Zapco ST-6X DSP amplifier review - no input gain!


Acoustics Engineer
Premium Member
5,000+ posts
10+ year member
May 1, 2007
Over the past two weeks I have both installed and uninstalled the Zapco ST-6X DSP six channel amplifier. My access to the amplifier is due to my having installed it (and a companion Zapco ST-2000xmII) in a friend’s car, only to remove it six days later. I’ll try to keep this review objective, despite having wasted two days and then spending additional time testing to verity the published specifications are not accurate and this amplifier lacks any input gain adjustment. I’ll say that again because it’s the most important part of this thread; this newest offering from Zapco lacks input gain adjustment and, based on my testing, can only handle a 0.5V RMS input before clipping.

Input signal voltage on left and output voltage on right (both DMM and scope).

0.741V Input

0.588V Input

0.467V Input

PC view with 0.467V Input

PC view with 0.588V Input

The review:
The cosmetics of the ST-6X DSP amplifier are really nice. The outward impression of build quality is high, which is expected of Zapco products.

Amp view

Amp ends

The input section has four pairs of RCA jacks, gold plated and well made. There are connections for fiber optic, a high-level input jack, USB, and a COM port. The output connections are typical set screw and the amplifier comes with the necessary tools. A USB cable is provided along with a fiber optic cable. You have to download the manual.

The main feature of this amplifier is the DSP, since that’s why you bought it. There are two screens; the main screen that shows all 8 channels and the input setup screen. The input screen lets you route input signals to outputs, and is flexible with summing and routing options. There is no input signal gain or sensitivity adjustment in this screen.

Software Input Screen

The main screen has all 8 DSP channels labeled, but you cannot change the labels. You’re stuck with front L&R tweeter and woofer, rear L&R, and preout L&R.

Software Main Screen

The color coding is also fixed. Grouping companion channels is possible. There is a flexible crossover section for each channel including polarity and channel mute. **Note: if you mute a channel when tuning, that mute setting is saved when you disconnect the computer. This ended up being annoying.** You also get phase and level per channel and a master output level. Note that neither of these are input gain and I tested this by introducing a 0.58V input signal and as I adjusted either level adjustment, the signal only changed in amplitude and remain clipped.

Time alignment can be time-based or distance-based for those who don’t have a calculator handy. Time-alignment is entered in the graphic of a car with front 2-way and rear speakers. The EQ section is a 15-band parametric. You can drag the gain slider and enter frequency and Q. You never really know when the software is linked to the amplifier because of the way the button is labeled (either “Link” or “Not Linked”). When we tried to save the program to the PC (Surface 4) it would crash, never letting us be confident the program saved, so we’d have to re-open and check the device. The firmware update process is akin to a beta test level of development. You have to install separate programs and run multiple windows simultaneously… and the supplied USB cable didn’t work for updating firmware. After trying 3 different cables, one worked on the third try… we were rewarded with a turn-on pop with the new firmware.

Firmware update process:

The Story:

My buddy bought a 2017 Malibu LT brand new, and it was immediately apparent that it needed a real sound system. The previous car was totaled by insurance and all he could keep from the vehicle was the Audison LRx5.1k and Hertz ML-2500 sub. We went with the HSK-165 speakers this time and assumed we could make the pairing work well. We started with the stock HU and a MiniDSP CDSP6x8, we didn’t like the input voltage limitations, noise concerns, and then moved to the Dayton Audio DSP. Still unhappy with the stock HU, we swapped to a Kenwood eXcelon with a 5V output. Given the input limitations of both the MiniDSP and Dayton, we looked for a solution that would be a good match for the head unit and eliminate separate pieces of equipment. The newest offering from Zapco seemed to be the perfect fit – an amplifier with an 8-channel DSP and a published input range from .25V to 5V. The price point is really attractive, at only $600 it’s a good deal for a Zapco amp, let alone a 6 channel amp with DSP. Even better, the ST-2000xmII monoblock amplifier without DSP is a perfect pairing. For $1,100 you can have a 3-way front stage fully active plus outputs to a healthy sub amp. This pairing is what many of us have been looking for, and seemed like the perfect solution for our application. After spending an afternoon removing the Audison and Dayton, we made room for the pair of Zapco amps. A few hours later we were still trying to figure out how to adjust gain and match the amp to the head unit. We couldn’t figure out how to adjust input gain because the manual refers to physical dials similar to the non-DSP version yet dials don’t exist. The ST-2000xmII is a perfect matching amplifier, the cosmetics are similar, and even though the amplifier came with four 25A fuses that it cannot accept, it seems like a nice amplifier. Sadly, if the ST-6X DSP isn’t a suitable product, it doesn’t make sense to keep its companion either (since the sub amp was only purchased to match the DSP amp). Both amplifiers are being sent back to the retailer because the buyer was sold components that did not match the specifications listed, and thus did not ship what was advertised.



Dec 4, 2018
its has an auto sensing voltage circuit. see if you can get the schematics from zapco id like to look at it..


Acoustics Engineer
Premium Member
5,000+ posts
10+ year member
May 1, 2007
  • Thread Starter Thread Starter
  • #3
Zapco removed the input sensitivity ratings from their website after I informed them of my results. If it were auto sensing it wouldn't clip at 0.5V. It can have an auto turn-on, but that's different from input sensitivity.

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