PT03 DIY hifi pre amplifier

09 Aug. 2011 by Thanaporn, 573 views.
PT03 DIY Class A Stereo solid state Preamplifier.
PT03 is a Preamplifier that was designed to be successor of PT01 vacuum tube preamplifier. I intend to design it with mounted most electronic devices on the PCB except the transformer.
So electronic components, potentiometer and RCA jacks will be installed on the same PCB. PT03 is a transistor-based preamplifier.The amplifier schematic can be divided into 3 stages.PT03 schematic
1st stage: the first is the fully complementary differential amplifier circuit. It's been bias to operate in class A.Both positive and negative side of audio signal of can be amplified by both upper and lower pairs of transistor. All low noise transistors are used in order to get quietness of sound.
2nd stage: is a voltage amplifier stage. It is designed to amplify the signal from the first stage output , it's also complementary circuit. Because of high linearity is needed. It is designed to integrate the cascode transistor pair in this section.
3rd stage: is a current amplifier circuit only. No necessary of the voltage amplifier in this section. Emitter follower has the advantage of having a high input impedance but low output impedance. The advantages of this one is compatible with any size of power amplifier. Dynamic changes of power amp input impedance will not be a problem.
DC offset detection circuit was designed to prevent the preamplifier with interfere dc offset to the power amplifier. Without matching of electronic devices such as transistor's gain and resistor's resistance cause the dc offset at the output. This circuit acts as a low pass filter amplifier for the error voltage at amplifier output. The error voltage sent back to the difference stage to adjustment.
DC offset detection circuit or DC servo circuit can manage to have zero offset voltage at output point. In practice, I can measure this offset only 0.1mv.
 

Construction:
I ordered a printed circuit board, then it is completed in 4 days.PT03 pre amplifierPT03 board

I take some time to choose some matched transistor pairs, they are used in each stage and each channel. BC550C and BC560C transistor are been selected as the same gain(hfe) or as similar as possible. Power supply circuit was made first to ensure functioning correctly.
Eventually I tested and got the output correctly at +15 V and-15V.

 

All components were installed for both L+R channel.

PT03 finish

 
Test results.
I connected a Marantz CD-5003 CD player with PT03 through JPS SuperconductorQ RCA, then output connected to the TH-200se DIY power amplifier via AudioQuest KingCobra. Response2.5 speakers connected through the Monitor Kable AtmosAir309.
It provides a clean, natural tone. The bass is much deeper. Accurate and fast rhythm.
 
I'm not yet installed it on the chassis because I have plans to build a new preamplifier with separate power supply. Then let compare results in term of sound quality.