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Old 22nd September 2007, 05:29 AM   #41
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It's what i am thinking about Maousse !
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Old 22nd September 2007, 07:01 AM   #42
Maousse is offline Maousse  France
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already done for a test, buddy!
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Old 22nd September 2007, 08:21 AM   #43
ALDO is offline ALDO  France
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Hello
what there is like French here!
Francois, summons us uncovered!!!
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Old 24th September 2007, 10:48 AM   #44
matejS is offline matejS  Slovenia
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Default Re: UGS Power : it's U.P.

Hi, Cheff!

Amazing work and good design (best I can think of - at least for DIYers).

Quote:
Originally posted by CheffDeGaar
I've added an on board DC protection, but I must confess I'm amazed by the offset performances : under the mV on diff offset, and under 5mV on absolute offset...
About the DC protection... is calculating diff really necessary (also it is nothing wrong with it)? I mean if you check them separately and they have no DC offset can their difference have an offset. I didn't yet found a mathematical proof to support my assumption.

What relay do you use for it?

Thanks,
Matej
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Old 24th September 2007, 12:01 PM   #45
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Default Re: Re: UGS Power : it's U.P.

Tiens, les français se sont réveillés

Quote:
Originally posted by matejS

Amazing work and good design (best I can think of - at least for DIYers).

About the DC protection... is calculating diff really necessary (also it is nothing wrong with it)? I mean if you check them separately and they have no DC offset can their difference have an offset. I didn't yet found a mathematical proof to support my assumption.

What relay do you use for it?

Thanks,
Matej
Thanks Matej ,

Well, what I want is no offset across the speaker. I could have go the way you mention, but it would have make things more complicated.
I assumed it is very unlikely that both sides of bridge go wrong at the same time, so a differential offset makes it perfectly.
And if you own a Son of Zen, you can use this DC protection, since both output voltages are non zero (Vsupply/2), but their difference is zero
The relay is a 24V DCPO (RTE24024 from Tyco/Schrack)

And to reply your email, no I do not have gerber files. I'll post a pcb later (I'm still prototyping), but only the way I use it, i.e. pdf or ps format.

Regards,
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Old 24th September 2007, 12:38 PM   #46
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Hi all,

I spent some time listening and comparing the global/local feedback configuration, and guess what...

How to put words on that... I must mention here that all the following is purely subjective, and that it involves pretty tiny and diffuse impressions

Compared to the original global feedback scheme, I feel the local feedback comes closer to the AX. I know it's not the same topology, but the the sound becomes a bit more lusher and warmer in the bass/mid part of the spectrum, and that the highs are rolled off a bit. What I call the "AX fuzzyness" comes back again, but with much less noticeable than in the AX's case, and it narrows the scene a bit (it's a very very thin difference). The sound seemed to have lost some air.
Since the output mosfets are no longer in the FB loop, I think they bring their own character to the sound, but may be I'm wrong...
But don't get me wrong, the sound remains quite superb, and some of you may prefer the local FB configuration for its warmer aspect. It all depends on the rest of your system, so feel free to compare and choose

These tests were made with an open loop gain of about 60dB (sorry, I haven't had the time to measure it).

BTW, thanks to Mr Pass and Ian MacMillan - and their breadcrumbs in the AX builders thread, this aspect of the OL gain made me think a lot
So I decided to test and listen what was happening with a lower OL gain. I didn't want to decrease the current mirrors gain, since first it required a heavy manipulation (removing the pcb to access the solder joints), and second since I feel that the current capabilities for driving the output mosfets should be left as is, or quite close.
So I just changed the value of the "ugs" output resistor (10K at the start) by paralleling socketed lower values resistors.
A 5K equivalent resistor didn't changed the things, at least ihmo.
So I tried a 1K equivalent resistor. It lowered the OL gain by 20dB (theroretically), i.e. an OL gain of a rough 40dB.

