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DIY Purifi Amp builds

mocenigo

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How do you explain such big differences? There is nothing "magical" in TomC's buffer that could cause this and EVAL1 already measured very well.

Well, this chimes also with my - admittedly subjective - impressions: the Neurochrome Universal Buffer sounds better than the buffer on the FE02. I think this is because it can deliver a bit more current, and this drives the Eigentakt module better. I can live with people believing this is pure delusion (I spent money to purchase it, so I must be satisfied by it). It is after all no different than believing that the choice of measurements present here tell the whole story about the performance of an audio component.
 
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JimB

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Balanced from a preamp or from a DAC.
No obvious basis for a difference then - balanced-in to balanced-out through the EVAL1. No potential power limit. Something else to make an audible difference???
 
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JimB

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Well, this chimes also with my - admittedly subjective - impressions: the Neurochrome Universal Buffer sounds better than the buffer on the FE02. I think this is because it can deliver a bit more current, and this drives the Eigentakt module better. I can live with people believing this is pure delusion (I spent money to purchase it, so I must be satisfied by it). It is after all no different than believing that the choice of measurements present here tell the whole story about the performance of an audio component.
Nothing wrong with preferring Tom's board for any of several reasons. I just don't understand audible sonics, under any normal conditions, being one of them, especially if driven by a balanced source. Are you also driving both types of input boards from a balanced source?
 
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mocenigo

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Nothing wrong with preferring Tom's board for any of several reasons. I just don't understand audible sonics, under any normal conditions, being one of them. Are you also driving both types of input boards from a balanced source?

Yes.
 

tomchr

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Hi Tom, could you please demonstrate these two assertions?
While I agree with your reasoning on the CMRR when the EVAL1 input stage is bypassed, I have to admit that I have hard time to follow your reasoning in the other cases.
The EVAL1 buffer is the front-end of an instrumentation amp. You can find a good treatment of it in just about any opamp text. Personally, I recommend Sergio Franco, "Design With Operational Amplifiers and Analog Integrated Circuits". There's a 4th edition out now, though I doubt it's significantly different from the 2nd or 3rd.

You can see the common-mode rejection simulation below. You'll notice the circuit has 0 dB CMRR, i.e. no CMRR.
Screen Shot 2021-04-14 at 18.18.26.png


Tom
 

tomchr

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I suppose I need to add some precision in my messaging regarding the single-ended vs differential. The output is differential, even with a single-ended input, but it's not balanced. If you drive the EVAL1 with a balanced source, the output becomes balanced.
Screen Shot 2021-04-14 at 18.35.09.png

Screen Shot 2021-04-14 at 18.51.55.png


In case of the Purifi/Hypex Buffer, I use a differential driver for the output, so the output is differential and balanced even with a single-ended input. That could explain why many have found the Purifi/Hypex Buffer to sound better, though I'm not aware of any actual study confirming this.

Hope this clears things up a bit.

Tom
 

tomchr

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Here's the resulting output common-mode voltage (Vcm) of the EVAL1 when driven by a single-ended source.
Screen Shot 2021-04-14 at 19.03.02.png


Tom
 

mocenigo

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In case of the Purifi/Hypex Buffer, I use a differential driver for the output, so the output is differential and balanced even with a single-ended input. That could explain why many have found the Purifi/Hypex Buffer to sound better, though I'm not aware of any actual study confirming this.

The Neurochrome UB has essentially the same circuit and properties. Correct?
 

