nervine reference phono 9

The nervine reference phono 9 is handbuilt with cost no object components and it is designed with three inputs:

50dB; 63dB; 70dB

All three inputs have continuously variable loading value using a special bulk metal film trimpot for the 63dB input and a wirewound multiturn for the other two inputs.

All inputs are commutable by means of front panel illuminated push buttons.

It is a true double mono built in two independent enclosures connected by an umbilical screened cable.

PSU-R

Inside the PSU-R we have two 120VA transformers, high speed soft recovery rectifiers and special four pole 15000uf smoothing capacitors.

There is also a small toroidal transformer that feeds the power supply for the control board.

In the back of the RSU-R enclosure we have a high grade mains switch, one EURO IEC mains female inlet, a fuse holder and a Ground lift toggle switch.

In the front we have one push button that controls the ON / Standby states.

While in Standby mode, only the control board psu is running enabling the user to switch between inputs. This mode is implemented so the user can verify input loading values by means of a DMM.

REFERENCE PHONO 9

Inside the reference phono 9 enclosure we can find two main boards with a dedicated shunt regulator and RIAA amplifier for each channel.

The third smaller board contains the logic circuit that controls the switching push buttons and signal relays.

In the back panel we find the three input RCA pairs numbered from 1 to 3.

1 - 63dB for medium output MC carts (0.2 ~0.5mV) loaded with a bulk metal film trimmer

2 - 70dB for low output MC (>0.2mV) carts loaded by one multiturn wirewound pot placed on the secondary of the SUT.

3 - 50dB for low output MM (1.1 ~2.5mV) carts (B&O) (Soundsmith) loaded by one multiturn wirewound pot.

The other two RCA are for output and should be connected to a preamplifier with minimum 10kohm input inpedance.

The gold terminal serves the purpose of GND terminal where the TT ground should be connected.

TECHNOLOGY

We chose to use a low parts count high speed two stage amplifier section with a passive RIAA network between the two stages. The first stage is a cascoded jfet amplifier and the second stage a common source amplifier terminated by a buffer.

To cope with the low PSRR of such a circuit we developed a very low output impedance shunt regulator that provides very clean, noiseless and fast power to the circuit.

To avoid intermodulation distortion we use one shunt regulator for each channel.

RIAA TOPOLOGY

In a typical active RIAA equalisation the related filter forms part of the feedback circuit in a phono correction amplifier. The feedback type of RIAA correction filter by nature is prone to induce Transient Intermodulation Distortion (TIM) changing sound according to (experienced) listeners. The active RIAA equalisation circuit is relative simple and is also cheap.

The passive RIAA equalisation circuit is built in-line as a series filter section between the input circuit and the output amplifier. The total signal gain is the same, but the effect on the replay sound quality is less because there is no TIM to degrade it. Also the sound is evaluated as being “faster”.

Implementation of the passive RIAA as we use in all nervine models is much more complex than the normally found in active RIAA designs but this way we can use only two amplifying stages reducing total active parts count and obtaining an uninverted output signal.

There is no global negative feedback so we eliminate TIM and have a faster more dynamic soundstage as a result.

The heart of the nervine is always the equalisation filter that follows the RIAA standard.

Every build is unique because due to manufacturing processes we can not find exactly matched pairs of components with repeated values. So we first build the input and output stages, measure it's output and input impedances (that form integral part of the filter) and then start selecting the corresponding passive parts based on individual calculations made using the formulae of Stanley P. Lipshitz.

To enhance spatiality and true stereo imagery we need to minimize gain and phase differences between channels so every component must be found in precisely matched pairs.

To be able to find a pair of matched input jfets we must collect and measure 100 components in average. To find a pair of matched capacitors we need to measure 30 components in average.

The parts cost and work involved for each build increases the final production cost.

VARIABLE CART LOADING

The majority of phono preamplifiers use a fixed value of 47kohm for MM cart loading and one 100ohm loading value for MC carts.

Some more complex designs use several fixed values for MC cart loading that the user can choose by means of internal or external dipswitches.

After many years experimenting with several types of cart loading techniques we found that the only way we can reach the true "sweet spot" that varies with each cart and strongly depends on the system (TT, preamp, power amp and speakers) is by means of using a continuously variable cart loading device.

In fact, the loading resistor acts as a shock absorber and as we increase it's value, the high frequency response increases while the low frequencies flow with lesser control.

reducing the cart loading resistor value tightens the bass but limits high freq detail so we must search for a precise value where everything snaps into focus. The only way to do it correctly is by means of a continuously variable resistor.

FINE ART OF AUDIO AMPLIFICATION DESIGN

Building audio amplifiers can be considered a fine art. We use a blend of "voicing techniques"  and well proven calculating methods and simulators.

We start with the pencil and paper basic schematic layout, than use a simulator to determine the ballpark values as well as gain and distortion goals and finally build a series of p2p protos to confirm it's viability. Only after that we start laying out the final boards and design the enclosures that will constitute the final products.

Once the board layout is ready, every amplifier is recalculated according to it's component's available values and all boards are hand populated. After assembly, each amplifier must pass a measuring test and listening test before it is delivered to the final user.

OTHER MODELS

The nervine statement phono 8 shares  the same psu and basic circuit layout but is implemented with less costly components in order to reduce production costs.

It is available with two inputs and two gains:

1 - 63dB for medium output MC carts (0.2 ~0.5mV) loaded with a bulk metal film trimmer

2 - 43dB for normal output MM carts (2~5mV)

The nervine pure family also shares the same psu and basic circuitry but has only one input and one gain:

1 - 63dB for medium output MC carts (0.2 ~0.5mV) loaded with a bulk metal film trimmer

2 - 57dB for high output MC carts (0.5~1mV)

2 - 50dB for low output MM carts (1~2mV)

2 - 43dB for normal output MM carts (2~5mV)

A new entry level model will soon be available with a smaller psu that, while retaining a double mono configuration, only uses a smaller transformer.