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Old 31st January 2006, 02:36 AM   #1
suzyj is offline suzyj  Australia-Aboriginal
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Default ETI 5000 MOSFET Power amp

Hi guys,

I recently inherited a gorgeous pair of Infinity RS-5b speakers (mid-80's vintage), and have dutifully replaced the surrounds on the woofers, and brought them up to scratch. My current amplifier (part of a ghastly Sharp 3 in 1 bookshelf system) is patently not up to driving them, so I've been looking for something a little better.

Looking at the specs of the current commercial Rotel, NAD, etc amplifiers in the 50-100W range, I see they all have really disappointing distortion figures, around 0.02-0.03% THD.

Then I remembered a series of articles in ETI (crappy Australian electronics magazine) on the ETI 5000 amp. It was published way back in 1981, and I remember as a teen being pretty blown away by the vanishingly low distortion figures (<0.005% or so THD).

I dutifully dug up the original magazines from the work library (I'm an RF/microwave engineer by trade, and have ready access to most things), and had a looksie. Unfortunately, it appears there's no easy source for the 2SK134/2SJ49 drivers, but I figure I can simply substitute EC10N16 and 10P16s. The BF469/470s also come up as obsolete, but I'm thinking the BF722/723 pair would be a reasonable substitution.

What are people's thoughts on this amp? I seem to recall Tilbrook did a follow-up amp (for AEM?), but we don't have AEM in the library, so that's the most up-to-date design I can easily lay my hands on...

I'm particularly impressed with the differential gain stages, all the way out to the power FETs. That's clearly the ducks guts for suppressing 2nd harmonic distortion. However, it does appear to use a hell of a lot of overall feedback, and methinks the proliferation of small caps kicking around the schematic indicate that it had some stability concerns.

Still, I'm betting that many of the foibles could be ironed out with a decent board layout, with more modern (SMD) passives, for example, and a proper 2 layer PTH board.

Cheers,

Suzy
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Old 31st January 2006, 03:07 AM   #2
Leolabs is offline Leolabs  Malaysia
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Default Re: ETI 5000 MOSFET Power amp

Quote:
Originally posted by suzyj


Looking at the specs of the current commercial Rotel, NAD, etc amplifiers in the 50-100W range, I see they all have really disappointing distortion figures, around 0.02-0.03% THD.


Dont get fooled with distortion figures,they are good amps.
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Old 31st January 2006, 03:08 AM   #3
lineup is offline lineup  Sweden
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Default Re: ETI 5000 MOSFET Power amp

Quote:
Originally posted by suzyj
Looking at the specs of the current commercial Rotel, NAD, etc amplifiers in the 50-100W range, I see they all have really disappointing distortion figures, around 0.02-0.03% THD.
Even if these 0.02-03% THD figures are done with a dummy load 8/4 Ohms resistor
(sometimes low inductance resistance)
I would not feel bad about them.

If I build such an amplifier and get those figures
I would be very very happy



... and now we are not talking real life numbers
... as they are on the other side of loudspeaker
...
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Old 31st January 2006, 07:01 AM   #4
AKSA is offline AKSA  Australia
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Suzy,

I built one of these in the early eighties. It was very ordinary, with a shrill, metallic sheen to it. Didn't like it at all. They were, as you say, extremely unstable if not built well, a sure sign of compromised design.

Suppressing H2 is not the way to go. You want to suppress H4 and beyond, and you need to really control the crossover event well to reduce listener fatigue.

High feedback is fine as long as the voltage amp, driver and output devices are fast and lag comp is spot on. Slow devices are more difficult to extract good performance. You need at least 100MHz voltage amps and 30MHz output devices.

The double diff design is not wonderful for bandwidth. SE designs will give much more speed.

The AEM6000 from Tillbrook was a much better sounding amplifier.

Hope this is helpful,

Cheers,

Hugh
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Old 31st January 2006, 07:14 AM   #5
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Lightbulb Nad

NAD make Great Amps, we have one, and it has fantastic sound

Try The Silicon Chip, Studio 350 Power Amplifier Module It also has some very good figures. Only Problem is that it has 8 $15AU each transistors(4x MJL21193 and 4x MJL21194). It has a very impressive heatsink on the back of it.
I am Actully in the process of make one at the moment.
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Old 31st January 2006, 12:56 PM   #6
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Question 5000 What?

Where does those "heroic" figures come from... was 6000 SUX (Robocop) the inspiration source, or was it the opositeClick the image to open in full size.

Do we have any schematic we can view?

Cheers Michael
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Old 31st January 2006, 01:42 PM   #7
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Default Re: ETI 5000 MOSFET Power amp

Quote:
Originally posted by suzyj
The BF469/470s also come up as obsolete, but I'm thinking the BF722/723 pair would be a reasonable substitution.
Why do you think, Suzy ?

The BF469/470s can still be found here, with difficulty. If the others don't work out, give a howler.

Gents,
we could at least say welcome to our latest Sheila with audio brains addition.
HI Suzy.
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Old 31st January 2006, 04:56 PM   #8
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I have heard the ETI5000 amp, built by a friend with the original PCB, Noble output resistors, heat-sink/face plate all supplied by Jaycar from Australia; in other words, the amp was as nearly as good as the one made by Dave Tillbrook himself.

As to the sound, nothing to write home about.

In this very forum, try the SymAssym, one of the Pass amps or Hugh's kits - you will not be disappointed.

If you have the skills and competence, you could make the Stochino amp. It can be rated among the very best. I have made a few of these.
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Old 31st January 2006, 06:57 PM   #9
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Suzy,

I have scanned copies of the 6000 articles, and status monitor (much thanks to the hard work of Janusz) and also the 6006 dc servo board (thanks to Rherber1).
The scan qualities are good, but as a result the files are not small.


If you would like me to email them to you?
then let me know

poor_lackey@hotmail.com


Also
If you want to avoid the hassel of making the PCB's.
I think they might still be avaible from
www.rcsradio.com.au

Tony
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Old 1st February 2006, 12:20 AM   #10
suzyj is offline suzyj  Australia-Aboriginal
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Hi all,

Ultima Thule wrote:

> Do we have any schematic we can view?

I've done a quick and dirty scan of the schematic, and bunged it on my homepage.

http://www.atnf.csiro.au/people/Suzy...on/ETI5000.jpg

It's not the sharpest, but is readable.

ASKA wrote:

> I built one of these in the early eighties. It was very ordinary,
> with a shrill, metallic sheen to it. Didn't like it at all. They were,
> as you say, extremely unstable if not built well, a sure sign of
> compromised design.

> Suppressing H2 is not the way to go. You want to suppress H4
> and beyond, and you need to really control the crossover event
> well to reduce listener fatigue.

Aye - I can see what you're getting at. Sounds like yours might have had some nasty higher order components. Certainly I would think keeping good phase margin over three stages isn't a trivial task (especially with the sort of dubious parts one buys from the local Jaycars), and the output MOSFETs are plenty fast, so I'd imagine they'd not be at all forgiving of poor layout or excessive inductance.

That was really why I was thinking of effectively repackaging the amp (with a much better board layout) and using modern parts.

Just a thought, anyway.

Cheers,

Suzy
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