TimoJ

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In case of the Purifi/Hypex Buffer, I use a differential driver for the output, so the output is differential and balanced even with a single-ended input. That could explain why many have found the Purifi/Hypex Buffer to sound better, though I'm not aware of any actual study confirming this.
But what explains differences when using balanced input signal? Output level difference or just vivid imagination? Or does your buffer actually measure worse than EVAL1/FE02A and listeners are hearing that and thinking it's different so it must be better?
 

tomchr

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The Neurochrome UB has essentially the same circuit and properties. Correct?
The Universal Buffer is a stereo buffer. It has unity gain (with options to increase the gain by adding a resistor). It features a single-ended output as well as a differential one and you can use both outputs at the same time. The Purifi/Hypex Buffer is optimized for the Purifi 1ET400A and Hypex NC500. If you were to connect a Universal Buffer to a 1ET400A or NC500 and configured it for the same gain as the Purifi/Hypex Buffer, you'd get about the same performance as with the Purifi/Hypex Buffer.

But what explains differences when using balanced input signal? Output level difference or just vivid imagination?
Probably neither. The gain of the EVAL1 is actually 1.5 dB higher than that of the Purifi/Hypex Buffer, so if level differences are to blame, the EVAL1 should win.

I have to push back on your notion of "vivid imagination". There are many psychological effects that can explain why someone would perceive a difference even when presented twice with the exact same stimulus. Those are real differences in perception and are the result of listeners being human.

Or does your buffer actually measure worse than EVAL1/FE02A and listeners are hearing that and thinking it's different so it must be better?
As you can see in the Performance Graphs tab on the Purifi/Hypex Buffer product page, the Purifi/Hypex Buffer provides better performance than I can measure with my Audio Precision APx525. But don't let facts get in the way of a good opinion... ;) I've included a couple of teaser plots below. You can read about the test conditions in the Performance Graphs tab on the Purifi/Hypex Buffer product page.

Purifi_Hypex_Input_Buffer__Multi-Tone_IMD_AP_32-tone_Vin_2.0_V_RMS_Ref.__9.12_V_RMS.png

Purifi_Hypex_Input_Buffer__THD_N_vs_Input_Level_Gain_1_kHz_20_kHz_BW.png

Purifi_Hypex_Input_Buffer__THD_N_vs_Output_Level_Gain_1_kHz_20_kHz_BW.png


Tom
 

TimoJ

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I have to push back on your notion of "vivid imagination". There are many psychological effects that can explain why someone would perceive a difference even when presented twice with the exact same stimulus. Those are real differences in perception and are the result of listeners being human.
But there is nothing measured that could explain earlier reported differences? For example wider soundstage, how could that be possible? Unless a buffer causes phase shift and that varies with each unit. I thought ASR was about measuring things, not about psychological effects... I guess it's that blue PCB that causes this. I do admit it looks very nice.
 

diyaudnut

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I updated regulators to hypex hxr on my nc500 eval buffer. Having spent the money, I wanted to be sure if it made any difference. I put on a familiar song late at night and listened intently. Voila! I heard differences! subtle background hums and tones which I noticed for the first time.

You know why I heard them? because I really wanted to and paid a lot of attention with closed eyes on a quiet night. The regulators did nothing.
 
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tomchr

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But there is nothing measured that could explain earlier reported differences? For example wider soundstage, how could that be possible? Unless a buffer causes phase shift and that varies with each unit. I thought ASR was about measuring things, not about psychological effects... I guess it's that blue PCB that causes this. I do admit it looks very nice.
I thought ASR was Audio SCIENCE Review. So why should the science of psychology not be included?

I think it's naive to ignore the human brain part of audio, just as it is naive to ignore the measurements. Maybe that's why I ended up with two engineering degrees and a psychology degree. :)

Tom
 
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TimoJ

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I thought ASR was Audio SCIENCE Review. So why should the science of psychology not be included?

I think it's naive to ignore the human brain part of audio, just as it is naive to ignore the measurements. Maybe that's why I ended up with two engineering degrees and a psychology degree. :)

Tom
But Amir is trying to prove things just by measuring. Is he missing the point (half of it) doing so?
 

tomchr

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Measurements don't prove anything in a scientific sense. A measurement or experiment can show support for a theory or fail to show support for a theory. It doesn't prove anything.

