Weak ROTEL phono section


Hi all,
Have a Rotel rc1090 preamp...former top of the line...with balanced ins/outs.
Despite using a very powerful 6.5mv cartridge, the output seems a weak and flat. The volume knob on the Rotel has to go far (almost 2 or 3 o'clock) on some recordings to get a satisfactory volume.
I have no problem with cd playback, plenty of power using that input.
I'm using the Rotel with a McIntosh 252 amp. Again, sounds great(and powerful) with CDs.
Any insights or advice greatly appreciated.
Andrew
dordevic
You're welcome! Another contributing factor is that the power amp, according to the specs in its manual, has an input sensitivity (for full power, and for the unbalanced inputs) of 1.6 volts (3.2 volts for the balanced inputs), while the specified output of the preamp is 1 volt. That results in having to set the volume control higher (for both phono and cd) than you would have to if the preamp had a higher output and/or the power amp had a lower sensitivity number.

Using the balanced inputs won't change that -- you'll probably have 2 volts out of the preamp (between the "hot" and "cold" signal polarities, instead of 1 volt between signal and ground in unbalanced mode), but the power amp sensitivity changes correspondingly (from 1.6 volts to 3.2 volts).

Regards,
-- Al
Thanks all.
Thanks Almarg,
I'm wondering if an outboard phono preamp might help. Success would depend on whether the phono output is higher than that of the Rotel.
Yes, a properly chosen outboard phono stage probably would help.

The phono section of your Rotel presumably amplifies a 2.5mv input up to the specified line-level input sensitivity of 150mv. That corresponds to a gain of 20log(150/2.5) = 35.6db, which is somewhat less than the 40db or so that typifies moving magnet phono stage gains.

The line stage of your Rotel provides a gain of 20log(1/0.150) = 16.5db, which is fairly typical (although the gains of preamp line stages vary widely from model to model).

So if you were to obtain a phono stage that provided gain in the low to mid 40's in db terms, while also providing 47K input impedance to be compatible with your moving magnet cartridge, you could reduce the volume control setting by the difference between that gain and 35.6db. Keep in mind the rough rule of thumb that a 10db volume increase corresponds to a subjective perception of "twice as loud."

Audio Advisor has a selection of phono stages you may want to consider.

Regards,
-- Al