Marantz 2325 W/ BA312 Modification
Today were showcasing a Marantz 2325 Receiver a customers brought in for restoration along with our exclusive BA312 to OP-Amp modification along with a few additional Marantz **see recent 2325 posts 8/17**. The 2325 is a 125 Watt rated receiver and amongst my personal favorites right up there with the 2230 and 2275 units. The 2325 has excellent shielding installed, separated drivers with a SE regulated PS with a coupled 30,000MFD of filtering.
P800 Power Supply
The 2325 power supply also houses the protection circuit which protects agains over current, dc deviation detection and initially inrush at power up. The P800 is a regulated single end design using two full-wave centre taped rectifiers with a final regulated 35V and 14/19V+ SE rails.
The two initial rectifiers were replaced with state-of-the-art Ultra-Fast, Low Leakage TO-220 common cathode, MUR On-Semi devices with high grade Pansonic ECW polypropylene films capacitors. The initial 220 and 470 smoothing capacitors were increase to 330/1000MFD Nichicon PW low impedance capacitors for decreased ripple potential, along with remaining circuit capacitors to Nichiocn PW’s with an increase in operating voltages. A new precision Bourns adjustment potentiometer installed. All new small signal TO-92L Fairchild devices for better dissipation installed as well as both TO-220 heatsink mounted regulators to a more robust, reliable TIP device with new performance Wakefield thermal compound applied. A new low noise/indicator type Omron MY2 relays were installed.
Both bridge rectifier filter capacitors were increased from 15,000 to 18,000 with new computer-grade Nippon filter capacitors installed.
**NOTE THAT ONE OF THE LARGE 15,000MFD CAPS HAD ACTUALLY FAILED AND RUPTURED ITS MATERIAL VIA VENTING OUT THE BOTTOM LEAVING THE RAILS TO SUFFER MAJOR NOISE AND LAG
PH01 & PE01 Tone W/ OP-Amp Adaptive Circuit Upgrade
Proceeding the the function switch the AF path is converted here to low output impedance sources
PE01 W/ BA312 UPGRADE
The PE01 assembly is mirrored in many Marantz models, all of which utilize the monolithic BA312 SIP7 pre-amplifier IC. The BA312 is a obsolete, out-of-production chip used for differential gain stages. We are proud to recently release the first solid, clean Op-Amp adaptive circuit module to replace the original BA312 ICs to a high performance, audiophile grade single end OP-Amp which has been installed on these two 2325’s. We are currently employing the Sparkos Labs SS3601 SE discrete OP-Amp, you can read additional details and performance reports at the link below to our upgrade;
P400 RIAA EQ
Through testing we see these P400 have an excellent curve response, as well as a massive following for their wonderful sound, why many initially purchase their first Marantz! One flaw to the P400 stages is the implementation of the 2SC458 TO-92 NPN transistor which have a tendency to become noisy, their are often changes to initial coupling stage MFD value from 1-3.3MFD. These tantalums should be a polypropylene 1MFD installed.
For these two models both input/output coupling capacitors were replaced with high grade MKP polypropylene film capacitors. The DC Filter axial capacitors was replaced with a preferred radial type audio grade KT Nichicon and increase to 220MFD for better filtering capacity, KT’s replaced the remaining electrolytic as well with an increase in operating voltages. The VD type epoxy diode was replaced with a series of general Vishay axial diodes and remaining diodes to axial Vishay type. The clipping adjustments were measured and replaced with fixed resistors rather than potentiometers for a more symmetric response. All the NPN devices including the 458 were replaced with low noise, modern TO-92 Fairchild devices. The resulting output is a 40dB gain stage.
RIAA Curve Response
The 2325 uses a differential gain stage and current stage which drives the eventual pre-drivers, the drivers are tied to the dynamic bias circuit.
The differential PNP pairs were matched and thermally coupled together (beta mapping see below Mirrored for X4 Channels.) The 714/715H bias transistors were updated to modern On-Semi TO-126 devices and current stage to modern TO-92L Fairchild devices for better dissipation. These large frame potentiometers tend to hold very well, I recommend a proper cleaning/lubricating as their as good as the Bourns available today.
**matched with prior 2325's groups BETA testing, mirrored results**
The electrolytic capacitors and tantalum 33MFD were all replaced with a audio grade KT Nichicon with an increase in operating voltages as well as all films including 1MFD input coupling to high grade MKP polypropylene and WIMA film capacitors. The SB was applied with new axial Vishay diodes.
New custom SMD LED indicators and fresh new Vellum filter paper installed (LED already present prior to rebuild)