Hi all,
I recently bought an Onkyo A-850 rather cheaply from a thrift store, and am quite happy with it (I am living abroad, so am restricting myself from acquiring too many expensive possessions for the moment) 🙂
I recently bought a bluetooth unit (Fusion MS BT-100, which has good reviews) and I intend to mod the amp, so that hitting the "Tuner" switch activates bluetooth.
I have plenty of experience with soldering, so stripping back the RCA leads and soldering them directly to the back/inside of the RCA terminals will be a piece of cake. Its locating the 15v rail which is annoying me a little. Please see pic from service manual:
As you can see, there is both a +VE and a -VE 15v rail. I assume this has something to do with the Op Amps higher up in the circuit, but I am not going to waste too much more time trying to understand where the -VE voltage is being generated.
My intuition tells me to solder directly to the +VE rail at the indicated point, and then ground to the GND terminal at the 50v step-down transformer? From what I understand, grounding to the -14.7 rail should give me something like ~+30v, which will more than fry the little bluetooth unit.
Any thoughts or suggestions welcome!
J
I recently bought an Onkyo A-850 rather cheaply from a thrift store, and am quite happy with it (I am living abroad, so am restricting myself from acquiring too many expensive possessions for the moment) 🙂
I recently bought a bluetooth unit (Fusion MS BT-100, which has good reviews) and I intend to mod the amp, so that hitting the "Tuner" switch activates bluetooth.
I have plenty of experience with soldering, so stripping back the RCA leads and soldering them directly to the back/inside of the RCA terminals will be a piece of cake. Its locating the 15v rail which is annoying me a little. Please see pic from service manual:
As you can see, there is both a +VE and a -VE 15v rail. I assume this has something to do with the Op Amps higher up in the circuit, but I am not going to waste too much more time trying to understand where the -VE voltage is being generated.
My intuition tells me to solder directly to the +VE rail at the indicated point, and then ground to the GND terminal at the 50v step-down transformer? From what I understand, grounding to the -14.7 rail should give me something like ~+30v, which will more than fry the little bluetooth unit.
Any thoughts or suggestions welcome!
J
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Either what PRR wrote or ground at the RCA terminal, whatever works best.
The 15 V supply stops working at about 30 mA, and it also has to supply the op-amps from that. Does your Bluetooth receiver draw much less than 30 mA?
The 15 V supply stops working at about 30 mA, and it also has to supply the op-amps from that. Does your Bluetooth receiver draw much less than 30 mA?
Really? It delivers that little?
Tech manual says bluetooth receiver draws 20 mA. I'll test it now to confirm. Maybe that's a little too close to the bone, huh?
I never considered grounding to RCA terminal. Looking at the schematic, RCA goes straight to chassis ground via a 0.022uF line-to-ground capacitor. If that's the case, I might as well just twist/solder together the RCA ground and -VE of the Bluetooth, and take +VE from a single point in the rail (assuming it can handle 20mV) right?
Use a scrapped cell phone charger, our phones get upgraded / damaged, and we all have extra chargers...
5.1V clean and stable, and use it to externally power the Bluetooth.
Connect that output to Aux or CD inputs.
Enough juice, but on older units the voltage may be higher. Current on those chargers is enough for a Bluetooth.
Some routers also use that voltage, the supplies for those will work as well.
Provided of course the Bluetooth will work off that.
Or else use a 78 series regulator, or a LM317, or whatever else strikes your fancy and is handy.
5.1V clean and stable, and use it to externally power the Bluetooth.
Connect that output to Aux or CD inputs.
Enough juice, but on older units the voltage may be higher. Current on those chargers is enough for a Bluetooth.
Some routers also use that voltage, the supplies for those will work as well.
Provided of course the Bluetooth will work off that.
Or else use a 78 series regulator, or a LM317, or whatever else strikes your fancy and is handy.
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Indeed, that most likely won't work because of the current needed by the op-amps.
The 15 V is derived from the 45.8 V rail by a Zener diode circuit that works as a shunt regulator. A bit less than 30 mA flows from the 45.8 V via R911 and R913. Whatever current the load doesn't need is shunted to ground by Zener diode D917. Load it too much and the Zener runs out of current and the voltage drops and becomes unregulated.
The 45.8 V rail can deliver many amperes, so you could supply your receiver from there. If it can't handle 45.8 V + tolerance, say 51 V, you could build your own resistor-Zener diode circuit to reduce the voltage.
20 mV?
I assume that the outputs of the Bluetooth receiver are referred to its ground, if not then the whole scheme won't work. If so, then to minimize pick-up of hum due to varying magnetic fields, you have to connect the ground such that you don't get large loops. That is, the path from the Bluetooth receiver ground to the ground of the line input that you connect it to has to be close to the path from the Bluetooth receiver output to the line input.
i'm a bit intrigued by D911 and D912. what reasoning would be behind these ?
reducing the supply by 0.5V? sense the period of the waveform ?
reducing the supply by 0.5V? sense the period of the waveform ?
That
It can´t.
Datasheet states "10 to 16V @ 20mA" so that´s it.
To avoid overloading the current +/-15V supplies, which are already in use by the Op Amps, add a little extra supply for the Bluetooth module.
For a 15V 30mA capable supply (so you have 10mA "surplus" just in case) tap the +45V main rail supply (as PRR said, straight from C912 positive terminal) through a 1K 2W resistor (it will actually dissipate about 1W so you want the next higher size) into a 15 or 16V Zener 1W dissipation, in parallel with a 100uF x 25V capacitor.
BEWARE the point circled in green and labelled "ground" is NOT ground but the PT centertap, here NOT the same thing.
Real Ground is C912 negative terminal and anything connected to it, notice the Earth/Ground symbol on that connection.
@ basreflex: I see no logic or usefulness either.
