VTV Purifi EVAL1 teardown and rebuild

[boXem]

Surely you are not suggesting that the fuse be higher than the typical (US) house circuit breaker?

Indeed .

 

[NTK]

Do not want nuisance blows, but want it to go fast went it shorts. The SMPS fuse is already a slow-blow one. Ideally could have measured but that stresses equipment and somebody else’s amp.

My understanding is that a 15A fuse blows at 15A. Surely you are not suggesting that the fuse be higher than the typical (US) house circuit breaker?

Don't know what fuse you are using. For example, here is the datasheet from Littelfuse's 5x20 mm (216 series) fast acting cartridge fuse. They provide the average time-current curves for the users to estimate how much time it takes a fuse to blow at a given current (see lower right graph in page 2).
https://www.littelfuse.com/~/media/...s/fuses/littelfuse_fuse_216_datasheet.pdf.pdf

There is also the i^2 t rating (aka melting point) for fuses. But of course we need to know how long the in-rush current surge is.
https://forum.digikey.com/t/fuse-i2t-rating-melting-point-and-breaking-capacity/1271

 

[beren777]

The fuse in the Hypex 1200 SMPS doesn't look like it'd be very easy for a typical end user to replace. An easy to replace upstream fuse that blows first seems like a nice addition.

 

[DonH56]

It has been years since I dug into specs and standards for fuses and breakers. I know back when I was looking at this that it was common to use 1.5x~2x or higher fast-blow current ratings when replacing a slow-blow fuse with a fast-blow fuse. Fuses, and breakers, will easily handle 2x to 10x their design rating for a short spike (few ms to maybe 100 ms), since the design rating is for long-term (seconds or more) current flow. Breakers more so than fuses IIRC. With high in-rush current a fast-blow fuse would not last as long for me as a slow-blow of about half the amperage rating, based upon foggy memory. Of course it is very application- and circuit-dependent... And even if it is a series of short high-current pulses you have to watch average heating (duty cycle) and all that jazz. I remember discovering a fuse was a complicated beast when I dug deep into how they worked and blew.

So based upon no research save that of a vague old memory, I would stick a series 20 A or even 25 A fast fuse in series with a 12-A slow-blow fuse with the expectation that it would last a good long while (whatever that means ).

But here is the big caveat: in this situation, you probably want the new easily-replaceable fuse to blow first, so maybe sticking on the low side is a reasonable choice.

Blah, glad I have no dog in this hunt - Don

 

[Rick Sykora]

Fusing can be a bit of an art, but I chose a 15A fuse cause it is in the middle between the amp fuse and the house circuit breaker. I did try it in my amps with 4 ohm dummy loads and low impedance speakers without it blowing. It may not be exhaustive, but as was mentioned, it is the easier fuse to replace if it does blow…

As you can see from my posts, I do check my work. This particular thread would not exist had the original builder checked his.

 

[ironhorse128]

I ordered a power plug with internal fuse to add a fast blow fuse to the VTV amp. Seems to be the easiest fix. These plugs are frequently used in the UK but they also work in Germany with an adapter.

https://www.amazon.de/gp/product/B08PVV48L2/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

 

[phoenixdogfan]

Based on Amir’s VTV amplifier teardown, I had posted some reworks on my used Hypex one. @Rottmannash liked what he saw and, after a brief conversation, decided to have me evaluate his VTV Purifi amp. The goal here is to improve build quality and safety - not to alter the any of the Hypex or Purifi modules. I use a Quantasylum QA401 audio analyzer and, while not Amir’s AP, helps identify major flaws and validate that any mods are beneficial rather than harmful.

So, here the first pass. Here is my markup of the BEFORE pic that was posted in Amir’s teardown thread:

View attachment 143373

First, I should mention that before opening this amp, I ran some quick checks and no major noise issues were identified. At this point, the main focus became improving any identified safety concerns. Please note my use of different colors is more for contrast than any other purpose. Starting with the Red X in the upper right, this is meant to note the lack of a case fuse. Some might say, the EMI filter is an issue too, but not as best as I can tell with my analyzer. So, will just be adding a 250V 15A fast blow fuse. The red circle to the left of the X is some slightly exposed AC wiring and will eliminate that. The yellow arrow points to the case ground that lacks a knurled washer and will add one of those.

