Taking the Guesswork out of Phonostage Gain
The March issue of TAS includes a sidebar on matching a phono cartridge output level to phonostage gain. To summarize, if the phonostage has insufficient gain for the cartridge’s output level, the signal will be noisy, dynamic contrasts will be reduced, and you’ll have to turn up the preamp’s volume control near maximum to achieve a normal listening level. Conversely, if the phonostage has too much gain for a particular cartridge, the phonostage could distort on musical peaks, and the preamp’s volume control will be insensitive; tiny movements in the volume control produce large changes in volume.
The article gave some overall guidelines for how much cartridge output level is required for a specific phonostage gain. But a reader post on our Forum prompted me to write this blog detailing a precise method of matching cartridge output to phonostage gain:
I am starting to second guess our cartridge/phonostage match. Presently we have a Benz "Wood H2" (2.5mV) cartridge running through a Graham Slee V Era Gold without the elevator (41.5dB). Is this a good match? Should we be running the low output version of the cartridge (0.4mV) along with the elevator? I'm not clear on the best possible match. Hope someone can direct me.
The post is here: http://www.avguide.com/forums/cartridge-output-vs-phonostage-gain
Does this reader’s phonostage have enough gain for his cartridge? Let’s look at how we can determine this without resorting to guesswork—for any system.
We’ll first determine the phonostage’s optimum output level (in volts) for your system. If you play CDs and find that you set your linestage preamp’s volume at about a third of the way up for normal listening levels, your system is typical. Most CD players output 2V at full-scale, and most preamps operate at unity gain (input level equals output level) with their volume controls at about “2 o’clock” (about two-thirds of the way up). This makes sense, because power amplifiers typically require about 1.2-1.5V at their input to drive the amplifier to full output power (a spec called “input sensitivity”).
We’ve determined that we need about 1V output from the phonostage to adequately drive the preamp and power amp. But how much phonostage gain do we need for a given cartridge-output level? Let’s take the reader’s example above of a Benz Wood H2 with an output level of 2.5mV.
We first find the ratio between the desired output voltage (1V) and the cartridge under consideration (2.5mV). In this case, the ratio is 400 (1 divided by 0.0025).
Using a calculator with a “log” button, we find that the logarithm of 400 is 2.602.
We next multiply 2.602 by 20 to get the number 52, which is the gain in dB required of the phonostage to amplify a cartridge with 2.5mV output to 1V.
The formula is:
NdB = 20 log V1/V2
The formula stated in words is: Number of decibels equals 20 times the log of the ratio of the two voltages.
V1 is the desired output voltage, V2 is the cartridge output voltage. Number of dB is the phonostage gain required.
Getting back to the reader’s situation, 41.5dB of phonostage gain is on the low side for his 2.5mV cartridge. A gain of 52dB would be better, which would increase the system’s signal-to-noise ratio.
Keep in mind that a doubling of cartridge output is equivalent to increasing the phonostage gain by 6dB.
You should keep the phonostage output voltage well under 2V because some preamplifiers with a volume-control chip at their input will clip and introduce distortion if driven by signals over 2V.