How to say... with this 1K resistor (in global FB scheme), I felt I was half way the previous local/global FB test : The sound got lusher and warmer, but without getting fuzzy and without loosing clarity, air and accuracy in the highs.
Always with the 1K eq. resistor, I tested the local FB, but didn't like the result this time.... I lost the "aerial" impression, got a darker sound with a feeling of slow basses. Of course these are tiny impressions, but they're mine

I also tested a lower resistor value (700R), but I didn't feel any differences.

To sum up, I think I'll stick to the 1K output resistors with global feedback. But your mileage may vary, so feel free to make your own tests, that's pretty easy, and that's the fun with DIY

Anyway thanks to NP and Ian for pointing this out to me

Cheers,
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Old 24th September 2007, 03:39 PM   #47
gl is offline gl  United States
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Thank you Cheff. This is an excellent project and makes a large contribution.

Graeme
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Old 24th September 2007, 07:00 PM   #48
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Hi Cheff,

Sounds like you are making good progress and the results of your investigations are certainly interesting. The differences between local and global feedback are pretty much as expected based on what others have said (I’m sorry to say I haven’t yet tried it myself). Note quite sure why local feedback should reduce the air but the rest is most likely down to the reduced damping factor with local feedback and as you say, no feedback correction of output device non-linearity.

Your comments on reducing the amount of feedback are particularly interesting as I am not aware of anyone who has performed this experiment and reported on it in the forum. Of course at present it is speculation on whether the reduced feedback is responsible for some of the effects of whether the additional loading on the “ugs” output may also be in some way responsible. I’d be interested in a similar experiment that varies the diff pair source resistors instead (to reduce the amount of degeneration). Discovering whether the gain in practice is the same as predicted will also be illuminating but I take your point about it requiring time.

BTW, I agree with you about it not being a good idea to reduce the current capabilities of the mirror although I don’t know what value you have set it to. I calculate you have a current gain of about 6 so I guess the “ugs” output current will be in the range of 25 to 30mA depending on the bias of the diff pair. Most people seem to use around 5mA per FET for this with the 2SK389.

Ian.
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Old 24th September 2007, 08:40 PM   #49
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Quote:
Originally posted by Ian Macmillan
Hi Cheff,
BTW, I agree with you about it not being a good idea to reduce the current capabilities of the mirror although I don’t know what value you have set it to. I calculate you have a current gain of about 6 so I guess the “ugs” output current will be in the range of 25 to 30mA depending on the bias of the diff pair. Most people seem to use around 5mA per FET for this with the 2SK389.
Ian.
Hi Ian,

I used the same bias as in the linestage ugs for the Jfets, i.e. around 3.5 mA for the BLs I had. I had to make compromises between bias and current mirror gain, and as this value worked perfectly (for me) in the linestage, I just applied it to the amp. Maybe it's an error, and I should have increased the bias...

The current mirrors output 19/20mA last time I measured. I just made a rough balance between input capacitance of the mosfets, the linearity and gain of the mirror BJTs, the dV/dt I could afford, the input stage, bias, etc... Perhaps it is not optimum, but it seems to perform the way I planned, as I do not have the feeling of a lack in speed. BTW, reducing the current mirror gain will not lead to a drastic reduction of the OL gain, as I don't feel comfortable reducing it by 2 (6dB) for current driving reasons...

For the source degeneration testing, I can't promise anything. I have to make some preliminary calculations to see how it impacts the whole front end, since the bias point will change.

I first want to test the "class A opamp biasing" adding some SE operation with output resistors to supply. That's pretty easy, so that's the next test (along with the OL gain). Just gimme some time, I'm a bit busy for now

Cheers
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Old 25th September 2007, 02:47 PM   #50
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Quote:
Originally posted by CheffDeGaar


I first want to test the "class A opamp biasing" adding some SE operation with output resistors to supply. That's pretty easy, so that's the next test (along with the OL gain). Just gimme some time, I'm a bit busy for now

Agreed, this will be very interesting. Take your time, there is no hurry.

I don't think there is anything wrong with your choice of current to drive the number of output MOSFETs you are using. I need a little more for my application as I am aiming for 100W in class A and will need more output devices.

I understand that the source degeneration is hard to change once built but I feel it may be a better way to loose OL gain than loading the output of the "ugs". Just a feeling...

Ian.
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