Ashhar Farhan tells it as it is … the problem of the TDA2822

Ashhar Farhan VU2ESE, designer of the µBITx tells us the sorry saga of the TDA2822:

“Here is the story. This is going to cause a lot of heartaches. I chose TDA2822 after listening to a lot of bad press about the LM386. Upon looking at its harmonic distortions et al, it was found to be a reasonably good device. In, it went.

“Now, unfortunately, the TDA2822 production has entirely stopped. None of the standard suppliers, including mouser, newark, element14 have no stocks left. We tried to find other source of remaining stock from our reliable supplier who has been supplying to us in the past as well.

“He turned up with a batch. We tried the ICs in burn tests for. Ten of them on ten boards at full volume for a whole day. In retrospect, we should have tested every individual IC. That batch had these WX ICs as well. About 100 to 150 of these must have shipped, there is no way to know. The way it works is like this : Not all the ICs turned out by a factory are good. So work very well, some not so well, some don’t work at all. These wafers make it to the silicon industry underground where they are cut out from the wafers and packaged and sold as low grade versions of the same parts. We got a a hundred or so of these lemons.

“We finally located a source of brand new, high quality TDA2822s that are currently being shipped. They cost almost five times as much as we were paying while this was in production. Well, such is life.

“In the meantime, we have to look for alternatives to the TDA2822. I am highly inclined towards making a discrete device amplifier. We will never run out of discrete devices, the audio would be so much better. Does the gang have any ideas?”

Initial feedback suggests that there will not be a consensus reached on a replacement.  Some want to stick with the TDA2822 (assuming a reliable source of affordable devices can be found), some would prefer the ability to add in a module of the builder’s choice, and others would like to see the device replaced with discrete components.  David N8DAH suggests the drop in replacement –  NJM2073D-ND

Reference

KD8CEC minor firmware update v1.04

Ian Lee, KD8CEC, has released a further minor update of his firmware (v1.04).  You can download it here.

The changes since version 1.01 are as follows:
– Reduce cpu usage
– Change BFO Calibration step (50Hz to 5Hz steps)
– Change CW Frequency Display (frequency is more accurate when in CWL, CWU Mode)
– Optimized source code and reduced program size (97% -> 95%)

WA6ISP uBITx replacement Raduino

Mike Hagen, WA6ISP has previously supplied Raduino X and Raduino XP alternatives to the builder community for the BITx40 units.    He has been asked by many builders whether a µBitx Raduino replacement was planned.

He now has designed, built and tested a µBITx Raduino replacement.  This comes with the extra feature of having an i2c 16 port I/O expander on board. The Microchip Expander IC uses the Adafruit Library MCP23017 to create 16 more Digital Pins.

This replacement board is slightly bigger than the standard Raduino and has the Ardunio Nano facing towards the left rather than to the right.

Email Mike for further information or to order.   The cost of the bare board is US$12 and the built up board is $46.  Note that the bare board will require you to source all parts and  mount some fairly small surface mount devices.

µBITx constructors now have several alternatives to choose from in considering a replacement Raduino board: the WA6ISP uBITx Raduino, the W0EX Radi2cino and VK4PLN Raduino replacement.

An independent audio output for your computer

 

Walter W9KJO in writing to the list says,

” The yellow wire on the volume control does not have enough audio for my Signal Link to work with.  I have to reconnect to the 3.5mm Audio Out jack and turn the volume up about half way.  which is too loud for my headphones so I need to install a second 3.5mm for my Head Phones.  

“I would be much better if I could find a way to get enough audio separate from the actual audio out to the 3.5mm jack.”

Clark Martin, KK6ISP replies “You could use the second audio amp in U1.  The TDA2822 is a dual amplifier.”   He provides the following guide [with some additions from the editor for the sake of clarity].

Modifications to the µBITx to add a second audio channel for computer use 

  • Remove R75
  • Connect U1 pin 6 to VOL-H.  This will give you an auxiliary sound output that is independent of the volume control.
  • Connect the + terminal of a 470 µF, 16V electrolytic capacitor to U1 pin 3.
  • Connect the – terminal of the above cap to your auxiliary out mini jack.
  • Earth the auxillary out mini jack sleeve connection.
Additional Notes
  1. The 470 µF could very likely be much smaller, depending on the input impedance of your Signal Link.   Experiment, it won’t hurt.
  2. You may also want to add a resistor divider between the cap and jack to reduce the signal strength, depending on how much your Signal Link can tolerate.
  3. Considering the problem others are having with the TDA2822 you probably should add a resistor in series with the cap, unless you implement the resistor voltage divider, that will provide the same protection.
  4. Rather than use a voltage divider on the output of your second audio amp, you may want to incorporate a trimmer potentiometer between VOL -H and pin 6 of U1 similar to the primary volume control to set the auxillary audio output level.
Reference

Audio Pop Fix

Don Cantrell ND6T saw a suggestion from Wayne Chang, VA7AT for the audio pop problem in going between RX and TX (and TX and RX).  This was the first solution that seemed sensible to Don. He suggested rewiring the preamplifiers to be permanently powered and inserting serial gating in each of the inputs, controlled directly from the TX and RX power busses.

