I've been meaning to get around to updating this by folding in the improvements to the diamond buffer stage made during development of the Sapphire 3 headphone amplifier. Here is the first look of the bboard v2 under LTSpice.
I've gone back to simple emitter resistors on the input, running under much lower current to keep the input impedance high. The output is simplified to a basic Sziklai compound transistor pair with the bulk of the bias current running in the second transistor.
In terms of distortion, for line level output level, CCS loaded input has no advantage. I'll have to double-check PSRR and a few other things before signing off on this version though.
The VSPS was a side-project that grew out of the Phonoclone, and actually ended up first out of the gate as a working circuit.
Apart from the general design philosophy (low parts count, simplicity, careful layout and grounding) it has no particular link to 47 Labs. While the concept of a non-inverting op amp active phonostage is nothing original the circuit is mine, particularly the configuration and values of the RIAA filter which I calculated and simulated myself. The rest is an amalgam from a dozen or so different sources, textbooks, datasheets and application notes &c. All the values are quite carefully chosen.
That said I've always put the circuits and everything else on the internet, with source attribution as I felt necessary. The boards and kits came...
On seeing the Gilmore circuit again the thought process re. a Sapphire+Gilmore went something as follows,
"Toss out op amp, convert the Gilmore dual-LTP front end to bipolar, bolt the Sapphire3 buffer stage to the back, and substitute in the Sapphire3 current sources. Wrap in a mild feedback loop."
The result is shown attached. The Vbe multiplier is still a simple resistor (R33) ... that may need to be refined to add thermal throttling. The offset servo is not shown, but the action is shown as Vadj. Alternatively a trim pot would be placed between R30 and R32 to provide a small measure of offset adjustment. Most of the open loop gain is controlled by R14,R15 ... it seems to me that some work could still be done in that area. Despite going...
Posted 22nd February 2015 at 01:24 AM byrjm (RJM Audio Blog)
Updated 28th February 2015 at 06:17 AM byrjm
I've added an additional RC filter stage (R3, C4 in the schematic below) before the Zener diode, substantially reducing the amount or ripple on the transistor base by cleaning up the voltage applied to the Zener reference. (The original Z-reg is described here.)
Circuit shows C2 with a value of 300 uF. Typically much larger values are used. I kept the filter capacitance to a minimum here to show circuit working with a reasonably high ripple (1 V p-p) on the input. The rectifier diodes used here are of no particular consequence, I just wanted the simulation to generate a realistic sawtooth for the input.
OK, this doesn't do as much as I originally thought. The improvement is mostly below 100 Hz, whereas the ripple is mostly in the 100Hz-1kHz band. There's perhaps 3 dB less output ripple, but that's about it. You can verify this yourself in LTSpice, just cut the wire between C4 and the junction or R1-R3 and rerun the sim.
Posted 31st January 2015 at 12:28 PM byrjm (RJM Audio Blog)
Updated 18th March 2015 at 01:52 AM byrjm(add photo of finished amp)
A couple of years ago I built a standard op amp + diamond buffer headphone amplifier, called the Sapphire.
My original circuit (Sapphire 1.x) was the simple four transistor four resistor diamond buffer of the LH0002. Later small resistors (Sapphire 2.0) were added to the emitters of the driver transistors to boost the output bias current.
In this next go-round (Sapphire 3.0), I've replaced the emitter resistors with current sources. This provides a significant improvement in PSRR, over 20 dB in simulation. The output pair has been reinforced in a Sziklai configuration for lower distortion, and the primary output transistors five-way paralleled for improved thermal stability. The output impedance is 1~2 ohms, limited primarily by the output resistor.
It simulates to <-100 dB harmonics for 0 dB (1 V rms) output into 60 ohms. The total circuit standing current is less than 50 mA per channel.
Posted 3rd October 2014 at 12:40 AM byrjm (RJM Audio Blog)
Updated 3rd October 2014 at 09:27 AM byrjm
What we are looking at here is the Fast Fourrier Transform (FFT) of the line output from my b-board buffer recorded at 24 bit, 96 kHz by an Onkyo SE-200PCI sound card. Upstream from the b-board is the Phonoclone 3 MC phono stage, connected to a Denon DL-103. The tonearm is Denon DA-307, and the deck is a Denon DP-2000.
Four recordings, taken 1) with music playing, 2) with the tonearm raised 3) with the phonoclone powered off and 4) with the b-board and all upstream components powered off.
True 24/96 data was obtained, measurements out to 48 kHz are possible, with -130 dB noise floor. (I was using Digionsound 6 to do the recording as Audacity truncates 24 bit recordings to 16 bit in Windows due to licensing issues. The FFT was generated in Audacity however.)
The soundcard's line input may have an impressive-looking low noise floor, but it's still useless for measuring line level audio devices like the b-board because the noise of the preamp/ADC...
Posted 3rd April 2014 at 01:24 AM byrjm (RJM Audio Blog)
Updated 3rd April 2014 at 11:07 AM byrjm
A set of Sapphire boards gave the proper V+, V- voltages out of the Z-reg, providing about 10.5 and -10.5 to op amp power pins. The output offsets were unusually high however, apparently at about 2 V in one board, and somewhat less in the other. Typically the offsets are in the order of +/-15 mV.
Changing out transistors and op amps did not help, and to all inspection the passive components were installed correctly and working properly. The offset voltages were extremely temperature sensitive. Measurements for the various circuit voltages were just screwy enough to be inconclusive.
I could ask for no more tests, so requested the boards be sent back to me. I found the circuit basically worked as expected, but the offsets were indeed high on both boards, though I measured 0.6 V max rather than 2 V.
***** stop here and make a guess *****
Blowing on the board through a soda straw, the offset shot up when I blew on...
Posted 17th December 2013 at 10:10 PM byrjm (RJM Audio Blog)
Updated 20th December 2013 at 10:14 AM byrjm
Straightforward transplant. Out with the old (anyone want them?) in with the new. Re-used the OPA134 op amp and my dog-eared pair of 0.47uF Multicaps.
On powering up I discovered that with the specified 10 ohms in R9,10 the output bias current was upwards of 200 mA and things were getting a bit toasty. I paralleled a second 10 ohm resistor, dropping the resistance to 5 ohms and dialing back the output bias current to about 70 mA. Latest schematic revision has R9,10 values edited to match.
Currents stable. Heatsink temperatures around 50 C. Output offsets around 15 mV. No noise or hum.