Cjss103

TuchAudio 103

Discrete, simple op-amp phono preamp

Purpose

The most simple discrete op-amp, with just 4 transistors per channel -- a 3-transistor design was tried, but performed poorly.

Design quirks

An op-amp this simple doesn't have too much of open-loop gain, thus the approximate formulae usually used are no longer valid. For this reason, bass performance is just acceptable (but within DIN 45.500 specs).

DIN and RCA jacks are provided for signal input. Since the native input impedance is around 110 kΩ, space for suitable load (Rx and Cx) is provided for perfect adaptation; but for added flexibility, it is possible to use the free input for a suitable RC combo in parallel, to match the appropriate cartridge load.

Originally intended to be connected thru the 501 Input Selector board, both output pins and power input are at compatible locations within each board. An aditional RCA output jack can be installed in order to use the circuit as a stand-alone preamp.

The use of an asymmetric power supply (and its associated feedback DC-blocking capacitor) means that it takes a few seconds for the preamp to operate after powerup (or a noticeable supply voltage change).

Stability

As any other amplifier with asymmetrical power supply, the PSRR is very poor, thus a well filtered, stable power supply is essential. Increasing (within reason) R115/R215 may help a bit.

Suggested mods

Standalone mode

Despite being designed around a 12 Volt power supply, this can be run from a single 9 Volt battery, with minor changes and similar performance.

• R115/R215: use 220 Ω instead of 330.
• R103/R110/R203/R210: use 8.2 kΩ instead of 12K.

IEC1976/Neumann ammendments

The stated values are designed around these ammendments to the RIAA curve; but since they're currently deprecated, you may get improved bass response by turning back to RIAA correction:

• C104/C204: do not mount (Neumann ammendment)
• C106/C206: use 470 nF or more instead of 100 nF (which made a -3 dB point at 20 Hz over 108 kΩ of the input on the original design of the 501 board)

Specs

Measured thru a 501 input board (tape rec. output), 12 V supply

• Power supply: 9-12 V, asymmetrical, max. 6 mA
• Input connectors: 2x RCA plus DIN (in parallel)
• Gain at 1 kHz: 47x (+33.45 dB)
• Input sensitivity: 3.8 mV (for 180 mV output)
• Input impedance: 110 kΩ; adjusted by Rx/Cx, either internal or external.
• Output level and impedance: 180 mV / 3.3 KΩ
• Overload margin: at least +15 dB
• Signal-to-noise ratio: TBD
• Frequency response: 40 Hz - 20 kHz (+0.25/-3 dB deviation from RIAA)
• Distortion: