My brother gave me a used turntable (Technics SL-D1) along with a cheap DJ preamp. I didn’t have any vinyl records to play, so he loaned me a few as well. I was once a bit of an audiophile back in the day (early 80’s) but after CD came out I ditched vinyl as a medium of choice because of how cumbersome it was to play. (not to mention the limited dynamic range, etc.)

Having not listened to vinyl for a few decades, I was curious to playback some familiar tunes – and I was surprised as to how clean the sound coming out of the vinyl was – much better than I remembered.

Slowly I started to listen to more vinyl and started feeling the limitation of the phono amp that I was using. So I looked around the Internet for phono preamp circuits, and formulated the idea of designing a discrete transistor phono preamp.

Late 70’s HiFi Phono Preamp

I found a schematic for Sony TA-F6B. The phono preamp section used a differential FET input stage, current mirror, and class-A gain stage with constant current, among other things that were popular at the time. Pretty much the maxiest form of phono amplifier of the time.

I took the basic concept of using a differential input stage and class-A gain stage with constant current source, and put together a prototype.

I also added a headphone amp built into the same board so that I can easily use headphones. 

(Only one channel shown)

Assembled Rev2 Board

I simply combined parts from different circuits I found everywhere, without much verification.

This prototype managed to produce decent sound (after some tweaking). So I refined the circuit for the 2nd version.

(Only one channel shown)

I added a current mirror to further enhance the performance. This version produced good sound, but I goofed on the power supply circuit that was incorporated into the same board. The AC current ran too close to the output transistors and produced a faint hum. Although this was only noticeable using a very sensitive pair of headphones, I decided that this board was a failure.

While I was fixing the PCB design, it occurred to me that I should simplify the circuit. Using many transistors to more or less emulate the op-amp seemed pointless.

There was a simple amplifier topology that I liked called “current feedback amplifier” (I have made a small power amp using this topology and liked the sound before), so I decided to use it for a phono amp.

(Only one channel shown)

This simpler circuit produced unexpectedly good sound.

Q2R was included just so that I can experiment with the “beta-enhancer” for the class-A gain stage. I also assembled a second board without it to compare. It turned out that it wasn’t really needed.

After much tweaking of component values and adjusting the gains of each stage, this amplifier started sounding great. Using high-gain, low-noise transistors for the Q1 and Q4 (BC560C and BC550C) made a huge difference especially in very high frequency representation.

One drawback of this (and all single power supply amplifiers) is that they are very sensitive to the power supply noise. I was testing this amp with battery power (which is the best power supply for low noise), but I didn’t want to use battery forever. So I added an active filter circuit to eliminate ripple from the AC adapter that I was going to use. This filter can reduce the ripple voltage to at least -60dB. Upon hearing I simply can not detect any hum on the output.

Here’s the final circuit.

Assembled Current Feedback Phono + Headphone Amp Board

This deceivingly simple circuit produces amazing sound. And it’s more than reasonably low noise.

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Added a new project to Instructables on headphone amp. This is something that I have made to improve my music listening experience during my subway commute.

- See the instructable

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Now those who are not into surface mount soldering can build Aurora color organ (or VU meter)!

The circuit is exactly the same as the original surface mount version of Audio Interface.

You can watch the video here.

The interface goes between the power cable and Aurora, using Molex 3 pin connectors. The interface has a 3.5mm stereo audio jack, so you can connect any audio output. There’s a potentiometer to adjust the sensitivity level as well.
Aurora 12 bar, Aurora 9×18 mk2 and Aurora 18×18 have audio or “color organ” modes, which you can select by either pushing the on board button or using a remote control (remote not on Aurora 12 bar).

> purchase the kit or PCB

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You can watch the video here.

The interface goes between the power cable and Aurora, using Molex 3 pin connectors. The interface has a 3.5mm stereo audio jack, so you can connect any audio output. There’s a potentiometer to adjust the sensitivity level as well.
Aurora 12 bar, Aurora 9×18 mk2 and Aurora 18×18 have audio or “color organ” modes, which you can select by either pushing the on board button or using a remote control (remote not on Aurora 12 bar).

- Schematics
- Audio Interface Part Placment Guide

Complete kit includes the 3 pin connection cable pictured (other cables are not included).

*** Purchase Here ***

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You can watch the video here.

The interface goes between the power cable and Aurora, using Molex 3 pin connectors. The interface has a 3.5mm stereo audio jack, so you can connect any audio output. There’s a thumb-wheel style trim pot to adjust the sensitivity level as well.
New Aurora 9×18 mk2 and Aurora 18×18 have audio or “color organ” modes, which you can select by either pushing the on board button or using a remote control.

*** Please check out the through hole version of Audio Interface here ***

- Download the soldering/assembly guide: Audio_Interface-PartPlacement

*** This item has been replaced by through hole version – see it here ***

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