Posted 19th April 2013 at 11:24 PM byrjm Updated 20th April 2013 at 06:04 AM byrjm
Changed the collector load on the voltage amplifier stage to a current source (Q3), as per the Marantz SR2285B circuit.
Also increased the resistance of the feedback connection, R6+R8, there seemed to be no obvious advantage in making it much smaller than the typical load (>10k). The compensation capacitor C2 is increased to match, to flatten the HF response.
The circuit can drive light loads to +20 dB. Of course that's not much of a challenge for this general class of circuit.
I'm a bit stumped as to what the next logical step is from here. Seems to me to depend on what you actually want the circuit to do.
Posted 16th April 2013 at 02:18 AM byrjm Updated 16th April 2013 at 01:23 PM byrjm
Over the last couple of years most of my interest in audio has been with transistors. I've been slowly teaching myself to read and understand the circuits.
Circuits like this one for example. Not hard, but still a bit too complicated for me to understand without the helpful wikipedia markup attached.
Instead I've looked at primarily at the schematics I have for discrete audio preamplfiers, 1970's vintage typically. Based on what I've learnt so far, I've done up a "test mule" in LTSPICE, shown below.
It's not a circuit you should build. It's for pedagogical purposes, though it does actually work reasonably well - in simulation anyway. Its just a simple starting point to observe how the different parts interact under simulation.
If you don't think about it too hard, you'd imagine that the signals in the phono stage would be smaller than the signals in the line level preamplifier stage that follows it. Or that the signals in the DAC/CD player would be about the same level or slightly lower than the signal in the preamplifier.
As usual, the answer is "it depends". It depends on the sensitivity of your speakers, how loud you are listening, and the voltage gain of the amplifier. It also depends on whether we are talking about a MM phono stage or low output MC.
My point is simply this: the volume control is an attenuator, and at the typical "9 o'clock" position the input signal is reduced in magnitude by about 35 dB.
That cuts it back down to being comparable to the output of a moving magnet cart, and much, much smaller than anything found in a DAC stage.
It means you absolutely, definitely, positively must spend as much effort and care...
Posted 19th March 2013 at 07:02 AM byrjm Updated 20th March 2013 at 02:54 AM byrjm
Mid-range 1970's stereo receiver.
I was curious to find out a) what the phono circuit was, and b) how tight the RIAA response might have been.
The answer is "four transistors" and "pretty damn good", respectively.
We are impressed. These Japanese engineers knew a thing or two. I would like to see some of these old circuits resurrected as discrete phono stages with modern components to see just what they are capable of.
Results shown below, together with the LTSpice .asc file you can use to play around with this yourself.
First attached image shows FFT for the rectified DC (green), reference voltage (red) and X-reg output (blue) for the designed-for 10 mA output (top) and a more punishing 100 mA (bottom).
Second image shows an LTSpice screengrab for the LT1086 with bypassed adj pin under comparable loading. Input voltage in blue, output in green. This is a reasonable approximation of a "good" IC regulator.
Last image shows a plot of the exported LTSpice FFT data for the X-reg and the LT1086-12V (Cin 1000uF, Cout 100uF) both at nominal currents of 10 mA. The LT1086-12V is a reasonable substitute for a generic LM7812, i.e. a "bad" IC regulator.
A typical op amp will have sufficient PSRR to mop of the residual noise from the bypassed LT1086. The fixed LT1086-12V, on the...
Posted 14th January 2013 at 12:29 PM byrjm Updated 28th January 2013 at 01:33 AM byrjm
I couldn't even build it myself for the price they are selling it at, $50 on ebay. It caught my eye for the linear, split power supply. I love the chrome 3-pin power connector, too.
As is usual when you buy cheap ultra-Chinese audio gear via eBay some adjustments are needed, however.
The basic problem seems to be a little mix up with the input coupling capacitors. It came with polar electrolytic capacitors, following the markings on the circuit board. If you lstudy the schematic I sketched up below you will notice the input signal and DC offset can swing in both positive and negative directions relative to the op amp inputs - polar capacitors in this position are a bad idea.
I recommend anyone buying this to replace the input caps with Nichicon Muse ES or similar 4.7 uF or 10 uF bipolar electrolytic caps. (I have some extra I can mail out. pm me if interested.)
Other than that it's a pretty solid circuit as far as I can see....
Posted 9th January 2013 at 11:41 AM byrjm Updated 21st January 2013 at 08:12 AM byrjm
The $50-from-China-via-ebay NJM4556-based headphone amp, pictured below, fails the "Is thing thing on?" test. With 16 ohm, 104 dB/mW headphones there is a faint-yet-audible background hiss when the unit is powered up and the volume is turned all the way down. The hiss increases only slightly as the volume is turned up (input disconnected).
The output noise of an op amp is normally estimated from the input-referred voltage noise density multiplied by the gain, and the current noise density multiplied by the gain and the sum of the input impedances on the inverting and noninverting inputs. The NJM4556 datasheet does not give a figure for current noise, but a reasonable guess is 1 pA/sqrtHz. The voltage noise density is estimated at 8 nV/sqrtHz.
Posted 7th July 2012 at 01:19 AM byrjm Updated 29th September 2012 at 03:58 AM byrjm
I always seem to end up optimizing my headphone amplifier circuits for higher impedance headphones, this mostly happens because I own a pair of 300 ohm HD-600s and it is tedious to design for both the voltage requirements of high impedance headphones and the current requirements of low impedance headphones.
Not impossible, just, for the class-A designs I seem to be building recently, increasingly large, heavy, and impractical.
Complimentary transistor circuits, however, offer the opportunity to swap voltage for current at something close to the same design cost. They are therefore a practical topology for efficient class-A power delivery into low impedance headphones. As a design experiment, my aim is to discover how far I can leverage an ultra-low-voltage, unity gain circuit for compactness without sacrificing sound quality.
Ok. Back-of-the-envelope calculations:
A typical 16 ohm in-ear-headphone has a sensitivity of 100-105...
Despite the (catchy) name I'm thinking pre-amplifier rather than amplifier applications.
update: I have have a quick and dirty sim up and running in ltspice. Curiously, the output distortion is 15 dB lower when the buffer runs open loop than when it is included inside the feedback loop. Intrigued. Currently under investigation.
update: refined the sim slightly, achieved -85 dB distortion levels at 0 dB / 1 kHz / 600 ohms running the output buffer open loop. Bandwidth is just under 1 MHz, adjusted by changing the feedback resistance. As before, performance sims out notably worse with the buffer
inside the feedback loop.