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DIY: Summer Projects

Silou

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#1
Since I will have a lot of spare time in the next weeks, I want to build myself something nice :D

First Project: passive filters for my HD58X Jubilee/Amiron Home/K712 Pro (by solderdude) built inside a case with one input socket for the headphone and one output for the amplifier. I want to wire the three filters using this thing, so I can switch between them and only use one pair of sockets. I want to add three LEDs so I know which filter is selected.

Second Project: This is just an idea of mine. I am not an engineer and electronics is not my main knowledge area, but I like to build things myself.
I want to use two (or do I need 4?) objective2 boards to built a headphone amplifier with a 4-pin-XLR output. The idea was to use one board per channel so I can double the output power. Please comment if this works this way or if it is nonsense :D

Third Project: First + Second Project combined with Tone Board as the DAC in one nice looking chassis
 

solderdude

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#2
Balancing 2x O2 will quadruple the power of HD58X and Amiron. For the K712 it will be slightly less, probably around 3x or so.
2 boards will be needed for stereo and you need a phase shifter or change the circuit (quite do-able and can tell you how)
The biggest challenge will be the volume control. You would have to use the volume control of 1 amp and route one channel to the other board.
Possible though.
You could also use 1 charger circuit and 1 set of batteries with some modifications.
 

Silou

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#3
Good to know that my idea is not crap and it is doable.:) I found this thread about symmetrical headphone amps and I think it is quite useful for amateurs like me. The phase shifters you mentioned are probably the square parts
pushpull.jpg
 

solderdude

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#4
the phase shifter in the above circuits are the lower op-amps in the left and right part (the ones driving the 'red' signal)

As the output stage of the O2 is similar to the blue signal output part you would have to add the resistors (these could be 10k because they 'load' the volume control so they cannot be much smaller)
Some PCB traces on the O2 board would have to be cut and rerouted using the resistors.
Not impossible to do.
 

Silou

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#5
Nice, I will tackle this project after my vacation next week. I would like to expand my knowledge to get a better understanding of what I am going to do and why it needs to be done. Any suggestions?
 

solderdude

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#7
Push Pull is another technique where each half of a sine is handled by a complementary power device.
But there are designs that use phase-shifters or use transformers to perform the same task.

When bridging amps 2 amplifiers both handle the full signal but opposite in phase.
 

solderdude

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#8
Any suggestions?
Build the filters first and take into account that it must also be used balanced.
You will also need balanced cables for your headphones.
 

Silou

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#9
Build the filters first and take into account that it must also be used balanced.
You will also need balanced cables for your headphones.
The parts for the filters are already on their way to me and should arrive tomorrow except for two capacitors. I would like to test them unbalanced first, because I do not have a balanced source or amp at the moment. I just have to "split" the negative cable into R- and L- to achieve a balanced connection, or am I wrong? Balanced cables are no problem for me. I have two spare 4-Pin XLRs at home for the HD58X and Amiron Home. The K712 will be modded soon so I can use the Amiron cable.
 

solderdude

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#10
I just have to "split" the negative cable into R- and L- to achieve a balanced connection
correct, the filter in the + side and the ground for each channel becomes the - side (when split)
 

Silou

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#11
I would like to order the PCBs from headnhifi. Is their parts only kit fine or should I order the needed parts separately with lower tolerances?
 

solderdude

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#12
After thinking about this a bit more it would be easier to use a different approach for signal routing as that does not require any cutting of traces.
I suggest to use 1 board for + outputs L and R and 1 board where the input op-amp circuit is converted to a phase shifter (easy to do) and the outputs of that board then become the - outputs for L and R channel.
Tolerances of parts are much less important this way as well.
Only the + channel boards have the volume control.
 

Silou

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#13
Sounds good to me. Does the symmetrical approach require two seperated power supplies or can I limit it to one? It is propably a lot easier to just use the stock power supplies that usually come with the O2.
 

solderdude

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#14
One power supply is fine.
As you are using high impedance headphones the transformer doesn't need to be able to supply more power.
This also means that you do not (should not) add the power supply parts on one of the boards.
Everything before Q1 and Q2 can be omitted in one board.
That is the parts on the left and above Q1 and Q2 in the original schematic, as well as 1 volume potmeter and the gain switch on the negative signal board.
 

Silou

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#15
I do not know if I got it right. Only the bottom right corner is important for the negative board?
IMG_20190731_230437.jpg
 

solderdude

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#16
In the attached schematic all components on the left and above Q1 and Q2 (including Q1 and Q2) are not needed.

On the board these parts are right below and on the right side.
 

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Silou

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#17
OK I got it this time. The O2 seems to overload quite easily with higher input sensitivitys than 2.8V. Most balanced DACs measured on ASR exceed the 2.8V mark quite easily. Best Gain settings would probably be 1X and 2X, but 2X already overloads the amplifier.
 

solderdude

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#18
That's the drawback of having lower noise at lower listening levels in pursuit of nice numbers.
The Atom, operating on the same principle, has the same 'problem'.
Which isn't a real problem just something to take into account.

You can change that by putting the volpot before the amp but not wise if you want to use high sensitivity IEMs (which you don't)
I can tell you how to make the changes that alleviates this issue.

An option is to adjust the volume on the source or use a switchable input attenuator.
 

Silou

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#19
As you mentioned I could change the volume of the DAC, but I do not know if my future DAC will be able to do that :D
I have got some IEMs, but there is no need to use them with the new amplifier since I have the Atom as a "backup"

Parts List:
- 2x PCB
- 2x Parts only Kit (I know that I do not need two full kits, but I guess it might be good to have some backup parts)
- Input and Output sockets (2x 3-Pin-XLR female, 1x 4-Pin-XLR female)
- power supply
- parts for the modification of the negative board

Did I forget something?
 

Silou

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#20
The filter parts arrived today. How exactly did you put them together? These are the parts for the Amiron. I lined them up like this. Is that correct?
IMG_20190801_194918.jpg
 

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