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ETI 477 article
ETI 477 article
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Old 30th November 2020, 08:36 AM   #161
suzyj is offline suzyj  Australia
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Originally Posted by janusz View Post
All picofarad caps are and should be silver mica, not ceramics.
How so?

IME there is no discernible difference (aside from price) between NP0 and C0G ceramics and mica.
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Old 30th November 2020, 10:57 AM   #162
janusz is offline janusz  Australia
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True, new ceramics such as x7r are much better but still micas I have in my measurements show better ESR and Q. Besides, ceramics also have some piezoelectric properties. I got a few friends who play guitars and do not like ceramics in their amps as they exhibit a tendency for harshness. Besides, if I'm not wrong, neither Altronics nor Jaycar do not have through hole x7rs and alike so one has to buy them over the net.

When it comes to the price then one can get cheaply ex USSR military standard micas on EBay. I bought and have used many of these. Also occasionally one can get cheap Cornell-Dubilier micas from some US sellers. When just a few pieces are needed EBay is also cheaper than Mouser or DigiKey.

And on top of this when I built my first eti477/5000 in the late 1980s it oscillated until I replaced kit ceramics with silver micas from RadioSpares. Yes, micas cost a few dollars even then but the amp did not oscillate in megaHz. With bjts maybe that would not have been the problem but with laterals there was. So I got used to micas in such applications but I use x7rs as bypass in PS lines.

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Old 30th November 2020, 11:31 AM   #163
Leinster Lad is offline Leinster Lad  Australia
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Originally Posted by janusz View Post
OK, you got 3 versions with THD at 10kHz shown. .

Thanks so much Janusz, some really great help and advice right there.


Lots to consider and parts to order !!!
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Old 1st December 2020, 08:37 AM   #164
janusz is offline janusz  Australia
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Do not forget that simulations are only approximations.

In my current eti477 I use larger gate resistors (470 and 330 ohm 1W) but 270/220 ohm resistors are good enough and my original eti worked with 100ohm/1W gate resistors well. In VAS I use Sanyo 2sa1249/sc3117 transistors, which I bought long time ago and are not available any more from Mouser or DigiKey.

Fukushima mpc74 0.1 ohm/5W resistors I'm using in sources have inductance of 17 to 18nH, not as good as Caddock (inductance below 10nH) but much cheaper.

I would also recommend checking the amp after mods for oscillations on a good scope.

cheers,
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Old 2nd December 2020, 03:09 AM   #165
Leinster Lad is offline Leinster Lad  Australia
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Thanks Janusz, great advice.


Suzyj kindly offered to "measure it to death", so would be great to take up that offer


Regarding the power supply.
What is considered an acceptable level of ripple on the DC rails?


I hooked up my scope yesterday and got approx 55mV RMS and 180mV p2p on each rail. (@100hz)
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Old 2nd December 2020, 04:10 AM   #166
Leinster Lad is offline Leinster Lad  Australia
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Originally Posted by janusz View Post
Fukushima mpc74 0.1 ohm/5W resistors I'm using in sources have inductance of 17 to 18nH, not as good as Caddock (inductance below 10nH) but much cheaper.
cheers,

Just in reference to the non-inductive resistors.
Lead spacing will not fit the eti477 board by a long way.
If I was going to fit these with bent leads, would it be best to have the leads evenly bent or leave one lead straight and bend the other ?
If so, which way around would you go ? ie. resistor element closest to the mosfet or closest to the output inductor ?
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Old 2nd December 2020, 05:09 AM   #167
suzyj is offline suzyj  Australia
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I’ve sent you a pm. Regarding acceptable ripple, it depends on the power supply rejection ratio of the amp. You can have truly horrid ripple and not hear anything with a well designed amp.
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Old 4th December 2020, 06:36 AM   #168
janusz is offline janusz  Australia
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Originally Posted by Leinster Lad View Post
Just in reference to the non-inductive resistors.
Lead spacing will not fit the eti477 board by a long way.
If I was going to fit these with bent leads, would it be best to have the leads evenly bent or leave one lead straight and bend the other ?
If so, which way around would you go ? ie. resistor element closest to the mosfet or closest to the output inductor ?
Original eti477 pcb layout is terrible. Caps too close to hot heatsinks, input transistors far away from each other so they cannot be joined to improve thermal tracking etc. Overall pcb not designed to minimize layout caused distortion. That's why I cut path connection of the output with LTP and took it from the output before Zoebel and using wire connected nfb resistors to it. Tiny improvement, at least theoretically. Redesigning the pcb layout following certain rules would certainly be beneficial.

When it comes to resistors it does not matter, just in case try to keep them further away from heat sources - in this case VAS as the main heatsink at normal listening levels stays almost cool. It is not as essential as in the case of electrolytics but all components more or less change parameters with temperature.

You might also follow Suzyj's approach and reduce N channel mosfet caps to 220pF - 330pF and add P channel RC 22 ohm with 18-22 pF. In simulation it shows better stability and the cap across 22k nfb resistor can be removed.

cheers,
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