HA-PRO2

If you need excellent mains operated headphone amp you should try this kit. It’s reasonably priced and it needs only mains transformer and suitable amp case. Simple regulated PSU with high quality Rubycon elcos is onboard. PCB is of excellent quality and is double sided. All resistors are metal film with copper leads. There is mute circuit onboard using 12V relay so you won’t hear any annoying turn on and turn off pops. Enough space is provided on the pcb for ALPS Blue Velvet 27 mm pot if you think that you can not live without expensive high quality pot. I used 10k 16 mm log pot instead of 20k 30-position dented log pot provided with the kit. I shall use that pot for some other project.

Schematics are not available but I can describe circuit to you. It uses 4 opamps per channel. First one provides all voltage amplification and is driven from the pot wiper. Voltage gain is 5,5 times. Three remaining opamps are parallel unity gain buffers used as highly linear output transistors providing necessary current for headphones. Original kit was supplied with NJM4580 opamps which can give 50mA output current. I used high performance NJM2114 which has 60mA output current. I also have NJM4556 at hand but in this configuration they are not really necessary. Each output unity gain buffer has 10 Ohm series resistor at the output.

Sound quality is excellent. I tried this kit with both my Sennheiser HD424 (2000 Ohms) and AKG K141 Monitor (600 Ohms) headphones. I don’t think that headphone enthusiasts using modern low impedance headphones will have any problems with this kit. There is plenty of output current available to drive such headphones. (I don’t think that anybody has Koss K6LC 8 Ohms headphones in working condition to chalange this headphone amp design.)

This headphone amp kit from China will outperform many expensive western HiFi industrial products.
 
I have just assembled HA PRO2 Headphone AMP which I got it from aliexpress. It started working fine. But my 32 Ohm Headphone suddenly got some crackling sound and the board have not output any audio now. what could be the issue here? I am a newbie for electronics and like audio electronics very much.
 
If you need excellent mains operated headphone amp you should try this kit. It’s reasonably priced and it needs only mains transformer and suitable amp case. Simple regulated PSU with high quality Rubycon elcos is onboard. PCB is of excellent quality and is double sided. All resistors are metal film with copper leads. There is mute circuit onboard using 12V relay so you won’t hear any annoying turn on and turn off pops. Enough space is provided on the pcb for ALPS Blue Velvet 27 mm pot if you think that you can not live without expensive high quality pot. I used 10k 16 mm log pot instead of 20k 30-position dented log pot provided with the kit. I shall use that pot for some other project.

Schematics are not available but I can describe circuit to you. It uses 4 opamps per channel. First one provides all voltage amplification and is driven from the pot wiper. Voltage gain is 5,5 times. Three remaining opamps are parallel unity gain buffers used as highly linear output transistors providing necessary current for headphones. Original kit was supplied with NJM4580 opamps which can give 50mA output current. I used high performance NJM2114 which has 60mA output current. I also have NJM4556 at hand but in this configuration they are not really necessary. Each output unity gain buffer has 10 Ohm series resistor at the output.

Sound quality is excellent. I tried this kit with both my Sennheiser HD424 (2000 Ohms) and AKG K141 Monitor (600 Ohms) headphones. I don’t think that headphone enthusiasts using modern low impedance headphones will have any problems with this kit. There is plenty of output current available to drive such headphones. (I don’t think that anybody has Koss K6LC 8 Ohms headphones in working condition to chalange this headphone amp design.)

This headphone amp kit from China will outperform many expensive western HiFi industrial products.

B: yes. HA - PRO2 is very simple, but it can play a full performance of the operational amplifier.
And I think it do not need to be more expensive operational amplifier.
I'm in the new production. Socket with a 3.5, by the way, mostly I feel more convenient.
In addition to the test data. To facilitate the actual contrast.

If someone need to other test data can email me. I really very slow landing site.
Three minutes maybe can open web pages.
 

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I have just assembled HA PRO2 Headphone AMP which I got it from aliexpress. It started working fine. But my 32 Ohm Headphone suddenly got some crackling sound and the board have not output any audio now. what could be the issue here? I am a newbie for electronics and like audio electronics very much.

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.
 

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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?
 
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.
 

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I have one of these kits waiting to be assembled.
Does anyone know what's the gain on it?

Also looking for a suitably small transformer (12-0-12 volts, 15-30 VA).
The case I ordered doesn't leave very much space:

s-l1600.jpg


I will report back on how it sounds when assembled.

Right now I'm short on time and trying to finish with another project (Smart Kit 1215).
 
Hello again.

I finally found the time to assemble this LJM head amp and I am glad I did.
Compared to my other head amp, the Smart Kit 1215, I think this sounds better.

I've been running it for a couple of days to open up and I actually enjoy it quite a bit.
It's more musical than the Smart Kit and a bit warmer too, without sacrificing details.

I also like the fact that it uses a simple circuit.
Easier to understand and less things to go wrong.

I will post some pics a it later..

-----------------

One question for the designer(or anyone else):

Dear LJM, (All),

Is it ok if i lower the gain of the inverted opamp?
Right now it has a 10kOhm over a 2.2kOhm resistor (equal to 5.5 gain if i'm correct).

Since this is a lot for my preference, could I change the 10kOhm to a value of 6kOhm or 7kOhm?

Would that be ok, or I am risking oscillations by lowering it?
 
Change R3 to 5k or 10k.

Thanks PRR.

I don't have any 5kΩ or 10kΩ at hand right now, but i have a couple of 7.5kΩ.
So, i might try those on positions R3 and R4.
Hope this is fine too.

Also, resistors R8, R9, R10, R11, R12, R13 are all 10Ω.
I was wondering, do they actually affect output impedance?
My hunch is that they don't, but maybe I'm wrong.

I also wonder what's the output impedance on this circuit.


Thanks again and please forgive my ignorance.
 

PRR

Member
Joined 2003
Paid Member
> do they actually affect output impedance?

NFB is taken before R8..R13. So certainly the 10r Ohms count.

So does the opamp. While 4580 has no Zout spec, we see it is a BJT pair probably working near 1.5mA, so about 20 Ohms when naked. GBW is 15MHz, so anywhere below 15KHz we have 1000:1 NFB, making <0.020 Ohm in the opamp.

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.)

Make it lower? Consider cross-current. The DC offset voltage is specced 3mV (either way). Say one amp is +3mV and another is -3mV. We have 6mV across 20 Ohms, just sitting there doing nothing. Hmmm, 0.3mA cross-current, far less than the estimated 30mA-60mA that each opamp can deliver. You could maybe go down to 2r or 1r. It will idle hotter and I'm not sure there is any advantage.