Bush DAC90A - Adding Bluetooth - UK Vintage Radio Repair and Restoration Discussion Forum

DonaldStott · 14th Feb 2021, 1:58 pm

I have been considering adding a Bluetooth module to my DAC90A for some time now especially given the dearth of AM stations available! This would need to be implemented in such a way that it is non-invasive and reversible if required. Of course it is appreciated that the DAC90A is an AC/DC set with all that that entails and there is no provision for a Gram input or similar - so doubly challenging.

I also appreciate there are as many different ways to do this as there are Bluetooth modules but my approach would be to use the Bluetooth KRC-86B V4.0 board with the implementation following the circuit recommended in the excellent DAC90A Restoration 'ebook' from Yannis Karalis:-

https://www.yanniskaralis.com/bush-d...sy-restoration

For copyright reasons I will not Post details from this PDF but what I can mention is that he recommends using two small impedance matching isolation transformers rather than the usual simple solution of resistors in series with the output. I have constructed a rectified, smoothed and regulated 5V DC supply for the BT board based around a small encapsulated transformer (9V 133mA) from ESR Electronics and this works very well. So far so good ...!

I have never used these small impedance matching isolation transformers and wondered if any Forum members had any experience of these. Yannis recommends ones from Mouser UK (42TL013-RC) but I was a bit put off by the steep p&p charges so have ordered some LT700s instead as they appear to have similar specifications and are more widely available.

If you have implemented the Yannis Karalis Bluetooth mod for the DAC90A then please advise accordingly and also if you have any experience of the LT700s.

Radio tirana · 14th Feb 2021, 2:42 pm

Hi Donald, I will be interested to hear how this works out I have also downloaded Yannis's excellent book.
I have just fitted a Bluetooth KRC-86B V4.0 board to one of my DAC90A sets
Along with a Capacitive dropper and 10VA 12V mains transformer to power the dial lamps as well as the Bluetooth module via a rectifier regulator board. Since all this is contained within the set I just broke the connection at the top end of the volume control and connected the module output to it then fed the screened lead to the modules input (left and right channels were joined via 47 Ohm resistors) Despite the impedance mismatch This works remarkably well Switching is automatic as soon as you pair to the module it mutes the radio input. I would think you only need the isolation transformers if you plan to make a wired connection to an external device.

Kind regards RT

DonaldStott · 14th Feb 2021, 4:01 pm

Thanks RT for your quick and helpful response.

I've got to admit that I did try the direct connection method you describe before I came across the Yannis Bluetooth mod. It did seem to work quite well and there was automatic switching when the BT board was paired with my iPhone. I did find, however, that there was some breakthrough from the RF/IF signal which could still he heard faintly in the background. This is what I would expect though, as the the audio stage will inevitably be receiving audio from two sources - RF/IF and Bluetooth?

As a relative novice here I assumed that the small impedance matching isolation transformers recommended by Yannis were essential in this AC/DC set?

ms660 · 14th Feb 2021, 4:08 pm

The transformers you mentioned are not suitable for live chassis isolation.

Lawrence.

14th Feb 2021, 4:37 pm

Quote:

The transformers you mentioned are not suitable for live chassis isolation.

Does that matter for a bluetooth connexion?

ms660 · 14th Feb 2021, 4:42 pm

Quote:

Originally Posted by merlinmaxwell

Quote:

The transformers you mentioned are not suitable for live chassis isolation.

Does that matter for a bluetooth connexion?

I wouldn't know, what I'm saying is they are not suitable for live chassis isolation eg: WRT Earth.

Lawrence.

Radio tirana · 14th Feb 2021, 5:28 pm

I would caution against making an external connection to a live chassis radio via this
type of transformer its not what they are designed for, I don't see any problem with using them inside the radio as long as an isolating mains transformer is used to power the Bluetooth module. I found that any breakthrough could be dealt with by tuning off into a quiet section of the waveband Sadly that is most of it where I live.

