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Old 16th Feb 2021, 12:23 pm   #41
DonaldStott
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Default Re: Bush DAC90A - Adding Bluetooth

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Originally Posted by wd40addict View Post
The cap will stop a bit of scratchyness as the volume is adjusted in BT mode and will stop any effect on the cathode bias of V2 & V3 when the volume is set low in BT mode. 47uF is rather large though
More insightful information - thanks!

If 47uF is rather large, how would you go about calculating what it should be?
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Old 16th Feb 2021, 12:47 pm   #42
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Default Re: Bush DAC90A - Adding Bluetooth

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Originally Posted by wd40addict View Post
There are different ways of doing this, the best is obviously thermal tweezers. Alternatives are two small irons used simultaneously, or where a component is as small as this one hopping from end to end with a well tinned bit until it starts to move and can be flicked off with the iron.
As you only have one pair of hands, you could perhaps lightly clamp the BT module with a spring loaded clamp to the bench top to hold it in place, (or maybe just weight it down?), then using two irons - one either side of the resistor - could use them like thermal tweezers and slide off/lift off the resistor when both end are melted, as depicted in this rather manky sketch.

Good luck with it!
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Old 16th Feb 2021, 6:09 pm   #43
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Default Re: Bush DAC90A - Adding Bluetooth

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If 47uF is rather large, how would you go about calculating what it should be?
The reason I ask is that in the earlier Cossor 464 bluetooth Thread, Post #24 you mentioned "I used 1uf because I had it to hand. The 3M resistor is between one of the inputs and ground. It could equally go on the BT output before the 1uf cap."

I also noted that the capacitor shown in Post #14 that links the L and R outputs isn't an electrolytic - just proving that I read the whole Thread again.

So still a bit confused and hoping you can clarify things for me in relation the 3M resistor and 1uF cap - thanks.
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Old 16th Feb 2021, 8:20 pm   #44
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Default Re: Bush DAC90A - Adding Bluetooth

If you're overdriving you need the reactance low to avoid bass breakthrough from the radio signal so say 1 - 10uF. If you're switching the radio signal then 0.1uF would be fine.

The attached are my two suggestions, note in the switched case the cap needs to be non-polarised as bias across it depends on mode. All assume 100k resistors removed.
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Old 16th Feb 2021, 8:55 pm   #45
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Default Re: Bush DAC90A - Adding Bluetooth

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Originally Posted by wd40addict View Post
If you're overdriving you need the reactance low to avoid bass breakthrough from the radio signal so say 1 - 10uF. If you're switching the radio signal then 0.1uF would be fine.

The attached are my two suggestions, note in the switched case the cap needs to be non-polarised as bias across it depends on mode. All assume 100k resistors removed.
Thanks WD for taking the time to respond and to attach details of your two suggestions - very helpful.

I'm going to try both as they are easily reversible starting with the overdriving option. I note your points about capacitor types and the need to remove the 100k resistors.

On the switching option I'm not sure where the top end of the 470k resistor is connected to IFT2 on the DAC90A?
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Old 16th Feb 2021, 10:38 pm   #46
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Default Re: Bush DAC90A - Adding Bluetooth

Quote:
Originally Posted by DonaldStott View Post
Quote:
Originally Posted by DonaldStott View Post
If 47uF is rather large, how would you go about calculating what it should be?
The reason I ask is that in the earlier Cossor 464 bluetooth Thread, Post #24 you mentioned "I used 1uf because I had it to hand. The 3M resistor is between one of the inputs and ground. It could equally go on the BT output before the 1uf cap."

I also noted that the capacitor shown in Post #14 that links the L and R outputs isn't an electrolytic - just proving that I read the whole Thread again.

So still a bit confused and hoping you can clarify things for me in relation the 3M resistor and 1uF cap - thanks.
The 1uF capacitor is post 14 isn't very clear, but it looks to me like a tantalum electrolytic, as small value ones often are.
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Old 16th Feb 2021, 11:40 pm   #47
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Default Re: Bush DAC90A - Adding Bluetooth

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On the switching option I'm not sure where the top end of the 470k resistor is connected to IFT2 on the DAC90A?
Circuit is from the Trader sheet 1161, top end of 470k is connected to the line that formally went from the IFT to the top of the volume control, but now connects to the input of the KRC.
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Old 17th Feb 2021, 1:00 pm   #48
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Default Re: Bush DAC90A - Adding Bluetooth

I've included this Post to summarise my recent experiences and hopefully it may save others time and expense?

This Thread started off as a simple request, or so I thought, to see if anyone had any experience of using the small impedance matching transformers as recommended by Yannis Karalis to add Bluetooth functionality to a Bush DAC90A. Due to my own lack of technical expertise in this area I decided to give the Yannis approach a try and purchased some EE14 transformers - these are small and very cheap and maybe this was a warning sign that things might not end up as expected.