This modification, although the most complex of those that Don had tried, actually works!

It only requires 9 components, all quite common, and costs less than 25 cents. It requires 2 traces to be cut and 2 small jumpers to be placed across traces. The new circuit is then connected using 6 jumpers from the new circuit board; The connection to the T7, 2 control inputs from the TX and RX busses, 2 outputs (one for the microphone preamp, one for the receive preamp), and a jumper to a common ground point.

For full documentation of this fix see his blog posting here.

Don’t be too scared off with the surface mount technology and the miniature circuit board.  This circuit can be built with regular hole through components to a much larger scale!

AGC Mod

Challenges with AGC circuits

Finding an AGC mod that works, and works well (with sufficient AGC range, that does not impact on the dynamic range of the receiver, and that does not distort) is proving to be difficult.  Nobody has probably tried out more AGC mods than John VK2ETA.

See his thread here for his experiences with a range of AGC mods.

MAX 9814 Circuit

John VK2ETA has now settled on using the MAX9814 circuit for his AGC.

He used the Adafruit MAX9814 board but there are a few variations on eBay with some probably requiring less hacking than the Adafruit design. John had to solder a wire on an SMD component to access the CT (time constant capacitor) pin of the MAX IC, and remove the Electret capsule.

Refer to the schematic and a few pictures of the AGC circuit. The 5VDC required for the Adafruit board is taken from the Raduino.

John took two sets of measures, one with the AGC turned down low and one with the AGC turned one third of the way up.

He used an A/B comparison with an FT-817, with the pre-amp off, receiving  a carrier at 1,500Hz from local radio stations (with attenuation). The FT-817 S-Meter seem non-constant in the steps between the S-units, but nevertheless, this was John’s reference for calibrating the AGC. The AGC voltage was taken on the CT pin of the MAX9814.

The AGC voltage fluctuates quite a lot, so he used the average value shown over time.

To determine whether any saturation was coming from the AGC circuit or the uBitx upstream at high signal strengths, he would bypass the AGC and keep the volume down to prevent the audio circuit after the volume pot from saturating. If harmonics of the audio disappeared, the AGC alone was producing distortion, otherwise it appeared at least prior to the AGC, and possibly from the AGC circuit as well.

Results

“Medium” AGC: input pot turned to about 30% of full scale.

FT817         AGC
S-Meter     voltage(mV)       Notes
S0                  0
S1               300
S2               350
S3               400
S4               460
S5               510
S6               650
S7               750
S8             1,700          Large variation. FT-817 S-meter S8 plateau issue?
S9            2,200          Some saturation of AGC noted (starts to appear in audio FFT, not noticeable)
S9+10     2,460           Saturation of AGC audible, but not unpleasant.
S9+20     2,460          Audible saturation of both uBitx and AGC (harsh sound).

The AGC kicks in early and keeps the volume pretty constant until saturation occurs. Saturation of AGC does limits the dynamic range of receiver.

“Low” AGC: input pot turned to about 7-10% of full scale.

FT817          AGC
S-Meter      voltage (mV)       Notes
S0 -S4               0
S5                50-200                  (100mv avg)
S6                  200
S7                  360
S8                  500
S9                1,260
S9+10         1,800
S9+20         2,300                 Saturation of both uBitx and AGC  (visible in audio FFT, but not really audible)
S9+30         2,400                 Audible saturation of uBitx and mostly of AGC.

This is the most “FT-817 AGC” like, from my perspective, and is what I have settled for.  I want to use the AGC voltage as an s-meter input and this setting does produce a gap at the bottom end, but this is not critical IMO.

In both cases I noted some small “clicks” when the AGC kicked in on strong sudden signals.

The maximum gain of the MAX9814 as set in the schematic attached is of 50dB and requires screened cables in the audio circuit. I originally had the input and output of the circuit fed to a two core “stereo” screened cable and I would get feedback. I had to use two single screened audio cables.

Reference