Kind regards RT

DonaldStott · 14th Feb 2021, 6:33 pm

My intention here has always been to fit a small PCB inside the radio comprising my 5V DC supply and the BT board - this power supply uses a small transformer from ESR Electronics - 1.2VA 0-9V (1 x 133mA).

I may have confused matters by referring to the 'small impedance matching isolation transformers' recommended by Yannis - these are only used in the output stage from the BT Board to the radio.

I have read elsewhere - "These are not meant to be isolation transformers. They are impedance matching transformers."

Does that help?

AmateurRepairer · 14th Feb 2021, 6:38 pm

Wouldn’t recommend fitting any sort of external casing or component to a live chassis model.

ms660 · 14th Feb 2021, 6:48 pm

The transformer isn't really needed then.

Lawrence.

murphy512 · 14th Feb 2021, 7:15 pm

I seem to remember that the Marconiphone T26A dealt with the breakthrough when the gram was in operation, by fitting a .02 capacitor between the anode of
V2 and ground which came in operation when the gram plug was inserted in the
gram socket and I believe that muted the radio.

I am not sure if that could be applied to the DAC90A but someone in the forum more knowledgeable than me could point out any possible problem with that.

Aldo

DonaldStott · 14th Feb 2021, 7:20 pm

Quote:

Originally Posted by ms660

The transformer isn't really needed then.

On the assumption that you mean the impedance matching transformers in the output stage can I ask if anyone has experience of using, for example, the LT700s?

I was quite content with the audio quality using the direct connection method described above, so why the need for impedance matching transformers?

Apologies if we appear to be going round in circles!

ms660 · 14th Feb 2021, 7:45 pm

Those transformers won't match the impedances, matching isn't required anyway because the input impedance at the top of the volume control is I suspect much higher than the output impedance of the Bluetooth device.

Lawrence.

DonaldStott · 14th Feb 2021, 8:28 pm

Quote:

Originally Posted by ms660

Those transformers won't match the impedances, matching isn't required anyway because the input impedance at the top of the volume control is I suspect much higher than the output impedance of the Bluetooth device.

The configuration that Yannis has shows the output from the BT board going to the Secondaries of these impedance matching transformers and the Primaries are then connected to the top of the volume control.

ms660 · 14th Feb 2021, 8:49 pm

I've no idea what a Bluetooth can output and into what impedance.

Lawrence.

Radio Wrangler · 14th Feb 2021, 9:01 pm

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 begn with, but the difference could be noticeable even with a table radio.

You might be better off with one known to work.

Also the LT700 is not intended as a high voltage isolator. Be sure you don't wind up relying on it in this way.

David

ms660 · 14th Feb 2021, 9:10 pm

Here's the specs. for the Mouser one:

https://www.mouser.co.uk/ProductDeta...pwpGuhuA%3D%3D

Lawrence.

DonaldStott · 14th Feb 2021, 9:18 pm

Quote:

Originally Posted by ms660

Here's the specs. for the Mouser one:

https://www.mouser.co.uk/ProductDeta...pwpGuhuA%3D%3D

Lawrence.

This caught my eye: -

'Frequency Range: 300 Hz to 3.4 kHz'

Which seems to confirm what David (Radio Wrangler) said "Use in higher impedance applications needs more inductance or else you lose bass." I'm not expecting hi-fi quality audio of course but if we have very little or no signal below 300Hz then that doesn't bode well!

Quote:

Originally Posted by Radio Wrangler

You might be better off with one known to work.

Any suggestions?

ms660 · 14th Feb 2021, 9:25 pm

Quote:

Originally Posted by DonaldStott

Quote:

Originally Posted by ms660

Here's the specs. for the Mouser one:

https://www.mouser.co.uk/ProductDeta...pwpGuhuA%3D%3D

Lawrence.

This caught my eye: -

'Frequency Range: 300 Hz to 3.4 kHz'

It was the first thing that caught my eye earlier on too, any chance of the Bluetooth specs? As I know virtually zilch about these devices.

Lawrence.

Radio tirana · 14th Feb 2021, 9:31 pm

Hi Donald
I think that Yannis uses the transformers because he also details a line input conversion (3.5mm Socket) in his document. He obviously thinks that they provide sufficient isolation.
I think you can dispense with the transformers in the Bluetooth only version. The Bleedthrough problem can probably be solved with some bypassing as sugested in #11

Kind regards RT

14th Feb 2021, 2:42 pm	#2
Radio tirana Pentode Join Date: Nov 2015 Location: Oxfordshire, UK. Posts: 171	Re: Bush DAC90A - Adding Blueooth Hi Donald, I will be interested to hear how this works out I have also downloaded Yannis's excellent book. I have just fitted a Bluetooth KRC-86B V4.0 board to one of my DAC90A sets Along with a Capacitive dropper and 10VA 12V mains transformer to power the dial lamps as well as the Bluetooth module via a rectifier regulator board. Since all this is contained within the set I just broke the connection at the top end of the volume control and connected the module output to it then fed the screened lead to the modules input (left and right channels were joined via 47 Ohm resistors) Despite the impedance mismatch This works remarkably well Switching is automatic as soon as you pair to the module it mutes the radio input. I would think you only need the isolation transformers if you plan to make a wired connection to an external device. Kind regards RT

14th Feb 2021, 4:01 pm	#3
DonaldStott Octode Join Date: Aug 2015 Location: Glasgow, UK. Posts: 1,842	Re: Bush DAC90A - Adding Blueooth Thanks RT for your quick and helpful response. I've got to admit that I did try the direct connection method you describe before I came across the Yannis Bluetooth mod. It did seem to work quite well and there was automatic switching when the BT board was paired with my iPhone. I did find, however, that there was some breakthrough from the RF/IF signal which could still he heard faintly in the background. This is what I would expect though, as the the audio stage will inevitably be receiving audio from two sources - RF/IF and Bluetooth? As a relative novice here I assumed that the small impedance matching isolation transformers recommended by Yannis were essential in this AC/DC set? __________________ BVWS Member

14th Feb 2021, 4:08 pm	#4
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bush DAC90A - Adding Blueooth The transformers you mentioned are not suitable for live chassis isolation. Lawrence.

14th Feb 2021, 5:28 pm	#7
Radio tirana Pentode Join Date: Nov 2015 Location: Oxfordshire, UK. Posts: 171	Re: Bush DAC90A - Adding Blueooth I would caution against making an external connection to a live chassis radio via this type of transformer its not what they are designed for, I don't see any problem with using them inside the radio as long as an isolating mains transformer is used to power the Bluetooth module. I found that any breakthrough could be dealt with by tuning off into a quiet section of the waveband Sadly that is most of it where I live. Kind regards RT

14th Feb 2021, 6:33 pm	#8
DonaldStott Octode Join Date: Aug 2015 Location: Glasgow, UK. Posts: 1,842	Re: Bush DAC90A - Adding Blueooth My intention here has always been to fit a small PCB inside the radio comprising my 5V DC supply and the BT board - this power supply uses a small transformer from ESR Electronics - 1.2VA 0-9V (1 x 133mA). I may have confused matters by referring to the 'small impedance matching isolation transformers' recommended by Yannis - these are only used in the output stage from the BT Board to the radio. I have read elsewhere - "These are not meant to be isolation transformers. They are impedance matching transformers." Does that help? __________________ BVWS Member

