DIY Purifi Amp builds

[diyaudnut]

@tomchr
I am confused by some entries in the 'NC500 OEM evaluation board' datasheet.
I see that VSIG+/- inputs (J12.1/2, Faston) are electrically connected to J3.25/26 outputs to the NC500 amp module interface.

Does this mean the amp module analog/modulator circuitry is directly working on the unregulated VAUX from the SMPS, instead of the regulated version from the buffer board?
If true, this doesn't seem ideal.
It also seems to be different from older version of the NC500 EVAL buffer datasheet which showed that the regulators on the buffer board fed regulated VSIG to the amp module over J3.25, J3.26.

Does this make sense to you? Please share your thoughts.

I see that your buffer will pass the onboard regulated voltage (when installed) to the amp module as does the EVAL1, EVAL2 purifi boards.

 

[JimB]

... I had a surprise when the kit came with a fused, filtered mains switch/socket! ... I haven't evaluated the values of the filter, so I don't know how useful it is. ...

As noted earlier, use of "Y-caps" is discouraged for audio work to avoid adding noise current on the ground line, and so voltage fluctuations on the chassis. I have successfully "lifted" the connection to ground of the two Y-caps in this specific unit, with them still connected to each other, so there is some filter benefit, but no more ground connection. This is the easiest change I could make to break the grounding, but it still involves nontrivial mechanical and soldering equipment and skills, so I'm not sure I want to detail it here. If requested by PM, I'll write it up to share, individually.

 

[JimB]

I have been contemplating the input buffers for a multichannel Purifi amp.
I remember a post from Bruno commenting that a high quality application of Purifi modules might want to look beyond the EVAL buffers.
Not sure what exactly are the possible improvements he was referring to. Can anyone clarify?

Some thoughts on readily available buffers:
A. Purifi Eval1:
Pros
* On board regulators
* EMI/RFI filters, GBW limiting
* On board XLR, Speaker out with feedback tap
Cons
* Not sure about the channel isolation. Havent seen any measurement or specification, but not sure if any inter-channel crosstalk is serious enough in my multichannel application.
...

Just one point on Case A. I expect channel isolation is already extremely high as only the SMPS and local regulator are shared and I thought PS isolation (PSRR >90dB @ 1KHz) is quite good on the 1ET400A modules. Why do you want the absolute highest possible channel isolation? I can't imagine a use case where it could matter even a little. For all other buffers, would you still use a single power supply? If so, the expected benefit is only from separate local regulators.

There could be other reasons for different buffers, according to what is needed in an application, including input and output connections, and gain.

 

[phoenixdogfan]

I know this is a silly question, but I'm going to put it to the group anyway. If for some reason, the Eval 1 was fed signal from an RCA to XLR cable, would it damage or significantly degrade the performance of the amp? I know Bruno does not recommend operating the amp with anything but balanced end to end, but is that just because he wants it to always perform at its optimum, or because it would be significantly damaging to either performance or the circuitry itself.

 

[JimB]

I know this is a silly question, but I'm going to put it to the group anyway. If for some reason, the Eval 1 was fed signal from an RCA to XLR cable, would it damage or significantly degrade the performance of the amp? I know Bruno does not recommend operating the amp with anything but balanced end to end, but is that just because he wants it to always perform at its optimum, or because it would be significantly damaging to either performance or the circuitry itself.

An RCA to XLR adapting cable? Wired as recommended? It is THE way to use single ended input on these amps. I'd guess that a majority of them are used this way. I can't see any way that is a problem, let alone a cause of damage.

 

[phoenixdogfan]

An RCA to XLR adapting cable? Wired as recommended? It is THE way to use single ended input on these amps. I'd guess that a majority of them are used this way. I can't see any way that is a problem, let alone a cause of damage.

I'll be using the Purifi for a couple of use cases. In one case, two channel, the input will come from a balanced DAC, but for multi channel, surround, the output of my multi channel source is single ended RCA, so it would need to be an RCA to XLR cable with something little a Nobu switch to toggle between the two inputs. Unless of course, I fork over the cash for an Octo DAC 8 Pro!

 

[tomchr]

@tomchr
I am confused by some entries in the 'NC500 OEM evaluation board' datasheet.
I see that VSIG+/- inputs (J12.1/2, Faston) are electrically connected to J3.25/26 outputs to the NC500 amp module interface.

Does this mean the amp module analog/modulator circuitry is directly working on the unregulated VAUX from the SMPS, instead of the regulated version from the buffer board?