Continuing with the AC wiring, there are several improvements planned. One is to shorten it by a bit more than half. This will be done by remounting the power supply so the heat sink is on the left. This also eliminates all the wiring running over the heat sink as identified by the yellow oval. Next will encase the AC wiring in nylon braid to double insulate most of it from the case. Finally will add some wire tires to tidy it up a bit too.

So here is a pic after I remounted the power supply:
View attachment 143375

As you can see, this left a lot of excess AC wiring and now only the LED wire needs to go over the heatsink. Will add some heat shrink tubing for the LED wire to double insuate. At this point, I reran my tests and found a very marginal improvement in low frequency noise. This may improve more once I shorten the wires. Along with shortening the AC wiring, plan to solder the power wires to the input connectors. This will prevent them from falling off and causing potential loss of power or shorting.

Definitely agree with this, Rick. For the life of me I don't know why they put the SMPS in ass-backwards and ran the mains over the heat sink. Not the way Ghent suggested in his DIY for the case (and he even now shows a YouTube video on how to properly assemble it from parts), so no excuse for what VTV did, unless it involved heavy alcohol consumption. Thankfully, not that hard to correct.

 

[Billy Budapest]

Then I am puzzled why the VTV EVAL-1 does not measure as well as the reference design! Any idea why?

Amir measured two VTV Purifi amps using VTV’s input buffers with two different sets of discrete opamps. To my knowledge he didn’t measure VTV’s EVAL1 amp. Did someone else?

 

[Rick Sykora]

I ordered a power plug with internal fuse to add a fast blow fuse to the VTV amp. Seems to be the easiest fix. These plugs are frequently used in the UK but they also work in Germany with an adapter.

View attachment 144635

https://www.amazon.de/gp/product/B08PVV48L2/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

Did not find these for US power. Could use for my NC252 amps. The AC wiring is so short, cannot do an inline fuse. Then again, the wiring is really short…

 

[ironhorse128]

Did not find these for US power. Could use for my NC252 amps. The AC wiring is so short, cannot do an inline fuse. Then again, the wiring is really short…

Well you can find them on Amazon.Com. Just a standard UK power cord.
https://www.amazon.com/StarTech-com...&keywords=uk+power+cord&qid=1628973784&sr=8-3

Then you need to have an adapter to use it.

https://www.amazon.com/LENCENT-Grou...uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

I use this setup for my German power plugs.

 

[Rick Sykora]

Well you can find them on Amazon.Com. Just a standard UK power cord.
https://www.amazon.com/StarTech-com...&keywords=uk+power+cord&qid=1628973784&sr=8-3

Then you need to have an adapter to use it.

https://www.amazon.com/LENCENT-Grou...uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

I use this setup for my German power plugs.

Thanks, but hard to get used to big 240V plugs when you are used to tiny 120V ones. Is not very power strip friendly either!

 

[fraa]

I wonder how people turn on/off this unit. Use the switch on the back every time?
I gather power supply standby is tricky because of the need for an external voltage. But how about real power on/off? Would it be possible to put another power switch on the front panel? Which wire should then be routed to the switch?

 

[Rick Sykora]

I wonder how people turn on/off this unit. Use the switch on the back every time?
I gather power supply standby is tricky because of the need for an external voltage. But how about real power on/off? Would it be possible to put another power switch on the front panel? Which wire should then be routed to the switch?

How you handle power just depends on how you integrate it into your system…

1. For those with a front end with a trigger output, then spending for the trigger input makes sense.
2. If no trigger output is available, many use a smart power strip. I have a voice/app controlled one on my dedicated listening system
3. Some suppliers do offer front panel switches. The better ones use the standby circuit as it can be easily integrated with a trigger input. If no separate power is available to do the power supply standby, then can implement amp standby with a front panel switch…
4. If you are handy, adding a standby switch is simple and usually worthwhile rather than running AC power through the amp. For the VTV, it already runs the AC wiring to the front, so adding a power switch is easy if you mount it on the underside. Otherwise, you need the right tools to drill a clean hole in the thick aluminum faceplate. Some have replaced the front faceplate to avoid the machining effort.

