Retro computer : RETRO II, a clone of the APPLE IIe computer.

Among other subjects, I have been assembling for a few weeks an APPLE 2e compatible board, The RETRO II.

I did manage to get the system start page but it wasn't easy. 

More details here: https://sillycony.blogspot.com.

Audio Direct Box prototype.

EasyEda is really an interesting PCB creation tool. I often use it for fairly simple circuits, and it does its job very well. I am currently working on the prototype of one board of my futur multi direct box :

Selection of one of the three unbalanced sources and electrical isolation and symmetrization of the output signal. 

Obviously, it's a little more complicated than performing this function with an audio transformer. And then I would probably have some modifications to make to the schematic based on the measurement results of this prototype...

FNIRSI-1014D and audio direct box.

Back to business after a few weeks of vacation. After a lot of buzz about FNIRSI's entry-level oscilloscope, the Model 1014D, I ended up buying a copy. 

The main reason is that on the one hand I did not notice too negative comments on this device, but above all three essential points: speed; clarity of the screen; silence.

I specify that I am absolutely not sponsored by FNIRSI.

In addition, I am currently looking for an audio mixer to mix the few electronic devices I own. Because I do not have a lot of space, I am forced to choose a rack version. So I have two models in view, the Yamaha TF-Rack, and the Behringer X32 rack.

Yamah TF-rack

Behringer X32-rack

In fact, for reasons of lack of flexibility in auxiliary inputs / outputs and overall the closed aspect of the Yamaha console, I rather want to try the Behringer model. In addition, while the audio qualities of the Behringer console are known to be very good, it costs however half the price of the Yamaha console.

But, of course, the input type of these two consoles is quite different. The inputs of the Behringer are symmetrical. Yamaha has therefore chosen to place unbalanced inputs directly compatible with synthesizers. This would have the immediate effect of choosing this type of console for a studio, rather than the Behringer console. 

Because in fact with the Behringer mixer it is necessary to convert the unbalanced output of synthesizers to the balanced inputs of the Behringer mixer. And that at a cost, typically between $ 100 and $ 120 per conversion box.

Also consider the fact that even in the studio, the multiplicity of materials inevitably generates a 'ground loops', generating noise and hum.

The price of 16 'direct box' being nevertheless rather high cost, I decided to develop a solution allowing to regroup in a single rack all the desired functions.

I therefore plan to build a rack allowing the selection of three line-level inputs per audio channel. Each audio channel will be isolated from the ground of the rack, and therefore from the ground of the console. Finally each Audio channel will provide a balanced output for a direct connection to the Behringer console. Each Rack will provide 8 audio channels to the console while having the possibility of selecting three sources per channel.

It is possible to use audio transformers specially made for this function in order to ensure the electrical isolation between the audio input and the audio output. But the price per unit of these transformers is too high for this type of realization. to be connected to a console of about $ 900.

https://www.audiophonics.fr

Only the active solution remains with the help of electronic circuits. The degradation of the signal will therefore be greater than the transformer solution, but the use of quality components should allow the transfer of a quality signal. 

So I carried out a preliminary test on a breadboard :

And the results obtained with the 1014D oscilloscope :

The yellow signal is the signal generated by the oscilloscope's internal generator.
The blue signal is the signal obtained after passing through the electrical insulation stage. 

At 20KHz, the signal is very correct and remains very good  on the lower frequencies. The square signal is a little worse, but it is difficult to get a real idea of ​​this type of signal with a flying wire assembly.
I'm waiting to have made a first prototype to get a real idea of ​​the performance of the circuit.

And now, the two signals in phase opposition after simetrization. Here too, the signals obtained are good, with one of the two with the slightly greater amplitude. The capacitors and resistors used are not very precise, which can explain this phenomenon.

Conclusion: in fact, the system works very well. The results are in line with expectations. I did not estimate the cost of this realization, but the NE5534s used are not very expensive and not the ssm2142 either.

Conclusion on the use of FNIRSI 1014D: First of all, it is not a professional device. But this, everyone knows. The biggest concern is the signal reference error. The zero volt on the display does not correspond to the reality. There is a 1V to 2V offset on each channel. Sometimes the trigger level control becomes unresponsive. The operation of automatic measurements works but most of the time, adaptation is not made to the display. Signals are out of frame. And a few other minor annoyances.

But this is compensated by the very fast operation of this oscilloscope. Even if you decide to turn it off and on again, the operation only takes a few seconds. The commands respond very well which allows a 'reflex' use of this device. Therefore, It is never boring to use. Transferring screenshots is extremely easy and efficient. The screen is really very pleasant to look at, even at an angle. 

And above all, it's small, light AND quiet! This last aspect allows me to spend hours on a montage without having that very distracting fan noise present in most oscilloscopes. Suddenly, I can allow myself to think quietly about the modifications to be made to the assembly while looking at the result on the oscilloscope, without having to turn it off between each measurement session. It's a real pleasure.

For around $ 160, this oscilloscope allows real experiments, without too many worries, even if you have to pay attention to the ground of the two channels. This oscilloscope is not made to take measurements in the field, nor in a hazardous electrical environment, nor to make precise measurements. But it is perfectly suitable for testing analog or digital assemblies up to a few tens of MHz and under a few tens of Volts (40V max). 

The logical procedure, with this type of oscillosope is quite simply to use another more professional machine to validate the circuit once the prototype has been made. So, I am not at all disappointed with this 1014D. For the moment there does not seem to be an update on the manufacturer's site. I will look from time to time to check if small issues are fixed by future updates.