Current Feedback Amplifiers are well-known among electronic society with their high slew-rate and small distortion.
The Amplifier proposed here is Current Fedback (CFA) type with input stage based on the 2SK170BL/2SJ74BL JFET matched pair, Baxandall super pair in the Voltage Amplification Stage (VAS) and the Diamond Buffer as an Output Power Stage (OPS).
A simplified block-schematic is shown on the pictures below.
Schematic is DC connected – there are no blocking capacitors in the signal path. That’s why in order to preserve the Zero on the Output a servo circuit is implemented (also shown on the block-drawing).
In order to raise the Power Supply Rejection Ratio (PSRR) the Input stage and VAS are fed via capacitive multipliers.
The Diamond Buffer at the OPS works in class A and utilize two power transistors in parallel at each rail in order to raise the quiescent current. Each output transistor is biased at ~ 65 mA which yields overall quiescent current of 130 mA.
Schematic should be powered from regulated power supply with +/- 18 to 24 V output voltages. The overall current drawn from the PSU is 350 mA (for both channels).
An in-depth explanation of the principles and the schematic’s evolution can be seen in my thread in diyaudio.com: http://www.diyaudio.com/forums/headphone-systems/271545-cfa-headphones-amplifier-2.html#post4647661
PCB dimensions: 115×180 mm.
PCB thickness: FR4 1.55 mm. Copper thickness: 35um (1oz). Surface finish – Nickel Gold (ENIG). PCB is two-layers design (Top and Bottom). Both sides have solder masks. Silkscreen is placed on Top layer.
There are ground planes on both layers. Special care has been taken in the ground plane’s design – as it’s seen on the photos, each ground plane is separated to 3 areas – input, middle and output in order to minimize parasitic current flows between amplifier stages.
3. Fully populated board
The board is fully populated, tested and measured. As can be seen from the pictures, only high quality components are used:
– the input stage is based on a matched JFET pair 2SK170BL/2SJ74BL
– the other low-power transistors are low-noise BC550/560 and KTA1268/KTC3200
– the output transistors are 2SA1930/2SC5171
– all resistors in the signal path are Vishay Dale CMF55/RN55
– The big electrolytic capacitors are Nichicon KG 4700uF and Nichicon KZ MUSE 1000uF
– the bypassing capacitors are polypropylene Roederstein MKP 1387
– the capacitors in the frequency correction networks are Silver Mica type.
For ease of cable mounting Molex Fixed Terminal Blocks 3.5mm/5 mm are used for input/output connectors as long as for power supply connector.
4. Schematics measured performance:
– Voltage gain: ~ 8 (can be lowered to 4-5)
– THD at 1 kHz: not more than 0.001%
– S/N ratio: ~ 110 dB
– Slew-rate: ~ 242 V/uS
– Bandwidth at -3 dB: not less than 1.5 MHz
In order to prove the quality of design one board was sent to Mr. Richard Marsh (http://www.diyaudio.com/forums/members/rnmarsh.html) for measurements and evaluation. You can see some measurement results in the diyaudio’s thread mentioned above. I’ll add here only one of his statements:
There are some very good HP amps around…. this one is amazing. A CFA, with high slew rate and ultra low distortion at any frequency and output level and load impedance.
I had a chance to measure it…
In the pictures below you can see his measurements results:
– 5KHz, 22 Ohm load, 3 v rms: 0.00013% THD
– 10 kHz, 8 Ohm load, 5v rms: 0.01% THD
– 15 kHz, 60 Ohm load, 2V rms: 0.00029% THD
– 15 kHz, 60 Ohm load, 5V rms: 0.00057% THD
Full schematic will be sent to the buyer’s PayPal address after purchase. If for some reason a different e-mail address must be used, note it in the process of checkout.
What the buyer will receive:
The package will contain 1 pcs. FULLY ASSEMBLED and tested CFA Headphones Amplifier PCB.
The unit will be shipped with Priority AirMail Registered (or equivalent courier service where applicable). The tracking number will be sent to the customer after shipment.
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