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Super Voyager

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2013 at 8:23am
Thank you for your suggestions highfell - they are welcomed and useful.

The OPA 627 op-amp uses common base bi-polar transistors which do not have what is called "miller effect" and that may help in it not detecting as much RF. The Voyager op-amp is J-FET and is much the same in that respect, except that I need to have around 3V r.m.s. to drive the range of headphones people will use, and the OPA 627 would do a little less at 2V r.m.s. which could make it less attractive to the many customers who choose the Voyager to drive high impedance headphones.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2013 at 10:13am
Correcting my mistake above, the OPA627 is J-FET input and not bi-polar.

So let's take a deeper look into the op-amps voltage amplifier stages...



Fig.1 OPA627 voltage amp stage



Fig.2 Op-amp voltage amp stage as used in Voyager

Here we need to look at the drain/gate capacitance. What is a drain? Look at the terminal of J1 in the fig.2 that connects to R42 - that is a J-FET drain. What is a gate? Look at the dot on the terminal of J1 (same picture).

OK, you see no capacitor symbol. The capacitance is intrinsic that's why.

Look at the same terminals in the OPA627 diagram, fig.1 (look at the bit labelled +in 3). A capacitance exists between those terminals too.

The drain load in fig.2 is R42 with top end connected to positive supply. Signal applied to the gate causes the drain to swing in the opposite direction. The amount of signal gain determines how much the intrinsic or parasitic capacitance becomes - this is called miller capacitance. A large amount of gain results in a large miller capacitance and that causes instability which in turn causes radio detection.

So how much gain has the J-FET in fig.2? If you look at the voltage reference to the top right in fig.2 you see "VBE +0.3V". VBE is 'consumed' by Q61 leaving 0.3V. J-FETs don't have as much gain as biploar transistors - and that results in something near to 20 x the drain load voltage which is 6, or possibly less. The frequency of instability should be much greater than the op-amp bandwidth one would think, and by compensating the op-amp for its noise gain within its bandwidth it should be well out of the way?

Looking at fig.1 OPA627 will there be any voltage gain at the drain? It can be seen that the drain operates into a common base transistor. A common base transistor has no miller capacitance so that can be discounted. However, there is a fixed base emitter capacitance so at some high frequency the drain resistor will see a fixed supply, and that will result in the J-FET having miller capacitance. Somewhere within that transition there will be the possibility of oscillation (instability) and radio detection. But here again, the frequency of instability should be much greater than the op-amp bandwidth.

So, for me, that begs the question of who exactly said that some op-amps are more susceptible to interference than others?

Maybe a uA741 is, but just because we're a non-American small manufacturer doesn't mean we  know diddly squat! (as most other forums love to peddle!)



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Post Options Post Options   Thanks (0) Thanks(0)   Quote highfell Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2013 at 11:32am
Hi Graham

I will try and understand your technical post -:)

I am no expert - as you can probably tell, but I am enjoying trying to learn the science behind the art and to ignore bull sh*t and marketing spiel on the various forums.

As regards the comments about interference actually they were taken from commentary from a technical designer of portable headphone Amps around the pros and cons of rolling different Op amps, pasted below which first drew my attention to susceptibility of different devices. As an aside, the OP627 is noted as having a proprietary input stage called a Difet - yeah that doesn't mean much to me -:)

"Traits of a good op-amp :

As an amplifier designer, there are a few key electrical traits I look for when selecting op-amps for a portable amplifier. It should have low output noise, it should be able to drive sufficient output power to a wide range of headphones with low distortion, and it should be able to do these things while drawing minimal power from the power supply. The goal is to have a "transparent" amplifier which faithfully reproduces the input signal without adding anything of its own. Because we are talking about a battery-powered amplifier, we also want to minimize the total power consumption.

The Analog Devices AD8610 was originally my standard op-amp for both Amp. It excels in all the key electrical traits, and it is well-known in the hi-fi headphone amp community. A few years ago Texas Instruments released the OPA209. It also performs very well, and it even beats out the AD8610 in some measurements. I began using it as my standard op-amp and it really seems to be gaining some fans.

In addition to the AD8610 and OPA209, I also added a few more popular op-amps to the list, such as the OPA627 and AD797. Both of these parts have large numbers of fans in the hi-fi community. Using these op-amps requires accepting some trade-offs, such as higher power consumption.

Interference - The pitfall of portable audio :

While the OPA209 has some great characteristics for portable audio, it turns out it also has a weakness: susceptibility to interference from GSM phones and other mobile devices. Many op-amps have this problem. You've probably heard the result: it's the buzzing or clicking sound that occurs when a mobile device is placed near an amplifier or speakerphone or some other type of amplified audio device. The OPA209 uses a type of transistor called BJT which is particularly susceptible to this type of interference. Op-amps which use JFET transistors, such as the AD8610, are much less susceptible to the interference. The OPA627 has a proprietary input stage called "Difet" which is similar to JFET, so it also has relativity low susceptibility."

BTW Another thing to consider with the Super Voyager is that Hifi Adicts like to try different equipment or combinations all the time and therefore the ability to roll Op Amps is some thing that they might like to do as well.

Cheers

Nigel
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2013 at 12:31pm
Originally posted by highfell highfell wrote:

Another thing to consider with the Super Voyager is that Hifi Adicts like to try different equipment or combinations all the time...


I aspire to produce equipment for the music listener rather than for Hifi Addicts who will never be satisfied with anything because it is their pastime to try as many things as possible and argue about it on forums.

In saying the above I'm not trying to be awkward nor do I want to sound arrogant - I know I will never please all the people all of the time.

It would be good if some of these hi-fi addicts would properly learn about audio electronics and settle down to making great sounding products for those who simply want to listen and not fiddle about, for when people like me cease to exist... when its gone its gone.

As for what the designer wrote I would give him 5 out of 10.

What is a Di-Fet? Di- is interchangeable with Bi- meaning dual, but in an op-amp Bi-FET is used to properly describe the combination of bipolar and FET transistors.

Looking at the diagrams it can be seen that both op-amps are bipolar and FET combinations.

Both have FET inputs - both inputs then drive a folded cascode bipolar transistor voltage amplifier stage.

As a designer I also look at the 'traits' - that's what designers do!

So by his own argument the op-amp used in the Voyager has the same susceptibility to interference as the OPA627.

That should be the end of the story but I'm sure it won't.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote less Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2013 at 12:49pm
Originally posted by highfell highfell wrote:

Hi Graham



BTW Another thing to consider with the Super Voyager is that Hifi Adicts like to try different equipment or combinations all the time and therefore the ability to roll Op Amps is some thing that they might like to do as well.

Cheers

Nigel


I think Nigel, that we should draw a distinction between those of us who regard Hi-Fi as the enjoyment of listening to Music with the highest possible fidelity viv a vis a live performance, and those who prefer to tweak and listen to the
Equipment.

I am not suggesting for a moment that there is anything wrong with latter. It is just that Graham has a level of knowledge, understanding and experience that the vast majority of tweakers probably lack. So the idea that someone without Graham's level of understanding and skill can somehow improve a circuit designed by Graham is, frankly, without merit.

If the circuit cannot be improved then what would be the point of tweaking it, other than the pleasure presumably gained from the tweaking activity? Again, nothing wrong with that in principle but I for one would prefer to just sit and listen to the Music, and if that makes me a music addict and not a Hi-Fi addict then so be it!

One last thing, this is not a personal attack on you or anyone else, just my point of view. I do find it very odd that some people spend a decent sum of money on a perfectly good component and then set about trying to Improve it without the knowledge or understanding of the circuits design parameters!

Regards

Les
I don't do mediocrity!

Les Sutherland
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Post Options Post Options   Thanks (0) Thanks(0)   Quote LOINER Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2013 at 1:24pm
Well said LES
It is actually an insult to the designer IMO
In this instance a more than competent one

Edited by LOINER - 01 Dec 2013 at 1:26pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote less Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2013 at 2:00pm
What I was trying to say in my last post was that Graham designs his products for those of us who want to listen to music.

I am sure, though I stand to be corrected, that Graham would love to sell more product to more people, but would prefer that purchasers trusted to his expertise. Tweakers should perhaps look elsewhere.

Hope I haven't upset Graham by saying that!

Regards

Les
I don't do mediocrity!

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