Discrete Audio Products for Audio Production and Live Recording

The basis of the entire Tonelux designs centers around our two new discrete op-amps, the TX-240 and the TX-260.  Why did we call them that?  Who knows.

In designing the TX-240 op-amp, I came across some very interesting things.  The op-amp sounded better without a power output stage, plus it required quite a bit less internal compensation for stability.  Of course, this raises the problem of how to drive a transformer.  This is where the second op-amp came in.  It is not actually an op-amp as such, as it has no gain and is only a non-inverting power stage.

THE GAIN OP-AMP  TX-240
TX-240

Starting with the TX-240 op-amp, I began with some parameters, such as overall open loop gain, output drive and bias.

OVERALL OPEN LOOP GAIN
My feeling is that most of the op-amps designed today have more gain than they really need.  The increase in gain makes the distortion lower, but in all applications, the op-amp never really sees more than 50dB of actual gain.  The rest is fed back.  These op-amp designs can be in the range of 120 to 180 dB of gain.  For the TX-240, I chose 85 dB.  The op-amp stays very stable, and requires little internal compensation to keep it that way.  This also seems to add to the "open depth" that some engineers have said about the Tonelux products.  To me, too much fed back gain is like pressing on the gas and the brakes at the same time.

OUTPUT DRIVE
For proper internal use and limited external use, I decided that being able to drive 60 ohms was plenty.  This is more than enough to drive level pots, panners, solo/mix busses and sends.  It will not drive a transformer, but keeping the drive abilities more conservative, I was able to use small signal transistors for the output stage instead of larger power transistors.  This has one big advantage.  The dies in larger output devices have an increased capacitance and are generally slower.  This forces the designer to make sure that the input and gain stages that come before it are slowed as well, which insures stability.  Although this compensation is not in the audio spectrum, it appears that the op-amp sounds more open with the smaller devices and less internal compensation.  Currently, the TX-240 has only ONE capacitor for compensation.  There are only SIX transistors for the audio and one for the current source.  This simple design insures that the op-amp does little but amplify.

BIAS
We all know the terms CLASS A, CLASS AB, etc.  The older Neve op-amps were CLASS A biased and the 2520, 990 and variations are CLASS AB biased.  Class A biasing simply means that the output device(s) are biased in such a way that the device swings the entire waveform.  This creates little if no distortion because there is only one device and it is not switching from one device to another as the signal passes from negative to positive.  The down side of a CLASS A op-amp or power amp is that when it is idle, it is drawing 50% of its power just sitting there.  It has to so it can swing positive and negative.  If it is powered by a single power supply, it will also need a large output capacitor that can handle a constant voltage on it.

CLASS B is a type bias where you use TWO devices, one for the positive going part of the wave form and one for the negative going wave form.  The advantage of a CLASS B amplifier is that when idle, it is drawing almost zero current, so it runs cool.  The problem is that because of the nature of a transistor junction, it has to have around .6 volts before it conducts, which means that there is a dead gap as the signal approaches zero and starts to go in the other direction.  This is called "crossover distortion".  To minimize this problem, the designer will "leak" a small amount of current into each transistor, biasing them into the range of the other transistor slightly, which causes both transistors to overlap slightly and there is no longer a dead gap.  This is called CLASS AB because it is a B amplifier with a little A bias.  There is still some distortion, but most or all of it is eliminated with a lot of negative feedback in the op-amp.  It gets canceled.

What I have done in the TX-240 is to bias the devices more into CLASS A than is usually done.  It is not 100% CLASS A and not 100% CLASS B.  In experimenting, I found that there is a point where the distortion really doesn't go down any lower as the bias goes up, once you reach that point, the rest is just current and heat.  The TX-240's distortion is less than .007% with the bias which I call Differential CLASS AAB, since it also uses a pair of output devices so there is minimal offset when it is idle and the output cap can be smaller and it is biased harder into CLASS A than a normal CLASS AB amp.

THE OUTPUT DRIVER STAGE  TX-260TX-260


The TX-260 is the power output driver.  The idea was to design a discrete power stage that could power a transformer and all the abuse that goes with it.  It is a typical push-pull power output stage, similar to most of the power op-amps out there.  With a pair of input transistors, the TX260 has a total of FOUR transistors.  The op-amp is biased the same way as the TX-240,  so the distortion is very low.  Because there is no gain, the slew rate is also blistering. 

There are two advantages with this design.  Having the gain stage separate from the output stage allows a level control to be put between the two amps for gain control, like a fader, without having to put it across the transformer secondary, which changes the tone as it is adjusted because the load is changing.  Another advantage is the power op-amp stage is not in the feedback loop of the gain stage, so the reflected distortion of the transformer does not become part of the feedback path.  I have found that the distortion of the transformer is much more consistent from low level signals to high level signals.  There is a good amount of low end distortion, which I prefer to call "love", and it is very even.  The graphs under the EQ4P info page show the effect of this.  The distortion of the transformer is a very "familiar" kind of distortion, with more stability.

The whole signal chain in the Tonelux product line is not a copy of anything that is out there now.  Both of the op-amps were fresh, ground up designs based around my theory of splitting them up and keeping the transformer out of the gain loop, and keeping the slower power devices out of the feedback loop also.  This idea draws from much older designs, where each stage was it's own and there was no overall loop that gets fed back.  Because of this, I have been able to design a very clean circuit with all the harmonic tones coming from the transformer in a very controlled manner.  It all sounds very warm and very consistent over all levels.

And of course, the sausage:

YUK