Zapco ST-6X DSP Amplifier Review - Spoiler - NO INPUT GAIN!?!?

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 review; this newest offering from Zapco lacks input gain adjustment and, based on my testing, can only handle a 0.5V RMS input before clipping.

[@K-Effect]

I think I just found my new favorite KZbin channel, this man is Mr. Knowledge!!

[@richardvedvik979]

Thanks! I'm happy to answer any questions.

[@AS-gp9kg]

You just saved me so much time and money. THANK YOU! Lot of reviews out there that sound like they know what theyre talking about but clearly dont know what they dont know

[@richardvedvik979]

Good deal. I like Zapco, this amp was just a miss. Their engineering techs have tried to argue they did this on purpose... which proves my testing and my point. Cheers! I love the JL VXi series and MVi series. They have DSP amps done right!

[@WBG123098]

Any recent updates on the SS channel videos of your most recent KHA acoustic treatment install? Ever since discovering your build log months ago, I've been excited to see the videos you mention throughout the thread that you plan on uploading about the install. I would very much like to see those videos!

[@AS-gp9kg]

Zapco’s stance as well as many in the sq community dont believe in raising amp gains as it reduces headroom and makes whatever noises that may already be in the system louder. The proper way to raise amp gain is from the line driver. Some music has as much as 20db of range in the recording. By default turning up your gains borrows from headroom.

[@richardvedvik979]

The omission of input gain has been corrected on newer models. The amps came out with the gain essentially at "MAX", which is the opposite of what we would do. You could not add a line driver with this amp. We could not use more than 20% of the head unit volume. They inverted the gain structure. If you're trying to make a case that what Zapco did is correct, then you're saying that all amps could come out with a fixed input gain set at maximum, allowing for minimal input signal level. What you're saying is that the amp gain should be fixed at minimum, and then input signal should be boosted with external means, but that's not what Zapco did.

[@MrLs1racer]

The new one is out. Different chassis with fins on top and fan cooled. Also has the gains on it. Looks like they only go to 4 volts though. Color is diff too, looks white or silver.

[@richardvedvik979]

Good to know that they took notice.

[@MarcCook]

I have a quick question for ya. Since the amp has high level inputs, could you test the clipping voltage of those? If it can go to the 4v better input voltage soldering up a high level to RCA adapter might mitigate the issue. Worth a shot.

[@richardvedvik979]

I like your idea and had the same after the amps were shipped back to Zapco. I had a limited amount of time to test before having to send them back or incur really high fees (25%). I'm hoping someone else with one can (I have a buddy I can ask).

[@jorgeplaza3406]

I just saw another video of this amp and it says u can control gain from the dsp

[@richardvedvik979]

You can control output level, not input gain. Those are two very different controls. Zapco has confirmed that they did not include input gain adjustment and do not plan on adding that function.

[@rebelguevara1]

Can a Audiocontrol matrix or any line driver solve this problem?

[@jonseeley8263]

What was zapco's answer to "no input gain controls" ??? Did anyone reach out to them ??

[@amazonAndy]

Are all zapco amps nowdays made in china or is it just these cheaper line models? I was thinking of this model but now i think i`m going with something else, maybe mosconi.

[@soundcatcher9088]

the new ST-4X DSP III now have Analog input gain controls

[@richardvedvik979]

That's great, and an example of how important that feature set is and how it was neglected upon release.

[@harunjonatan9239]

their newest ST-6x DSP V3 have analog input gain now

[@richardvedvik979]

Great! I'm glad the community was able to create change within the organization.

[@SotonyaAcckaya]

Just to clarify, if i use only high level and optical inputs, it should work ok? or high level inputs are also subjected to clipping issue?

[@richardvedvik979]

Optical is certainly OK because that isn't tied to the analog inputs. I didn't test the high-level inputs, but I should have before I sent the amp back. I only had a few days to test and make the video before sending it back for a refund.

