Here is an analysis of approximately how much of the amp’s power capability can and can’t be utilized in this situation when playing LPs, based on the cartridge’s rated output and the gain structure of the system:
Stereophile measured the gain of the SIT-3 as being 11.3 db, which corresponds to a voltage multiplication of 3.67x. For an amp having that gain to provide an output of 30 watts into 4 ohms, which corresponds to 10.95 volts, an input of 10.95/3.67 = 2.98 volts is required.
Mytek’s description of the Brooklyn Bridge states that “Brooklyn Bridge is identical to Brooklyn DAC+ in terms of audio circuits and performance but has the addition of a built-in Network Streamer.”
Stereophile’s review of the Brooklyn DAC+ states that “the DAC+’s feature set, which includes MQA and a built-in moving-magnet/moving-coil phono stage, is largely the same as the original Brooklyn’s.”
Therefore it appears likely that for MM phono cartridges the Brooklyn Bridge provides gain that is similar to what Stereophile measured for the Brooklyn DAC, namely 55.6 db. That corresponds to a voltage multiplication of 603x.
As stated above Murph’s cartridge has a rated output under the standard test conditions of 3 mv. A gain of 55.6 db will boost that to 0.003 x 603 = 1.81 volts, **if the volume control is at max.** That of course falls well short of the 2.98 volt input the amp requires to be driven to full power.
Since for a given load impedance power is proportional to voltage squared, the percentage of the amp’s 30 watt/4 ohm rating that is being used when the cartridge is supplying its rated output, and the volume control is at max, is:
(1.81/2.98) x (1.81/2.98) = 0.37 = 37% (!)
The corresponding power supplied by the amp when the cartridge is supplying its rated output is 0.37 x 30 watts = 11.1 watts (!!). Meaning that almost 2/3 of the amp’s power capability can’t be used for many recordings without additional gain being provided.
The peaks of various recordings can of course be either somewhat greater than or somewhat less than the standard LP test conditions. But this analysis appears consistent with Murph’s observation that there isn’t enough gain in the setup to support LPs which aren’t “hot.”
And given his earlier statement that he is “on the cusp of where I want to be” it seems to me that replacing the speaker or the amp is not necessarily called for, despite the mismatch in power capability. If neither of those things is done, what is called for is providing additional gain between the Brooklyn Bridge and the amp. 12 db or so should be adequate. The simple one-tube line stage I linked to earlier provides a gain of 22 db.
Best regards,
--Al