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Old 4th December 2014, 04:01 PM   #1
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Default eti 466 HUM

Hi, I have made the eti 466 amplifier years ago , recently was playing with it again made some differences to the box and wiring yet ther is still HUM , much less than before but havent completely got rid ot if , the transformer is far away its in a 5mm thick metal cage and many other things , checked ground wiring made sure all wires are thick and as short as possible , also checked for any loops anywhere nothing.

maybe there is someting wrong with the input ground ? the original pcb I made looking from this site

http://users.otenet.gr/~athsam/power_amp_300w.htm

in the schematic at the input the 10 ohm resistor to ground was disabled but maybe it would be better whne the common ground would be attached to the input via this 10 ohm resistor ?

maybe someone has any feedback on this old amp circuit just in case.

thanks.
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Old 4th December 2014, 04:08 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by crazymechanic View Post
in the schematic at the input the 10 ohm resistor to ground was disabled but maybe it would be better whne the common ground
would be attached to the input via this 10 ohm resistor ?
Yes, that 10 Ohm resistor was there for a reason.
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Old 4th December 2014, 04:21 PM   #3
osscar is offline osscar  Latvia
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Hi,

This option works well for me:
Attached Images
File Type: jpg ground.jpg (168.9 KB, 149 views)
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Old 4th December 2014, 05:14 PM   #4
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but in your picture you have bypassed the r10 resistor as it is not there just like in my case.

if I understand correctly what rayma says the input ground shouldn't be connected directly to the common and output ground but it should be connected in series with a 10ohm resistor to the common ground , correct?
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Old 4th December 2014, 05:21 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by crazymechanic View Post

if I understand correctly what rayma says the input ground shouldn't be connected directly to the common and output ground
but it should be connected in series with a 10ohm resistor to the common ground , correct?
Yes, this reduces ground loop currents. This is commonly done in audio gear.
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Old 4th December 2014, 08:45 PM   #6
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The PCB is obviously isn't the original ETI466 PA amp design. See here: eti300
The schematic though was lifted from ETI (Aust.) Magazine. It seems Sam Electronics decided to use it for their PA300WP product, though.

As it stands, the output and input grounds on Sam's PCB would have been tied together before taking a common connection to power supply star ground. That would be a noisy mistake, regardless of whether you fitted the ground lift resistor. I built the original on my own draft of the PCB way back in time and I certainly had hum and noise problems before I learned much about hum from ground loops and noise from lousy PCB layouts.

Agreed, the mod. shown with separate ground leads to star power supply ground would be much better but the rail bypass caps should also have been returned to PS star ground. Really, all measures should have been implemented but how quiet did a DIY PA amplifier have to be, back in the late 1970s?
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Old 4th December 2014, 09:13 PM   #7
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so i did a little solderwork and now there are two thick ground wires coming from a common point connection at the filter capacitors to each of the two channel ground traces, the input ground is now connected with a 10 ohm resistor in series and only then soldered back to the common ground on the pcb.

as for my expectations , nothing changed , the hum is still there. when i first built the amp years ago i had much bigger hum , now when i took a second look after years of being put away i found some ground loops , so i repaired them and rewired the grounds , and the hum now is much less but it's still there and when connected to more sensitive speakers makes a silent yet audible low frequency hum in the background that pisses me off to the extreme


yes the pcb is self made , i have a single thick ground trace on it and every component that has a ground conection is attached to it , even though the pcb is not " high tech" so to speak of I dont think its the fault here , i made one other amp from the Elliot sound pages , a 100w HI FI amp and it too has lousy self made pcb's yet its dead silent and works great , a guy made a third channel pcb of the same amp using pc and a pro looking circuit board and that channel is in the same amp as the two first channels i made myself and the pro looking channel hums while my channels don't.

