Bush DAC90A - Adding Bluetooth

[ColinB]

Is it worth measuring the current that the module needs when run from a stand-alone power supply? This will give an insight into weather the current output of the regulator is high enough, and what it's power dissipation is likely to be (Vin - Vout x I)

Decoupling in the nF range seems very low, I would expect around 100uF min at the input and the output....

[wd40addict]

Donald, try fitting the 470uf. The spikes without it may be kicking the regulator inro foldback.

Continuous consumption of the KRC in receive mode is about 30mA, but it is very spiky, you need the big capacitor.

[Ambientnoise]

I think that for the L05 if you have say 11v in and 5v out, 125 degrees junction limit and 200 degrees/watt thermal resistance, you can probably only get 50mA before hitting problems. Even this is pushing it. I suggest a straight 7805 with a clip on heatsink.

Ken

[robreddog]

i used a transformer out of a roberts radio with half wave rectification and had drop out problems, i dropped the input volts down from 9.3 to 7.5 ,and also used a 470uf electrolytic cap
i had a steady 5.07v output.
wish this great thread had been going when i did mine.
ps it also powered a lm386 ok but not sure about its performance though.

[DonaldStott]

Apologies for the delay in drafting this update but I've been down the Rabbit Hole and Through the Looking Glass trying to resolve the Bush DAC90A/Bluetooth issues! Fortunately my trusty guide David G4EBT was on hand to help me through and I can't thank him enough for staying the course and persevering with this!

A few weeks ago I had managed to fit the Bluetooth PCB, which includes the KRC-86B module, under the chassis of the DAC90A with the only obvious external change being three additional small holes for bolts to hold everything firmly in place. The PCB connections to the set were as per the advice from wd40addict in Post #44 above - thanks again for your valuable input which really did represent a turning point in this Project. Initially it all seemed to work well and I managed to pair with my iPhone and listen to several online radio stations and podcasts. Unfortunately the Bluetooth connection was lost and these drop-outs became more and more frequent until I eventually disconnected the PCB from the DAC90A again - back to square one!

It was then a case of systematically working through the various components on the PCB and checking the connections to the set in an attempt to locate the problem(s)? I also disconnected the KRC-86B module from the PCB to isolate these and test in turn.

Too many variations and options to detail here were tried, suffice to say that all is now well and Bluetooth has been working flawlessly for a number of days with no drop-outs or other lost connection issues.

I ended up populating (yet) another PCB but this time using a 7.5V transformer from ESR, a 7805 regulator with 100nF cap on the output side and connecting a new KRC-86B module (100k SMD resistors removed). David G4EBT was extremely helpful in relation to these changes and once again his technical advice, guidance and support were indispensable.

Output from the 7805 Regulator is around 4.96V while the current drawn by the KRC-86B module is 26-30mA. Left it running all day and after 8 hours the temperature of the metal tab on the 7805 was 36.8°C - no change to the current and no drop-outs of the Bluetooth connection over all that time! Audio quality is very good when linked to a variety of radio stations and podcasts while loop-through to and from the audio from the radio is also good. I still have the ubiquitous DAC90A background hum, but that's another story!

So what went wrong - one line of inquiry I'm going down is that connections had been made numerous times to the KRC-86B module including removal of the SMD resistors so I may have applied too much heat at some point causing thermal stress to the board? I know that the PCB itself was starting to show signs of deterioration due to numerous connections/re-connections being made! As mentioned above, both the PCB and KRC-86B module were replaced.

I think that at last we have a result and a winning combination of components and no ongoing issues with the voltage from the transformer and rectifier, the temperature and output voltage of the 7805 regulator or the current drawn by the KRC-86B module.

What a relief after all this time and can't thank everyone enough for staying the course and persevering with this! It's just a pity we started off down the transformer impedance matching route when a simpler solution was to remove a few SMD resistors from the KRC-86B board - a bit of lateral thinking there from wd40addict. Thanks also to Gabe001 for his ongoing interest, support and suggestions.

Happy to answer any further questions but I think we can now sign-off on this Bluetooth project?

I've attached a picture of the aftermath and clean-up - no words required! Still very glad I don't have to make an economic return on these projects!

