Zapco ST-6X DSP 6-Channel 900W RMS Class AB Amplifier with DSP

The Zapco ST-6X DSP and likely the ST-4X DSP…Those of us working as engineers have learned to read and interpret specifications such that we can design and implement solutions that support the project goals. But what happens when the specifications/instructions given are not representative of the actual product?In today’s market, its important to be the first. I can understand the pressure to introduce a new product when someone has already beaten you to market. If you come to market with a similar product shortly after the competition and you accomplish the same goal at a cost-effective price point, then what will the consumer choose to purchase? I’d bet the more cost-effective solution, wouldn’t you? What is the old saying, caveat emptor, let the buyer beware…?In their haste to get their product to market, Zapco has missed the boat. At first glance the product and specifications look great, but at closer inspection - or upon implementation- you will find that is not the case with the new ST-6X DSP.If you ask an audio engineer, “What is important when setting up an audio system?” you will likely get a lesson in gain staging. For those of you who don’t know, gain staging is the process of managing the relative levels in each stage of the audio signal path to prevent unnecessary noise and distortion. Ideal gain staging, or gain structure, is achieved when each component in the signal path is transmitting or receiving signal in the optimal region of its dynamic range. Improper gain staging results in unwanted, and sometimes excessive, noise and/or distortion. Correctly setting the input sensitivity maximizes the signal-to-noise ratio by matching the input sensitivity of the amplifier/DSP to the output voltage of the source.At a quick glance on their website, as well as the manual, you will see that the new ST-6X DSP has an input sensitivity range of 0.25V to 5V. This range may be true for the ST-6X SQ, as it has a variable gain pot at its input stage to adjust the pre allowing for a range of input voltages at its input; however, it’s NOT true for the ST-6X DSP. I’m guessing the thought process at Zapco was likely something along the lines of “all we are doing is adding a DSP to the front end of this amp; let’s just copy the specs from the ST-6X SQ.” At a close look of the ST-6X DSP, you notice that the input stage is lacking any sort of manual gain pot on the amplifier chassis. Suspiciously, on page 9 of the manual there is a section dedicated to gain adjustment that describes how to adjust the gain pots on an amplifier… for an amplifier that has no gain pots. That could be OK if, for example, the input sensitivity was able to be adjusted in the new software packaged with the amplifier. Sadly, input gain/sensitivity seems to have been forgotten and is simply not available. In my book that makes the input sensitivity FIXED. If it’s FIXED, what exactly is the true input sensitivity? We do not know because this information is not available. With the appropriate information, the consumer could make an informed decision if this solution meets their needs. But in this instance, the consumer cannot use the published specs or manual to make their decision.If you’ve ever visited Zapco’s website and perused their literature, I’m sure you will find information on the importance of gain structure along with their commitment to excellence: so how could a manufacturer committed to sound quality have forgotten the basics? The FIXED input sensitivity is clearly not 5V because my personal experience is that there is distortion present with a 5V source at approximately 60% of the volume range. When a 9dB pad is inserted at the amplifier input there is still distortion (indicative of input clipping) present in the signal at the higher end of the usable volume range, which leads me to believe the input sensitivity is approximately 2V. Further testing will verify this assumption.For those of you that have chosen a higher quality head unit that supports a 5V+ output, this is not the amplifier for you. Beware of the specifications touted on Zapco’s website, as they are not an accurate representation of this amplifier’s configuration. This lack of accuracy also puts into question all the specifications detailed for the ST-6X DSP as well as the ST-4X DSP, both on Zapco’s website and within their literature. Can they be trusted with specifications if something so basic has been missed?The question now becomes, does this problem have a solution? After using the software to program the amplifier, which by the way is clearly in beta and has problems of its own that I will cover later, you will find no option to adjust the input voltage sensitivity. Is the input sensitivity truly FIXED, or could it be set between 0.25V to 5V? Was the ability to adjust the pre just forgotten by the GUI programmers? Or perhaps, are the analog gain pots somewhere on the board and not accessible to the user without voiding the warranty by opening the chassis? Or perhaps, since the specifications may have been copied from another amplifier, it was never a consideration. What can Zapco do to make this right for the consumer? Can they? This is my first impression with a Zapco product, as it is the first Zapco amplifier I have had the pleasure of purchasing. I’ll tell you, after being duped, I would never buy another Zapco product again.For those of you who would like to run an active 3-way system in your car, I’m sure you have some expectations of the software that would support your workflow beyond the basics like matching your input sensitivity. Installers can forget about naming channels and making notes so others can take over where they left off. The naming on the channels are fixed and permanently labeled for use with a two-way system in the front and LR speakers in the rear. Additionally, the ability of the software to scale based on your PC’s resolution is limited. When adjusting your gain settings -- if you like to work blind, you’re all set as there are no level meters or simple red light to indicate clipping. The lack of active signal levels is a substantial miss within the software.After searching Zapco’s website in hope of finding better directions for the software, a firmware update was posted for the amplifier. Thinking the firmware update would provide added functionality, we performed the upgrade… which is a polite way of saying that we spent two hours trying to complete the upgrade. After numerous failed attempts with what was clearly not a commercial solution for the upgrade process, we found that even the USB cable packaged with the product was too long to complete the firmware upgrade. The amplifier would not connect to the PC with a 2-meter cable. By the way, for those of you who don’t know, USB 2.0 has a length limitation of 5 meters. That means a cable up to 5 meters should support communication between the amplifier and PC. However, for some reason, the amplifier was finicky and required a short 1-meter cable to complete the upgrade. This was determined after two failed attempts using the same 1-meter cable.For some reason the third time was the charm, and the firmware upgraded successfully using the 1-meter cable. Our efforts were rewarded with a turn-on pop that was not clearly audible and no additional functionality.The latest firmware provided no solution to our immediate problems, and the software failed to support the adjustment of the input sensitivity; therefore, we saved our work to the amplifier. When it came to saving our back-up file to the local PC, the software would crash and restart. Hence, the ability to save your settings to file is a Hail Mary at best. Perhaps your PC will be more reliable. My personal suggestion is save your money and go with another manufacturer until Zapco can get their S**T together with this “beta version” that is clearly not ready for sale. I’m not a fan of companies that use their clients to test their products. We shouldn’t be responsible for finding their mistakes at the expense of our hard-earned cash.

The positives (only reason 2 star instead of 1):+Plenty of great output from the 6x100+Looks really nice+Doesn't get overly warm (4 ohm all channels for me. Don't push it too hard. I don't like my ears ringing!)+DSP interface is actually pretty nice and organized (though, not the greatest to use)Like the other review states, this amp has a lot of issues...-No gain control-DSP causes random "static" sound once in a while, must power cycle to rid of-Output channel send signal ~2 seconds before amp output signal-Turn on thump on output of output channel-Firmware update is a terrible process and fixes nothing-Not that it needs 0/1awg inputs, but would have been nice (had 0/1 awg already, bought 4awg adapters to fit)-After several days, customer service from Zapco has yet to contact me on the issues.This amp might be great in the future, but right now, it's a bit of a turd and really is a blemish in the Zapco name. If Zapco can't find a fix for all these issues ASAP, I'll be forced to return this product and go with another (likely stick with the Zapco brand, probably the ST 6X-SQ, but not their DSP as that's the real issue here). Woofersetc (the seller here) were wonderful in returning an email on my concerns immediately, but they can only do so much.