Can anyone explain why this works?

[JimmyJet]

Thought I'd ask here since this is an Audio "Science" group and figured someone would be able to give me a technical/scientific reason why a phono stage I just acquired works with my cartridge when the numbers say it technically should not work.

My MC cartridge has the following specs:
Output Voltage = 0.5mV
Impedance = 50 Ohms

My new phono stage, Line Magnetic LP-33, has the following specs:
Input Sensitivity MC: 1mV(H), 0.5mV(L)
Input Sensitivity MM: 6mV
Gain MC: 66dB(H), 72dB(L)
Gain MM: 50dB
Input Impedance MC: 100Ω(H), 47Ω(L)
Input Impedance MM: 50kΩ
Output Impedance: 10kΩ

The optimum gain for the output of my cart is around 56dB and loading is in the 500 to 1000 ohms range. I had been using this cart with my old solid state preamp's built in phono stage with fixed gain at 60dB and the loading set at 1000 Ohms and it sounded great.

This new phono stage is tube driven and only has two settings (High/Low) for an MC cart as listed in above specs. After trying both settings, it was clear the Low setting sounded best, and in fact, it sounds fantastic! - much better than my solid state phono stage. But technically speaking, this new phono stage should sound horrible because at the low setting it's providing 72dB of gain which should be producing clipping and distortion. And with providing only 47 Ohms of resistance, it should sound muffled and lifeless. While I'm enjoying the sound, it really bugs me how these numbers can be so off but yielding such great sound.

The only numbers that line up between this phono stage and the cart is that both the cart's output voltage and the phono stage's input sensitivity are both 0.5mV, other than that, the numbers are way off. I'm wondering if the tubes interact differently with a MC cart than that of a solid state phono stage? (I would think this shouldn't be the case)

Anyway, I'm hoping someone here can explain why these "off" numbers are working so well.

--cheers!

 

[mhardy6647]

Well 47 ohms is about the same as 50 ohms (your cartridge spec, per your post), so you're good there.
More gain will typically sound "better" (i.e., louder), until and unless one hits overload. It's certainly possible that none of the recordings you've listened to are "hot" enough to overload the preamp.

 

[antcollinet]

72dB and 0.5mV will give almost exactly 2V output. Why will that result in clipping?

Unless your subsequent amplification stage cant take a 2V input - which would be odd, because it is pretty much standard for unbalanced RCA.

 

[StephanB]

I will attempt to answer your questions.
There is nothing inherently wrong with having an input impedance similar to your source. It's just not the case with line-level connections where we have low impedance sources connected to high-impedance inputs. Microphones, guitar pickups and microphone pre-amps will fall in the same scenario as phono cartridges.

When the impedances are similar you have calculate the insertion loss and factor it in the gain.

• For the H input: loss is 20 x log_10(100/150) = -3.5dB, your total gain is 66 - 3.5 = 62dB. This should give similar results as the solid state pre-amp.
• For the L input, loss = -6.3dB, total gain = 72 - 6.3 = 65.7 dB

Clipping will occur when you reach the headroom of the preamp, I doubt that 0.5mV is a specification of max. input level, most likely a value for nominal input.

Muffled sound would be caused by the source being primarily inductive, like some guitar pickups, but IDK if that is the case for phono cartridges.
Sauce: Robjohns H. (2003) Understanding Impedance Sound on Sound

 

[egellings]

I will attempt to answer your questions.
There is nothing inherently wrong with having an input impedance similar to your source. It's just not the case with line-level connections where we have low impedance sources connected to high-impedance inputs. Microphones, guitar pickups and microphone pre-amps will fall in the same scenario as phono cartridges.

When the impedances are similar you have calculate the insertion loss and factor it in the gain.

• For the H input: loss is 20 x log_10(100/150) = -3.5dB, your total gain is 66 - 3.5 = 62dB. This should give similar results as the solid state pre-amp.
• For the L input, loss = -6.3dB, total gain = 72 - 6.3 = 65.7 dB

Clipping will occur when you reach the headroom of the preamp, I doubt that 0.5mV is a specification of max. input level, most likely a value for nominal input.

Muffled sound would be caused by the source being primarily inductive, like some guitar pickups, but IDK if that is the case for phono cartridges.
Sauce: Robjohns H. (2003) Understanding Impedance Sound on Sound

Phono carts use a heavily equalized preamplification, while guitar amplification is relatively flat. Moving magnet carts have relatively large coil inductance due to the many turns of wire on the coil, while moving coil carts have few turns, simply out of necessity of keeping the moving mass acceptably low. The same RIAA EQ is used for both types of cartridge. I suspect that higher inductance of guitar pickups is not that big an issue since the guitar does not have to reach the upper end of the audible spectrum.