[@scottwelch6637]

I may have missed it but are you driving a 4-ohm load? If so wouldn't that be in the neighborhood of 160+ watts per channel(V^2 / impedance) from a rated 100w per channel amplifier?

[@richardvedvik979]

Not in the video but I did drive a speaker and turned down the output levels to observe input clipping regardless of output level. Full rated power is achievable with a 0.5V RMS input signal. 18V out is 81W @ 4 ohms. The amp can do rated, clean.

[@scottwelch6637]

@@richardvedvik979 Thank you for the information

[@adriandragos6445]

Hey I just bought two of these and also a 12 channel pxe 0850x alpine dsp. You think I can by pass the dsp on the zapco and just use the alpine dsp? For my gain setting and fix whatever problems you were talking about?

[@richardvedvik979]

Since you have output control on the PXE, you can use that adjustment to control the signal into the amps so you the lack of gain adjustment isn't as much of a deal. You'll have more DSP than you need so you can just bypass the amp DSP And use the Alpine. You can use level output in the amp to tune, which is my recommendation, so you keep all of the amp input signals the same. It's a weird way to do it but they made a weird amp. They have corrected the lack of input gain in the newest models.

[@PRINYATIP]

Oh My God!! My Zapco dc 1100 came in and I just realized no physical gain knobs! Soo used to old school amplifier!! Besides software calibration wise. Can I just just plug this in my car and play it without any calibrations?

[@richardvedvik979]

You need to coordinate the input sensitivity (gain) setting on the amplifier with the output voltage of your source. That process hasn't changed over the decades. You need to delve into the software to find that setting, if it exists. if it doesn't exist, then you need to figure out what the max usable volume is on your source so you don't clip the input.

[@marckendrick3163]

Im looking at the st-6xsq to run my front. A good choice? 6.5, 3.5 and 1". Looking at the audio control DSP. Your opinion on an external DSP? Thanks!

[@richardvedvik979]

I run JL VXi amps with internal DSP and love them. I've actually sold all of my external DSPs. Moving forward, integral DSP will be my approach for space-saving and seamless integration with a reduction in cabling.

[@marckendrick3163]

A friend is running an sq system and runs masconi now, though, switched from JL, and said I'd be doing myself a favor by going that route. Funny, this popped up a few days after. After reading more on zapco they still make great amps but not like my old Studio series from way back. JL, like a few others, has the long-running standard of being the bar or above. Greatly appreciate the advice!👍

[@808drgreenthumb]

Wow I’m shocked . I thought zapco is suppose to be one of the best . Are you saying that audison and Jl amps are better ?

[@richardvedvik979]

Zapco does make really good amps, they just missed a few key items when rushing this design. The JL amp is far better for both form factor and the DSP quality. The Audison doesn't have a DSP front end.

[@hinchadahe]

Hi, I'm new subscriber. I have a 3 way FOCAL ES165KX3, I going to run active, already have a Alpine pxe-0850s dsp. With choice do you recommend me? Jl audio xd600/6v2 or zapco ST-6X SQ ?

[@richardvedvik979]

Either are good choices but they each are different sizes and layouts. Can you fit either? The non-DSP Zapco has normal gain adjustments and is a nice amp, just bulkier.

[@hinchadahe]

@@richardvedvik979 thanks for your answer Richard. What is your personal opinion about the dsp alpine pxe-0850s?

[@richardvedvik979]

@@hinchadahe I've had Alpine processors since the 90's including the ERA-G320, PXA-H701, and PXA-H800. The newest model has even more flexibility with 12 output channels. And I like the interface options and wired remote.

[@romannavarro3137]

I got hte same amp with dsp and my software does have gain set up.

[@richardvedvik979]

I've been told that all of the new amps have improved capability and it's possible the software has been updated since. Note that output level is not input gain, and the model I tested only had output level.

[@troyus111]

has this been updated in the newer version / 2021 model?

