I suggest connecting three (110 ohm, 0.6 watt, 1%, metal film) resistors in parallel with each other and in parallel with the headphones. these units at Mouser.com would be good choices.
The impedance seen by the ACA+ is 1/[(3/110) + (1/300)] = 32.67 ohms, which is exactly what Nelson Pass recommends. And the power handling capability of the three parallel resistors is 1.8 watts, which ought to be plenty more than what you require.
Headphone Impedance Matching
I also have headphones that are not 32 ohms so I modified my ACP+. As Nisbeth and Mark mentioned, paralleling resistors with the headphone out to get 32 ohm net impedance is the way to go.
However, I found that since the output selector switch is incorporated into the headphone jack, it was easier said than done. The headphone positive is always connected to the output signal positive.
See pictures below for how I adjusted for headphone impedance. I have included calculated resistor values for some common headphone impedances. Choose resistor values that are closest to the calculated values.
This previous post of mine also shows the added resistors:
Amp Camp Pre+Headphone Amp - ACP+
I also have headphones that are not 32 ohms so I modified my ACP+. As Nisbeth and Mark mentioned, paralleling resistors with the headphone out to get 32 ohm net impedance is the way to go.
However, I found that since the output selector switch is incorporated into the headphone jack, it was easier said than done. The headphone positive is always connected to the output signal positive.
See pictures below for how I adjusted for headphone impedance. I have included calculated resistor values for some common headphone impedances. Choose resistor values that are closest to the calculated values.
This previous post of mine also shows the added resistors:
Amp Camp Pre+Headphone Amp - ACP+
Attachments
Very clean and tidy layout.
A pity perhaps that you did not give the high current tracks a bit more width.
150mA bias is 2x that of the quiescent current of a LM3886 !!
Best regards,
Patrick
Here is my finished ACP+ - just got the cartoony labels in for it.
The case came from ebay.
The sound is incredible!
Many Thanks to Nelson for a very cool design!
And thanks to Nisbeth for his PCB's.
Bruce
Bruce, is it possible to share ebay link to the enclosure?
Here's the one I bought but it's out of stock now.
You could see if the seller can order more.
2607B-2 Aluminum HIFI Amplifier enclosure /DAC case/ preamp chassis AMP BOX | eBay
Here's an ebay case that is close.
New aluminum power amp chassis home audio amplifier case size 308*250*70MM | eBay
withmatt - Your PCB layout is really spiffy. The 120 x 160mm size is so nicely compact. Thanks for sharing the Gerber files! I'm just learning Eagle myself. Would you be willing to share the Eagle source files with me? I guess I'm just curious to play around with them a bit. In any case, I'll let you know how it turns out. I'm going to try JLCPCB for the first time on this.