This God damn Humming thing and trying to eliminate it feels like caching a vision of Jesus in mid day on a crowded street.
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Old 4th December 2014, 09:39 PM   #8
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What are you trying to do with this amp, crazymechanic? Is it for PA, musical instruments or home audio? Personally, I have only used mine as a single PA amp for band use where it had great sound and was appreciated because it was tough and protected well enough with current limiters. I would not call it a Hi-fi design by any stretch of imagination, though. The output stage even has gain via its CFP topology to get the extra power from a low voltage front end but that is not so good for sound in that configuration.

However, if hum is your principal objection, that is mainly a PCB layout and wiring problem. Whilst PSRR (the designs ability to reject power supply noise) is important in the electronic design, it was not a great problem to reduce it to low levels as the design stands - i.e. inaudible to an audience at a small band gig etc. Again though, if you are trying for hi-fi performance with a 45 year old DIY design, you are way behind the times to begin with.
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Old 4th December 2014, 10:23 PM   #9
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no no you got me wrong , this eti466 is not intended for HI FI, I mean I'm crazy as my nickname but not plain stupid :d

the 100w amp from Elliot pages sounds really great in a room connected in series with a luxman l3 that I got for free from a electronics waste site , it was both in excellent technical and visual condition , only the fuses were blown once i replaced them it works now for 4 years without any problem whatsoever.

this eti466 I built as sort of a subwoofer low frequency driver , not exactly for PA I was playing it together with the amps I mentioned previously.
I built it years ago for fun and learning so to speak of.

Ofcourse in a party where people are loud and some are drunk the last thing one would notice is a small hum in the background since the background from people talking alone is at the db levels at which i normally listen music in my house.

Yet I think we can both agree that while it is no HI FI ampplifier , background hum is not exactly a thing that any amplifier should have.
That's like saying if you dont drive a ferrari or Mercedes you might as well have the engine compartment with all the noise and smell in the saloon.

So your amp has been humming all these years right? Did you solve it or just left it what it was ? have you had any measurments , like milivolts of the hum at the outputs or something ?

when i first built it it had something like 20mV in the output.

I also must say I'm using and self wound transformer but the transformer itself is from an old tube tv , its kinda big and the geometry is not probably the best for audio enclosures.
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Old 5th December 2014, 04:59 AM   #10
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OK, there is no disagreement here because I fiddled with the design about 4 times with serious mods like a new PCB twice. The second PCB was a professional one which was sold as part of the ETI 300 kit, as the drawings and docs show in the above post #6. This was very much quieter on hum and noise. I even added a shield around the input stage, changed emitter resistor types to film, moved the power supply off-board. I also shielded the feedback return and I had already rerouted and split the ground return wiring up. Together, these made a lot of difference to the hum and noise.

The overall problem I had was that I didn't understand PCB design and layout much so I made a lot of stupid mistakes because my friends had no clue either and there were no understandable textbooks to read. This was from 40+ years ago, so things have changed a lot from then and the point was that this design is even older - that makes it a bad starting point for a party amplifier but again, nobody partied with cheap PA amps then. PA amps all sounded bad on recorded music and were noisy so we built our own 100-150W stereo hi-fi amps or borrowed big domestic ones from rich guys. That's how how amps like Elliott's and ETI480, DIGI 125 and many others came about too.

Designs like P101 are newer and have been revised and improved several times by Rod Elliott over the years and that design was intended for Hi-fi too. I'm sure I did a lot of changes to the ETI166 design for myself and for friends who needed a cheap PA. I was no expert though but I did get it much quieter than my first - better than a couple of pro. PA amplifiers the band owned or borrowed too.

Here is the original and verified test spec from the designer: The ETI300 was a different board and range of power capabilities but the same electronic design as The ETI466.
http://www.solaris.no/electronics/po.../300w_spec.gif
It states the audio bandwidth noise for the original design was measured at -105 dB wrt full power. I measured 2 amplifiers I built for 150W/4R at -101 dB on a Kenwood audio millivoltmeter VT176 and I was happy with that for band use. KENWOOD VT-176 - Service Manual Immediate Download
I didn't measure the noise in my first build - I didn't even know what it meant, that many years ago and I can hardly remember, so go easy on me.
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