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Voltage Regulators for Line Level Audio. Part II : The Pass Transistor

Posted 16th January 2014 at 04:14 AM by rjm
Updated 20th January 2014 at 10:21 PM by rjm

Part 2 of a series.

Instead of pulling the output current through R1, we add an npn pass transistor, Q1. The output current now "passes" through the transistor, while the Zener diode still regulates the output voltage by being connected to the transistor base.

The output impedance falls to a few ohms, but the ripple rejection improves only slightly.

This is a useful basic circuit block for audio, but the ripple rejection can easily improved by the addition of a couple of additional components, as we'll see shortly.
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Voltage Regulators for Line Level Audio. Part I : Zeners

Posted 15th January 2014 at 11:34 PM by rjm
Updated 16th January 2014 at 04:40 AM by rjm

This is the first of a series, where I will be investigating the output impedance and ripple rejection of various voltage regulator circuits using LTSpice.

Today, for the first "lesson" (I'm teaching myself, as much as anything) we will look at the very simple zener voltage regulator.

The load is 1 kohm, and the Zener breakdown voltage is 12 V. The load current is about 10 mA, and to avoid gross inefficiency we will limit the current flowing through the Zener to about 5 mA, by adjusting R1 accordingly. The input voltage is fixed at 18 V.

To measure the ripple rejection, we perform an AC analysis with the voltage source AC set to 1 and the current source AC set to zero. The ripple rejection is the negative value of the signal at Vout: so -20 dB means 20 dB ripple rejection (1 V ripple at Vin generates 0.1 V ripple at Vout at a given frequency.).

To measure the output impedance, again the AC analysis function is used but...
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Hakko FX-888 : ebay unhappiness

Posted 29th May 2013 at 12:12 PM by rjm
Updated 29th May 2013 at 11:59 PM by rjm

Oh, for Heaven's sake...

Just got the FX-888 soldering station I bought on ebay.

It does not power up.

Do you know why it doesn't power up?

So glad you asked...

It does not power up because - pause for effect - the fuse board that fits on the power transformer is inserted the wrong way round. That's right, the full "rotated 180 degrees" deal.

Fortunately I have another soldering iron. You know, so I can fix my soldering iron...


I would just return it, but the shipping would cost me half again what I paid for it. Perhaps ebay will refund me anyway. We'll see.

I think I know what happened: The people selling these are modding them by changing the voltage and power cord. The soldering stations are officially bound for the Chinese domestic market, 220 VAC. The box, when it came, had "110V" hand-written on it, though the instructions...
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A catalog of headphone amplifier circuits.

Posted 19th May 2013 at 10:45 AM by rjm
Updated 21st May 2013 at 12:12 AM by rjm

Lots of circuits out there, but most are variations of a small set of archetypes. Let's see if I can put together a list:

1. Dedicated headphone amplifier IC. e.g.(lme49860)

2. Battery powered, single stage, generic audio op amp. The ever-popular mint tin cmoy.

3. Op amp + buffer (complementary transistor pair, diamond buffer, unity gain op amp, etc, in either integrated or discrete package.)
a. closed loop connection or "compound amplifier" configuration as developed by Walt Jung.
b. open loop, two stage circuit, e.g. nwavguy o2 and my sapphire amp.

4. simple 2 or 3 transistor "introduction to electronics"-style amplifier

5. The "little big amp", a scaled back version of a transistor or vacuum tube power amplifier design. (Zen, DoZ, transformer coupled SET amps)

6. The power follower. Single-ended MOSFET or BJT, with or without CCS load, voltage...
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Audacity sucks.

Posted 9th May 2013 at 02:50 PM by rjm
Updated 10th May 2013 at 03:35 AM by rjm

Under windows at least, Audacity is unable to record audio at bitrates above 16 bit.

It will seem to, all right, but the data is quantized at 16 bit (30 microvolt LSB), regardless of the settings chosen.

The attached images show the same source, the first recording is made in Audacity, supposedly 24 bit, but actually only 16 bit, while the second is recorded with a program than actually supports 24 bit, exported, and imported into Audacity. The data is amplified +70dB in both cases to make the difference visible.