That's probably a better question for Hypex. I seem to recall that the max supply voltage for the NC500 opamps and modulator section is ±12 V ±5 %. Note that the SMPS1200 has on-board regulators as well. It ships with those enabled and provides ±12 V on the AUX supply.

Tom

 

[NTSOS]

When I used an existing case for my Purifi build, the power cord and fuse holder holes were already in place, so when I machined the back plate for the amp modules, the left channel speaker outputs ended up right above the power cord and fuse holder. If I had to, I could move the power cord and fuse holder to the center of the case side panel, which would look kind of lame or I could run it and see if the close proximity of the power cord to the speaker outputs causes left channel noise. I would assume that arrangement is bad practice, should I move them now or wait? Thanks!

 

[tomchr]

The speaker outputs are low impedance, so the mains voltage will have a very hard time inducing any error voltage on the outputs. So don't worry about it. I do recommend keeping the inputs away from both the outputs and the mains, however.

Tom

 

[NTSOS]

The speaker outputs are low impedance, so the mains voltage will have a very hard time inducing any error voltage on the outputs. So don't worry about it. I do recommend keeping the inputs away from both the outputs and the mains, however.

Tom

Hi Tom, great....that is good news/information and also quite a relief.

Yes sir, the inputs are not real close to the outputs and mains.

Thank you for taking the time to help.

John

 

[diyaudnut]

That's probably a better question for Hypex. I seem to recall that the max supply voltage for the NC500 opamps and modulator section is ±12 V ±5 %. Note that the SMPS1200 has on-board regulators as well. It ships with those enabled and provides ±12 V on the AUX supply.

Tom

Thanks Tom. Have sent Hypex an email.

The datasheet for nc500 recommends typical of +-15V for VOP and Purifi recommends +-12V.

 

[tomchr]

Ah. That's right. That's one of the few differences between the two. Hypex runs them on ±12 V, though. The SMPS1200 provides ±12 V. As does the Hypex NPR and HNR regulators.

Tom

 

[diyaudnut]

Yeah, agree 12V makes sense.
But I noticed the default regulators on NC500 oem input buffer produce +-15V (7815, 7915). Technically violating Purifi recommended operating range. But suspect it's not a real problem.
Purifi protections kick in at 20v for VOP

 

[tomchr]

You're probably right. However, it's generally bad form to operate electronic components outside of their absolute maximum specs. At least if you want a reliable circuit. It's entirely possible that running the Purifi amps at ±15 V would result in some devices being over-voltaged, for example during startup, power-down, or error conditions, thereby lowering the long-term reliability of the amp module. Many modern MOSFETs have pretty tight restrictions on how much gate voltage you can apply, for example. Some of the low on-resistance types only allow 8-10 V, for example.

You can always ask Purifi how much margin there is and what prevents you from running the modules at ±15 V.

Tom

 

[diyaudnut]

Yes Tom. Agree with you assessment I was not happy when I saw this... long term reliability being my first concern. I have dropped an email to Purifi.

Replacing the 7815/7915 with HxR should take care of this.

 

[cooldude]

Did anyone implement microcontroller based monitoring for multichannel purif amp?

 

[tomchr]

Replacing the 7815/7915 with HxR should take care of this.

Or 7812/7912.

Tom

 

[Matias]

So the DIY bill of materials is:
- Ghent case + harness: 250 usd
- Purifi EVAL-1 kit (I/O board + 2 amp modules): 760 usd
- Hypex SMPS1200A400: 215 usd
Total: 1,225 usd

While an assembled amplifier costs:
- Apollon: 1,780 usd
- Audiophonics: 1,240 usd
- March: 1,600 usd
- VTV stock buffer: 979 usd
- VTV Sparkos Pro opamps and regulators: 1,350 usd

All prices converting currencies in Google, and excluding both VAT and shipping (DIY has to pay for 3 shippings).

Notice that the VTV amplifier uses essentially the same case, SMPS, amp modules but with a stock buffer, and is 20% cheaper than DIY.

I guess it makes sense to DIY for the experience and pride of assembling, or to add any special parts that the manufacturers don't have. Because considering price and effort, it is not worth it, at least in my opinion.

VTV now has EVAL-1 inside a Ghent-like case for 960 usd!

https://vtvamplifier.com/product/vtv-amplifier-purifi-eigentakt-stereo-amplifier-based-on-eval-1/

This makes it even harder going DIY as it is much more expensive.

 

[Matias]

 

[JimB]

It IS the Ghent case, but with VTV custom front panel. But wait! They've used an inline mains filter with Y-caps! (Or did they?)