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Old 17th August 2012, 09:51 AM   #1
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Default Balanced preamp connexion ?

Hi everyone.
I am finishing an amplifier which now is expecting a preamp and I have there two options:
* balanced, in case of differential ouptut
* unbalanced in case of input signal from Gnd to +V.

My amp has a double PSU with +Vcc, Gnd, -Vcc.
my preamp has only +Vcc , Gnd.
Here is my question: if connected together, my preamp feeds the amp with a signal from gnd to V, only positive. In that case, the neg supply never is used and the out signal is also from 0 to +V.

Does this mean that I should make a preamp with also a double PSU, so it outputs from -V to +V, to get the full swing ?
In this case, since my input signals to the preamp are also from gnd to +V, how should I connect the source to the preamp ? I have only +signal and Gnd, no -signal.

Thanks
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Old 17th August 2012, 10:12 AM   #2
DF96 is offline DF96  England
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A coupling capacitor may solve your dilemma.
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Old 17th August 2012, 12:29 PM   #3
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Hmm, not so sure, that is what I use currently but the signal appears still as from Gnd to +V, it is not symetrical versus Gnd.
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Old 17th August 2012, 12:30 PM   #4
DF96 is offline DF96  England
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Add a ground leak resistor to the other end of the cap.
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Old 17th August 2012, 02:04 PM   #5
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There is one, which also defines the input impedance.
But can you explain: am I wrong somewhere, does a sym PSU amp need a sym PSU preamp or not ? if no, I'll find a solution out of all suggestions I 'll get.
Thanks for all.
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Old 17th August 2012, 05:58 PM   #6
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it is important to clarify the issue. if you are talking about a preamp within the box of the amp, that is, a subcircuit fed from the same supply, then it is a matter of internal design and level shifting might be needed in case of asymmetrical supplies. However, if you are talking about different equipment then it is completely irrelevant what the particulars of each are. at any rate, never feed the DC of one equipment to the input of another. always use capacitors or transformers. hope it helps.
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Old 17th August 2012, 06:42 PM   #7
AndrewT is offline AndrewT  Scotland
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If there is only AC signal at the output of the pre-amp, then that is exactly what the input of the receiver is expecting.
Check for no DC offset at the pre-amp output. Then you can couple the two bits of equipment together.
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Old 18th August 2012, 02:11 AM   #8
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Or you can DC couple if the the power amp's input range is large enough to permit it and the amp's feedback loop has sufficient CMRR to maintain acceptable DC offset on its output.

IMO there's really no reason to build a preamp with unbalanced outputs, though. My default is to a differential output DAC to an LME49724 using dual supplies---the trafos I use have two windings, so I might as well use the other one to get a Vee rail. Susumu's 0.5% RR series resistors are pretty cheap so one gets at least 46dB CMRR in the DAC output buffer, implying around 13mV max common mode on the DAC outputs. Do the same again in the power amp and the DC offset into the speaker is on the order of 60uV, maybe less if the amp is DC servoed.

Last edited by twest820; 18th August 2012 at 02:25 AM.
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Old 18th August 2012, 08:10 AM   #9
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Ok thanks.
the preamp has AC signal with no continuous V in it. the modulation goes from 0 volts to +6 volts.
If this goes through a capacitor then it comes out the same. the signal s still not centered aroung Gnd.
I would like it to swing between say -3V to + 3 Volts, around Gnd.
In all this I am not speaking of a continuous Voltage on the output. Only about trying to modulate symmetrically between a + and a - centered around Gnd.
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Old 18th August 2012, 09:03 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Billy60300 View Post
,,,,,,the preamp has AC signal with no continuous V in it. the modulation goes from 0 volts to +6 volts.........
this is physically impossible.
AC = Alternating Current.
That means the current alternates by flowing in opposite directions in alternate halves of the waveform.

If you did have a waveform that varied from 0V to +6V then that means there is never any reverse flow current. That would be a DC current = Direct Current. The current, when flowing, all flows in the same direction. The fact that the driving force varies (from 0V to +6V) just means you have a varying DC signal.
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Last edited by AndrewT; 18th August 2012 at 09:06 AM.
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