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Old 1st May 2013, 01:16 PM   #11
iggy111 is offline iggy111  Croatia
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Thanks, I will try this in a week or so, I'm moving so everything is a mess...
I will first put back all the components I took out or changed, and remove the regs so I can properly test the amp with and without them, and post the conclusions.

In the meantime, anyone who has the "bloated" sound problem or similar, or has implemented the decoupling AndrewT mentions, please post here.
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Old 1st May 2013, 01:23 PM   #12
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
So many questions I do not have time to detail all you ask for.

A pair of radial 0.1" xr7 100nF 100V caps make good HF decoupling.
Cut a lead on each very short. Solder these short ends together.
Cut the other two leads to about half length. This soldered junction is a Power Ground
Solder one half length lead to +ve pin.
Solder the other half length lead to the -ve pin.
You now have the power pins connected to an HF power ground.
Keep going, build an MF Power Ground using electrolytics.
I see a few errors and omissions.

A pair of radial 0.1" xr7 100nF 100V caps make good HF decoupling.
Cut a lead on each very short. Solder these short ends together. This soldered junction is a Power Ground, the HF PG.

Cut the other two leads to about half length.
Solder one half length lead to +ve pin.
Solder the other half length lead to the -ve pin.
You now have the power pins connected to an HF power ground.

Keep going, build an MF Power Ground using electrolytics.
Connect the HFPG to the MF PG with a short wire or a short trace.
A short trace that is wide has much less impedance than an equal length wire, no matter how thick you make the wire.
At HF impedance rules. Everything that can be done to reduce inductance is important.
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Old 1st May 2013, 10:39 PM   #13
iggy111 is offline iggy111  Croatia
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A short wide trace instead of wire - how's a piece of copper shielding from a satellite cable here?
But why use a trace or wire anyway, when you can just solder the pins directly?
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Old 2nd May 2013, 02:46 PM   #14
AndrewT is offline AndrewT  Scotland
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The electros are far too big to fit in close to allow their MF PG to be attached to the HF PG.
You need to assemble the electros in a location where they can "fit". Then connect the MF PG to the HF PG.
That connection should be of low impedance to get great performance from your decoupling.

The copper foil from a coax screen may well be a good trace substitute. Nice idea. Shame most of mine are aluminium metalised onto plastic film.

That foil strip could also be used to solder/bolt an IEC filter to a chassis. Wide thin strip connection is far better than a wire or a bolt connection. I have been reading Ott, does it show?
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Last edited by AndrewT; 2nd May 2013 at 02:51 PM.
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Old 16th May 2013, 10:10 PM   #15
iggy111 is offline iggy111  Croatia
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Ok, settled to the new appartment now...
A few questions, AndrewT, if you're still following this:

1) what would be the best value for the MF decoupling caps?
2) would 100-330pf ceramic caps be a good choice for the HF decoupling?
I'm trying to do this with stuff I have at home, so it's either that or some really small xfk caps, which are multifoil if I'm not mistaken.
3) shouldn't I connect this HF + MF + Zobel ground to the star ground? You didn't mention it, so I have to ask.

Unfortunately I still have to find that copper shielded satellite cable again, so no copper trace for now. But a visit to a specialized satellite shop should solve that...

PS someone suggested that the 47uF of Ci should be increased as high as 330uF because of the -3dB point being too high. I calculated it to be 4.98Hz for 47uF (with 680R), 2.34Hz for 100uF, and 0.7Hz for 330uF. I doubt I would hear any difference, but what would this change achieve?
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Old 17th May 2013, 09:14 AM   #16
AndrewT is offline AndrewT  Scotland
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MF decoupling is usually from 47uF to 1500uF. A big spread to play with.
HF decoupling is usually 100nF. I see very little variation around this value.
I standardised on 100nF 50V or 100V x7r 0.2"pin pitch.
I have recently bought 100nF smd for lower inductance where a circuit layout suits.

The Zobel Ground + both decoupling grounds are the amplifier's Power Ground. Yes, this goes to the Main Audio Ground whether it is on board, or near the board.

I did quite a few experiments with that input cap RC and found that very approximately when near 90ms I could not hear any further changes.
This was for a couple of amplifiers that had a very wide pass band. Soon after I did this, I discovered on THIS FORUM that the amplifier pass band should ALWAYS be wider than the signal being applied. The ratio suggested was at least sqrt(2).
Combining the 90ms input filter and the sqrt(2) factor gave the NFB RC >=130ms.
I tried this and sure enough I could not hear any difference compared to the wide pass band.
But, I then tried the narrow passband that is very often shown by builders and found I could hear the bass change again. I moved it up another octave above what I thought I could now hear and the bass became unnatural. That then confirmed my conclusion that the suggestion was right. I have stuck with this ever since.
It gives deep pleasant bass.
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Old 17th May 2013, 01:52 PM   #17
glenv6 is offline glenv6  United States
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[QUOTE=AndrewT;3473684...If you want a DC coupled amplifier then my advice is learn how to protect your speakers. And learn why DC detection and DC servo and speaker isolation and maybe input muting, should all be implemented. That is a lot of learning and a lot of board space and a lot of cost to avoid AC coupling that can give very good sound.[/QUOTE]

I too blew up a pair of speakers testing my stripped down AudioSector LM3875 amp in the first few hours of its life. I assume I had DC at the input from my Microsoft Zune MP3 player that was amplified and sent to the speakers, which blew them up. This happened a second time to another pair of speakers, but they were more resistant to blowing up and allowed me to study the problem for a few seconds before I shut the amp off. So far my NAD 1600 Tuner-Preamp has been a good match with this amp.

I don't plan to risk my good speakers on this amp though until I replace R1 (shown on the amp board here http://www.audiosector.com/images/lm3875_se_pcb.gif) with a capacitor and add that Zobel network, Rz and Cz.

Iggy111, with all of that said, my amp does sound fantastic - no "bloat" in the audio at all. I have run it through 4ohm and 8ohm speakers and aside from being a little hot with the 4ohm speakers, it sounds great, when it is not amplifying DC that is

-Glen

Last edited by glenv6; 17th May 2013 at 02:17 PM.
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Old 17th May 2013, 02:18 PM   #18
minifly is offline minifly  South Africa
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I remember seeing some scope tests that showed that even a small capacitor between the + and - signal inputs caused the amplifier to take a while to stabilise on HF input signals - perhaps try removing that to see if it goes away?
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Old 17th May 2013, 02:39 PM   #19
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Quote:
Originally Posted by glenv6 View Post
I too blew up a pair of speakers testing my stripped down AudioSector LM3875 amp in the first few hours of its life. I assume I had DC at the input from my Microsoft Zune MP3 player that was amplified and sent to the speakers, which blew them up. This happened a second time to another pair of speakers, but they were more resistant to blowing up and allowed me to study the problem for a few seconds before I shut the amp off. ...


Before Andrew freaks out:

Light Bulb Tester

I think there's a more thorough description, technical explanation and schematic of the Light Bulb Tester ("light bulb limiter")in one of the recent threads if someone with more google-fu can find it.

Last edited by AudioLapDance; 17th May 2013 at 03:09 PM.
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Old 17th May 2013, 03:15 PM   #20
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Quote:
Originally Posted by AudioLapDance View Post


Before Andrew freaks out:

Light Bulb Tester

I think there's a more thorough description, technical explanation and schematic of the Light Bulb Tester ("light bulb limiter")in one of the recent threads if someone with more google-fu can find it.
When properly used, it would have saved your speakers.
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