I think the input resistance of the current driven phono stage is in parallel (not in series as per your equation) with the internal impedance of the cartridge, but I am open to discussion about that. Anyway, my net experience with two very different current driven stages and three different LOMC cartridges is that the input impedance of the stage, which must be some finite but very low value above zero, plays into the resulting sonics. So, it's not just whether you have enough net gain but also how the cartridge can drive that input impedance. The result is that the SQ is not always predictable. Every pairing of cartridge with current driven stage is a new experience. Where data are available, I found the input impedance of various units to vary between 2 ohms and up from there to about 10 ohms.

Hagerman Piccolo Zero: New transimpedance head amp

I recently received my brand new (recently released) Piccolo Zero "transimpedance" MC head amplifier. Its sounds awesome! And it already completes for my favor with some good SUT’s - and perhaps more. Pretty certain I already prefer it to voltage-mode JFET head amps. Jim Hagerman commented he might not prefer this head amp for (say) Earth Wind and Fire, but thus far I find no need to qualify its performance by genre or complexity of music. For $250, this product is simply awesome. Maybe all the folks here raving about this transimpedance approach, and eschewing SUTs (Raul), were correct after all 😅

BUT, how to make sense of Piccolo Zero’s specified "gain" ratings? And how to predict matching to a given cartridge? The manual states the following gain levels:

- 83uV/uA (0dB)
- 133uV/uA (+4dB) - EDIT: my unit actually shipped with this level pre-selected, not +6. The internal switches are tiny and very confusing!
- 166uV/uA (+6dB)
- 332uV/uA (+12dB)

Those +db number are not "absolute gain" values. They’re gain relative to the (0dB) base level’s 83uV/uA sensitivity (read that as: 83 micro-volts output for every 1 micro-ampere input). The actual gain must be calculated against the actual current generated by a given cartridge. So how to do THAT? I can guess to start with the basic kiddie equation:

V = I * R

And let’s start with a "typical" MC example: Koetsu Platinum (any stone) at 300uV output for 5ohm DC coils (for simplicity, we’ll ignore the inductive component of impedance). Then:

300uV = I * (5ohms + Rp)

Where Rp is the input resistance of the Piccolo Zero. It "should" be close to 0, but I can’t find Hagerman’s specification. Perhaps 1 ohm or less? This value becomes more important for ultra-low impedance coils (e.g. My Sonic Labs cartridges, Benz Ebony TR, some vintage Ortofon MC’s). For now we’ll conveniently ignore it and assume 0 ohms :)

300uV = I * (5ohms + 0) => I = 60uA

So 5 ohm coils gives us 60 micro-amperes. Note that the generated current is **always** going to be inversely proportional to the coil ohms (plus the input resistance we ignored)! Also note that we can simply multiply this current value against each gain level’s listed sensitivity. So for the 83uV/uA (0dB) level, we get:

83uV/uA*60uA = 4,980uV ~= 5mV

Wow! That’s right on the money for what signal level you want hitting the MM phono stage. We can also easily calculate the traditional voltage gain of this transformation:

4980uV / 300uV = 16.6x (step-up ratio) = 24.4dB

And it follows that the 166uV/uA (+6dB) level yields:

166uV/uA*60uA = 9,960uV ~= 10mV

4980uV / 300uV = 33x (step-up ratio) = 30.4dB

It’s interesting to note that the highest 332V/uA (+12dB) level yields a whopping 66x step-up ratio for 36.4dB of gain! It seems you can very easily push into some MASSIVE voltage gains, given a cartridge with ultra-low impedance coils. In those scenarios, one must exercise caution to not overload a downstream MM stage.

Furthermore, the above calculation can be generalized quite simply:

Voltage gain (ratio) = Sp(uV/uA) * (Vc / Rc(ohms)) / Vc = Sp(uV/uA) / Rc(ohms)

Where Sp is the sensitivity of a given gain level, Vc is the rated output voltage of a cartridge, Rc is the DC ohms of the cartridge coils. The cartridge output voltage cancels itself, and we’re left with:

Voltage gain (ratio) = Sp(uV/uA) / Rc(ohms)

So basically, we just take a given cartridge’s DC ohms and divide it into a sensitivity for a given gain level. That’s our net voltage gain ratio. Super easy :)

For clarity, here’s the full lineup of gain levels for our **Koetsu Platinum**:

