Stereo Phono Equalizer Amplifier
|Features||Phono EQ amp for MC Cartridge, op amps (LT1115 x2, LT1010 x2) are used|
|Outline specifications||Circuit type: NF (closed-loop) type. Gain: 60 dB (@1 kHz). Input impedance: 100 ohms. Power supply: Lithium-Ion Batteries.|
|Dimensions||Amp unit: 300mm(W) x 100mm(H) x 150mm(D), Weight: 1kg. |
Power supply unit: 200mm(W) x 60mm(H) x 150mm(D), Weight: 1kg.
|History||Designed in 2012-13, planned to start building in 2018. The design is currently being reconsidered.|
This amp was planned as a phono EQ amplifier to be used in Gaudi II, when
the system design of Gaudi II was still in the early stage.
At that time, I regarded the digital media as the main source of Gaudi II. So I planned the separate phono EQ amp instead of one to be built in the preamplifier.
After thorough consideration, I changed my mind and chose the analog disc
as the main source of Gaudi II. I've decided to bulid a preamplifier with
a built-in phono EQ stage.
I'm not going to use PE-114 in Gaudi II, but build it for use in the subsystem in the study room. And, I'd like to compare the sound quality of PE-114 with that of the phono EQ stage built in the preamplifier, because I'm going to design a different circuit for the latter one.
Nickname and Theme Music
I named this amp Petit. It means small or cute in French. It has nearly the same meaing of English
word "little". The Japanese counterpart "Puchi" is
often used in Japanese too. "Puchi zeitaku" (meaning "a
little luxury") is a typical usage. While seeing the model number
(PE-114) of this amp, I came up with this word.
The theme music was Mozart's Eine Kleine Nachtmusik. I chose this because the German "klein" is the word for petit. The recording is performed by the famous French orchestra, Piyar Chamber Orchestra (CD, Classic Library - Haydn/Mozart, BMG CPL 3002B).
--- Eine Kleine Nachtmusik ---
The following contents were copied from the page of PE-114 on my previous homepage 'Tonochi's Audio Room', and edited to fit in the new format. The content was written in 2018.
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.
|Circuit type||Closed-loop type|
|Input||PHONO x1 (RCA jack)|
|Input impedance||100 ohms|
|Output||EQ OUT x1（RCA jack)|
|RIAA deviation||+/-0.3dB (20-20,000Hz)|
|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|
For the amp unit, I chose the op amp, Linear Technology LT1115, and the closed-loop type
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 (PE-114_MainBoard.BMP)]
The simulation result of frequency response is shown below (simulator:
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 (PE-114_Sim_Gain.png)]
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 (PE-114_PsuBoard.BMP)]
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.
Tuning & Improvement