Aleph-X builder's thread.

[still4given]

Sorry if this has been asked.

The input is set up for balanced line. I have some equipment with balanced outputs but most of my stuff is unbalanced. For testing I shorted the - input to ground and just used the shield terminal and + inputs. If you set up the amp to use both, do you need to short the - input to ground when using RCA's? Seems like I got a hum without doing that.

Do you use a switch to ground it when going with an unbalanced source?

Thanks, Terry

 

[wuffwaff]

Hi Terry,

yes -IN needs to be grounded (via the same value of input cap you use on the +IN side).
You can either use a switch or shortign plugs between PIN 1 and 3 on your XLR inputs.

William

 

[still4given]

Hi Terry,

yes -IN needs to be grounded (via the same value of input cap you use on the +IN side).
You can either use a switch or shortign plugs between PIN 1 and 3 on your XLR inputs.

William

Not sure what you mean by the input cap. Could you explain a little more?

Where would I find shorting RCA's?

Thanks, Terry

 

[wuffwaff]

Hi Terry,

to keep DC from entering the amp you need input capacitors in series with +IN and -IN. (around 10uF will do nicely).

To use the amp unbalanced you connect the -IN cap to ground.

You don´t need shorting RCA´s (wich are very easy to make yourself) but a shorting plug for the balanced input connectors or a switch that connects -IN to ground

William

 

[Netlist]

To avoid input caps I corrected the DC with the preamp connected. This of course only works well if you don’t plan to change preamps at a regular basis.

/Hugo

 

[still4given]

Hi Terry,

to keep DC from entering the amp you need input capacitors in series with +IN and -IN. (around 10uF will do nicely).

To use the amp unbalanced you connect the -IN cap to ground.

You don´t need shorting RCA´s (wich are very easy to make yourself) but a shorting plug for the balanced input connectors or a switch that connects -IN to ground

William

Do you mount those caps on the input jacks themselves?

Why would I use a shorting XLR? If I'm using a balanced input I wouldn't want to short the - input to ground would I? Or are you talking about a 1/4" jack that shorts when nothing is plugged in?

Thanks, Terry

 

[BobEllis]

The caps could mount directly to the jacks or anywhere convenient between the jacks and boards.

Only short the - input to ground when using unbalanced sources.

Depending on how you implimented it you have some choices:

An RCA to XLR adapter cable that shorts your - input

A switch to short the - inputs to ground when using the RCA jacks

An XLR plug that shorts the - input inserted into the jack when you use the RCA input.

 

[hieroglyphics]

Couldn't you also use some sort of inverting op amp between the + and - sides? By shorting the - side, aren't you only using 1/2 the amp?

 

[BobEllis]

Both halves of the amp get used through the "magic" of the differential, but the gain is lower with a single ended source.

 

[wuffwaff]

Hi Bob,

like you said both halves are used but the gain should be more or less the same balanced and unbalanced.

William

 

[protos]

If you use a rca to xlr adaptor which shorts the - signal to ground at the connector then the - input cap is short circuited since the signal and the dc will be routed to the gnd wire connection on the board.
So the -signal to ground connection should be after the input cap .

Ideally -signal----input cap----gnd---- -input pcb
however.... any gnd connection at the -input of the pcb will likely destabilise the dc balance carefully achieved by pulling one side to ground so destabilising the other side.The inputs are essentially floating with the dc balance of the x circuit.

 

[LineSource]

Sorry if this has been asked.

The input is set up for balanced line. I have some equipment with balanced outputs but most of my stuff is unbalanced.

Thanks, Terry

I've also been looking for a discrete circuit that can convert single-ended to differential with no DC offset. It seems that a servo'ed buffer will be necessary to avoid DC blocking caps since even a servo on the main amp cannot remove any DC sum between the + and - buffer outputs.


http://www.diyaudio.com/forums/showthread.php?s=&threadid=65896

DRV134 buffer Burr Brown is one possible solution. Blocking capacitors are still recommended in the app note.

