Companion regulator to the S-Reg. Same board dimensions and connection layout. For line level audio.

Same idea as before rectified 2x12 VAC input, +/- 11 V output. 50 mA max output current unless the transistors are heatsinked.

There is a small amount of over-current protection afforded by R3,4 but do not short the output for all but the shortest of transients.

Eagle/Gerber files attached, so you can go get this made yourself, optinally with whatever modifications you need.

I've use this regulator circuit in my Sapphire headphone amplifier. It's a simple and modest circuit but I feel it works really well in practice as long as the audio circuitry it powers has reasonable PSRR. No feedback means no out-of-band noise or instability, even as the output impedance remains low.

S-reg voltage shunt-source regulator for line-level audio circuits.

This is designed to accept rectified 12x2 VAC input, producing +/-12 V output split rails.

Features soft-start and over-current protection, standard configurations for load currents up to 50 mA. High performance: 80 dB ripple rejection up to 100 kHz.

update: BOM uploaded
update: web page (stub)

This is either an ingenious interfacing of the output buffer and current feedback amplifier by adding second arm to the central current mirrors ... or it's another really bad idea(tm).

It simulates nicely though.

[What's happening here is current output of collector Q9 is no longer being asked to bias and drive the buffer Q16. Instead Q2 and Q17 do that job. The change lowers distortion, improves bandwidth, and even raises the PSRR a little.]

(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...

This is my first op amp design which doesn't completely suck.

Now, it's a terrible op amp... don't misunderstand... (Nat Semi will not be making me any offers)... but it does perform the job I want it to do reasonably well: remain stable while providing 6-20 dB of line-level voltage amplification with low distortion, decent PSRR, and sufficient bandwidth.

The main limitation is the distortion at high frequencies rises to -70 dB. The circuit needs less open loop distortion, or more open loop gain above 10 kHz, or both.

You will note the circuit has no current sources. This is intentional. I wanted to see how far it was possible to get without them. Obviously headroom takes a big hit, but distortion and PSRR ended up better than I imagined.

This is a simulation. No guarantee it will work, and there are no safeties (current limiters, input voltage clamps, etc) shown.

PS. Frequency response in image is open loop, while...

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

Consumer audio standard line level output is -10 dB, 0.316 V rms [dB = 20 * log (V/1V)]. Some devices like computer sound cards can boost that at the max volume settings, my Asus Xonar can do 6 dB or 2 V rms. Quite a lot of digital audio produces 2 V rms output, DACs and CD players and not just computer sound cards.

The amount of output current required by the line driver is the signal level divided by the load impedance, so to estimate the worst case scenario we have to consider the smallest practical load and the largest likely signal. The input impedance of consumer audio is typically 10k to 100k. 10k is the lowest design point, but sometimes people do strange things like drive two components at once which halves the value, or headphones, or pro audio gear with 600 ohm inputs.

The long and short of it, though, is that consumer audio inputs are never normally going to draw more than 1 mA. For pro audio the maximum is meanwhile 3 mA. 5 mA bias current through...

A [just my opinion, bro] post...

I actually had occasion to try this the other week. I had a box with a volume control followed by the bboard unity gain buffer and in preparation for replacing it with a similar buffer with voltage gain (a power-derated Sapphire 3) I removed the buffer and briefly used the box as passive preamp, i.e. just the 47k stepped attuator, with 1 m interconnects to the amp and 2 m interconnects back to the phono stage. Sure enough the system noise increased, depending on the position of the volume control, with some nasty low level buzzing interference.

Why does this happen? It's pretty simple really. Noise is usually induced as a current, and the larger the resistance (impedance) this noise current is forced to flow through to reach circuit common, the larger the noise voltage since by Ohm's Law, V=IR. Noise induced between the volume control and the amp is faced with the high impedance of the amp (47k) or the output impedance of the...