- 0db level => 83 / 5 = 17x step-up ratio = 24dB gain
- +4dB level => 27x = 28dB gain
- +6dB level => 33x = 30dB gain
- +12dB level => 66x = 36dB gain

Clearly, one should stick to the 0dB or +4dB levels on a Koetsu. I can verify the +4dB level with Blue Lace sounds awesome :)

Now let’s look at the **Van den Hul Crimson XGW Stradivarius**, with 0.65mV from 12 ohm coils:

- 0db level => 83 / 12 = 7x step-up ratio = 17dB gain
- +4dB level => 11x = 21dB gain
- +6dB level => 14x = 23dB gain
- +12dB level => 28x = 29dB gain

That works out pretty well! I’ve also tried out this combo, and it sounds awesome. These numbers roughly gel with what I’m hearing versus a SUT. The +4dB level also works very nicely here, perhaps helped by the fact this VdH’s output seems weaker than its rating, relative to other brand cartridges rated 0.4mV - 0.8mV.

Now let’s look at the **Van den Hul Colibri XGW Stradivarius**, with 0.38mV from 36 ohm coils (we lose much output from the monopole design):

- 0db level => 83 / 36 = 2.3x step-up ratio = 7dB gain
- +4dB level => 3.7x = 11dB gain
- +6dB level => 4.6x = 13dB gain
- +12dB level => 9x = 19dB gain

At this point, we’re clearly struggling to achieve enough gain from those 36 ohm coils. However, the +12dB level should still be sufficient. I listened to this briefly, and indeed found its output level to be significantly lower than that of a 15x - 30x SUT. However I do not favor this combination yet - I’ll have to give it another shot later. The SUT or voltage mode approaches may have the clear advantage here, due to 36 ohm coils. EDIT: I had the internal gain level switch positions confused. They are very tiny and confusing! I was initially hearing the Colibri at 0dB level when I thought it was +12dB lol no wonder it sucked. I'll try the real +12dB shortly.

And finally, let’s look at a **My Sonic Labs** cartridge. I dunno, **Eminent Ex**? This cartridge wasn’t my jam (*I no longer have it on hand for testing here), but serves as a good example of an MSL configuration. 0.4mV from 0.9 ohm coils:

- 0db level => 92x step-up ratio = 39dB gain

That’s crazy. And it only gets more ludicrous above that. So clearly the head amp’s input impedance is important at this point. If we assume 1 ohm, this drops to an almost reasonable 44x step-up ratio. Still way too much for 0.4mV, but it would be serviceable. Anyways, that begs the question: are My Sonic Labs cartridges "too much" for a transimpedance stage? Does such a head amp need special accommodation for these cartridges (i.e. a much lower gain level for MSL)? The Benz Ebony TR and vintage Ortofons (e.g. MC20, MC2000) don’t face this problem, because their output levels are lower in proportion with their coils.

** **I’ve used the Piccolo Zero with two fairly high-end standalone phono stages, so far: VAC Renaissance SE and Hagerman Trumpet Reference. Both have a tube-based MM stage (6x 12AX7 in the VAC and 4x 12AX7 plus 4x 12AU7 in the Trumpet), and I’m bypassing their own internal MC stages. Results with both have been equally exceptional.**

** *Please note I’m not posing the above as any kind of authoritative analysis. This is just me, an analog hobbyist, trying to get a better handle on this product. Please help improve my understanding if you have corrections or insights to share!*

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Interesting. Thanks @intactaudio , think I’ll just make a JavaScript calculator where I can plug in different parameters including cable + input resistance. 1-2 ohms seems awful high for 1.2m of cable, even at these small gauges (~ 24 - 26 AWG?). Is a lot of that from the physical contact points? Yeah I suspect you’re right on the matching. It’s not just about gain, though that is an important factor. I ran the Colibri at +12dB level last night (19dB "estimated" head-amp gain) and it was loud enough but still sounded bad lol. I’m not sure why the input resistance of a transconductance phono would be in parallel, though (e.g. loading resistors for voltage mode head amps are in parallel because it makes sense there). Does anyone else have a comment on that? |

The thick wire that nets a low DCR also restricts movement so from the cartridge to the din connector is typically 12" of #36 or #37 which puts you at about 1/2Ω per foot. I have an early Schick and a Schröder and both are in the 1Ω range from pins to RCA so that becomes 2Ω since they are in series. dave |

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