I commend Amir's approach. If you look at my websites you'll notice that I provide - by far - the most measurements of the performance of my circuits than any other vendor.

As I said earlier: I think it is naive to ignore that listeners are humans and that their brains play a role in the perception. Similarly, I think is naive to ignore the measurements.

My point (and Amir's too, I would guess) is that with the measurements you can make an informed choice when you go to buy hifi gear. Without measurements, all you have is the manufacturer's or (paid) reviewer's claims that the gear sounds good and has great PRaT (or whatever meaningless audiophile term). Measurements provide a way to sort the wheat from the chaff. Other factors then decide which grain of wheat you end up buying.

Tom
 

mocenigo

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The Universal Buffer is a stereo buffer. It has unity gain (with options to increase the gain by adding a resistor). It features a single-ended output as well as a differential one and you can use both outputs at the same time. The Purifi/Hypex Buffer is optimized for the Purifi 1ET400A and Hypex NC500. If you were to connect a Universal Buffer to a 1ET400A or NC500 and configured it for the same gain as the Purifi/Hypex Buffer, you'd get about the same performance as with the Purifi/Hypex Buffer.

Yes, as you know I have both the Universal Buffer and a pair of Purifi/Hypex Buffer prototype boards :)
So I am aware of the extra circuitry to allow a single-ended output to work in parallel to the differential one – and the fact that it is stereo and not mono. I was referring to the core circuit, the one that is on the Purifi/Hypex Buffer – that one seems to be the same, right?

There is also a difference in declared slew rate between the two: is this due to a different RF filter implementation or was the information in the Purifi/Hypex Buffer case given for the circuit without the filter?

Indeed I have connected the Universal Buffer to a pair of 1ET400A's, using the prototype boards as breakout boards. Just to complicate my life and because I already had the Universal Buffer...

Roberto
 

tomchr

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Ah. I remember you now. Sorry. I don't have a list of user IDs and orders committed to memory. My episodic memory is pretty good, though. :)

There are subtle differences in the designs that could explain the difference in slew rate as well. I'd have to go look at that. The challenge with the UB is that it should be able to drive fairly long cables, whereas the Purifi/Hypex Buffer should just plug into the 1ET400A/NC500 and be happy. This means I compensated the UB differently than the Purifi/Hypex Buffer.

I also seem to recall that the two Purifi/Hypex Buffer prototypes that you have (Rev. 1.1) do not have the balanced output. Only those two boards of that design exist in the world, so that's unique to your amp.

Tom
 

mocenigo

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Ah. I remember you now. Sorry. I don't have a list of user IDs and orders committed to memory. My episodic memory is pretty good, though. :)

There are subtle differences in the designs that could explain the difference in slew rate as well. I'd have to go look at that. The challenge with the UB is that it should be able to drive fairly long cables, whereas the Purifi/Hypex Buffer should just plug into the 1ET400A/NC500 and be happy. This means I compensated the UB differently than the Purifi/Hypex Buffer.

I also seem to recall that the two Purifi/Hypex Buffer prototypes that you have (Rev. 1.1) do not have the balanced output. Only those two boards of that design exist in the world, so that's unique to your amp.

Tom

They do not have the unbalanced input to balanced output circuit. Yes, they are the version without that feature. That's why I stick with the UB. It also sounds better in my humble opinion. But of course the production model would be essentially the same as the UB.
 

tomchr

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But of course the production model would be essentially the same as the UB.
Yeah. I doubt you'd gain anything by switching to the Purifi/Hypex Buffer. If you pluck C19, C20, C22, and C23 from the UB and replace R7, R9, R23, and R25 with 0 Ω resistors you end up with the same circuit as is used in the Purifi/Hypex Buffer Rev. 1.2 (production version). If SMD soldering is not your thing, it's sufficient to just pluck the capacitors. Leaving it alone works too. Ain't broken. Don't fix. :)

Tom
 
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