Spark Amp Headphone Output

Sep 29, 2020

The Spark Amp features a headphone output connection but no lineout. This article describes the headphone output level and noise characteristics which may be useful for those wishing to recording into analog recording equipment, as an alternative to using the USB connection as a recording interface.

The picture shows the setup used to measure the headphone output level. A splitter at the guitar input jack was used to input a variable level 500Hz audio sine wave from the lineout of a computer soundcard. This input level and also the headphone output level were simulaneously monitored using a dual channel Digilent Analog Discovery oscilloscope. The input signal level was increased until the headphone output level reached the highest available level without clipping.

The output impedance of the headphone output connection was also measured by comparing the output signal level measured directly with the oscilloscope with no loading (1Mohm scope input impedance) and then loading the headphone jack with a 10ohm resistor. Comparing the 2 voltage levels shows the headphone output impedance is about 1ohm.

The dual scope traces below (Ch1 orange is input level; Ch2 blue is headphone output level) show that the maximum headphone output voltage available is about 520mV peak before clipping at the output occurs (2nd diagram):

Next, a full amplitude 16bit/44.1kHz 500Hz sinewave audio clip was played over the Bluetooth connection from an Samsung S9 phone. This should indicate the maximum headphone output level available with music streamed to the Spark amp via Bluetooth. The scope trace below (with the "Music Volume" control on the Spark amp set at 100%) shows the maximum headphone output level via Bluetooth input is 300mV peak:

Finally, to provide a fairly simple noise assessment of the Spark amp headphone output, the headphone output was used as an analog recording input to a Tascam DP24 multitrack recorder:

The input gain level on the Tascam unit was adjusted so that the signal used for the Bluetooth test created almost full digital recorded signal -1.1dBFS on the Tascam. Then a 16bit/44.1kHz recording was made of the headphone output, with various connections configured during the recording. The results (as confirmed by simple listening to the output) show very low noise background at the headphone output (about -80dBFS) even with a guitar (Fender strat .. middle pickup ... all controls at max) connected to the input with default high-gain preset 2 and the OUTPUT level on the Spark amp set at 100%. There was a noticeable increase in noise floor, to about -65dBFS at 8 sec, as the MUSIC VOLUME control on the Spark amp was increased to maximum . The images below are the same except for the high zoom level in the 2nd image to show the noise floor details. At about 15 sec, the guitar was switched in (with no noticeable noise increase) and at about 17 sec, the guitar was picked up and strings touched gently creating the spike. At about 21 sec, the Bluetooth signal was applied for reference:

These basic results show that the headphone output of the Spark amp can be used to achieve fairly good low noise performance guitar audio recordings with analog recording gear, possibly mixed with SmartJam tracks or local backtracks streamed over Bluetooth from the Spark app. Certainly with any realistic electric guitar connected to the Spark input, the noise floor characteristic of the Spark amp alone would likely never be noticeable. Compared to standard lineout connections, the headphone output, because of its intended main use to drive low impedance headphones, has a relatively low signal level (~ 0.5V) compared to typical lineout levels of over 1V.

More detailed RightMark Audio Analyzer measurements for the Spark amp of the headphone output performance indicates reasonably good quality, for a guitar amplifier.

A comparison of audio performance of a high-quality Creative X-Fi Elite Pro sound card, a Tascam DP24SD multitrack recording input characteristics and the Spark amp results may be of interest.

The Spark amp can be connected to any standard analog equipment such as audio mixers, amplifiers and recording consoles with suitable 1/8" stereo mini-phone-plug to either dual RCA or 1/4" phone plug adapter cables :

Spark Amp YouTube Demos: