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DIY Purifi Amp builds

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
 

tomchr

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I measure with 10Ω imbalance and report the worst case result.

Tom
 

Armand

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I measure with 10Ω imbalance and report the worst case result.

Tom
Nice.
You should write something about that on your measurement page. I assume that without the imbalance the results are even better.

On the buffer I make for the Vera amplifiers the CMRR matches yours. Here is a plot with 10 Ohm worst case measured on the output of a complete amplifier.
1618518298575.png


Kind of magical to insert a 4V 50Hz signal on the inputs and measure 70uV on the speaker output :)
 

mocenigo

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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
Oh no why should I “fix” something that is working incredibly well. I honestly think that the UB + 1ET400A is the best amp I have ever heard, and by a long shot. Again this is subjective but I know that it is also backed with good engineering.

I am in fact so happy with the UB that I would be tempted to build something around your Modulus amps (the only thing I dislike about them is that the names remind me of intel CPUs - and I am an Architect at ARM ;-) ). But this would probably induce a divorce…
 

boXem | audio

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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.
View attachment 124071

Tom
Sorry Tom but I have to disagree here.
The buffer doesn't have 2 outputs, it has one output that is the difference between OUTP and OUTN. So displaying the common mode gain (and not the CMRR) is done like this:
1618525618246.png

I introduced errors in the resistors because they are the main reason for common mode errors. Otherwise what is displayed is mainly errors from the simulator.
 

boXem | audio

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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.
View attachment 124079
View attachment 124080

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
Same comment here, we do not care about OUTP and OUTN individually, what is important is the difference between the 2.
1618526333915.png


Where your solution is superior is when the single ended input reaches values above 3.5 V RMS (10 Vpp). With EVAL1, the common mode voltage will reach values above the +/- 5 V recommended by Purifi in the 1ET400A datasheet, while staying close to 0 with your solution.
 

tomchr

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Very intriguing. That doesn't change the fact, that with a single-ended source, the output of the EVAL1 will contain a common-mode component that the 1ET400A/NC500 will have to reject, whereas the output of the Neurochrome Purifi/Hypex Buffer will have zero common-mode component under the same conditions.
This is a clear and quantifiable difference between the circuits.

Tom
 
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JimB

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Very intriguing. That doesn't change the fact, that with a single-ended source, the output of the EVAL1 will contain a common-mode component that the 1ET400A/NC500 will have to reject, whereas the output of the Neurochrome Purifi/Hypex Buffer will have zero common-mode component under the same conditions.
This is a clear and quantifiable difference between the circuits.

Tom
Intriguing indeed. There certainly are real differences in the characteristics of the two approaches, as well as useful differences in features. A question of interest is how the differences in implementation effect the total amp performance, in particular cases (the later amp stages matter). Several claims of sonic superiority have been made for balanced inputs and the 1ET400A. Here we are focusing on the single-ended case (more challenging?). What is the significance of the CM voltage on the 1ET400A when that CMV is a only fraction of the audio signal itself?
 

tomchr

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That's a great question. The common-mode component is a significant fraction of the signal. Without firing up the simulator and just looking at the plots I posted yesterday, I'd say the common-mode component is maybe ~15 dB (5x) lower than the differential signal. The CMRR of the 1ET400A is specified as ">60 dB", so the CM component will be attenuated ~75 dB. I don't know how audible that would be. If any of you would like to set up a scientifically controlled experiment with, say, 250 participants so we can tease that out I'm happy to help. :) But some claim to be able to hear -80 dB THD, so that's one "back of the envelope" figure that I keep in mind.
At least the CM component is of the same frequency and phase as the input signal, so I would think it would be hard to hear.

Another question is whether the Purifi 1ET400A suffers from common-mode distortion. That's an issue for many opamps, for example. That's why some prefer to use an inverting amp rather than a non-inverting one. The first half of my Super GainClone Review covers that in better detail (with measurements).

When I designed the Purifi 1ET400A/Hypex NC500 input buffer, I honestly didn't think that a differential and balanced output would make any difference compared to the instrumentation amplifier front-end used in the EVAL1. As pointed out earlier in this thread, both the EVAL1 buffer and my Purifi/Hypex Buffer measure well. And this is why @mocenigo's buffer does not have the differential and balanced output. Someone on DIY Audio pointed out that many claimed a differential and balanced output sounded better with the 1ET400A, and suggested that I offered that. I took that as an opportunity to offer something that is unique and different from the EVAL1. The output of the EVAL1 will contain a common-mode component when driven by a single-ended source (e.g., an RCA output), whereas the output of the Purifi/Hypex Buffer will not.
The differential and balanced output of the Purifi/Hypex Buffer increased the production cost by a few percent and there was a bit of room left on the board, so why not include it? Including it certainly won't harm anything and many say it sounds better. Sometimes you have to deliver what the market wants. There are many buffers out there that are basically copies of the EVAL1 with a few tweaks. I decided to be unique. I did no harm and I may have provided a benefit. What's not to like? I think we can all coexist.

Tom
 
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JimB

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That's a great question. The common-mode component is a significant fraction of the signal. Without firing up the simulator and just looking at the plots I posted yesterday, I'd say the common-mode component is maybe ~15 dB (5x) lower than the differential signal. The CMRR of the 1ET400A is specified as ">60 dB", so the CM component will be attenuated ~75 dB. I don't know how audible that would be. If any of you would like to set up a scientifically controlled experiment with, say, 250 participants so we can tease that out I'm happy to help. :) But some claim to be able to hear -80 dB THD, so that's one "back of the envelope" figure that I keep in mind.
At least the CM component is of the same frequency and phase as the input signal, so I would think it would be hard to hear.

Another question is whether the Purifi 1ET400A suffers from common-mode distortion. That's an issue for many opamps, for example. That's why some prefer to use an inverting amp rather than a non-inverting one. The first half of my Super GainClone Review covers that in better detail (with measurements).

When I designed the Purifi 1ET400A/Hypex NC500 input buffer, I honestly didn't think that a differential and balanced output would make any difference compared to the instrumentation amplifier front-end used in the EVAL1. As pointed out earlier in this thread, both the EVAL1 buffer and my Purifi/Hypex Buffer measure well. And this is why @mocenigo's buffer does not have the differential and balanced output. Someone on DIY Audio pointed out that many claimed a differential and balanced output sounded better with the 1ET400A, and suggested that I offered that. I took that as an opportunity to offer something that is unique and different from the EVAL1. The output of the EVAL1 will contain a common-mode component when driven by a single-ended source (e.g., an RCA output), whereas the output of the Purifi/Hypex Buffer will not.
The differential and balanced output of the Purifi/Hypex Buffer increased the production cost by a few percent and there was a bit of room left on the board, so why not include it? Including it certainly won't harm anything and many say it sounds better. Sometimes you have to deliver what the market wants. There are many buffers out there that are basically copies of the EVAL1 with a few tweaks. I decided to be unique. I did no harm and I may have provided a benefit. What's not to like? I think we can all coexist.

Tom
In this special case the CMV IS the audio signal. So, does that simply attenuate (or boost) the net signal in the 4th decimal place? Or, as you say, is it a challenge to the 1ET400A, producing some CM distortion? We don't see anything strange 70-80dB down in the results @amirm got when using the EVAL1-based Purifi demo. However, your implementation just seems a bit more fundamentally "sound" for marginal conditions. I don't think Purifi suggests that one cannot best the FE02 input buffer, just that it is adequate to demostrate/evaluate the performance of their amp.
 

tomchr

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Can I ask what the purpose/benefit of the optional voltage regulator is?
As far as I can measure, there is no advantage of using the voltage regulator. I measure the same performance when I use the regulated AUX supply from the SMPS1200 as I do when I use the optional regulator and I'm happy to tell my customers that. Some choose to geek out over voltage regulators and I support that. DIY is a hobby.

We don't see anything strange 70-80dB down in the results @amirm got when using the EVAL1-based Purifi demo.
True. But I think he used the balanced input, so you would not see a difference. At least not one that could be attributed to the common-mode voltage. I could be wrong. I didn't read that specific review, but in general my observation is that Amir uses the balanced input if one is available, so I would assume that he did so on the Purifi+EVAL1 as well.

Tom
 
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JimB

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... I think he used the balanced input, so you would not see a difference. At least not one that could be attributed to the common-mode voltage. I could be wrong. I didn't read that specific review, but in general my observation is that Amir uses the balanced input if one is available, so I would assume that he did so on the Purifi+EVAL1 as well.

Tom
Good point. I think you're right. With many(?) using single ended sources, it would be interesting to know what the performance difference is, if any.

Thoughts, @amirm ?
 
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JimB

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Good point. I think you're right. With many(?) using single ended sources, it would be interesting to know what the performance difference is, if any.

Thoughts, @amirm ?
I also wonder whether the amp output being single ended (not balanced) matters. We lack the necessary details to assess that. A direct test is likely needed to show the differences.
 

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Forgive me if its been mentioned but does anyone know if these amps produce enough power with the gain stage bypassed if you are using 94dB speakers? My pre has no gain, DCB1 buffer so source voltage only, room is 12x12. Also with the input impedance been low with no gain stage, I am wondering if the DCB1 would operate at its best? Oh it will be single ended, even less gain, maybe I should think about Toms input stage with my setup?
 
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JimB

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Forgive me if its been mentioned but does anyone know if these amps produce enough power with the gain stage bypassed if you are using 94dB speakers? My pre has no gain, DCB1 buffer so source voltage only, room is 12x12. Also with the input impedance been low with no gain stage, I am wondering if the DCB1 would operate at its best? Oh it will be single ended, even less gain, maybe I should think about Toms input stage with my setup?
The amp has the same power potential regardless of the gain of the input buffer gain stage. You need the SOURCE driving it to have the range to do that. I don't know what you have, so I can't comment on it's suitability to achieve full output. What is the nominal impedance of your speakers? With 94 dB sensitivity, I doubt you'd ever need to achieve the 200/400W these amps can produce.
 

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