# Zapco ST-6X DSP amplifier review with testing



## keep_hope_alive (Jan 4, 2009)

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.


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## metanium (Feb 2, 2007)

I've been hesitant to mention anywhere publicly, but I've been told by an extremely reliable source (and confirmed by another) that the new DSP offerings from Zapco basically don't function properly either. Something along the lines of the software not being ready for primetime.

Aside from the ST-1350XM II amp dyno debacle of last year, sounds like something stinky is going on.:shrug:


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## NoDestiny (Jul 28, 2019)

Excellent information. I bought this amp (And the ST-1650XT II) and have found a few issues with it myself...
-All the exact things you mentioned (gain, firmware update, etc)
-Turn on pop with my subwoofer
-Subwoofer outputs sound first, ~2 seconds later, the front end plays
-Sometimes I get a weird static-like sound from my front end. Power-cycle my deck and it's fine afterwards.

Otherwise, it outputs plenty of clean power and I love most of the DSP interface. Never seems to get beyond a little warm.


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## EnzoB (Jul 30, 2016)

Salve, I don’t know your name, I am Enzo, president of APEX, owner of the Zapco brand and manufacturer. Sorry for the late replay, but I am not usually in this forum. The ST-6X DSP dont need input regulation because with 0.5 Volt of input give you 100 watts of output for each channel in 4 ohm and 150 watts for each channel in 2 ohm. If the amplifier receive more than 0.5 Volt in the input it clip because the amplifier reached its maximum power. It is an amplifier with DSP, then different with conventional amplifiers without DSP, we prefer dont have an input regulation that can compromise the DSP correct functionality at 0 dB. Over 2000 customers today use that amplifier with HU that have output over 0.5 volt (normally go from 2 to 4 volt today) and are fully satisfied about output power and purity of the signal. If some is not clear in the my message please tell me. Warm regards


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## crdi_lover (Jul 19, 2018)

I presume there's a video on this by OP (IIRC). Here it is: 






Sorry for the OT in advance. I know this thread is particularly for the DSP variant but I have a doubt below. The HU which I'll be getting has an advertised pre-out voltage of 4V which will be fed to a Helix DSP MK2 PRO. Hope it shouldn't create an issue with input voltage to the Zapco amplifier?!



EnzoB said:


> Salve, I don’t know your name, I am Enzo, president of APEX, owner of the Zapco brand and manufacturer.


I'm on the verge of buying a Zapco ST-6X SQ + Zapco ST-1000XM II. 

Is the above issue pertaining ONLY to the DSP equipped amplifiers or for the non-dsp ones too?

Should I go ahead with the purchase of these two amplifiers for my audio setup?

Sent from my ONEPLUS A3003 using Tapatalk


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## EnzoB (Jul 30, 2016)

connecting a HU with 4 Volt output to a Zapco amplifier with DSP does not create any problem. The amplifier will achieve maximum power with the radio volume at 75% and you will still have a volume reserve for audio recordings made at too low a volume. with Studio Zapco DSP amplifiers can connect HU from 0.5 Volt to 4 Volt without any problem


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## thewinelake (Jul 21, 2020)

NoDestiny said:


> Excellent information. I bought this amp (And the ST-1650XT II) and have found a few issues with it myself...
> -All the exact things you mentioned (gain, firmware update, etc)
> -Turn on pop with my subwoofer
> -Subwoofer outputs sound first, ~2 seconds later, the front end plays
> ...


I have one of these - installed a few weeks ago by someone who seems to know what he's doing.
Overall I'm reasonably happy with it, but I'm no expert and don't have much to compare it to.
Newbie question - I presume that the clipping is only an issue when playing really loudly?
The main problem I have is the occasional static I get at the front. It's not always there, but is annoying (and can be heard even when the head unit is outputting at zero volume).
Installer suggested dialling down the master volume. I reckon that would only make the problem a bit less apparent - it's not a proper fix.


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## dumdum (Feb 27, 2007)

EnzoB said:


> connecting a HU with 4 Volt output to a Zapco amplifier with DSP does not create any problem. The amplifier will achieve maximum power with the radio volume at 75% and you will still have a volume reserve for audio recordings made at too low a volume. with Studio Zapco DSP amplifiers can connect HU from 0.5 Volt to 4 Volt without any problem


Sorry but this is not a good design imo, why would you not make it with a variable gain on the inputs like every other dsp in the world... mk2 should have a variable gain imo! Nobody I’ve spoken to likes the 0.5v fixed gain


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## EnzoB (Jul 30, 2016)

anuraagshenoy said:


> I presume there's a video on this by OP (IIRC). Here it is:
> 
> 
> 
> ...


Sorry for late replay but I see only now that message.
The ST-6X SQ is an amplifier without DSP and have input gain regulations. So can buy it and adapt the input gain regulation according your headunit output.
The ST-6X DSP dont have input regulation like DSP unit inside the amplifier work with an fixed digital level input. The amplifier will reach the maximum power just with 0.5 Volt of the source and any more voltage cannot create any problem, only that the amplifier make distortion because over the maximum power. Again about ST-6X SQ we suggest an regulation of the input gain at 0.5 / 1 Volt also using an head unit with 4 volt of maximum output. 6 or 9 dB of higher amplification from the supplier give you warranty that any musical signal can be reproduced well also when the file registration is lover than normally. Take note than 6 or 9dB that is the margin in the amplification that have the amplifiers when is connected with an 4 volt head unit correspond at -6 or -9dB in one digital volume of the head unit that is sane to have like reserve of volume.


