Posted 21st April 2017 at 01:05 AM byrjm (RJM Audio Blog)
Updated 21st April 2017 at 04:44 AM byrjm(revised to 2.0g (capacitors C9 and C10 misslabelled))
In hindsight the original CrystalFET suffered from overreach. To obtain the 55 dB midband gain required for an MC phono stage lots of other things I might have wanted to build into the design like degenerative feedback were discarded.
Degenerative feedback as it applies to JFET amplifiers is the practice of leaving the source resistor partially or completely unbypassed. It reduces the voltage gain, but linearizes the dynamic operation of the amplifier - coercing the gain to the ratio of the source and drain resistors rather than leaving it defined by the JFET transconductance. The circuit gain can then be fixed by the designer without worrying so much about matching devices.
The CrystalFET circuit is a traditional two stage passive photo preamplifier design, the only novelty here is the addition of a second JFET configured as a source follower to each amplifier stage. To obtain above 50 dB gain no degenerative feedback could be used, and the JFETs had to be not...
Posted 15th February 2017 at 11:11 PM byrjm (RJM Audio Blog)
Updated 14th April 2017 at 04:19 AM byrjm(BOM 40b8 and photo uploaded)
The Phonoclone 4.0b applies a major revision to the board layout, making it more logical with grouped, standardized I/O connections. The X-Reg has been replaced with the S-Reg shunt regulator circuit. Overall the BOM has been considerably simplified and the parts cost reduced.
For the moment, the boards are back-compatible with earlier Phonoclone revisions, with the anachronistic 2.5"x3.5" mounting holes. Future versions will move to 70x90mm mounting holes to be compatible with the Sapphire and CrystalFET boards.
The BOM and board connectivity checks, no issues.
The Bourns trimmer, R19, comes from the factory set to the middle of the range so it can be used as-is and the S-Reg will work out to be almost perfectly balanced.
I reduced R17,18 to 6.8k (and R9,10 to match because it makes little difference). This lowers the V+, V- regulated output to about 8.6 V instead of about 12.5 V...
Runs off 100VA or higher power transformer, 2x15 VAC secondary, bridge rectifier, 2200 uF filtering or more on the power supply rails in addition to what is on the main boards.
The outputs go to a mute toggle switch and then a 2A fast fuse before connecting to the speaker terminals.
In the original, Q9 and Q10 are taken off board and glued to the face of Q13 and Q14 packages. This provides overtemp control, as the output transistors heat up the Vbe of Q9, 10 decreases - reducing the output bias current.
C2 was not present on the original. I include this compensation capacitor as an optional tuning option, though my own tests show it shouldn't...
Posted 23rd December 2016 at 05:02 AM byrjm (RJM Audio Blog)
Updated 27th December 2016 at 11:46 PM 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 20th December 2016 at 04:47 AM byrjm (RJM Audio Blog)
Updated 7th March 2017 at 07:09 AM byrjm(BOM 16b2 uploaded)
I've come this far so I might as well complete the trifecta.
This is the X-Reg circuit, with a new layout and component numbering to match the new S-Reg and Z-Reg boards. Each is a drop in replacement for the other.
Like its siblings, the X-Reg is a low current voltage regulator for line level audio. The output is about 9 V (adjustable up to 12 V) and the maximum load current is 50 mA without heatsinks, 100~150 mA with small heatsinks on Q1,2.
It is not a true regulator as there is no fixed reference and instead the output voltage is defined relative to the input voltage. The high open loop gain of the op amp is harnessed for very low noise and very high ripple rejection. It is necessarily a high feedback approach.
So, there you are: three mix-and-match power supply options for all your low voltage, low current audio needs.
Posted 19th December 2016 at 12:55 PM byrjm (RJM Audio Blog)
Updated 7th March 2017 at 07:11 AM byrjm(BOM 11b2 uploaded)
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.
Posted 16th December 2016 at 01:21 AM byrjm (RJM Audio Blog)
Updated 16th December 2016 at 01:28 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...