This is from my personal collection, not sure on the # but less than 15-20 of these were made I believe. The Echo Amp is a small standalone audio amplifier with a built-in speaker that consists of a preamp into a simple echo circuit into an op-amp gain circuit with the output fed into a small Class D power amp. Overall it is a bit noisier than I was aiming for and the wood enclosure versions can vibrate too much at certain frequencies. Not a ton of headroom for clean guitar sounds and probably better for slightly distorted to overdriven guitar but can get relatively loud for the size. I think I might like it more for synths etc.. vs guitar but my brother uses his with a contact mic and a variety of instruments and really seems to enjoy it. It can be powered via a standard 9volt battery (included) or via a 9V DC Power Adapter (Standard Boss Style Center-Pin Negative) Controls for echo time, feedback, mix, and gain as well as master volume control. 1/4" Mono Input suitable for guitar, bass, synth, drum machine etc.. 1/4" Mono Speaker Output for plugging into external speaker cabs (not for plugging into another amp or active speaker) Echo Amp - Instructions 1/4" Input: Mono input, plug your instrument/synth/drum machine in here. 1/4" Output: Mono Speaker Output: For passive speaker only = DO NOT PLUG INTO AN AMPLIFIER or ACTIVE/POWERED SPEAKER. Note: The internal speaker will still produce sound when using an external speaker. Larger external speakers will draw more current. DC Power Jack: 9VDC Center-Pin-Negative Power Supply ONLY. Current draw is variable depending on the volume output setting and using a larger external speaker will draw more current than the small internal speaker, up to more than 600mA, recommend using a DC Power supply that can provide up to 1AMP of current, especially when using with an external speaker. 9V Battery Power: The unit can be powered with a standard 9V volt battery (included) which can be installed/replaced using the battery door on the bottom if you have a wood enclosure version or by removing the bottom plate on the aluminum enclosure version. Note: Depending on the volume setting the peak current draw may be too much for the battery and the audio may cut out momentarily if that happens. TIME: Turn clockwise to increase the length of the echo. Clean echo times up to around 500-600ms are available, echo times approaching 800ms are available but may result in increased noise/grit. FDBK: Turn clockwise to increase the number of times the echo repeats. The control is tapered to allow self-oscillation, so be careful when turning past halfway. MIX: Sets how much of the delayed instrument signal is mixed with your clean analog signal path. Turn clockwise to increase the mix of the delay signal. This control is tapered so that the echo level can be louder than the source signal. GAIN: Sets the output volume of the pre-amp/delay signal that is sent to the power amplifier, turn clockwise to increase volume and acheive overdriven/distortion tones. VOL: Sets the output volume that is fed into the internal speaker and external speaker output jack.