[Gabe001]

Great outcome Donald! For future reference, can you post a diagram of your final version of the Bluetooth powering module and radio connections.

I'm tempted to do this at some point too.

Gabriel

[David G4EBT]

Well done on staying the course! I'm happy to have had some input.

There have been two strands to this project which have impeded progress.

Initially, the rather neat option of using two LT700 which Yannis Karalis says have been successfully implemented never worked as expected. It involved Donald designing a PCB and ordering it from JLCPCB. That project didn't seem to work, so different transformers were obtained, and they didn't work either, so that option was abandoned.

Input from other forum members - WD40Addict et al - produced a workable solution.

A second PCB was designed, incorporating a small encapsulated 9V 133 mA PCB-mounting transformer from ESR Electronics, a bridge, and 78L05 regulator, adequate for the 30mA or so drawn by the Bluetooth module. It worked, but kept dropping out after a short while. It was evident that as the current being drawn from the 9V transformer was very modest, it's it's standing off-load voltage was much higher than 9V and above the maximum rated input voltage of the 78L05 so it was getting very hot and closing down.

I checked the ESR 9V transformer that I have at various current levels using a range of 'load' resistors. The results confirmed my suspicions that at low current levels, the output voltage of the transformer was indeed closer to the 'standing' (no load) voltage, (as is normal with any transformer with no load. or very little).

The mains Voltage here is 242 Volts. The results I obtained were:

Standing (no load) output voltage of the 9V transformer was 14 Volts.
Using a 390 Ohm load resistor, the current flowing was 33mA (close to what the KRC-86B Bluetooth module will draw). The Voltage was 13V.
220 Ohm load resistor: 55mA, Voltage was 12.4V
100 Ohm resistor: 105mA, Voltage 11V
56 Ohm resistor 165 mA, Voltage 9.3V.

That confirmed what I suspected.

Namely, that the lighter the load on the transformer, well below its rated output, the closer was the voltage to it's no-load standing Voltage. Though the 78L05 would cope with supplying the current, it wouldn't cope with the input/output voltage differential. The 7805 is much more rugged and will operate with an input Voltage from 7V - 25V, with an absolute maximum of 125C junction temperature.

A 7.5V ERS transformer (same PCB pin layout) was fitted by Donald to lower the standing voltage, which was then11.3V. Using that transformer and a 7805 without a heatsink produced the desired 5V regulated output, with the 7805 at a steady 35C, well within its spec. The DC Voltage into the regulators was 13V so the differential from the input to the output was 8V. Given that the minimum specified input Voltage for the 7805 is 7V and the maximum is 25V, 13V input is nicely in the middle - 6V above the minimum and 12V below the maximum.

To summarise:

The transformer is operating well within it's current rating, the 7805 operating conditions are well within all of its parameters, the regulator and the KRC-86B module are providing a good and consistent Bluetooth connection. So taking everything into account, I'd say that having gone down several rabbit holes, that's 'mission accomplished'!

The PCB that Donald designed, incorporating the power supply and KRC-86B Bluetooth module, fits neatly into the only space in a DAC90A where it can be fitted - on the rear chassis apron, and though the radio is a live chassis design, the on-board power supply is isolated from the chassis which means that there are no safety considerations and no external connections.

The radio can be used as normal on MW or LW, and when the Bluetooth module is paired to an external device, works on Bluetooth without any switching being necessary.

A look back at this thread and other threads of Donald's, show that his staying power (as he puts it, his 'stubborn gene'), shows no bounds. Examples that spring to mind are the 300Hz strobe project he built, his recent U122 (restore or scrap?), A KB MR10, and further back, a Bush AC91 that had been so messed about with by a previous owner that all the dodgy modifications had to be removed to get it back to as it should have been, before he could embark on the restoration.

None of those projects were straightforward - all were successfully completed.

(Don't know where he's going to squeeze a Bluetooth module into a DAC10, but I'll watch this space!).