Hope this helps. One of the Purifi DIY threads has more details on how to add a standby switch.

 

[fraa]

How you handle power just depends on how you integrate it into your system…

1. For those with a front end with a trigger output, then spending for the trigger input makes sense.
2. If no trigger output is available, many use a smart power strip. I have a voice/app controlled one on my dedicated listening system
3. Some suppliers do offer front panel switches. The better ones use the standby circuit as it can be easily integrated with a trigger input. If no separate power is available to do the power supply standby, then can implement amp standby with a front panel switch…
4. If you are handly, adding a standby switch is simple and usually worthwhile rather than running AC power through the amp. For the VTV, it already runs the AC wiring to the front, so adding a power switch is easy if you mount it on the underside. Otherwise, you need the right tools to drill a clean hole in the thick aluminum faceplate. Some have replaced the front faceplate to avoid the machining effort.

Hope this helps. One of the Purifi DIY threads has more details on how to add a standby switch.

Hi Rick, thank you for your reply.

Yes, some of the amp suppliers such as Apollon Audio and March Audio seem to have implemented nice standby switches in their nice amps. My problem with their amps is they have never been actually measured. So I am more and more inclined to buy the VTV Purifi EVAL1, which is said to be basically the same as the Purifi prototype Amir measured. That's why I asked this question in this thread.

While VTV Purifi EVAL1 is the least expensive, as you showed it might need some modification. One of my reservations about it was how I would turn it on and off. So I wanted to know how people are doing it.

Taking your advice I did some more search and using softstart seems to be the right solution. (Edit: I later found out that this is not a recommended solution for SMPS, according to the manual of the softstart.)
Alternatively, though at first it seemed silly to have to use another power supply just to put the SMPS into standby mode, I realized that such a small power supplies can be had cheaply. For instance, this one is sold for less than US$3 in Japan. So implementing SMPS standby turned out to be not that hard either.

Another feature I want but lacks in VTV is the ability to change the gain. As I use Topping Dx7 Pro with its balanced output of 4Vrms as the source, the optimum gain seems to fall between 13dB and 27 dB, the two possible values with the VTV. I wonder if a solution is still lurking to be found in these threads. The Purifi EVAL-1 Users Guide says it can be modified by changing the resistor R17 on the front-end board. But said resistor is so tiny I don't think I should mess with it!

 

[Rick Sykora]

Hi Rick, thank you for your reply.

Yes, some of the amp suppliers such as Apollon Audio and March Audio seem to have implemented nice standby switches in their nice amps. My problem with their amps is they have never been actually measured. So I am more and more inclined to buy the VTV Purifi EVAL1, which is said to be basically the same as the Purifi prototype Amir measured. That's why I asked this question in this thread.

While VTV Purifi EVAL1 is the least expensive, as you showed it might need some modification. One of my reservations about it was how I would turn it on and off. So I wanted to know how people are doing it.

Taking your advice I did some more search and using softstart seems to be the right solution.
Alternatively, though at first it seemed silly to have to use another power supply just to put the SMPS into standby mode, I realized that such a small power supplies can be had cheaply. For instance, this one is sold for less than US$3 in Japan. So implementing SMPS standby turned out to be not that hard either.

Another feature I want but lacks in VTV is the ability to change the gain. As I use Topping Dx7 Pro with its balanced output of 4Vrms as the source, the optimum gain seems to fall between 13dB and 27 dB, the two possible values with the VTV. I wonder if a solution is still lurking to be found in these threads. The Purifi EVAL-1 Users Guide says it can be modified by changing the resistor R17 on the front-end board. But said resistor is so tiny I don't think I should mess with it!

Would not change gain until you get and try with your DAC. If you can solder, removing a SMC resistor is easy. Putting it back takes the right tip and steady hands.

While objectively, I get preferring to know that that what you are buying measures well, AFAIK VTV does not measure his amps. So, versus the one Amir tested, you may get one that actually performs worse (or the same or possibly better). Unless you can test it yourself, you really do not know with certainty.

