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Old 1st May 2014, 03:54 PM   #1
iggy111 is offline iggy111  Croatia
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Default Bridged LM3886 + LME49720 + remote FB

Hi guys,
after reading about MyRef and MiniRef I stumbled upon this project, which should be similar to MiniRef I guess, but with bridged LM3886 and a remote feedback that is connected to the speaker terminals, thus taking cable impedance and inductance into consideration.

The guy that came up with this (Dragoljub Aleksijevic) wrote about all the advantages of a bridged approach as well as remote fb, but I presume that's all well known stuff, so no need to go into it.

My question is - is this similar to MiniRef design appart from the bridge and fb (I think so, but with different op-amps, chips and values I can't tell)? Also, do you think this approach could be an improvement?

scheme:
Click the image to open in full size.

and the pcb:
Click the image to open in full size.
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Old 1st May 2014, 07:59 PM   #2
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Quote:
Originally Posted by iggy111 View Post
I think so, but with different op-amps, chips and values I can't tell?
Take a look at the positive feedback paths, familiarize yourself with Howland charge pumps (if you're not already), and the topology differences from MyRef should be obvious. Which is better depends on the desired output impedance.
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Old 1st May 2014, 08:07 PM   #3
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Thanks for posting this schematic.
I have been looking for it again for some time and I couldn't find the thread where I had originally found it!!

jer
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Old 1st May 2014, 09:26 PM   #4
iggy111 is offline iggy111  Croatia
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The schematic was originally posted on a serbian forum I think, but here I found someone copied everything, and it's in english.

geraldfryjr, did you try to build such an amp or had a chance to hear it somewhere?
twest820 - thanks for the link, it clarified the schematic a bit, though I can't say I get it all.

After several versions of my LM3886 amp I'm very itchy to try this, for it combines three concepts that seem very beneficial - Howland pump, bridging, and remote feedback. Though I would definitely optimize the layout and decoupling further, regulate the front-end power supply, and probably go for separate ps board.
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Old 2nd May 2014, 02:35 AM   #5
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I have not built it yet but it is on my things to do list.
I want to employ the input stage to a paralled version with DC servos.

jer
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Old 2nd May 2014, 03:49 PM   #6
iggy111 is offline iggy111  Croatia
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What do you think of the power supply and the zener regulation? I think I would go for LM317/337 regulators and even make pads/pins on the pcb so they could be changed for something even better down the line.
The ps utilizes the benefits of the bridged configuration so there is only one big cap, and I would try that for the experience, but I would definitely go for a higher-quality ps like the concept danielwritesbac proposed for chipamps more than once. See here for example.
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Old 2nd May 2014, 07:43 PM   #7
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The Zener setup is fine, Personally I prefer the 3 terminal regulators as well.

I have on the drawing board a BPA200 that can be expanded to a BPA400 or larger possibly using the LM4780 for the larger versions.

I was looking for this schematic in order to finish the design.

Since I want to use a Quad opamps I am thinking about changing the design to a PA150 that can be bridged as well.
I will be employing a OPA1664 in it providing the Vos of each opamp stage is below 1mv or at least equal between the stages.

http://www.ti.com/lit/ds/symlink/opa1664.pdf

The Vos of the LME49720's I have are not what they should be, although they seem to be in spec with the data sheet.

On the same chip one is about 1mv and the other one is about 9mv!
I prefer them that be as close to the same as the other as they can be 1mv or less.

All three of my sample LME49720 ( maybe it was the LME49860's I forget at this time) devices shows this trait.
I may have to switch to a single opamp for the DC servos if I can't find any multiple devices that match up, I am hoping the OPA1664's fit the bill.

I like the bridged configuration as well as the transformers I have produce 49.5v which is too high for a bipolar supply configuration.

My requirements aren't for maximum power but to be able to drive the low impedance loads of my DIYesl designs well being able to maintain a 20v to 30v peak signal.
Typically no more the 20v peak under normal conditions as my desktop ESL produces +105db at that level!!

This thread is a must read in it comes to layout design,

LM3886 PCB vs Point-to-Point (with data)

I have been contemplating my design (up to 16 devices) since 2004 when the LM4780's First appeared.
But I never pursued it because of the mixed issues that some have reported about their chipamp designs through the years.

Even though many are using the exact same schematic many have had mixed results mostly due to layout design.

I still have the very same samples I obtained back in April of 2004 unused.
I do have plenty of both the LM3886's and Lm4780's to work with.

The thread I posted explains those issues in detail and I am now ready to start experimenting with the LM3886's and LM4780's now.

In fact the only thing that is holding me up is that I had misplaced my OPA1664's and I can't seem to find them right now !!

I have also had the thought of using some of the newer HV opamp's in order to not have to have the extra regulation circuits but I have not tested them yet and it may not be a cost effective solution anyhow.

jer

Last edited by geraldfryjr; 2nd May 2014 at 07:52 PM.
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Old 2nd May 2014, 09:45 PM   #8
iggy111 is offline iggy111  Croatia
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Yes, I follow that thread and I can witness that solving layout and decoupling issues takes the sound to a new level, as it did for my LM3886 amp.

My requirements are also not for the maximum power, but for best possible quality, as the chip is very dynamic as is, even on lower voltage, but with a good CRC supply.

Now this Macola guy (that's his nick) proved this amp to be extremely stable with capacitive loads, hooking up a 8uf capacitor while playing music, and the amp got warmer but continued to work flawlessly.

I can even find a photo of that, but I'm a bit tired right now. I could also translate what he wrote about bridging chips like in BPA amps and why he finds it's not as good as this approach.

I think I'll try to design another pcb for this amp, with regs and power supply on separate boards, so we can pay attention to layout and decoupling easier.
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Old 2nd May 2014, 10:21 PM   #9
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Yes, I do remember reading about that.
That is one of the main issues that I liked about the design.

And being of a composite design its THD performance would be that of the First opamp stage just like the My_Ref designs.

jer
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Old 3rd May 2014, 07:55 PM   #10
iggy111 is offline iggy111  Croatia
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Well now, I got a bit ahead of myself here (and I love it - there is a lengthy forum thread on another forum on this topic (in croatian though) so now I have to read all 79 pages to find out what's new

geraldfryjr, I'll write updates as I can, this has become way more interesting than I thought
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