Here is a picture of the initial 1300:8 ohm EE14 POS transformers I purchased:-

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These measured - Primary : 84.1ohms Secondary : 2.5ohms - I appreciate of course that resistance isn't the same as impedance. These were to be configured with the audio L & R channels and the AGND from the BT board going to the Secondaries with the Primaries output going to the radio. Two transformers were connected in parallel following connection information from Yannis. At this stage all seemed well and made sense to me (!) as a means of impedance matching the output from the BT board with the input to the radio?

I used my Breadboard to prototype the EE14 configuration and one of the main benefits of this approach is that it allows you to quickly change and test connections. The two EE14 transformers had to be configured correctly to ensure that there were no phasing issues with the L & R channels. So I rigged up a Heath Robinson arrangement involving my DIY 5V DC PSU, the Bluetooth board, my Breadboard with the resistors (I used 100R) and the small EE14 transformers - a real rats nest of wires, connectors and crocodile clips!

Plugged everything in, tuned the DAC90A to a quiet area of the LW band and paired my iPhone with the Bluetooth board - the good news is that it worked first time! The Volume was excellent BUT a major problem was the TONE - it was all TREBLE, extremely thin, tinny and sibilant - reminded me of a 1960s transistor radio! Of course I wasn't expecting deep thunderous BASS or hi-fi performance but I knew that when I connected the Bluetooth board directly to the DAC90A via two resistors the audio quality was very good. It's the same as the quality of the DAC90A when working as a radio with none of the AM background hash? The only change was the introduction of the EE14 impedance matching transformers!

Several days of pondering ensued with the assumption that I had incorrectly configured my connections, had managed to get the phasing wrong or had incorrectly identified the start and end of the windings on the EE14 transformers? So I tried every variation I could but no matter the configuration the audio output remained too bright, harsh and sibilant.

I then concluded that the problem must be the EE14 transformers which are simply not up to the job - is that why they are so cheap? Yannis recommended ones from Mouser UK (42TL013-RC) but I was a bit put off by the steep p&p charges I ordered some LT700s instead as they appeared to have similar specifications and are more widely available.

This picture shows the comparative sizes of the EE14 transformers and the LT700s:-

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Another assumption was that these 'better quality' LT700 transformers would have less of a negative impact on the audio output. Although these were perhaps marginally better than the previous small EE14 transformers the audio quality remained unacceptable, extremely thin, tinny and sibilant TREBLE, no matter the configuration!

I cant find any details online for the frequency range of the LT700 transformers but the Datasheet for the equivalent 42TL013-RC shows:-

'Frequency Range: 300 Hz to 3.4 kHz'

This seems to be in line with my own experiences and confirms what David (Radio Wrangler) said:-

"Transformers like the Eagle LT700 were designed with enough primary inductance to cover the audio band in lowish impedance circuits. Use in higher impedance applications needs more inductance or else you lose bass. They weren't hifi to begin with, but the difference could be noticeable even with a table radio."

As I mentioned, I'm not expecting hi-fi quality audio but if we have very little or no signal below 300Hz then that explains the extremely thin, tinny and sibilant TREBLE?

Hope that someone may find this information useful?

NOTE: I have to mention the help and support I have received from Forum member David G4EBT (David Taylor) throughout this Project. David's patience with my amateur fumblings have kept me on the straight and narrow and ensured on several occasions that I didn't throw in the towel!
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Old 17th Feb 2021, 1:15 pm   #49
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Default Re: Bush DAC90A - Adding Bluetooth

OK, now I'm baffled, did you try any of my transformer-less suggestions?
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Old 17th Feb 2021, 1:45 pm   #50
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Default Re: Bush DAC90A - Adding Bluetooth

If there's enough signal voltage coming out of the module you won't need any kind of impedance matching, that's because the input to the Bush audio circuit is high impedance, remember that all circuits are potential dividers and Ohms Law is Ohms Law, here's a thought exercise, the impedance of a voltage source is say 500 Ohms, you now connect across that source a 500k resistor, what effect on the output voltage will that have.....not a lot.

It was said earlier that the output impedance was 390 Ohms.

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Old 17th Feb 2021, 2:08 pm   #51
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Lightbulb Re: Bush DAC90A - Adding Bluetooth

Quote:
Originally Posted by DonaldStott View Post
I've included this Post to summarise my recent experiences and hopefully it may save others time and expense? ....
Donald, thanks for this. I am not familiar with yannis' circuit but I tried the same transformers in a raspberry pi audio project and had pretty much the same experience as yourself ie a lowering of volume and sound quality issues. I don't think they're up to the job.

