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PE-114 Petit: Stereo Phono Equalizer

 Concept of PE-114
Description: Solid state stereo phono equalizer for MC cartridges. Gain: 60dB(@1kHz). Op amps: LT LT1115 x2, LT LT1010 x2. Power supply: Li-ion batteries.
Dimensions: 300mm(W) x 100mm(H) x 150mm(D). Weight: 1kg.
Cost: approx. 40,000 JPY.
History: Designed in 2012-13. Under re-design now. Planned to be built in 2016.


This phono equalizer (EQ) is supposed to be used in my new audio system Gaudi II. In the old system, Gaudi, the main musical source was vinyl records. In Gaudi II, digital media are going to be the main source. So I thought an independent EQ was the better option than an EQ built in the preamplifier like PA-210 Simplicity.

I chose a chip based circuit design as in the crossover network CD-211 A-NET and the MC head amplifier HA-213. The chip I selected was the op amp LT1115 (Linear Technology), which I used for HA-213 and confirmed its high performance and sound quality (SQ).
I didn't consider a discrete circuit. I believe advantages of a chip amp, such as the smaller numbers of components and soldering joints and shorter signal paths, are crucial for high sound quality. Many of professional audio engineers still seem to insist on discrete circuits, but I think discrete circuits are a little too difficult for DIY constructors like me to design.

This EQ is the first NOBODY amplifier that is battery-powered. Nowadays, the AC power line is contaminated by noises emitted from appliances having inverters in them such as air conditioners, refrigerators, electric lights, etc. I've been using noise filters to avoid the noises, but it's not perfect. Not using the AC power is the ultimate option. The DC power (battery power) has one more advantag; it avoids voltage fluctuation caused by the impedance of the power line and the current disspation of other devices.

I named this EQ "Petit." It is a French word that means "little." I came up with it when I was gazing at its model number (PE-114).
I have named some of my works before. Compared with nameless works, the named works became more satisfactory because I had more affection to them and made many improvements for them. I've decided to give a name to every work from now on.
In addition, I selected a theme music for Petit. (Am I overdoing?) It is Mozart's Eine Kleine Nachtmusik. I chose it because the German word "klein" has the same meaning as "petit." The artist is a French orchestra, Orchestre de chambre Jean-Francois Paillard conducted by Jean-Francois Paillard (CD, Classic Library - Haydn/Mozart, BMG CPL 3002B).


The specifications of Petit is specialized for the cartridge of the record player PS-104: Audio Technica AT33PTG/II (gain: 60dB, input impedance: 100 ohms).
Petit has only one pair of input and output jacks. It's the simplest configuration like other NOBODY amps.

The specifications of AT33PTG/II are typical as an MC cartridge, so Petit is applicable to many other MC cartridges.
It's also applicable to MM cartridges by changing the gain and the input impedance. A switch is not equipped for this purpose. In order to change the gain and the impedance, pertinent parts on the board should be replaced. I know from my experience of the HA-213 project that switching very small signals is so difficult technically that the switch could degrade SQ.

Those parts can be replaced without removing the board from the enclosure. Only by removing the cover of the enclosure, the parts are accessible.

A +/-18V power supply is required for this EQ. So two 18V, or four 9V batteries are used. Primary (disposal) batteries are costly. Secondary (rechargeable) batteries are desirable. Lithium-ion (Li-ion) type, which doesn't have memory effect, is the best at this time. It is ideal that the batteries can be charged by Petit. But the method of charging a Li-ion battery is so complicated. I've decided to use a commercial battery charger.

The following table shows the specifications of Petit.

Specifications of PE-114 Petit
 Circuit type  Closed-loop type
 Input  PHONO x1 (RCA jack)
 Output  EQ OUT x1iRCA jack)
 Input impedance  100 ohms
 Gain  60dB (@1kHz)
 RIAA deviation  +/-0.3dB (20-20000Hz)
 Distortion  <0.01% (THD) (w/o A-curve filter)
 Residual noise  <30uV (w/o A-curve filter)
 SNR  >100dB (w/o A-curve filter)
 Max. input level  10mV
 Power supply  DC-powered (Li-ion batteries)
 Enclosure  Separate amp/PSU enclosures

Circuit Design

Circuit design

For the amp unit, I chose the op amp, Linear Technology LT1115, and the closed-loop type EQ circuit.
LT1115 is my favorite op amp for very small signal amplification. Its transparent sound is preferable for Petit. I confirmed its performace and SQ in the HA-213 project.
I selected the closed-loop type circuit because it's easy to change the gain for MM cartridges.

The schematic of the amp unit is based on the sample circuit printed in the datasheet of LT1115. I customized it in my way. The difference is the super servo circuitry added to avoid use of large capacitors.
The time constant for the low frequency band (LF) is not the standard 3180us but 3000us. It was determined from the simulation result.
[Schematic of amp unit]

The simulation result of frequency response is shown below (simulator: LTspice IV).
The upper figure shows the RIAA deviation and the lower shows the gain (the broken lines show phase).
The RIAA deviation fell narrowly within the tolerance level (+/-0.3dB).
[Simulation result]

The schematic of the power supply unit (PSU) consists of batteries and a smoothing circuitry. It seems a good idea that the batteries should be connected directly to the op amps, but I thought the smoother is necessary to suppress a surge current, which could activate a built-in protection circuitry of the batteries. It also decreases the voltage fluctuation caused by the internal impedance of the batteries.
[Schematic of PSU]

Selection of key parts

As mentioned above, I chose Linear Technology (LT) LT1115 as the key op amp. It is a super low noise op amp. Its BJT input circuit minimize the noise level, particularly when the signal source impedance is low. Since the internal impedance of MC cartridges is so low, LT1115 is suitable for EQ.
The maximum supply voltage to LT1115 is as high as 36V (+/-18V). This high supply voltage broadens dynamic range a little. In this point too, LT1115 is suitable for EQ.
Other than LT1115, LT1010, which has high driving capability, is used as the output buffer, and LT1097 is used for the super servo as in HA-213. Both are LT products too.

I have two reasons to choose LT op amps: 1) Petit's circuit design is basd on the sample circuit printed in the datasheet of LT1115; 2) I use the simulator LTspice IV.
All the electronic parts makers except LT offer Pspice models for simulation. They say LTspice models are compatible with Pspice models. I tried several times, but haven't succeeded to convert Pspice models into LTspice models as for ICs. I guess that's because most of Pspice models of ICs are encrypted so that they cannot be used for simulators other than Pspice linked to OrCAD (the most popular schematic CAD in electronics industry).
Now I choose an LT product as long as I can when I need an op amp. When I need to use other manufacturer's op amp, I pick one of LT op amps whose characteristics are similar and use its LTspice model for simulation with LTspice IV.

As for the battery, 18V type is ideal, and 9V type is the second best to create +/-18V power suuply. And high capacity is desirable.
First, I hit upon a secondary battery compatible with PP3 type primary battery. I googled and found a Ni-MH type at the website of Akitsuki Electronics. Unfortunately, Ni-MH is not suitable for audio equipment, because it has memory effect though it's not as serious as Ni-Cd. It shouldn't be recharged till it's completely discharged.
Then, I found PP3 type Li-ion battery at Rakuten. The capacity was 400mAh. It seemed good. The manufacturer and model number were ___.
400mAh was not large enough, so I decided to build two sets of battery blocks to supply to L/R channels separately. Eight batteries were going to be used in total.





Circuit Board Design



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