14th Feb 2021, 1:58 pm	#1
DonaldStott Octode Join Date: Aug 2015 Location: Glasgow, UK. Posts: 1,842	Bush DAC90A - Adding Bluetooth I have been considering adding a Bluetooth module to my DAC90A for some time now especially given the dearth of AM stations available! This would need to be implemented in such a way that it is non-invasive and reversible if required. Of course it is appreciated that the DAC90A is an AC/DC set with all that that entails and there is no provision for a Gram input or similar - so doubly challenging. I also appreciate there are as many different ways to do this as there are Bluetooth modules but my approach would be to use the Bluetooth KRC-86B V4.0 board with the implementation following the circuit recommended in the excellent DAC90A Restoration 'ebook' from Yannis Karalis:- https://www.yanniskaralis.com/bush-d...sy-restoration For copyright reasons I will not Post details from this PDF but what I can mention is that he recommends using two small impedance matching isolation transformers rather than the usual simple solution of resistors in series with the output. I have constructed a rectified, smoothed and regulated 5V DC supply for the BT board based around a small encapsulated transformer (9V 133mA) from ESR Electronics and this works very well. So far so good ...! I have never used these small impedance matching isolation transformers and wondered if any Forum members had any experience of these. Yannis recommends ones from Mouser UK (42TL013-RC) but I was a bit put off by the steep p&p charges so have ordered some LT700s instead as they appear to have similar specifications and are more widely available. If you have implemented the Yannis Karalis Bluetooth mod for the DAC90A then please advise accordingly and also if you have any experience of the LT700s. __________________ BVWS Member

14th Feb 2021, 6:38 pm	#9
AmateurRepairer Pentode Join Date: Dec 2020 Location: Chesterfield, Derbyshire, UK. Posts: 138	Re: Bush DAC90A - Adding Blueooth Wouldn’t recommend fitting any sort of external casing or component to a live chassis model. __________________ Working on a Murphy U198M, Marconi CR100, PYE VHF2D x2, P75A and 25C.

14th Feb 2021, 6:48 pm	#10
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bush DAC90A - Adding Blueooth The transformer isn't really needed then. Lawrence.

14th Feb 2021, 7:15 pm	#11
murphy512 Pentode Join Date: Mar 2008 Location: London, UK. Posts: 122	Re: Bush DAC90A - Adding Blueooth I seem to remember that the Marconiphone T26A dealt with the breakthrough when the gram was in operation, by fitting a .02 capacitor between the anode of V2 and ground which came in operation when the gram plug was inserted in the gram socket and I believe that muted the radio. I am not sure if that could be applied to the DAC90A but someone in the forum more knowledgeable than me could point out any possible problem with that. Aldo

14th Feb 2021, 7:45 pm	#13
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bush DAC90A - Adding Blueooth Those transformers won't match the impedances, matching isn't required anyway because the input impedance at the top of the volume control is I suspect much higher than the output impedance of the Bluetooth device. Lawrence.

14th Feb 2021, 8:49 pm	#15
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bush DAC90A - Adding Blueooth I've no idea what a Bluetooth can output and into what impedance. Lawrence.

14th Feb 2021, 9:01 pm	#16
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: Bush DAC90A - Adding Blueooth 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 begn with, but the difference could be noticeable even with a table radio. You might be better off with one known to work. Also the LT700 is not intended as a high voltage isolator. Be sure you don't wind up relying on it in this way. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

14th Feb 2021, 9:10 pm	#17
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bush DAC90A - Adding Blueooth Here's the specs. for the Mouser one: https://www.mouser.co.uk/ProductDeta...pwpGuhuA%3D%3D Lawrence.

14th Feb 2021, 9:31 pm	#20
Radio tirana Pentode Join Date: Nov 2015 Location: Oxfordshire, UK. Posts: 171	Re: Bush DAC90A - Adding Blueooth Hi Donald I think that Yannis uses the transformers because he also details a line input conversion (3.5mm Socket) in his document. He obviously thinks that they provide sufficient isolation. I think you can dispense with the transformers in the Bluetooth only version. The Bleedthrough problem can probably be solved with some bypassing as sugested in #11 Kind regards RT