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Old 21st December 2011, 07:36 PM   #21
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Nikon,
You could use a regulator on it.
Typically when using in SE, the (-) input is tied to ground. I have a jumper pin that I slip into the XLR to do that. You can also make a XLR to RCA conversion cord that does it for you. IIRC you tie 1&3 to ground.

As far as reduced voltage maybe Mr. Pass himself would answer that for you in the Pass forum. It has been many years since I built that circuit, but I do recal that he had a reason for the voltage that he ran at.
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Old 21st December 2011, 07:55 PM   #22
NicMac is offline NicMac  Italy
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Ciao Nikon,
did you have a look at the RelaiXed?
I'm using this pre most of the time and it has most of the features you are looking for. There is no SE output.
Cheers,
Nic
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Old 3rd January 2012, 02:13 PM   #23
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Funny thing, holidays is always a busy time. Should be the opposite...anyway.

I have two second end Jensen transformers on their way, just to try.

I also simulate the suggested Pass circuit with LTSpice. It looks like I could have decent results with 30 V supply, using IRF510.

Any suggestion for alternative mosfet to try in the simulation ?

One thing is still not clear: If used like SE to balanced converter, the circuit produces two outputs that are not 100% symmetrical. The negative has higher amplitude than the positive. This is kind of expected, as the circuit inverts. Does this matter in real life ?

Thanks,

Davide
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Old 3rd January 2012, 02:20 PM   #24
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Nic,

I checked the design you suggested, but I cannot see anything really attractive, at first look, is a opamp pre. But I am not an expert: what am I missing ?

D.
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Old 3rd January 2012, 02:34 PM   #25
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by Nikon1975 View Post
One thing is still not clear: If used like SE to balanced converter, the circuit produces two outputs that are not 100% symmetrical. The negative has higher amplitude than the positive. This is kind of expected, as the circuit inverts. Does this matter in real life ?
First, the asymmetry is a result of the use of resistors instead of current sources and the limited transconductance of the mosfets.

Does it matter? If it does not matter to your actual pre-amplifier circuit, then you did not need a SE -> Balanced converter to start with.

Ciao T
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Old 4th January 2012, 03:42 AM   #26
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Thorsten,

Thanks for the tip. I tried to replace the lower resistor with a current sink, the asymmetry goes away, but the distortion increase, not sure why.

D.
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Old 5th January 2012, 07:49 PM   #27
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Quote:
Originally Posted by Nikon1975 View Post
Dear All,

My electronic knowledge does not allow me to design cutting edge circuits, but I understand enough to combine and adapt other people designs to my needs.

So, the situation is the following:
1) I have some SE and some Balanced sources.
2) I have some SE and some Balanced amps.

I'd like to have the possibility to connect them all, switch between them without sacrifice in term of quality. I have various options:

a) install both SE and balanced output to all the sources, use two different pre amps, one for the balanced amps and one for the SE amps.

b) assemble a preamp that can combine both, but here I am not sure it is possible without double conversions, that I want to avoid.

Of course I can take shortcuts, and use only one phase of the balanced, but I feel like it is a waste.

I am sure I am not the first one to have this problem, and I see the high-end commercial amps (even HT) have these features, but this does not mean it is implemented in an optimum way.

Any hint ?

Thanks,

Davide
You can use line input and line output audio transformers for this purpose. You only need 2 of each for stereo.
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Old 6th January 2012, 12:58 AM   #28
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So, I have two of these:

http://www.jensen-transformers.com/datashts/11p1bn.pdf

As I understand, I can use them to convert a balanced signal to feed an SE amp, but I am not sure it will have good performances the other way around, as there is an RC network built in.

Am I correct ?

D.
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Old 6th January 2012, 01:18 AM   #29
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Quote:
Originally Posted by Nikon1975 View Post
So, I have two of these:

http://www.jensen-transformers.com/datashts/11p1bn.pdf

As I understand, I can use them to convert a balanced signal to feed an SE amp, but I am not sure it will have good performances the other way around, as there is an RC network built in.

Am I correct ?

D.
Ah, you didn't have to buy those! But yeah, there is an RC network built in but you don't ever have to reverse this transformer since it's 1:1 ratio. You just have to make sure there is a 15kohm load or greater on the secondary side, which is the one with the zobel.

You can use this transformer to feed either a single ended OR a balanced signal to a single ended OR a balanced amplifier. It is a high impedance input transformer. You need a different kind of transformer for the output side. You need a low impedance 1:1 transformer for the output (usually 600 ohms).
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Old 6th January 2012, 01:30 AM   #30
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I found them second hand, so they were cheaper.

I don't really see the need of the output transformer, if the source has already low output impedance.

D.
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