I had tried the Bluetooth overdrive solution recommended by WD in an ultra twin as a quick test a few months back, with the appropriate resistors removed as WD recommended to me in the cossor thread, and apart from some noise from the BT indicator LED (easily fixed) it worked absolutely fine the first time round. I used a sharp pointy scalpel to remove the resistors and they came off without much drama.

This thread is interesting and I'd also like to know whether WDs suggestions (it would be useful to know your name at this point ) worked in the DAC90a.

Gabriel

Last edited by Gabe001; 17th Feb 2021 at 2:35 pm. Reason: Shortened the quote
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Old 17th Feb 2021, 2:19 pm   #52
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Default Re: Bush DAC90A - Adding Bluetooth

I'm Paul
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Old 17th Feb 2021, 3:09 pm   #53
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Default Re: Bush DAC90A - Adding Bluetooth

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OK, now I'm baffled, did you try any of my transformer-less suggestions?
Hi Paul

Apologies for any confusion - this was just meant as a summary of where I'd got to before trying any of your transformer-less options.

The two options you detailed in Post #44 above are next on my agenda once I figure out how to remove the 100k resistors safely!

“Fortune favours the bold.”
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Old 17th Feb 2021, 10:22 pm   #54
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Default Re: Bush DAC90A - Adding Bluetooth

Some progress tonight as I managed to remove the surface mount 100k resistors - a combination of using the smallest solder bit I have, hopping quickly from end to end and flicking off with a sharp pointy scalpel!

So tomorrow I will try the overdriving option first.

Just one final question - should the AGND from the BT board be connected to the screen on the DAC90A volume control?
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Old 18th Feb 2021, 8:54 am   #55
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Default Re: Bush DAC90A - Adding Bluetooth

Yes, this should be fine.

All those miniature audio transformers you tried will have a low value of primary inductance which causes heavy loading of the source at low frequencies. Decent audio transformers from the likes of Jensen and friends are larger and much more expensive.

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Old 18th Feb 2021, 10:24 am   #56
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Default Re: Bush DAC90A - Adding Bluetooth

I've had more success using the isolating transformers designed for hum-loop eradication in car stereo systems. I can confirm they have great audio properties as I've used them in disco sound systems, not known for tinny reproduction. A 3 or 4 quid isolator yields 2 of these transformers.

Some of these modules have class-d / H bridge outputs designed to drive small speakers or headphones directly and don't seem to like a high impedance load, causing spurious RF emissions or strange whistling/ticking/hiss in the background.
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Old 18th Feb 2021, 11:40 am   #57
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Default Re: Bush DAC90A - Adding Bluetooth

I once measured a Jensen isolation transformer with an Audio Precision - it was stunning!

The KRC-86 is a conventional analogue single ended output, but keeping the spiky ground currents from polluting the audio needs some care. Hopefully Donald will be ok as he has a floating supply from his mains transformer.
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Old 18th Feb 2021, 12:37 pm   #58
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Default Re: Bush DAC90A - Adding Bluetooth

I'm still interested in where he is going to mount this in the DAC90a. Needs to be away from the heat and I wouldn't mount it close to the aerial either personally. Not much wiggle room.

Also, if the power lead is ever connected the wrong way round and aGND sees 240vAC, will it go bang?
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Old 18th Feb 2021, 3:04 pm   #59
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Default Re: Bush DAC90A - Adding Bluetooth

Spent the morning configuring the overdriving option as shown in Post #44 above from Paul (wd40addict).

I used a 2.2uF electrolytic as it's what I had along with twin core shielded signal cable. The positive lead was connected to the radio and negative to the BT board. The OUTL & OUTR from the BT board were combined and connected to the top of the volume control while the AGND was connected via the cable shield to the screen on the DAC90A volume control?

First impressions were that there was no apparent digital noise or any additional background hum, even with the LED fitted. Sound quality was very good although the volume was reduced when switching to Bluetooth. This was most noticeable on BBC Radio 3 when played from BBC Sounds on my iPhone? If the volume is increased when using Bluetooth it becomes far too loud when switching back to radio!

Unfortunately I still have the problem of the RF/IF signal breaking through and the radio can still be heard in the background. Of course I can 'fix' this problem by tuning the radio away from any stations but that really isn't a solution?

So good progress today but the overdriving option isn't really suitable due to the breakthrough problem ...

Any tweaks I can try before moving on e.g. the bypass cap suggested in Post #11 above?
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Old 19th Feb 2021, 12:11 am   #60
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Default Re: Bush DAC90A - Adding Bluetooth

The only tweak I can think of is to add a series resistor (try 100k, but you may need to experiment) between the IFT and the top of the volume control. It sounds like the output impedance of the KRC isn't low enough to completely silence the radio. I assume you have the volume on the Bluetooth source turned up to max? If you leave it on default the output level is only 90mV.

If a series resistor doesn't help next step is to try the switched connection.
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