[@richardvedvik979]

No, but I am aware that input gain has been added, which is the correct approach!

[@jorgeplaza3406]

So it sucks? Btw, u just got another follower from the end of the world, try to guess where

[@richardvedvik979]

I can't say that it sucks because I think it's really close to being great, they just missed a key aspect in design. If someone runs optical to it then they are fine and can just control volume with the remote knob.

[@jorgeplaza3406]

@@richardvedvik979 but not input gain?😏 Mi Pioneer stereo doesn't have an optical output, I'm not 100% sure, but I think it doesn't, I'll have to check

[@RodolphoDesign]

@@richardvedvik979 What kind of remote knob? Where can you connect a knob in?

[@raymondc9896]

Do you know if the zapco dsp-iv ii has this problem. Looking at using one with a 5volt output head unit. Thanks.

[@richardvedvik979]

I haven't tested it but I can say that the Zapco engineers have publicly (on DIYMA forum) defended their design and have said the lack of input gain was on purpose because no one should have a source with that high of a voltage output.

[@raymondc9896]

@@richardvedvik979 but there is. A kenwood 5volt output will do 4.7v with a 0db tone. They really should include some form of adjustability.

[@richardvedvik979]

@@raymondc9896 Totally agreed, we returned these amps to Zapco because we were using a 5V Kenwood head unit. I think this was an oversight and they aren't fixing it, instead, they are just saying it's a design feature.

[@bginvestor101]

So, is your recommendation .5 RMS volt or .5 peak in the input signal? thx!

[@richardvedvik979]

That DMM is reading RMS voltage so yea, 0.5V RMS is the RCA input signal.

[@bginvestor101]

Thank you! I bought one two months ago and have not installed yet .. I’m stuck with this unit..

[@richardvedvik979]

@@bginvestor101 we tried using some inline attenuators (FMOD) to knock the HU output down so we had more usable volume range. It worked well but was bulky.

[@rickylah69]

Can u adjust the master level DB in the dsp and reduce it to accept high voltage (4v) from HU so it wouldn't clip?

[@richardvedvik979]

Master dB is output signal, not an input sensitivity adjustment.

[@richardvedvik979]

To expand: no, you cannot prevent clipping with the master level slider, i demonstrate that in the video. Without a dedicated input sensitivity adjustment it is not possible to change the range of input signals the amp can accept.

[@bennyblanco14]

Isnt there a gain setting on the amp itself? Which is why there isnt an option on the dsp... or is there nothing?

[@bennyblanco14]

Nevermind you answered my question

[@richardvedvik979]

@@bennyblanco14 It's not what you expect, but the amp has no input sensitivity or gain adjustment, only output, which doesn't help us match a source to the amp. Someone could buy a HU with the matching input sensitivity unclipped, pad the input with FMOD, or use high level settings and maybe add a line driver to match the high level sensitivity. They overlooked it, sadly.

[@bennyblanco14]

@@richardvedvik979 you said in your video u spoke to zapco rep?...what did they say on the matter?

[@richardvedvik979]

@@bennyblanco14 The reps are not able to influence the manufacturer. They understand our concern and have no ability to make change.

[@richardvedvik979]

@@bennyblanco14 no response was given. Just today, someone from Zapco is discussing the lack of input gain on the Strictly Sound Quality group on FB. Zapco essentially maxed the gain knob then broke it off to prevent owners from matching sources.

[@voca164e]

I see that Zapco put out a firmware update (on 1/4/20) since the video was posted. Does anyone know if this input sensitivity issue was addressed in that firmware update?

[@richardvedvik979]

Good question, i'll ask my Zapco reference.

[@harishchandran6219]

@@richardvedvik979 did you check whether they add anything new?

[@richardvedvik979]

@@harishchandran6219 based on a reply from a Zapco tech on a DIYMA thread it appears they are standing by the design as-is and they don't think gain adjustments are needed.