 

[protos]

I think the easiest solution for the AX is a good balanced line transformer.
Might not be as cheap as a couple of chips but it is much simpler to implement and probably more transparent than any active solution.With a 1:2 ratio you could probably use a passive pre as well.
Otherwise just build a nice balanced tube or chip pre.For a simple but good tube one look at the Aikido which can output balanced as well
I have used the drv 134 on occasion and they are ok if you want the simplest pre/buffer.But... I would not add them to an existing pre.Too many stages for my liking.

 

[still4given]

If you use a rca to xlr adaptor which shorts the - signal to ground at the connector then the - input cap is short circuited since the signal and the dc will be routed to the gnd wire connection on the board.
So the -signal to ground connection should be after the input cap .

Ideally -signal----input cap----gnd---- -input pcb
however.... any gnd connection at the -input of the pcb will likely destabilise the dc balance carefully achieved by pulling one side to ground so destabilising the other side.The inputs are essentially floating with the dc balance of the x circuit.

Hi Protos,

I don't understand what you are saying. Are you saying that no matter what, the DC will be a problem when using unbalanced inputs?

I'm not sure where to ground the -input. Does it go to the star ground? I used a terminal block on the PCB for the input. For the test, I just ran a wire from the -input terminal to the one marked shield and then connected the shield on the shielded cable to the shield terminal as well, and the center wire to the +input terminal. It worked fine and was dead quiet. No hum at all. What is it that the "input cap" does and do I understand that there must be one between the -input and ground when using an unbalanced source? If these are necessary, why aren't they on the PCB?

Thanks, Terry

 

[wuffwaff]

STOP,

let´s try to keep it simple.

There´s absolutely no problem using the Aleph-X with an unbalanced source! No need for opamps, drivers, transformers etc.

1. Connect the +IN and -IN from the circuit board with the XLR connector using two caps in series (around 10uF will do)

2. Connect the ground from the circuit board to PIN 1 at the XLR connector

Now the amp is ready for Balanced input signals.

When you want to use a unbalanced input:

1. Connect a RCA female plug parallel to PIN 1 (ground) and PIN 2 (+IN) of the XLR connector. After the cap, directly at the XLR connector!!

2. Connect ground (PIN 1) and -IN (PIN 3) at the xlr connector. You can do this with a switch, a shorting plug or a RCA to XLR adapter.

3. Connect your source to the RCA plug .

Ready

William

You can omit the caps if your source is absolutely DC free and has no output resistors to ground (like a BOSOZ).
Or you can adjust the dc balance with the preamp connected as Hugo did.

 

[BobEllis]

That's what I thought I'd said, but much clearer. Thanks William.

 

[still4given]

Hi William,

Thanks, that is much clearer. The only think that is confusinign to me is where you say, "Connect a RCA female plug parallel to PIN 1 (ground) and PIN 2 (+IN) of the XLR connector. After the cap, directly at the XLR connector!!"

"After the cap, directly at the XLR connector"

If I make the connection after the cap then the cap is not in the circuit. Is this right?

When you say XLR connector, are you talking about the XLR connection on the PCB or on the XLR socket?

I guess what I'm asking is, do I connect the wires coming from the RCA jack to the pins on the XLR socket or tie them in past the input caps?

I will us a switch to ground the -input to ground.

Thanks, Terry

 

[tomchaoda]

XLR & RCA connection

A image is worth more than a thousand words...

XLR and RCA connection

Scroll down to the middle to see the connection illustration.

Adding the sw is to switch between UNBAL and BAL.[

 

[wuffwaff]

Terry,

connect the rca to the xlr socket, connect the switch to the xlr socket. Leave the caps in the citcuit all the time.

William (who is too lazy to make a drawing)

 

[still4given]

Terry,

connect the rca to the xlr socket, connect the switch to the xlr socket. Leave the caps in the citcuit all the time.

William (who is too lazy to make a drawing)

Perfect!

Hi Tom,

Thanks for the illustration. That is what I thought William meant but wasn't sure. You don't have the Input caps in yours though so without the explanation I still wouldn't have gotten it.

Thanks again,
Terry