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Old 5th July 2009, 03:16 PM   #1
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Default Balanced & Unbalanced input with LM3886 chipamp.com

Hi,

I purchased a kit from chipamp.com and was wondering if there's any possible way to have balanced and unbalanced inputs for it.
I want to use it with my computer system but my soundcard (M-Audio Delta1010) has 1/4 inch balanced outs. I was wondering if there is a circuit that can change the balanced out to unbalanced for the amplifier.
I'm new to amp building, but can understand the basics. Really it's the third rail that I don't know to do with. Thanks for your help in advance.... JohnnyP.
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Old 5th July 2009, 03:54 PM   #2
Ipanema is offline Ipanema  Malaysia
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There's a few solutions for balanced to SE :

1) from the balanced output, just take the (+)ve and GND. Left out (-)ve.

2) Use DRV134 http://focus.ti.com/docs/prod/folders/print/ina134.html

3) Read Aleph30 service manual.

4) Design your own discrete diff to SE converter. Any LTP stage in typical two stage amplifier. Refer to Douglas Self book.

5) Transformer.
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Old 16th January 2010, 03:08 AM   #3
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Default Best voltage for INA134?

Just going to bump a thread if that's ok.
I'm building a P101 that will need balanced input.
I decided to order a generic power supply from Mouser to power these chips, than to derive something off of the main DC lines. I read the pdf for the INA134 and, because I'm fairly new to electronic circuit building, slightly misunderstood the recommended voltage range and assumed a 12V supply was somewhere in the middle of the range. When, in fact, it's at the low end. (+/-4 to +/-18.) (actually, what I more misunderstood was that power supplies are not spec'd +/- with respect to ground but + to -!)

TI "guarantees" the performance from 8V to 36V total supply but is there any reason to believe it was really optimized for the middle of that range and I should aim to be closer to something like 24V? The standalone supply is 10W so should have plenty of power for 2 chips and the other component it will power.
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Old 16th January 2010, 12:19 PM   #4
AndrewT is offline AndrewT  Scotland
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Ti tells you the chip works between 8V and 36V.
Believe them.

The advantage of using higher supply volts is that it allows more signal voltage to pass through without voltage clipping of the signal.
The disadvantage is more heat in the chip.
Most specifications tend to improve slightly with higher supply voltage.

Your 12V supply will limit the peak to peak signal to about 10Vpp into a high load impedance and quite a bit lower into a medium or low load impedance. The datasheet will tell you what the losses are into various loadings.
Expect your maximum signal to be about 8Vpp~=2.8Vac=2.8Vrms.

This is comfortably above the maximum of 2.2Vac that a CD player and most other sources puts out.
What is the sensitivity of the P101 in voltsAC for maximum power? I recommend that the preceeding stages pass a +10dB to +20dB signal without clipping.
If we take the +10dB signal as 2.8Vac then if the sensitivy of the P101 is <=0.9Vac for maximum power then your 12V supply is perfectly adequate.
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Last edited by AndrewT; 16th January 2010 at 12:22 PM.
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Old 17th January 2010, 01:02 AM   #5
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Thanks. As a "noob" I appreciate the detailed response.
Presumably P101 is designed for consumer level signals. I was aware of the possible issue in using pro-level signals into it. The INA134 is the non-attenuating version of the balanced line receiver. For that reason I was going to install a level trimmer on the back of the unit...only applicable to the XLR inputs...that would follow this design. Using, of course, a blue ALPS 50K pot.
I think I'm just going to schtick with the 12V and see how it sounds. Funnily I feel like I'm already violating the "rules of the game" by having now become aware of a supposedly better part (the THAT-brand balanced line receiver, which is quite pricey but in stock at Mouser) and not immediately upgrading to it! Seems to me buying a balanced line receiver should be like what Philip Greenspun said about buying a macro lens: nobody is going to make a really bad one because it's such a specialized, demanding market segment. Besides it sounds more like the THAT chip is better for strange demanding situations like having to run 200' of cable from a vintage microphone that might put slightly different impendances on the two wires - in my case there will be 2, 3' Mogami cables coming out of either a 1) Carvin XC3000 (now) or 2) Behringer DCX2496 (soon).
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Old 17th January 2010, 02:33 AM   #6
lgreen is offline lgreen  United States
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Here is a balanced chipamp.com lm3886.
(mine)

http://www.parttimeprojects.com/audi...onewebpage.php
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Retired from DIY (2010) but still lurking now and again. My DIY audio projects- PartTimeProjects.com.
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