HA-PRO2

So we have three legs at 10.02 ohms, call it 3.3 Ohms.

This IS low enough to drive headphones satisfactorily. (It seems low to me.)

Thanks a lot for the detailed explanation.

I will not pretend that I understand much, but you gave me a nice start to do some more reading.

By the way, do you think I could use another (better?) opamp at stage position?
I have a couple of OPA1641 on DIP8 adapters.
 
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Are you using the HA-PRO2 to drive speakers?

I'm using it to drive my Sennheiser HD600 and HD559.
One is 300 Ohms and the other is 50 Ohms.

If you use it for speakers with an impedance of 8 Ohms, the current required would push that little headphone amplifier a bit too far..

Maybe you could use it as a preamplifier connected to a power amplifier to drive your speakers.
 
Just assembled one of these. Am i deaf, or it is something lack of bass on Sennheiser HD380 Pro? 54 ohm impedance or i did something wrong? Maybe i should increase capacitance in supply filter? I mean, use something more than 470uF

Everything else is ok - absolutely silent, crystal mids and highs, enough volume. I switched to Pioneer SA-508 (restored this beautiful amp yesterday) - and there is more bass, not much, though (with loudness switch set to off). Any advices?
First of all, I have to say something. This amplifier does not add bass.
But it will not reduce the bass. It is a pure DC amplifier. There will be no low frequency attenuation.
In theory. It can increase the volume. Or you can match enough current between the player and the headset.


I assmebled a kit bougnt on alixpr. Listened it for few hours on HD 380 PRO and found it very analitical. However it had no bass. It sounded very close to direct output from DAC I used.

I replaced JRC opamps with original TI NE5532 and headamp retain it's analytical clarity but got a nice amount of bass.


LJM - I'm impressed how such simple design can sound. Great work.
 
From my experience, this kit sounded balanced without lack of bass.
In terms of tonality it is pretty neutral i'd say.

Also, to my ears the 4 x NJM4580 included in the kit, sounded better than 4 x NE5532 i tried after.

Now, if you want a bit more 'weight' in the sound than either two above, I suggest you try 4 x NJM4556.

Another great little opamp that is cheap and available is the NJM2068.
It sounds better than all of the above in my opinion.
 
Just started reading so.....

1. What is the output impedance of this amp?

2. On the board there seems to be output/line out connector (One seller says this unit can be used as a preamp - I presume wiring these to a set of RCA out will be adequate?)

3. There is no output selector so can I presume the unit will direct output to RCA or vice versa when the headphone jack is unused/used respectively?

Thanks
 
It is a very simple circuit.

But must pay attention to the installation is correct. Especially the diode 4148 5 v must not mistake.

And important point. It must use AC 12-0-12 v transformer.
Can't a higher voltage. Because there is no voltage regulator IC relay. Higher voltage will damage the relay.

If you can. If you can measure the relay conduction. Or when the relay switch on.
If there is a gentle sound. Prove that no damage.

If damaged. You can consider to use wire, sub relay input, output.

Try to listen to sound output.


Hi, but after the input seems to be placed an 7912 regulator. If my transformer is not perfect regulated at 12v .... could be a problem? Normally the 7912 must be do it’s job... or not?
 
Hi, but after the input seems to be placed an 7912 regulator. If my transformer is not perfect regulated at 12v .... could be a problem? Normally the 7912 must be do it’s job... or not?

I’m asking because if a trasformer has no load (or a light load) on secondary, the output normally is higher. So if I buy a transformer 12-0-12 30vac is quite sure that the output could be of 15/16v.
Could be ok feed with this voltage this amp?

Thanks
 
Small EI transformers usually have secondaries that are much above nominal rating. 12V secondaries can be over 20V unloaded. Therefore it is better to buy 9V secondaries and hopefully your voltage at the input of regulator will not be too high under load, but will still have at least 15V to allow proper regulation. Relay coil is 12V and powered with regulator input voltage. That's why you must not have much more than 12V at the input of regulator.
 
Review on the transformer said:

Unloaded output voltage is 15.5v (For 12v declared)

Could the relay support it? Maybe when It is loaded with the amp, it could be less as well....
Normally the relay has a toleration about the max voltage operation.

What do you think?
 
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You will probably have at least 20V at the input of regulator. It's too much for the 12V relay. No 12V relay will work long with 20V coil voltage. 12V relay can accept 13V or 14V. You can measure the input voltage at the regulator to see, relay will work even with 20V for short period of time. It won't fail immediately.
 
That's what puzzles me too. My comments are based on LJM's comment because transformer voltage is important only if relay coil is connected to relay input voltage. The best thing to do is to connect transformer you have and measure the coil voltage on activated relay. If it's 12V than the coil is connected to regulator output voltage.
 
The voltage marked at PCB is "nominal" minimum voltage for the circuit to work. The input voltage for the 7812 regulator must be 3V higher for the regulator to work properly, meaning 15V. LJM does not know how each particular transformer manufacturer will wind their transformer. Therefore he marked nominal voltage. With my 12V transformer input voltage at regulator is 21V! That's 7V more than necessary. The simplest way to correct this is to use 9V transformer. Or if you have 12V transformer that actually outputs nominal 12V, everything will be fine.