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Chip Amps Amplifiers based on integrated circuits

LM3886 opinions sought
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Old 1st February 2018, 06:57 AM   #21
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by john blackburn View Post
Anybody got any thoughts on regulated supplies?
Quote:
Originally Posted by scottjoplin View Post
Unnecessary complexity and expense
Quote:
Originally Posted by tomchr View Post
2x25VAC is a bit much for a single LM3886 as you'll get about 35-36 V DC out of it. I explain my reasoning here: LM3886 Output Power. .........Then you get the bragging rights to having a regulated supply... Tom
Firstly, on Tom's site, he also mentions that 35-36v DC is perfectly fine as long as you are driving 8Ω.
Secondly, I would think a regulated supply, such as this one:
High current dual rail regulator kit for power amplifier or bench power supply ! | eBay
would definitely improve performance. If you disagree, please say why.
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Old 1st February 2018, 01:45 PM   #22
bullittstang is offline bullittstang  United States
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Here is the one I ordered - mainly because of the DC cap and coil, but I have no specific testing that would indicate it is any better than the one you ordered.

HiFi LM3886TF Mono 68W 4Ω Audio Power Amplifier Board AMP 50W/38W 8Ω Q5B5 | eBay
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Old 2nd February 2018, 12:45 AM   #23
john blackburn is online now john blackburn  United Kingdom
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Hi Andrew, am I understanding this correctly? I take the power ground from the decoupling caps to the speaker return terminal along with the other power ground connections (zobel etc) and join them there. The connections to signal ground are made at the chip and follow the same path back to the power supply.

This brings me to my next question which is where would be the best place to position the zobel and output inductors? Are they better close to the chip or out at the speaker terminals?

This is the diagram I am looking at, Tom gets a name check in his writings.

A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics

PS The speakers this will be used with are CSS FR125s which appear to be around 6 ohms at best, so I now realise 35 volts would have been pushing it.
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Old 2nd February 2018, 12:56 AM   #24
scottjoplin is offline scottjoplin  Wales
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That sounds about right, I think this would be a good link to add to your signature http://hifisonix.com/wordpress/wp-co...tober-2017.pdf
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Old 2nd February 2018, 01:03 AM   #25
dotneck335 is offline dotneck335  United States
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This fellow ----http://www.decdun.me.uk/----lost me with this statement:
"I'm no scientist or engineer but I can detect differences in interconnects, even as short as nine inches."
I'd be willing to bet that in a true double-blind test that he (or anyone else) can't hear diddle-squat difference between any two reasonable-quality 9" cables.
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Old 2nd February 2018, 07:48 AM   #26
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by john blackburn View Post
.............I take the power ground from the decoupling caps to the speaker return terminal along with the other power ground connections (zobel etc) and join them there.
Yes, where the Power Ground meets the speaker Return becomes the Main Audio Ground.
This is the reference voltage for other parts of the amplifier.
Quote:
The connections to signal ground are made at the chip and follow the same path back to the power supply.
I don't understand what you are describing. Have you read Leach's Lo Tim articles? He uses a low impedance connection on the amp PCB to make the reference voltage connection between Signal Return and Speaker Return. That low impedance connection MUST be SHORT and he inserts 10r. I understand from that, that his low impedance means low inductance. I read Lo Tim about 20years before reading D.Joffe's paper on HBRR/HBRL and realised they were addressing the same problem.
Quote:

This brings me to my next question which is where would be the best place to position the zobel and output inductors? Are they better close to the chip or out at the speaker terminals?
As I understand it the amplifier Output Zobel works by making an ultra low impedance connection using a loading resistor Between the output node of the output devices and the lowest impedance at the collectors/drains of the output devices. I have seen few comments on this conjecture and the very few that have commented have not given any explanation of why this could be wrong.
If the Output Zobel is indeed an ultra low impedance route from output node to device collectors/drains, then the Zobel must be buried inside the output stage, not at some more remote location.

The output (damped) inductor does not change the performance of the amplifier. In effect it is not part of the amplifier and thus it does not matter where it is placed. However, the air-core inductor can influence other electrical components and is affected by other ferromagnetic materials. I believe the best place for this output inductor is in the cable between the amplifier PCB and the amplifier output terminals. The damped inductor in this slightly remote location still has the effect of adding the resistor in series with any parasitic capacitances and thus reducing the load phase angle presented back to the amplifier. The downside to this is the possible effect of the cable capacitance in the twisted pair from amp PCB to remote inductor. I have NEVER seen a Designer comment on this.
Quote:

This is the diagram I am looking at, Tom gets a name check in his writings.

A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics

PS The speakers this will be used with are CSS FR125s which appear to be around 6 ohms at best, so I now realise 35 volts would have been pushing it.
I agree, 35Vdc might be a bit much for 6ohms speakers.
I have stated repeatedly that the National chipamps are current crippled. Other manufacturers are worse. 7Apk is not enough to drive an 8ohms reactive speaker. 6ohms and 4ohms will be even more demanding.

Tomchr has done an admirable job in getting his composite to work as well as it does. But the performance is down to the enormous gain of the composite and that increases the available feedback to hide the distortions of the chipamp.
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Last edited by AndrewT; 2nd February 2018 at 07:59 AM.
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Old 2nd February 2018, 08:06 AM   #27
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by john blackburn View Post
A very quick scan to see that this appears to be a re-write of Tomchr's articles, but this item jumped out at me
Quote:
The Zobel network is used to prevent oscillations caused by inductive loads. It also prevents radio frequencies picked up by the speaker wires from getting back into the amplifier’s inverting input via the feedback loop.
Is any of that true?

If one were to believe E.Cherry, then his conclusions are completely wrong.
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Old 2nd February 2018, 12:12 PM   #28
Puffin is offline Puffin  United Kingdom
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Quote:
Originally Posted by john blackburn View Post
A bit of an update.

Further biscuit tin rummaging turned up four LM338s that I bought years ago to try a regulated chipamp using CarlosFMs design. I didn't get round to it at the time so I think I will try those at 28 volts to start with, and maybe try discrete regulators eventually.

What are your collective thoughts on the best way and place to create a signal ground when point to pointing? I can't do it on the RCAs due to the distance.

I have used a one ohm resistor from the power ground as a signal ground point on a previous build which seemed to work nicely, so that is probably the plan unless I've missed something.

John
Hi John, I built 2 LM3875 Monoblocks some years ago using the schematic on Dec Dun and the regulated PS. As I only had 30vac tranny's I used those, making the dc output on the limit of the 3875's handling. I have never had any problem with them into a nominal 8Ohms.
Rob.
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Old 2nd February 2018, 07:48 PM   #29
john blackburn is online now john blackburn  United Kingdom
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Quote:
I don't understand what you are describing
Having re read it myself I'm not surprised. What can I say, it was late! All my questions are answered and construction is underway

I overlooked the clearly marked signal ground on the diagram when I scribbled it out on an envelope for the pencil layout stage
. Sorry for any confusion caused.

Also thanks for the grounding links I am working my way through it slowly.
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Old 2nd February 2018, 11:44 PM   #30
Mark Whitney is online now Mark Whitney  Netherlands
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Another good source of information that should help you with layout design.

http://www.updatemydynaco.com/docume...lemsRev1p4.pdf
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