That's right the No Headphone Amp!
It's very inexpensive because, well there's no headphone amp and it's very transparent because ... you guessed it, there's no headphone amp!
Looking at Blues' Cool Head 'amp' (The WarpSpeediER) pdf I noticed this:
View attachment WarpspeediER - The Lightning Bug Headphone attenuator.pdf
The output impedance of our Source/DAC will determine how much max current it is capable of delivering. For a given max output voltage (>/= 2 Vrms), the lower the Source/DAC Zout (</= 200 ohms) the more current is available, and is preferred for all phones -from 16 ohms through 600 ohms. A Source or DAC that touts very low Zout (<50 ohms) but smokes its output stage when accidentally shorting the output connectors is not a good design.
He makes a good point that we have all the voltage swing we need and really it's about can the output stage supply the current?
Well, how much power do headphones need?
To rock out on a 32-ohm/98dB/1mW HP we essentially will need lower than 600 ohms for Source Zout. For higher impedance HPs like 300-ohm Sennheisers (102dB/1V = 107dB/1mW!) or 600-ohm BeyerDynamics, higher Source Zouts(>/= 300 ohms) will still generate thunder in our heads. Remember that an 88dB SPL directly into our ears (a mere 0.1mW on 98dB/1mW HPs -that's 1.77mA on a 32-ohm HP or 245mV on 600-ohm HP!) is already very loud, albeit good quality loud through the Warpspeed. I hope not one of us aspires for 100dB SPLs directly into our ear canals –this number is acceptable with loudspeakers in a typical listening room volume with furniture, other sound absorbing materials, and we are 8 feet away. 90dB to 93dB peaks however are plenty loud for headphones.
Hmmmm ... I have some sort of optical volume around here ...
Aha!
http://www.ska-audio.com/diy/optivol.html
SKA's optivol is very basic: 28k in series and then LDR shunt. The output is off the LDR shunt. (so just a single matched pair of LDRs for stereo)
A single log pot drives a current source I think through both LDRs. It was my entry into LDRs for a nice $30!
I hacked it to make the No Headphone Amp: LDR in series then headphone element as shunt (no shunt resistor)! About as simple as you can get: a variable resistor then the headphones!
I have Sony MDR-EX71SL IEMs:
Driver Unit : 9mm
Frequency Response : 6Hz - 23,000Hz
Impedance : 16 ohms
Sensitivity (db) : 100dB/mW? (higher!)
Power Handling Capacity : 100mW
and AKG311 'ear buds':
Frequency Response 18Hz - 20.5kHz
Maximum Input Power 15 mW
Input Impedance 32 ohms
Sensitivity: 125 dB SPL
and old over-the-ear AKG K301s:
Sensitivity 94 dB SPL/mW
Frequency range 20 to 25,000 Hz
Rated impedance 120 ohms
THD <1%
Max. input power 200 mW
I measured the impedance with the 3 different headphones hooked up and regular listening was between 700 ohms and 1.5k.
I have the No Headphone Amp hooked up to a USB dac with 2V out and reasonable drive and ... it sounds good! On the AKG K301s (120 ohm) it sounds perfect but with the IEMs and loud volume I think I'm detecting some distortion (700 ohm)
And really, you don't have to use an LDR. You could use a pot, attenuator or even a few resistors (next test!)
No Headphone Amp really is best!
Cheers,
Jeff
It's very inexpensive because, well there's no headphone amp and it's very transparent because ... you guessed it, there's no headphone amp!

Looking at Blues' Cool Head 'amp' (The WarpSpeediER) pdf I noticed this:
View attachment WarpspeediER - The Lightning Bug Headphone attenuator.pdf
The output impedance of our Source/DAC will determine how much max current it is capable of delivering. For a given max output voltage (>/= 2 Vrms), the lower the Source/DAC Zout (</= 200 ohms) the more current is available, and is preferred for all phones -from 16 ohms through 600 ohms. A Source or DAC that touts very low Zout (<50 ohms) but smokes its output stage when accidentally shorting the output connectors is not a good design.
He makes a good point that we have all the voltage swing we need and really it's about can the output stage supply the current?
Well, how much power do headphones need?
To rock out on a 32-ohm/98dB/1mW HP we essentially will need lower than 600 ohms for Source Zout. For higher impedance HPs like 300-ohm Sennheisers (102dB/1V = 107dB/1mW!) or 600-ohm BeyerDynamics, higher Source Zouts(>/= 300 ohms) will still generate thunder in our heads. Remember that an 88dB SPL directly into our ears (a mere 0.1mW on 98dB/1mW HPs -that's 1.77mA on a 32-ohm HP or 245mV on 600-ohm HP!) is already very loud, albeit good quality loud through the Warpspeed. I hope not one of us aspires for 100dB SPLs directly into our ear canals –this number is acceptable with loudspeakers in a typical listening room volume with furniture, other sound absorbing materials, and we are 8 feet away. 90dB to 93dB peaks however are plenty loud for headphones.
Hmmmm ... I have some sort of optical volume around here ...
Aha!

http://www.ska-audio.com/diy/optivol.html
SKA's optivol is very basic: 28k in series and then LDR shunt. The output is off the LDR shunt. (so just a single matched pair of LDRs for stereo)
A single log pot drives a current source I think through both LDRs. It was my entry into LDRs for a nice $30!

I hacked it to make the No Headphone Amp: LDR in series then headphone element as shunt (no shunt resistor)! About as simple as you can get: a variable resistor then the headphones!



I have Sony MDR-EX71SL IEMs:
Driver Unit : 9mm
Frequency Response : 6Hz - 23,000Hz
Impedance : 16 ohms
Sensitivity (db) : 100dB/mW? (higher!)
Power Handling Capacity : 100mW
and AKG311 'ear buds':
Frequency Response 18Hz - 20.5kHz
Maximum Input Power 15 mW
Input Impedance 32 ohms
Sensitivity: 125 dB SPL
and old over-the-ear AKG K301s:
Sensitivity 94 dB SPL/mW
Frequency range 20 to 25,000 Hz
Rated impedance 120 ohms
THD <1%
Max. input power 200 mW
I measured the impedance with the 3 different headphones hooked up and regular listening was between 700 ohms and 1.5k.
I have the No Headphone Amp hooked up to a USB dac with 2V out and reasonable drive and ... it sounds good! On the AKG K301s (120 ohm) it sounds perfect but with the IEMs and loud volume I think I'm detecting some distortion (700 ohm)
And really, you don't have to use an LDR. You could use a pot, attenuator or even a few resistors (next test!)
No Headphone Amp really is best!
Cheers,
Jeff
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