Posted 23rd December 2016 at 06:02 AM byrjm (RJM Audio Blog)
Updated 28th December 2016 at 12:46 AM byrjm
Following on from this post and this post, we arrive in time for the holidays with Project Unity. Merry Christmas everyone.
Four circuits: Unity, Unity H, Unity B, Unity BH. line preamp, Headphone amp, line Buffer, Headphone Buffer respectively, all derived from a common base circuit called Unity Root. Unity Root is conceptual, it exists only as a reference so you can see more clearly how the four working variants relate to each other.
It's the "all for one, and one for all" approach to diyaudio, a single research and development line applied to a range of applications, feedback from any of the applications brought back to apply to the line in general.
This simplifies not just the circuit development, similar efficiency is also brought to the documentation, board layout design, and BOM... about which I'll have more to say in a bit.
Posted 16th December 2016 at 02:21 AM byrjm (RJM Audio Blog)
Updated 16th December 2016 at 02:28 AM byrjm
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.]
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...
Posted 28th July 2016 at 03:15 AM byrjm (RJM Audio Blog)
Updated 4th December 2016 at 11:55 PM byrjm(add measurement data)
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.
Posted 28th July 2016 at 02:05 AM byrjm (RJM Audio Blog)
Updated 8th April 2017 at 05:37 AM byrjm(added BOM)
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.