Any better electronics manufacturer does acceptance testing by measuring. While not perfect either, you are much more likely to get a good level of performance and reliability that is too easily taken for granted.

 

[fraa]

Would not change gain until you get and try with your DAC. If you can solder, removing a SMC resistor is easy. Putting it back takes the right tip and steady hands.

While objectively, I get preferring to know that that what you are buying measures well, AFAIK VTV does not measure his amps. So, versus the one Amir tested, you may get one that actually performs worse (or the same or possibly better). Unless you can test it yourself, you really do not know with certainty.

Any better electronics manufacturer does acceptance testing by measuring. While not perfect either, you are much more likely to get a good level of performance and reliability that is too easily taken for granted.

I was just looking at a youtube video showing how to replace those tiny resistors. The last time I had an inclination to do any kind of electronics is almost 40 years ago and it is startling to see how much everything has gotten smaller.

You are of course right that I wouldn't really know how good any amp I receive is. But VTV EVAL1 is just the Purifi Eval1 put into a box, isn't it?
I wonder why Apollon didn't send their amp to Amir, which they once said they would. Their mini is one nice looking amp, which I might still decide to buy. March Audio's P452 also looks solid. The common thing about them is that there is so little information on the net about them. Besides measurement, an independent review on youtube would go a long way to convince me to get one.

 

[Rick Sykora]

I was just looking at a youtube video showing how to replace those tiny resistors. The last time I had an inclination to do any kind of electronics is almost 40 years ago and it is startling to see how much everything has gotten smaller.

You are of course right that I wouldn't really know how good any amp I receive is. But VTV EVAL1 is just the Purifi Eval1 put into a box, isn't it?
I wonder why Apollon didn't send their amp to Amir, which they once said they would. Their mini is one nice looking amp, which I might still decide to buy. March Audio's P452 also looks solid. The common thing about them is that there is so little information on the net about them. Besides measurement, an independent review on youtube would go a long way to convince me to get one.

There are bad, middling and good ways to put amp electronics in a box. VTV despite decent measurements is a middling case. His Purifi amps measured fairly well, but Amir did not recommend because of quality concerns. At the risk of repeating myself, this thread only exists because of VTV’s marginal implementation. This includes safety questions like the lack an AC line fuse. He spent extra for questionable EMI filter but since he did, he could readily used one that had a fuse. He did not.

I will repeat that I am fairly confident that you could take a Purifi amp and mess it up slightly to reduce its measured performance to be the same (possibly worse) than a well-built and tested Hypex NCore. So, not sure why I would spend hundreds extra for the Purifi unless I knew I was getting the best implementation.

 

[fraa]

I don’t want to pay for unnecessary second power supply or discrete buffers. That restricts the choice. I would pay for a good build as Apollon and March seem to be. But right now there’s not enough information to convince me that they haven’t screwed something up (especially buffers). You say they measure but they are not telling us the results. I guess I should wait until some well-built Purifi-based amplifier is measured and recommended by Amir. Meanwhile I wish the manufacturers would put more information online.

 

[SCG]

I know that this is an older thread but Rick, great work and thanks for posting this! I ordered a VTV Amp Purifi Eigentakt on Sunday, it shipped on Monday, and will be here by end of the week via UPS.

As I mentioned in SubsonicLurker's Thread if it's a keeper after the 30-day home trial, I'm going to implement your changes with the only difference I plan to replace the LED cable with a longer version and reroute it instead of shrink wrap tubing the existing one and routing it over the heat sink.

All my reasons for ordering this unit and doing your mods are in the other thread but in the U.S. its the quickest & most cost effective way to get a good unit.

 

[SCG]

More rework pics…

Flexseal on the trigger board exposed AC contacts:

View attachment 143435

Ground now has (internal) knurled washer…

View attachment 143436

Tie wrap of control signal cables to keep the away from AC input wiring…

View attachment 143437

Do not want nuisance blows, but want it to go fast if it shorts. The SMPS fuse is already a slow-blow one.

Rick any recommendations on sheilding to use on the AC lines once the board is rotated and do you mind stating exactly which fuse you used and if it did meet your needs long term?