RIAA with balanced output?

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roger-k said:
RIAA with balanced output?
I want to build a RIAA-preamp with balanced output,
based on OP-amps.

Do you people know where I can find some schematics
or datasheets with any exemplifyes?

I have find many but not with balanced output.


That's because it's not something that's ever really required?.

If you REALLY want to do it (but please tell us why?), then simply use an RIAA preamp followed by a balanced driver - everything you need is here.

You could also use an active DI box, the Behringer DI20 has two channels, which could be used for this!.
 
roger-k said:
Why? Because my preamp has balanced input.

As 'Pjotr' has already said, you can feed an unbalanced signal into your balanced input - it simply requires one of the inputs connecting to signal ground. I would expect your preamp instructions already explain how to do this?. If your input uses stereo jacks (rather than XLR's, or as well as?), then inserting a mono jack plug automatically makes it un-balanced.

There's likely to be no advantage in using it balanced?, the low impedance ouput of the RIAA stage, and the high signal level, render it pointless - unless you have an EXTREMELY long connection betwen the two items?.
 
Correct me if I am wrong.....

A Phono cartridge is a balanced/differential source. There may be benefit in a completely differential RIAA stage from input to output plugged into a true balanced line input on a differential pre amp to differentail power amp?

I believe Jeff Rowland has or had one in their line up.
 
I agree with all posters here. Keep it balanced to the preamp. An MC cart is inherently balanced. IF you are going to use transformers for balancing, better spend the money on good or better step-up transformers. At least that's how I see it.

The cables following your phono preamp are the most crucial cables in your system. Keep them short, under one meter.
 

rjm

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Just for fun then:

I favor an unbalanced phono stage.

First, noise generated by or induced on the MC cartridge or phono cables does not dominate the noise seen on the line output of the phono stage. The noise on the output is rather generated by the phono amplifier circuitry itself. Or it is if you are doing things right, at any rate.

So its more important to have a low noise amplifier than an amplifier with rejects common mode noise at its inputs.

With a balanced circuit, you have two input stages, each one seeing half the input signal voltage that the input of an unbalanced stage would see. The gain of each input stage is the same of course. So, all things being equal you end up with twice the noise at the output. Which being random sums to 1.4 times the noise, but thats still a 3 dB hit on your noise floor.

I dont have an issue with balanced outputs though. Since a good way to set up your system is to have the amplifier next to the speakers and your turntable as far away from them as possible, a long interconnect is often needed. Given the added complexity this adds to the circuit - the joy of phase splitters - its not something I'd implement lightly.

/rjm
 
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rjm said:
With a balanced circuit, you have two input stages, each one seeing half the input signal voltage that the input of an unbalanced stage would see. The gain of each input stage is the same of course. So, all things being equal you end up with twice the noise at the output. Which being random sums to 1.4 times the noise, but thats still a 3 dB hit on your noise floor.

Absolutely true, unless you use a transformer before a non-differential input, in which case:

(1). A step-up transformer allows you to increase the voltage applied to your (noise generating) input stage, thus improving S/N ratio. This statement might not necessarily be true for a bipolar transistor input where current noise usually predominates.

(2). You enjoy the unparalleled common-mode rejection offered by a properly designed input transformer.

A transformer-coupled MC input really does make sense...
 
Onvinyl said:

Are you sure you don't mean 'the cables preceeding your phono amp'?
Rüdiger

If you use step-up transformers, yes. Hagerman writes this:

"Keep in mind here, that at 10:1 gain, every foot of output cable is equivalent to 100 feet of input cable."

But the cables from the turntable aren't that important, though more important than many others.
 
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phn said:
"Keep in mind here, that at 10:1 gain, every foot of output cable is equivalent to 100 feet of input cable."

I'm not sure what he's thinking of there, although it's clearly to do with impedances. However, it is important that the output leads of the transformer are kept as short as possible. Put the transformers in the pre-amplifier, not in a pretty box outside with extra leads that add capacitive loading to the secondary.
 
Hi EC8010,

I think he meant that a 1:10 voltage ratio means a 1:100 impedance ratio. I.e. if you load the tranny with 10Kohm, you will load the cartridge with 100 ohms. So the load at the secondary should be at high impedance where you need to avoid too much capacitance.

This is a bit of disadvantage of a step-up with some MC’s when they are more pleased with a high load impedance (> 1K).

Cheers ;)
 
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