Transformer center tap goes nowhere else.
Interesting discussion. Intriguing, educational 🙂
The BT receiver is 10–16v, so powering it off the 46v rail is a no-go.
Back in my drone-building days, I would use very small/cheap buck regulators like this one (DC-DC Step-down Buck Converter 1.5A - 5V output - DCDCSTEPDOWN5V) to power peripherals straight off the 21v LIPO pack.
Search as I might, I cannot find anything which can take 50v and put out 12v, but if I manage to locate one I think I will go with that.
Or, I could be a little ambitious and try to build am zenner/resistor circuit. That would be the next step of my education.
On a semi-related note, can anyone recommend some good software for playing around/designing small circuits? I have been using this site (CircuitLab - Editing "Unnamed Circuit") so something along these lines would be amazing. I typically learn through doing/messing around, but with real world electronics this is obviously not very sensible 😀
The BT receiver is 10–16v, so powering it off the 46v rail is a no-go.
Back in my drone-building days, I would use very small/cheap buck regulators like this one (DC-DC Step-down Buck Converter 1.5A - 5V output - DCDCSTEPDOWN5V) to power peripherals straight off the 21v LIPO pack.
Search as I might, I cannot find anything which can take 50v and put out 12v, but if I manage to locate one I think I will go with that.
Or, I could be a little ambitious and try to build am zenner/resistor circuit. That would be the next step of my education.
On a semi-related note, can anyone recommend some good software for playing around/designing small circuits? I have been using this site (CircuitLab - Editing "Unnamed Circuit") so something along these lines would be amazing. I typically learn through doing/messing around, but with real world electronics this is obviously not very sensible 😀
the LM5008 family can handle the input voltage (up to 75V) . it generates a triangular superposition on the output, so a post LDO regulator is advised. read the datasheet carefully..
A buck converter with post regulator seems a bit overdone compared to a resistor, Zener diode and capacitor.
Besides, when you use a buck converter and the Bluetooth receiver has a sigma-delta DAC, which it almost certainly has, any crosstalk of the switching frequency and its harmonics to the DAC reference or clock can mix out-of-band quantization noise into the audio band. Of course that can be solved with good post-regulators, filters, shielding and so on, but it seems a bit overdone to save about 1 W.
Hi guys, an update:
I screwed up!! (a little)
I build the circuit people here recommended, see image.
The problem is, the specs manual was a little bit misleading. The BT device only uses 20mA at idle; this jumps up to about 35mA once connected, and jumps to a further 120–50mA once playing music (with rare peaks @ 180mA).
Device seems stable when artificially constrained to ~100mA with a bench supply, but randomly disconnects etc below this point. Any advice about building a replacement circuit that can handle 200mA? I thought of just sticking to the same design, but running them massively in parallel. See design:
The problem with this, is that it will dissipate ~10w through the resistors alone, and a further ~3w through the zener diodes at idle. Is there a better way of engineering this, without resorting to a switching-type step down (buck) transformer, run through a massive filter?
Help much appreciated!
I screwed up!! (a little)
I build the circuit people here recommended, see image.
The problem is, the specs manual was a little bit misleading. The BT device only uses 20mA at idle; this jumps up to about 35mA once connected, and jumps to a further 120–50mA once playing music (with rare peaks @ 180mA).
Device seems stable when artificially constrained to ~100mA with a bench supply, but randomly disconnects etc below this point. Any advice about building a replacement circuit that can handle 200mA? I thought of just sticking to the same design, but running them massively in parallel. See design:
The problem with this, is that it will dissipate ~10w through the resistors alone, and a further ~3w through the zener diodes at idle. Is there a better way of engineering this, without resorting to a switching-type step down (buck) transformer, run through a massive filter?
Help much appreciated!
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Shunt-regulating with this much drop and this much load swing is not a happy answer.
A series regulator will only get hot on peaks. With a few more tricks the pass transistor can be heat-sunk on the case.
Attachments
1) mount it *outside* your amplifier chassis , you don´t want a Bluetooth receiver inside a metallic cage.
Instructions explicitely say so, sugvgest how to mount it to a wall or something, away from metal surfaces, and connect it toAudio system using RCA plugs.
At least glue it *outside* tge chassis or wooden/plastic cabinet.
2) VERY misleading soecs indeed and I am not sure there´s not something wrong with you Bluetooth receiver, can see no bechanism why a recever which output ¨*signal* , is not driving hdphones or something , needs going from 20-30mA (which is already beyond spec) to 100 or even crazy 180 mA.
No way.
In any case, pulling that from a +45VDC supply is inefficient (to put it mildly):
get a 12V plug in charger/wall-wart capable of 200-250 mA and that´sit.
You might mount both: supply and BT inside a plastic case, your own DIYVReceiver, which connects to Mains for power and sends Audio through RCA.
You can get 12V supplies for peanuts at Saalvation Army, Thrift stores, etc.
Instructions explicitely say so, sugvgest how to mount it to a wall or something, away from metal surfaces, and connect it toAudio system using RCA plugs.
At least glue it *outside* tge chassis or wooden/plastic cabinet.
2) VERY misleading soecs indeed and I am not sure there´s not something wrong with you Bluetooth receiver, can see no bechanism why a recever which output ¨*signal* , is not driving hdphones or something , needs going from 20-30mA (which is already beyond spec) to 100 or even crazy 180 mA.
No way.
In any case, pulling that from a +45VDC supply is inefficient (to put it mildly):
get a 12V plug in charger/wall-wart capable of 200-250 mA and that´sit.
You might mount both: supply and BT inside a plastic case, your own DIYVReceiver, which connects to Mains for power and sends Audio through RCA.
You can get 12V supplies for peanuts at Saalvation Army, Thrift stores, etc.