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Old 11th November 2013, 10:58 AM   #2951
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Hello,
i'm a first-time diyer and was hoping someone could help me to understand if what i want to do will work:

I'm planning to build LM4780 kit in bridged/balanced, dual mono confuguration and use it with Buffalo IIIse DAC + Legato output stage. From what i understand - Legato uses variable resistors to trim and zero out DC offset (no caps), but common mode DC remains (at balanced outputs). If I understand correctly - as long as there's no ground for it - common-mode DC is not a problem, or am I missing something?
I'm hoping to build a system with no decoupling caps (or bal>se opamps) in the path

Also, is DC offset for both halves of LM4780 is close enough that i should get very little DC in bridged mode, or it varies heavily?
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Old 13th November 2013, 11:10 AM   #2952
AndrewT is offline AndrewT  Scotland
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If the two phases of the balanced input have identical DC offset then the (perfect) balanced to unbalanced receiver will ignore the DC offset and output a plain unbalanced signal without any offset.

That raises two questions.
a.) is the DC offset alwaysbalanced?
b.) how close to perfect is the bal to unbal receiver?

That can be measured at the output. Measure the output offset at the speaker terminals.
Does it vary?
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Old 13th November 2013, 10:30 PM   #2953
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I don't have it all yet - just planning ahead...

I guess in my case "balanced to unbalanced receiver" is one half of LM4780 chip (3886 to be exact). So if I feed each 3886 with balanced signal with identical DC offset it will be zeroed-out inside the amp and i'll only get amp's own offset in the end which needs to match (more or less) offset of it's out-of-phase 3886 brother, right? It should still be better than resulting DC of unbalanced LM3875 amp for example...
Can I completely zero-out DC at dac's output? Manual says i can (except for common-mode DC).
I know Twisted Pear own LM4780 amp works with Legato I/V stage, just wanted something simpler with no additional opamps in signal path...
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Old 14th November 2013, 08:23 AM   #2954
AndrewT is offline AndrewT  Scotland
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each half of the dual chip gets it's own signal phase plus the DC offset.
The output is an amplified version of the signal plus offset.
The two opposite halves have a signal and an inverted signal, these add to give a double level signal to the output device.
The two DC offsets are in phase and these cancel in the combined signal fed to the output device.
It's the balanced to unbalanced combination that amplifies the differential input signal and cancels the common mode input signal (which will include interference).
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Old 10th December 2013, 12:55 AM   #2955
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Default LM4870 runs hot idle

Hi guys,

just putting together an LM4870 amp in parallel mode using Audiosector pcb's.

Parts are same value as suggested by Peter. DC is 35V. It runs much too hot idle, each rail on each chip pulls 0.5A!

I've checked and double checked, but can't find the reason. Any help is welcome!

Thanks,
Abel
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Old 10th December 2013, 10:00 AM   #2956
sangram is online now sangram  India
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Try a bigger resistor in the output. Mine runs warm too, though idle is only slightly higher than normal.
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Old 10th December 2013, 10:59 AM   #2957
AndrewT is offline AndrewT  Scotland
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0.5A through each supply rail = dead chip.
Quote:
DC is 35V. It runs much too hot idle, each rail on each chip pulls 0.5A!
+-35Vdc @ 0.5+0.5Adc = 70W dissipation into the dual chip heatsink.
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Last edited by AndrewT; 10th December 2013 at 11:14 AM.
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Old 10th December 2013, 04:02 PM   #2958
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Thanks guys!

Ya, that's right, after two minutes I can start cooking on it! 70W for two physical chips.

I've connected an oscilloscope and there's a 20Vpp sinus oscillation at around 250kHz. If I shorten the input, or even just connect a piece of cable to it, it stops. And current goes back to around 110mA per rail per physical chip.

The input consists of 22k to ground followed by 220ohm to each opamp in the chip.

Seems like in a normal setup the oscillation would not occur. But if it would not be there to begin with, I would be happier. Any suggestions?
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Old 25th January 2014, 06:46 AM   #2959
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Question NOS DAC

Click the image to open in full size. If you don't stock board can I pay board price to you and use this layout at say PCBXpress to build? Just for me no commercial resale of any kind. If patent infringement I understand. Thanks!
Steve
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Old 17th June 2014, 10:10 PM   #2960
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Smile Bal or SE?

I finally found suitable enclosure and thinking to order one of the kits to introduce myself to world of "simple" gainclones as well as convinient way to test balanced vs single ended outputs of my dac. But which one?

I have Buffalo III dac with discrete Legato output stage and wanted to try shortest signal path possible using integrated (ES9018) volume control to output directly to chipamp.
LM3875 is easiest choice, but it means i'll have to use dac's bal>se stage which consist of opamp followed by a buffer (lme49600) and their neg.feedbacks which doesn't make it as short and straight as balanced outputs to say bridged LM4780 which are also coupling caps free (If common-mode DC is not a problem, can null differential DC). Or maybe i can use paired LM3875 in bridged configuration?

My speakers are 95db and 6 ohm minimum so should be easy load for both?
I'd appreciate opinions/suggestions
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