Posted 19th January 2017 at 11:37 PM byrjm (RJM Audio Blog)
Updated 22nd May 2017 at 02:26 AM byrjm
This is an unfinished design idea.
The Sapphire4 and Unity (H) boards were based on the 47 Labs Treasure 0347 amplifier circuit, and the original was not so far away that I couldn't "de-modify" the Unity (H) board back to what is essentially a 0347 (but with neither the original transistors nor the original resistor values).
I changed just about all the resistor values slightly for one reason or another, and changed all the transistor types. I've run my own simulations to check my version basically works as it should. Output BD135(39),136(40) are shown on the schematic as being leftovers from my headphone design, they will work to play music but to meet high current output specifications something bigger in TO-220 or similar thermal package will be needed.
As I hope you can see from the circuit schematic, the design - for a power amplifier - is unconventional. There is no feedback...
Comparison of the noise baselines, measured at the circuit output using a NI USB-6215 DAQ. Unloaded for the preamps, and with a 6 ohm load for the 0347 amplifier.
B-board: -139 [300Hz-100kHz] 0 dB gain (-139 - 0 = -139 dB, 110 nV sqrtHz input referred). The actual B-board output noise is below this measurement threshold.
0247: -124 dB [300Hz-100kHz] 14 dB gain (-124 - 14 = -139 dB, 110 nV sqrtHz input referred).
0347: -109 dB [300Hz-100kHz] 31 dB gain (-109 - 31 = -140 dB, 100 nV sqrtHz input referred).
See the attached plot for the FFT data. Note the peak at 28 Hz is an artifact of the measurement apparatus.
By way of comparison, a typical audio opamp has an input referred voltage noise figure of 3-8 nV sqrtHz (-170 ~ -160 dB) and can be expected to return this datasheet specification in most well-designed circuits. In other words the output noise is going to be about -160 dB + the circuit gain.
Posted 22nd January 2012 at 11:52 AM byrjm (RJM Audio Blog)
Updated 25th January 2012 at 08:57 AM byrjm
So now we have the model 0247 DAC/preamp/headphone amplifier and model 0347 power amplifier in the main system. That's the old Denon DP-2000 TT, Phonoclone 3, and Onkyo D-605SR speakers.
The gain of the 0347 is 30 dB, so with the 0247 doing duty as a preamp the overall gain is a tad high, but it's perfectly useable. I'll look at the passive preamp options (including the model 0447) at a later date.
Anyhow, the big news is it completely destroys the non-inverting LM3875 Gainclone I have been using for the last decade, so badly, in fact, I'm starting to wonder if my Gainclone was busted. In comparison with the 0347 it sounds like a distorted, uncontrolled mess.
Since my speakers are really very modest in quality (though at 82dB and 6 ohms, a bit of a rough load) I wasn't expecting much difference one way the other, so this comes as a severe shock.
Unfortunately I'm not within reach of a good reference system to give the 0347 a real evaluation....
Posted 20th January 2012 at 12:48 PM byrjm (RJM Audio Blog)
And not even sure what a tribulation is, but I'm sure I've had at least one building this amp.
Like the 0247 it's not so much difficult as it is painstaking hard work. The output transistors have to be mounted "just so" so the boards screw to the case properly, the wiring of the thermal protection transistors and gluing them to the output devices is tricky and easy to mess up.
I triple-checked and was rewarded by, it seems, error free boards. I messed up and wired the mute switch incorrectly though (the photos show it wrong, in case you come across this blog post looking for help) and I think the switch has a fault somehow as L and R channels appear to me mixed together. Taming the offsets gave me considerable grief but I think its all OK now.
Tomorrow I'll hook it up to the speakers and see what happens.
Posted 17th January 2012 at 11:52 AM byrjm (RJM Audio Blog)
Updated 25th January 2012 at 11:21 PM byrjm
I like the concept of appearance in high-end audio. I mean, where the look of the equipment makes a statement about what it's all about inside.
The 0347 is proportioned like a vintage tube receiver, but in miniature. It even has the lit, transparent front panel where the radio tuning strip would have resided. The heavy grey hammertone and very simple, basic hardware are reminiscent of Heathkit or Dynaco. In short, the appearance evokes the spirit of a simpler, more innocent age of vintage hifi without being a slavish replica.
61,500 yen delivered, a tad more than the model 0247 headphone amplifier / DAC. Basically lose the volume control and DAC board, gain a bigger transformer and a thick aluminum bottom plate that serves as the heatsink... the amplifier circuit itself is almost identical, just with a slightly bulked-up output stage and the addition of certain safety considerations such as fine tune-able bias current and thermal protection for the output transistors.
Which makes an unusual circuit for a power amplifier: the output stage is a unity gain current buffer running open loop. For a headphone amp that's justafiable because the bias current is such that the amp will almost always be running in class A. For a power amp rated 40 W into 4 ohms, it's class AB.
Mr. Tsukahara seems pretty pleased with the result though, and I am anxious to try it...