(which is a current feedback amplifier)

Lately I have been looking again for a discrete transistor voltage gain amplifier for line level duty that isn't just another op amp.

I keep coming back to variations on this circuit. It's a diamond buffer input with current mirrors in the mid-section strapped around a voltage divider / feedback loop that provides the gain. This backs out into either another diamond buffer or, in the revised version below, a standard Sziklai output stage which can be more easily scaled up as needed for a headphone amp for example.

An offset bias adjust circuit could be added to trim the output offset voltage. Or use a coupling cap. It's a few hundred mV otherwise.

Circuit gain is R4/R6, approximately. The total value of R4+R6 should be kept about 20 kohms. C1 is a compensation capacitor. Circuit gain as shown is 14 dB, -3 dB at 250 kHz.

As with all CFA, the choice of the feedback resistance...

Introduction

This is my build log for relatively basic line preamplifier based on rev. 30f boards of the Sapphire3 headphone amplifier. I modified the circuit to run at lower currents (about 10 mA output bias) and adjusted the gain settings to 10/16 dB.

It is built in a Hammond 1550 cast auminum chassis, with an external Plitron 160VA 2x12VAC rectified power supply. The volume control is a 50k Goldpoint V24 stepped attenuator, while the RCA jacks are rhodium plated from Oyaide. The feature set is limited to two switchable line inputs and an output mute.

Chassis Layout Notes

Audio components are conventionally designed as rack-mounted equipment with all controls on the front panel and all connectors on the rear panel. To try and keep internal cabling to a minimum I'm modelling my preamp more like a recording console with both the controls and I/O on the top plate.

Build Notes

All components mounted...

I've had quite a few requests for the bboard buffer circuit without the built-in regulators, so here is a bboard 2.1 standalone 2-layer board, measuring 5x8 cm. Gerber files attached in zip file.

It is designed for +/-12 V rails, but the circuit will work with anything from +/-5 V to +/-18 V. A regulated power supply is recommended.

This is a line buffer. It intended to drive cables, not headphones.

Available for $15/pair shipped. Several people have asked me about kits. I figured the BOM was so basic it wouldn't be necessary but I can send you the boards with the parts to populate them for $50/shipped. You will still need to provide the power supply.

BOM attached. (updated to 21f4)

Although the original Sapphire headphone amp can be configured as a line stage, or use as-is as a line stage, I've gone ahead and made a new circuit variant with a new set of boards.

The Sapphire Line (in development) combines the shunt-series regulator, bboard 2.0 buffer and an op amp voltage gain stage. Same basic idea as the Sapphire of course, but with a much less beefy output stage so the low noise regulator can be added and everything still fits on the board.

rev 10e - now with support for 2520 op amp modules

Asus Xonar Essence STX, Audacity 2.1.2, VLC, Windows 10 [and DigiOnSound6 Express for 24 bit recording]

Purpose

To confirm the calibration of the sound card input and output gain. Also, to determine the relationship between the signal voltage, the recorded signal amplitude displayed in Audacity, and the signal peak and noise baseline levels in the FFT spectra.

Summary

* Setting the volume slider of the device output to 100 gives 1 V rms output for an amplitude 0.5 sine wave.
* Setting the volume slider of the device recording line input to 100 gives records a 1 V rms tone as an amplitude 0.5 sine wave, which is displayed in the frequency spectrum (FFT) as peak of magnitude 0 dB in Audacity when both channels are averaged.
* volume setting 100 needed for unity gain loopback.
* 0.5 amplitude sine wave = 0 dB FFT = 1 V rms.
* noise baseline in averaged stereo FFT is 3 dB lower than single channel measurement....

The original VSPS is getting up in years, and could do with a bit of an update.

The VSPSX adds the foursquare shunt-source voltage regulator and bboard v2.1 output buffer.

Development thread here.

[updated attached content to rev. 1.0a4]

Two mistakes in previous BOM:

C4 was a 5mm lead spacing Wima MKP but the board spacing is only 2.5mm. The part is changed to Wima MKS with 2.5mm lead spacing.

R22,23 was incorrectly given as 47.5k, it should have been 4.7k and has been since updated to 10k. This is not a critical value, the circuit will work with any of the resistances.

[further work]

The boards have been revised to 1.2a. The new design is posted to the development thread.

I'm not totally sure this would work as advertised, but I can't see any obvious reason why it would not...

It's pretty much the same circuit as I used in the CrystalFET, which started out in a previous blog post in the Voltage Regulators for Line Level Audio series, but here I've replaced the MOSFETs with bipolars. It is shown configured to deliver 20 mA @ 12 V, split supply. Enough to power an op amp phono stage for example, or a preamp, or the voltage gain stage of a headphone amplifier.

[developed further under the S-Reg name, with pcb layout.]

Development thread here. Web page here.

CrystalFET is a J113 jfet-based two-stage phono preamp, with passive equalization and on-board MOSFET-based shunt voltage regulator.

Jfets Q1 and Q3 should be matched between channels, and for best results the value of drain resistors R2 and R9 should be selected based on the jfet pinch off voltage of Q1 and Q3 respectively.

rev. 1.4e (final version) removes some unneeded resistors and tidies up the board layout a little. The resistors have been renumbered.

Measured at 24/96 with my Asus Xonar STX soundcard (~ -147 dB noise floor)

The Chromecast Audio output noise powered with the included USB wall wart supply is -130 dB at 1 kHz, rising gradually at lower frequencies and showing some switching power supply noise peaks at 4763 Hz and higher multiples, never exceeding about -120 dB.

This is respectable performance given its price point.

A while back I did a series of blogs on voltage regulators. Back with a new entry today: The Crystal M, configured here for 40 V DC output and a 25 mA load.

The circuit is based on two p-channel MOSFETs, the top one is a constant current source, the bottom one a constant voltage source. As the load current changes, the voltage source adjusts its current to balance.

It's lifted directly on the Salas shunt design (as reworked by me for my own jFET phono stage), but the circuit can also be considered a distant, DC-coupled relative of the Zen amp.

I trick, I discovered, to getting it to work nicely - the attached screencap shows it well-behaved while handling a full-swing output current pulse - is the source resistor R10. This resistance dials-down the current gain of the MOSFET, damping out the overshoot.

The ripple rejection is about 70 dB over the audio bandwidth. The output impedance is about 0.05 ohms over the same frequency...