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Discrete transistor line amplifier stage

Posted 16th April 2013 at 01:18 AM by rjm (RJM Audio Blog)
Updated 16th April 2013 at 12:23 PM by rjm

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.
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A poll. Basic electronics and DIY audio.

Posted 2nd April 2013 at 01:46 AM by rjm (RJM Audio Blog)
Updated 4th May 2013 at 10:54 PM by rjm

It came up at the help desk, but I want to put this before diyaudio.com members generally:

I feel strongly that people who build audio equipment as a hobby should take it upon themselves to obtain a basic understanding of both the practical and theoretical aspects of electronics. Take a trip to the library and read through the first couple of chapters of electronics textbooks, that kind of thing.

It's more than just the safety aspect, I think of it as a basic necessity...

So, how many people here are familiar with the following statement?

The impedance of a capacitor is -j/([omega]C)

Familiar as dirt? Never heard of it before?
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Preamps are not line level.

Posted 1st April 2013 at 04:57 AM by rjm (RJM Audio Blog)

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...
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Marantz SR2285B Receiver : Phono Stage Model Response

Posted 19th March 2013 at 06:02 AM by rjm (RJM Audio Blog)
Updated 20th March 2013 at 01:54 AM by rjm

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.
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X-reg voltage stabilizer LTSPICE file

Posted 15th March 2013 at 01:54 AM by rjm (RJM Audio Blog)
Updated 20th March 2013 at 01:47 AM by rjm

I did up the X-reg circuit in LTSpice.

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...
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RIAA Equalization Curve

Posted 5th March 2013 at 02:51 AM by rjm (RJM Audio Blog)
Updated 5th March 2013 at 03:37 AM by rjm

For reference and experimentation.

This excel worksheet will provide you with reference data that you can overlay and compare with the measured/simulated response plots of actual phono stage circuits.
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Phonoclone boards, soldering, and Q1, Q2.

Posted 7th February 2013 at 06:34 AM by rjm (RJM Audio Blog)

This is in response to several recent emails I've received, where people were having problems with, typically, one board having a bad V+ or V- regulated voltage output.

The number of cases relative to the number of boards shipped caused me to worry that a manufacturing error might have occurred, so at my request I had a customer return the phonoclone boards he had built to me for inspection.

I'm happy to report that the problem was traced to poor soldering technique, the boards themselves are fine. What had happened was solder had cooled before the component had fully settled, and pushing the component down to the board surface then tore the trace away from the bottom of the board, breaking the circuit.

Subsequently, thinking the transistors blown, he replaced them, doing a fair bit of damage to the pads of Q1, Q2.

Fortunately, I was able to fairly quickly set everything to rights, and the boards are now on their way back to him....
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Old

Modification of the S.M.S.L. sAp VI headphone amplifier

Posted 14th January 2013 at 12:29 PM by rjm (RJM Audio Blog)
Updated 27th December 2016 at 02:38 AM by rjm

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....
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Old

Headphone amps are hard.

Posted 9th January 2013 at 11:41 AM by rjm (RJM Audio Blog)
Updated 21st January 2013 at 08:12 AM by rjm

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.

Total noise is

Vn (output) = sqrt [(i_n R_s)^2 + e_n^2 + 4kT R_s] * Gain

Working from a source impedance of ~10k, I get about 18 nV/sqrtHz or -138 dB.

[Revised according...
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Old

Google+ DIY Audio community.

Posted 7th December 2012 at 11:06 PM by rjm (RJM Audio Blog)
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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