I've been thinking,( always a dangerous situation I know ) and pondering the question, why are there so few line level amplifiers ?

There are countless devices aimed at amplifying the tiny signal from our turntable cartridges. SUTs and pre-preamps for example.

However it seems to me that even the best of these are often plagued by issues such as hum  and rf interference.  Audio forums a full of this type of discussion.

So the question I am pondering is why don't we leave the cartridge connected directly to the preamp ( MM or MC ) with the minimum of wire and connections and implement some form of amplification, if required, at the preamp output to the main amp. ie at line level ? The signal is much stronger here and I would have thought less prone to interference.

For example, many people love the Denon DL110 but complain about the slightly low output when fed into the recommended MM stage.

A simple amplifier on the line level would only need to boost the output by say 1 : 2 or even slightly less to bring the level up to that of a 5mV  output cartridge.

Is there a reason that I'm not aware of that a slightly low signal output is never boosted at the line level stage ??

I'll chip in here Graham....

These days, with digfital sources, actually most 'preamps' are line level only.
Whereas in the older days, a 'preamp' would include a phono stage within, these days it is omitted.

Nowadays (in the audiophile market) a phono preamp is often separate - and often called a 'phono stage'.

Gain-wise, it typically works like this:

Phono stage (MM 40db) + Preamp (line level 10-20dB) + power amplifier (say 26dB for a 50W amp).
For a MC cart source, you add 20-30dB gain stage of some sort in front.

Traditionally, 'line level' preamps were designed for 0.3V input, but modern sources (CD, ipod, etc) put out 1-2V.
Which is why modern line stages are sometimes only 10dB gain - or even dispensed with altogether.

Which leads us to the main function of a preamp & that is as a 'control' amp - switching sources, volume/balance control, tone filtering, etc.

Re. the example of the Denon DL-110 cart, it is a high output MC, one of those things that was popular back in the 80s, but now odd because it's too high output for MC gain & too low for MM gain. ie. to get 1.6mV up to 0.3V (at least) line level, you need 200x (46dB) gain - a bit much for a MM input, esp if the following power amp is insensitive.

You can also see how the 'sensitivity' of the power amp can be a factor too - in the above example, if your power amp is sensitive (say it needs only 0.5V or less in for full output), then the DL-110 system could work OK. But if the power amp is insensitive (needs more volts, say >1V), then the system could be struggling.

The speaker system sensitivity (& room size & listening habits) also, determines how much power amp watts is required..

DarkLantern blog - darklanternforowen.wordpress.com

I have done away with a preamp as I only have one source and my power amp is very sensitive. Provided I have an approx 60dB gain phono stage with my Benz' .4mv output I'm good. The Zu Denon 103 may be another story with it's lower output.

Every preamp I've had adds it's own flavour and some were very nice flavours.

Wouldn't discount another one but seems silly to spend $$ on something I really don't need.

in the old days i never found a good passive to out- perform a good active line stage.

ditto having a preamp always sounded better than direct connection.

It depends on if you have enough volume to drive the speakers, taking into consideration the poweramp sensitivity (as described) and the cartridge output. If you need more gain, it's possible and most convenient to include it in a line stage, unless you can modify the phono stage to have more gain. From an RF pickup standpoint, the high impedance and low levels of a MM or a high output MC cartridge is very prone to interference. It's good to keep the cable from the tonearm as short as is reasonable for this reason. The impedance output from a phono stage is far lower and as you say less prone to RF, also being at a much higher level.
My experience with line stages is that they can provide more drive and dynamics, at the expense of the flavour of another active gain circuit in the signal path. I like the transparency of a passive line stage with very short, low capacitance cables connected to the poweramp. For the Denon DL110 specifically, I would add gain to the phono stage somehow and eliminate the line stage and its flavours. If it were me, I'd use a MC pre-preamp set to low gain of 2 or 3 and high input impedance, like 15k (to be determined empirically). And insert it before the MM phono stage. Because its operating at low level, 1.5mV, the circuit doesn't get to generate too much extra harmonic distortion, compared to if a line stage was added, operating at 300mV or so. So it's insertion would flavour the sound less than if a line stage were added.

AMR-iFi R&D