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## EnzoB (Jul 30, 2016)

dumdum said:


> Sorry but this is not a good design imo, why would you not make it with a variable gain on the inputs like every other dsp in the world... mk2 should have a variable gain imo! Nobody I’ve spoken to likes the 0.5v fixed gain


At the end we are going to add an gain input control in the next generation of the DSP amplifiers like also we have in the high end DSP (see HDSP-V that have separate and accurate for each channel). We did not put it at the bginning because there is the risk that an bad regulation can create big problem putting in saturation the ADC that need an fixed voltage of 1 Volt. Then the big distortion created from the ADC in this case is much much worse than dont use at the maximum volume the head units outputs. Again it is sane to use the head unit just at 75% of maximum volume and to take the rest of 25% for reserve when the signal is too low like often happen with high res files. So for the Zapco engineer an fixed input at 0.5 volt give 9 db of headroom, and 1 volt give 6 dB of headroom to the HU and that headroom is better to have that dont have like when the input level of DSP is regulated at the maximum output level of the HU . If market want an pots because for 50 years the analog amplifiers have and it is difficult understand that an DSP is different, then we will add pots also in the our entry level products and some leds to advice when the signal is too high. Not important if at the end all will find the best regulation from 0.5 volt to 1 Volt, but more important if some dont make mistake to give too much signal to put the DSP in saturation. For my personal opinion better dont have any regulation so nobody can make mistake, but for market reason we will add dont have critic like this one or worse disinformation like the test in you tube that say that the amplifier have distortion with more than 0.5 volt input without to say that the power in the output is over the maximum rated (so the distortion measured was not in the preampl, but in the final stage of the amplifier like it is normal when the maximum power is exceeded). We have in the front of our eyes only an result after we sold over 2000 units. That all the customer say the sound is plenty, clear and no distortion with an exceptional result that is expected only from amplifier with much higher power. To note also that this amplifier have an DSP at 96 Khz of sampling rate (in similar situation other have just 48Khz) and it is in class AB when others are in class D. And all of that for little more then 500 usd.


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## opekone (Mar 24, 2020)

Thanks for that careful, thorough, and transparent response. That's really good communication right there.


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## keep_hope_alive (Jan 4, 2009)

EnzoB said:


> At the end we are going to add an gain input control in the next generation of the DSP amplifiers like also we have in the high end DSP (see HDSP-V that have separate and accurate for each channel). We did not put it at the bginning because there is the risk that an bad regulation can create big problem putting in saturation the ADC that need an fixed voltage of 1 Volt. Then the big distortion created from the ADC in this case is much much worse than dont use at the maximum volume the head units outputs. Again it is sane to use the head unit just at 75% of maximum volume and to take the rest of 25% for reserve when the signal is too low like often happen with high res files. So for the Zapco engineer an fixed input at 0.5 volt give 9 db of headroom, and 1 volt give 6 dB of headroom to the HU and that headroom is better to have that dont have like when the input level of DSP is regulated at the maximum output level of the HU . If market want an pots because for 50 years the analog amplifiers have and it is difficult understand that an DSP is different, then we will add pots also in the our entry level products and some leds to advice when the signal is too high. Not important if at the end all will find the best regulation from 0.5 volt to 1 Volt, but more important if some dont make mistake to give too much signal to put the DSP in saturation. For my personal opinion better dont have any regulation so nobody can make mistake, but for market reason we will add dont have critic like this one or worse disinformation like the test in you tube that say that the amplifier have distortion with more than 0.5 volt input without to say that the power in the output is over the maximum rated (so the distortion measured was not in the preampl, but in the final stage of the amplifier like it is normal when the maximum power is exceeded). We have in the front of our eyes only an result after we sold over 2000 units. That all the customer say the sound is plenty, clear and no distortion with an exceptional result that is expected only from amplifier with much higher power. To note also that this amplifier have an DSP at 96 Khz of sampling rate (in similar situation other have just 48Khz) and it is in class AB when others are in class D. And all of that for little more then 500 usd.


Thank you for the reply. We appreciate the conversation and it is important for your side to be heard. 

The manual was incorrect, and the published stats were also incorrect until I brought this up to my reps and Zapco. If this fixed gain stage was truly intentional, the manual, the published stats, and the website should have made this clear. To have incorrect information published created additional confusion. 

The purpose of the video (which was made fairly quickly due to the return policy of the seller, and with limited equipment for the same reason) was just to show that buyers need to be careful with the application since most source units have 2V, 4V, 5V, or higher outputs. The 12V industry has made a push to have 4V or higher outputs on source units to improve signal to noise ratio (since the 1990's). In my application, a 5V source unit was chosen to specifically reduce amplifier gain on the input stage to reduce noise, and we saw saturation and clipping way before 75% volume, closer to 40% in our application. We also see many installations integrating with OEM equipment more frequently, which further increases the variety in source unit signal strengths. Measurements above rated power simply showed how easily it was to provide a 0.5V signal. 