[chompy1]

you could use 2 relays with their own supply with a separate switch to energise/de-energise them. One relay for the mains and one relay for the speakers. That would totally eliminate all breakthrough 🙂

[Gabe001]

The 5v relays I've used in a different project (not radio related) use up to 250mA each when energised. It would require a meatier transformer and other components rated for at least 750mA, which would make this much more expensive.

[wd40addict]

Did you fit the 470uf capacitor at the module end? This helps to smooth the very spiky current draw and minimise potential digital noise breakthrough.

Woop woop 1000th post!

[chompy1]

I've added in a stereo bluetooth receiver unit in my 1957 Invicta table top radio.
I've used two 12v relays with their coils connected in series with their own supply and a separate switch which I have mounted on the front plastic panel.

Reading through this thread, it sounds like there is very little available space inside of the Bush DAC90A to mount/install the bluetooth unit and other components and supplies if required, but very interesting though

[wd40addict]

The beauty of the KRC module is that it contains its own automatic audio switch - no relays required.

[chompy1]

cool! Is eliminating radio breakthrough straight forward?

[DonaldStott]

Originally Posted by wd40addict

Did you fit the 470uf capacitor at the module end? This helps to smooth the very spiky current draw and minimise potential digital noise breakthrough.

Woop woop 1000th post!

Well done on your 1000th post - although I've posted more (not a competition!) the vast majority of mine are requests for advice, information or suggestions.

I did try the 470uf capacitor but it didn't seem to make any difference whatsoever to the very spiky current draw which is around 26-30mA? I'm happy with the way things are, for the moment, but will revisit the Bluetooth module if the need arises.

[Gabe001]

I use the 7805 too, in a similar setting. The 100nf cap is recommended in the datasheet which is probably why David and Donald used this value

I do use the 470uf electrolytic which comes with the BT module, but use a diode in reverse across the regulator to safeguard it. I think this was recommended by Ed Dinning in another similar thread.

I've gone down to 10uf (maximum recommended with the 5v isolator module) with no I'll effect on BT performance, but never felt brave enough to go into the nanofarad range, so do update this thread if the need arises to increase capacitance in that position.

Gabriel

[Gabe001]

Not a dac90a, but this is a short video to show how well the krc-86b bluetooth module works using the setup Donald and Paul describe above. The cossor 464 has no gram input, so the krc-86b is doing all the switching work, as it is doing in Donald's radio.

In this video, the BT module is powered directly from the radios transformer, using a 5v isolator to minimise digital noise. You can see that the krc-86b mutes the radio very effectively when BT is connected, and unmutes it when BT is disconnected. No relays or extra switches are required. The 4 resistors on the module are removed as Paul describes above.

I hope someone finds this useful. No doubt that Donald's DAC90a behaves similarly.

https://youtu.be/jZj8Hhqr1X8

The background noise is the clock ticking!

Regards
Gabriel

[DonaldStott]

Thanks Gabe - short, succinct video which demonstrates the ease with which Bluetooth can be switched on and off with a vintage valve radio - excellent work!

This is exactly how both my DAC90A and DAC10 now behave although the DAC10 is the better performer in terms of background hum, audio breakthrough and digital noise. I'm experimenting with a Pi filter in the DAC90A today as I've implemented all the known mods for hum reduction but it persists!

Of course the KRC-86B module is Bluetooth V4 and I think we are now on to V5.2?

Excellent restoration job on the Cossor 464 especially the Bakelite finish and the clean and bright dial scale display.

[Gabe001]

Thanks Donald. That's the cossor from the original cossor 464 bluetooth thread (now closed) which I did with Paul's advice.

I don't think there is a v5 BT analogue to the krc-86b yet. I do have some bt5 modules which I'm tempted to try at some point, but they don't have the switching ability which the krc-86b has, so they're only suitable for radios with gram inputs. I don't think there will be any noticeable difference in performance though.

[wd40addict]

I think for routing audio to an valve radio the SBC CODEC used in the KRC is more than sufficient. Later models may bring AAC, APTx and LDAC to the party, but I suspect we'd struggle to hear the difference.

[DonaldStott]

My question about Bluetooth V5.2 wasn't really to do about performance or audio quality but more about if and when V4 is no longer supported.

We know the pace at which things tend to progress these days and that legacy formats are often the first thing to go!