Audacity will happily manipulate and save high bit rate data, but as a result of licensing restrictions and on account of it being freeware, it does not support the actual recording of this data.


Any internet search will confirm that the Windows version of Audacity is limited to 16 bit recording. And yes, it's more of a limitation of Windows than it is of Audacity. My irritation, however, is chiefly with Audacity...
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Google+ DIY Audio community.

Posted 7th December 2012 at 10:06 PM by rjm
Updated 14th April 2013 at 11:14 PM by rjm

There was no DIY Audio community on Google+.

So I made one.

Google+ has always been a "quiet room" where like-minded people got together to talk about things, away from the noise and chaos of facebook.

The problem was that it was difficult to find said like-minded people. They were there, but for the majority you had no way of knowing that.

Well, now Google has fixed that by introducing "communities". It's a user-created hub, a digital meeting room ~ salon ~ lounge ~ front porch, a designated gathering place associated with a certain hobby, interest, or topic.

This feature is brand new, so I have no idea whether it will work. It could, like facebook, devolve into noise and spam... or it might be useful and worthwhile. I think its worth a shot, anyway.

I created the group, but it is free and open. It's not mine in the sense I don't intend to use it as a platform. I have my own G+ page for...
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NwAvGuy odac 24/96 DAC review

Posted 23rd June 2012 at 07:02 AM by rjm
Updated 2nd July 2012 at 04:52 PM by rjm

I cased up my odac board (NwAvGuy via jtktam) in a small aluminum project box.

The DAC is a simple two-chip affair, with a Tenor TE7022L controller fronting the ESS ES9023 DAC and integrated line driver. A couple of voltage regulators, the clock oscillator, and an eeprom chip round out the principle component list.

I compared it with my Onkyo SE-200PCI sound card. This 24/192 (115dB S/N A-weighted, 0.003% THD 0dB 1kHz) PCI card sells for about $15,000 yen and is based on the VIA Envy24HT and Wolfson WM8740.

I'm listening to 16bit 44.1kHz .wav (CD rips), though VLC [sample rate converter set to sinc, best quality, resampling quality 8]. Windows 8 release preview [default format 24/192 (onkyo), 24/96 (odac)]. Line out though Oyaide PA-02TR interconnects to the Sapphire headphone amp, and Sennheiser HD-600s.


So, I was planning on writing up a big 'ol review with my impressions, but, well... there's not really a...
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Project "Originality" Headphone Amplifier by Tao Kurohane

Posted 11th June 2012 at 12:35 PM by rjm
Updated 11th June 2012 at 11:32 PM by rjm

It is certainly a little bit different. One might be tempted to say "gilding the lily", but come on, headphone amplifiers are just the right place for these indulgences.

Building your own long tailed pair (LTP) to bolt in front of an IC op amp has fallen out of favor in recent years. I must admit I couldn't see the point then, and still don't.

I've seen a number of headphone amp circuits with 3 paralleled pairs of output devices. I wonder if there any real advantage over simply using one pair at 3x the current, perhaps with slightly larger transistors?
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Stepped Attenuator

Posted 8th May 2012 at 12:20 AM by rjm
Updated 8th May 2012 at 12:24 AM by rjm

I bought a stepped attenuator (actually, two of them) from ebay seller hanshare-electronics. As always with eBay, I was a little worried about what would show up.

I'm happy to report that this is a nice job. The included resistors are clearly labelled, and the documentation, while spartan, is sufficient.

The only negative is the rotary switch is a bit "clicky". Better quality units I've seen are more damped.

On the positive side, though, the resistor values are very elegantly selected, to give a smooth response of about 4 dB per step from -60 to -40 dB, and 2 dB per step from -40 to -3 dB. Having been-there, done-that, I can say that I'd happily pay the asking price to get those resistors sourced and sorted for me. Next time, I'd also pay to get it assembled. Soldering was more painful than I imagined it would be, and the pre-assembled units much neater-looking than what mine ended up as.
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RJM B-board vs. 47 Labs 0247

Posted 5th May 2012 at 12:29 PM by rjm
Updated 6th May 2012 at 06:16 AM by rjm

B-board vs. 0247.

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.

The 0247...
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