Posted 3rd October 2014 at 01:40 AM byrjm Updated 3rd October 2014 at 10:27 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 23rd August 2014 at 12:33 PM byrjm Updated 27th August 2014 at 07:39 AM byrjm
I suppose everyone has at one point or another adjusted the volume sliders in Windows. The ones that go from 0-100, and you are never quite sure what whether its a boost, or an attenuation, or what.
Some years ago I measured the outputs and inputs using a fixed amplitude .wav file created in audacity and played back through the Onkyo SE-200PCI. I've taken another look at the worksheet I made and I've noticed that the volume settings correspond to very logical, even steps, namely:
100 0 dB
90 -1 dB
80 -2 dB
70 -3 dB
60 -4.5 dB
50 -6 dB
40 -8 dB
30 -10 dB
20 -14 dB
10 -20 dB
or for the mathematically inclined: 20*log(volume/100)
This scale is the same for both the output master volume and the line input, so its probably maintained throughout the operating system.
Posted 20th May 2014 at 01:01 AM byrjm Updated 20th May 2014 at 03:15 AM byrjm
The Technics SU-9070 is a 2U rack mount stereo preamplifier from 1977, matched to the SE-9060 amplifier. The preamp was sold as the SU-9070II in Japan.
The circuit is shown below, for educational purposes.
The power supply regulation is quite elegant, I hope to get to that in a future post. Here, just note that there are separate regulated lines for the MC stage, the input/VAS sections of the MM and line amps, and the output sections of the MM and line amps.
Posted 19th May 2014 at 01:04 AM byrjm Updated 20th May 2014 at 12:58 AM byrjm
There was a series of relatively slim (2U chassis rather than 4U), upmarket audio separates put out by Technics in 1977: the ST-9030T tuner, SU-9070 preamplifier, and SE-9060 amplifier. They are two degrees of separation from the top of the line models at the time, the range went 9600>9200>90x0.
I recently acquired the tuner (more on that some other time) and have the others in my sights.
For educational purposes, the amplifier schematic for the SE-9060 is shown below. The input stage and voltage amplifier was driven from regulated 55 V supplies (Va+ Va-), while the driver stage was powered directly by the rectified DC at about 50 V (Vb+ Vb-) filtered with 18,000 uF per rail per channel.
Note that there were 9060 and 9060II as well as 9070 and 9070II models in Japan, but the export model of the preamp which sold as the 9070 was actually the 9070II rather than the 9070 domestic version. Likewise the SE-9060 shown below (from a European...
Posted 25th April 2014 at 11:13 PM byrjm Updated 26th April 2014 at 08:19 AM byrjm
As a companion post to the GeminiPS I thought I'd throw the amplifier circuit out there too...
It's not something you'd have any reason to built today I think, but some of the ideas are worth revisiting.
The output stage is what is normally referred to as a complimentary Sziklai pair. The LTSpice circuit below uses the same output, but with the diamond buffer type bias, with it all scaled down to headphone-amplifier voltages and loads. It would be interesting to compare it against i.e. the conventional diamond buffer used in the Sapphire headphone amp. Maybe I'll get around to it. The simulation shows a bit more transient peaking than the straight diamond buffer, ideally there could be some way of adding compensation / reducing the bandwidth to more reasonable levels.
The GeminiPS is another discrete series voltage regulator, with a Zener reference and bipolar pass transistor. It's an old circuit, published in Practical Electronics in 1970-71, and written by D.S. Gibbs and I.M. Shaw. I happen to have a reprint, but there's a nice overview here.
For reference it might be worth checking back to the two transistor regulator. The GeminiPS circuit is related in the sense that it is a more sophisticated take on the same basic principle. With just a handful of components we have a stabilized, 30 W output with soft turn on and short circuit protection. The circuit can be scaled up and down relatively easily, and the complimentary (negative output) version is an easy modification.
The pass transistor (TR2/3, Q2/3) is between the circuit common and the rectifier anodes. This may seem odd, but it was relatively common back in the day when high voltage transistors were both expensive and rare. The...
I've been meaning to take up shunt regulators for some time. I've never got around to building one myself to try, so I'll have to make do by playing in simulation.
Today's circuit is the shunt analog of the Z-reg series regulator: no feedback, Zener reference, single transistor regulation. The output impedance and ripple rejection-characteristics are similar too, with about 40 dB of RR and an output impedance of just a few ohms. It can be built equivalently from either an pnp or pnp transistor. (See attached LTSpice .asc files.)
The difference between shunt and series regulation can best be explained by considering the upstream power supply: In a series regulator an increase in current demand by the load causes the regulator to increase the current to compensate. In a shunt regulator an increase in current demand by the load causes the regulator to decrease the shunt current to balance, so there is no net change in current flowing...
Posted 3rd April 2014 at 02:24 AM byrjm Updated 3rd April 2014 at 12:07 PM 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...
So you have a small handful of parts and want to build a (simple) discrete voltage regulator instead of using an IC. What to do?
For line-level audio circuits, especially op amp based (IC or discrete) preamps with high PSRR, something like the Z-reg is generally sufficient. Robust, works well, has enough ripple rejection to cut power line noise from the preamp output.
If you add just a couple more parts, however, you can add feedback to the Z-reg circuit, a simple error amplifier in the form of an additional transistor Q2, with the output-sampling voltage divider R1,R2.
The ripple rejection is not vastly superior to the circuit without the feedback unless some additional bypass capacitors are added as shown in the first version of the circuit below. The output impedance, however, improves from a few ohms to a few tenths of an ohm as a result of the feedback. Which could, in principle, be of use.
I'm not going to spend too much time on this one. The idea is to increase the input impedance of the pass transistor by buffering it with a jFET so it will support a high-impedance passive CRCRC filter section that generates a low noise reference voltage. The reference is defined not by a Zener or diode stack, but by a simple voltage divider. There is a LM317 pre-regulator on the front, but it is traditionally configured and works independent of the following circuit so it is omited here together with the additional transistor that speeds up the charging of the reference voltage filter capacitors.
The basic problem is that lowering the noise of the reference cannot lower the output noise indefinitely. After a point the output noise is defined by the performance of the pass transistor instead.