To say that users have 75% of the usable volume range with a fixed, maxed input gain is not accurate and ignores the fact that there is a wide variety of source units on the market and the users of this product would typically have and choose a higher output voltage source unit. That specific head unit had a flat EQ, and had any EQ bands been boosted we would see a stronger signal at some frequencies - and we know that users like to play with EQ boosts.

If the stated design intent was to eliminate adjustable gain, why have adjustable gain on any of the other Zapco products? Why not turn the pot up to 0.5V (near max) on every other product and cover up the port? Is the recommendation to turn all Zapco amplifier input gain stages to 0.5V regardless of source? If not, please explain why this one product is the exception to the others.

Users like to have a wider volume range, which is to say that they don't like large jumps in output volume with one click of the volume control. When you fix the input gain at maximum (which is basically what has been done), you eliminate the ability to take advantage of the entire volume range on many products. 

External pots aren't required with a DSP amplifier. Your competitor, JL, achieves an adjustable input gain through the Tun software (with signal strength indication on the input) to allow for matching the amplifier input stage to the source unit.

Thanks again for engaging the group and sharing!


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## EnzoB (Jul 30, 2016)

keep_hope_alive said:


> Thank you for the reply. We appreciate the conversation and it is important for your side to be heard.
> 
> The manual was incorrect, and the published stats were also incorrect until I brought this up to my reps and Zapco. If this fixed gain stage was truly intentional, the manual, the published stats, and the website should have made this clear. To have incorrect information published created additional confusion.
> 
> ...


Thank for the opportunity to explicate better because we did not put gain control in the our amplifier with DSP when, in the same time, also the brother amplifier ST-6X SQ have that control. ST-6X SQ like all the amplifier we produce from ever have pots control and it was the reason that who make the manual of the ST-6X DSP making copy first of the brother made the mistake did not erase that functionality no more existing in the DSP. Sorry for the mistake in the manual, again. Now why an amplifier with DSP is better dont have an input gain control? Exactly because it is not an amplifier but an DSP first of all. The amplifier come after the DSP stage and all we see from the HU is just an DSP.
Then just to make an example our DSP-Z8 IV II dont have any regulation of the input sensitivity, but many other DSP dont have too. In the same time all the amplifier we can find, all, have an input regulation.
The reason is that the DSP input that is an ADC converter need an signal of 1 Volt. No more, not less. If more the ADC have an incredible distortion, if less the S/N of the DSP become worse. It is like work an digital signal. Digital signal pratically dont have volume and need to be more that is possible costant at 1 volt to have the better performances from the system. The lower and higher volume created from the our HU is just an information that the DAC (after the DSP) need to transform again in one analog signal with volume to be directed the amplifier. 
All that is called work at 0 dB. 0 dB is the maximum volume of the HU with digital control. DSP have to work at 0 dB, mean the maximum level that is possible for the digital signal. Now we have an HU that have 4 volt output or more like maximum signal and we have an DSP that have pot regulation for input sensitivity (see our HDSP-V for example), what is the regulation that the customer have to do? At 4 volt the input? It is what normally do the majority of the installers. For us it is wrong so we removed the pot of the gain to avoid the risk to have an bad regulation. Products have to be more that is possible simple and dont need to have much control that at the end can generate only confusion in not expert peoples. Any control added need to have an specific necessity. Why we dont need in this case? if you give me 4 volt input at the DSP amplifier I need to reduce it at 1 volt before the DSP for the reasons I said before. If I dont do it the DSP will have an incredible distortion. When I say incredible I mean much more than normally we have in the final stage of the amplifiers. So an mistake in the regulation, in this case, create big problem. We eliminated the problem taking off the pots and regulating the input sensitivity fixed at 0.5 Volt. Should be also 1 Volt. One fixed input from 0.5 Volt to 1 Volt can be the optimal for us. Now any HU i connect that have 0.5 volt or 4 volt can work effectively. The matter is the amplifier reach is better power perfomances at that input voltage and the HU still have 6/9 dB of head room to compensate low level signals often in High resolution music files. It is true that never I can utilize the maximum output of the HU, but why I need. Probably the HU at lower volume have also less distortion, in the same time than an opposite effect in the DSP will create an high distortion. But if the signal in the HU is low and I want to listen that signal in the amplifier at the maximum power, I still have 6 or 9 dB (depend if input is regulates at 1 or 0.5Volt with 4 volt HU) to amplify the signal to arrive at the amplifier maximum power. 
Then an my suggestion to everybody. If you have an regulation on the input of the your amplifier with DSP, regulate it around 6dB less than the maximum output of the HU. 
Regulation of the volume inside the DSP reduce normally the DSP sound quality performances introducing higher level of noises. Please use less that is possible volume in the DSP and dont equalize over 6 dB more or less, if possible. DSP have to work at costant volume of 0 dB to have maximum quality of the signal.
Warm regards


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