[@bbfoto7248]

@@richardvedvik979 I'm late to the party, but WTH were ZAPCO thinking?!?! Their amplifiers have all traditionally been geared towards SQ and accepting strong input voltages for low noise, even going back to the balanced Symbi-Link system. Like you, I just cannot understand their logic and also maintaining the stance that the input sensitivity adjustment is not important or needed! WTH??? Even budget head units have 2 volt preamp outputs, and obviously most people who normally buy Zapco products would be using a head unit with 4v-5v RCA preamp outputs. :so confused: This just makes no sense whatsoever, and I'm completely dumbfounded that a "High-End SQ" company such as Zapco would even THINK to design and release such a product, LOL. Major FAIL IMO. I'd be really interested to know just how many of these were actually sold, AND how many were returned....AND how much time and money was wasted by producing these pretty much useless DSP/amps? Who would even approve this design for production? I'm just so baffled that this DSP/AMP even existed. I also agree regarding the JL Audio VXI series DSP/amplifiers. Excellent features, great DSP Software, and clean output quality, though the output power is a bit on the low side IMO. HOWEVER, I really dislike all of the GIANT Connectors and I/O pigtails on the VXi's! At first the amplifiers seem tiny (and they are)! BUT, once you make all of your necessary connections to the amplifier, the GIANT connectors & pigtails essentially make it 3" wider! :( You really have to be careful when laying out your amp rack to account for the extra clearances for these connections! And I agree regarding that it's just silly that JL Audio did not also produce a non-DSP VXi subwoofer amp to go with the multichannel VXi amps. Thanks for the video, and hopefully you're settled into the new house by now.

[@romannavarro3137]

I have the same amp with dsp and i can set up my gains from software.

[@Tsmixedup]

why didn't you swap it out for the st-6x? I'm having distortion issues when the volume is turned up...I've tried fixing it by adjusting output signal through the H/U, It seemed to help some, but there is still some distortion at low volume which tells me that that the input level is set too high and you can't fix it at all... I have also been hearing (on occasion) a hissing sound at low volume or no volume and at times have to turn the amp off and back on to get it to go away...It's loud even @ 0 volume...very concerning...I am thinking about returning this amp for something else...I don't think it will be zapco again. The Orion amp that I had before the zapco sounded way better, but it was a clipper as it was 400 bucks cheaper.

[@richardvedvik979]

We already had an Audison LRx5.1k that the Zapco were supposed to replace, and the owner was done with Zapco after this experience. My testing showed clipping after 0.5V input on the Zapco, which is stupid low. Adjusting HU outputs is one way, but we still felt like it was really quiet when we kept the signal low enough to not clip. Owner considers the amp a fail. Thanks for sharing!

[@Tsmixedup]

@@richardvedvik979 I was almost talked into keeping this amp until i watched your video again, and I decided not too and sent it back. it has too many issues for me including the gain adjustment. Do you have any suggestions on good amps around this same price? I'm looking at the AudioControl LC-6. Customer reviews seem to like it...I'm not sure after the zapco disaster.

[@richardvedvik979]

@@Tsmixedup the JL Audio VXi series is awesome. I've enjoyed them and am very impressed. Very flexible.

[@Tsmixedup]

@@richardvedvik979 cool, thanks man!

[@DirtySouthFlorida3]

Would you say the reguler st 6x sq is a good amp? Also curious what is the best sq amp youve heard ans how far off is the zapco st sq?

[@richardvedvik979]

Yea, Zapco makes good stuff normally, this is an abnormal miss on their part. The new JL VXi are awesome DSP enabled SQ amps, but not in the "best" overall category. I see a wide variety of amps in both IASCA SQ and MECA SQ, so there really isn't a "best" but a variety of preferences. McIntosh, Genesis, and Mosconi tend to be popular with expensive builds. Once you're spending a couple grand per amp you're into some nice stuff. Under a grand, you have a variety of compromises that the buyer needs to weigh for themselves.