provided by the preamp at the volume control setting being used (12db being the gain corresponding to the maximum setting of the volume control).

You can put a 10db attenuator at the amp's input or just remove the PL3 and drive the DNA1 from the Dac ... which has Tube output and a volume control.

If the PL3 is used, putting a 10db attenuator at the amp's input won't work well. Bass rolloff will result, as I indicated in my previous post. It also won't be a sufficient amount of attenuation.

Good point about the volume control on the DAC, though. Assuming it is the D1 model, I note that it also includes source switching, for both digital and analog sources. I don't know what its output impedance is, but if it is low enough to work well with an in-line attenuator, perhaps ditching the preamp AND putting a 10db attenuator at the amp inputs would do the trick. The OP might want to ask the DAC manufacturer what its worst case (maximum) output impedance is, across the audible frequency range.

If it were me, though, I would seriously consider Herman's suggestion of a 300B SET amp, or other low powered tube amp.

Regards,
-- Al

It seems the gain of the amp is not a published specification. I can only see it inside the Stereophile review under measurement section.

As a rough approximation, you can calculate the gain by using the input sensitivity and output power numbers, and then adding a few db to the resulting number:

First, convert the 8 ohm power rating into volts, based on

P = (Esquared)/R,

where P is power in watts;
E is the voltage corresponding to rated continuous power into 8 ohms;
R is 8 ohms.

In this case, P is 150; R is 8. Therefore E = square root (150 x 8), or 34.6 volts.

Using the 1.5 volt sensitivity rating that was provided above by Dave, the voltage gain is 34.6/1.5 = 23.1.

Next, convert that gain to db by taking 20 times the logarithm of the voltage ratio:

20 x log(23.1) = 27.3db.

As I indicated, the actual gain will be somewhat larger than that number (in this case by 5db), because the continuous power rating will be lower than short-term power capability, and because of margin that is provided in the specs.

I would like to try the SET amp.... Does the requirement to have input impedance > 100k Ohm still apply?

Yes. That is a consequence of the preamp's output impedance characteristics. As for specific amp recommendations, you might want to research past threads to see what amps people have used with the particular Zu model, and/or start a separate thread on that question.

I can lower my DAC input so I can make the volume know of the preamp within a usable range. This is similar to if I lower the volume in iTunes. My question is, how would this affect the sound? Will I run into the bass roll off problem like using attenuator?

IMO reducing the setting of that volume control is a reasonable thing to try. The bass rolloff effect I described has no relevance to that. As Ralph (Atmasphere) indicated, the sonic effects of reducing the setting will depend on the specific design, and only you can make that judgment.

Regards,
-- Al