Ta101bc

TuchAudio 101b

Discrete Phono preamp -- reedition of my very first phono preamp, circa 1987...

Purpose

Fully symmetrical discrete op-amp phono preamp -- loosely inspirated by Elektor designs. RIAA correction is achieved thru two different ways: low frequency boost is implemented on the feedback network, while the high-frequency rollof is done with a passive low-pass filter, for improved signal-to-noise ratio.

Design quirks

The preamp itself is fully DC-coupled, for optimum bass performance; however, that means that the input stage offset is multiplied by the low-frecuency gain (+34 dB), which may result on stage saturation. To compensate for that, each channel provides room for R+ or R- resistors, connecting the inverting input (feedback) to the power rails, in case some manual adjustment is needed.

Just like the original build, a subsonic filter is integrated into the board. This is a fourth-order high pass filter (24 dB/oct) with a cutoff frequency of 11.8 Hz; again, based on an Elektor design (Summer Circuits 1984). However, since it is located after the amplification, the high gain of the stage at low frequencies might saturate the preamp itself, in extreme cases.

DIN and RCA jacks are provided for signal input. Since the native input impedance, being a symmetric op-amp design, is expected to be extremely high, 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.

Both direct and subsonic-filtered outputs are provided, and properly selected thru the 509 Input Selector board. An aditional RCA output jack can be installed in order to use the circuit as a stand-alone preamp.

Suggested mods

Subsonic output

As previously mentioned, both direct and subsonic outputs are provided, and selected thru the 509 Input Selector Board; in case a non-selecting board is used (like the 501 Input Selector board), you can break the JP1 and JP2 jumpers to route the subsonic output to the bus-style/RCA jack output instead (with suitable external bodge wires).

DC-block

While being fully DC-coupled is definitely desirable from a sound quality standpoint, it may be troublesome in some extreme cases, especially with warped records. If this is a concern, adding a suitable capacitor (~100 µF) in the feedback loop will prevent this. Just lift one pin of R9/R109 and bodge the capacitor in series. There is no need to use R+/R- in that case, and you should remove them in order to check the offset polarity; mount the extra capacitor consequently.

Specs

• Power Supply: +/- 9 V (symmetrical), just a few mA.
• Gain at 1 kHz: x50 (+34 dB)
• Input sensitivity: 3.6 mV (for 180 mV output)
• Input impedance: determined by Rx/Cx, either internal or external.
• Output level and impedance: 180 mV / 1 KΩ
• Overload margin: at least +15 dB
• Signal-to-noise ratio: more than 80 dB (estimated)
• Distortion: TBD