Zhaolu DAC headphone amp tweaks

ashok

Member
2002-06-06 4:43 am
3RS
Has anyone tried to tweak the headphone amp in the Zhaolu V2.5-A DAC ?
I pulled out the board and had a look at the circuit.
It has a unity gain input buffer running wide open followed by a 100K volume pot and an opamp buffered complementary emitter follower stage . Overall gain being 3 .
I suppose the input unity buffer can go and the input coupling cap on the following stage can be changed for a film type . Size would be a problem unless one just puts it above the board.
The 100uF non polar cap in the NFB leg could go too unless the output dc offset is unacceptable. Will try this tomorrow. Any one else tried any tweaks here ?
I have OPA2604's istalled at present.

I had to remove the DAC board and 'clean' it with isopropyl alcohol to remove what appears to be flux residue on some components. It had tarnished the solder on those parts and looked like that section would go defective with time. The board was also not cleaned well , especially the bottom. The headphone board was pretty clean. I didn't remove the output or power supply boards.
Cheers.
 

ashok

Member
2002-06-06 4:43 am
3RS
Input stage circuit.

Here is the input stage of the headphone amp.

The 100K resistor is the volume pot.
 

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kevinkr

Administrator
Paid Member
Hey Ashok,
Welcome to the wild and wooley world of Zhaolu dac tweaking.

From the looks of things I think you can eliminate that entire first stage. I would however keep/upgrade that 100uF cap in the feedback loop in order to prevent the headphone amplifier from amplifying dc offsets from other places.. IMO it is unlikely that the op-amp will contribute more than a couple of mV on its own, if you are sure nothing else is generating significant offsets this one can probably go.

Take a very careful look at the power supply board and read my posts the other Zhaolu thread relating to the loops I found - you'll want to fix these as they will have an impact on headphone amplifier performance.

Finally I am convinced that the last change I made to the audio output board made a very substantial improvement and you should make those changes too.. I plan to post a picture with some instructions to cover what can't be seen on the bottom of the board. (You can even retain the stock parts, they aren't that bad - the whole change mostly requires the removal of existing parts and the addition of two 2.21K resistors in places where they didn't previously exist.) You might want to consider the servo version if you want to go capacitorless in the headphone amplifier

FWIW I found the standard headphone amplifier to sound very uninvolving and at one point they did have a much better version on offer. It was apparently discontinued due to relatively low order volumes, but there might still be some floating around.