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acl0056  
#1 Posted : Tuesday, November 16, 2010 7:04:23 PM(UTC)
acl0056

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Is there any way I can use a DACT balanced stepped attenuator with the Legato board as a sort of passive "pre-amp", to control both the level of a balanced output and the level of the SE output. It says the balanced and the SE can be used at the same time, but I want to make sure I can control the volume of both of them at the same time. The idea is that the balanced would go to main left and right, and the SE would go to a sub.

Here is the DACT website:

http://www.dact.com/html/attenuators.html

Any help is appreciated.


thanks,

Adam
Dingo  
#2 Posted : Tuesday, November 16, 2010 7:13:24 PM(UTC)
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Having a pot(stepped pot, etc.) on the output of the dac is a bad move, as you'll be driving your cables from a high impedance source.

I would suggest using some sort of level control with the sub amp, not at the source.
Brian Donegan  
#3 Posted : Tuesday, November 16, 2010 7:55:05 PM(UTC)
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You would be much better off using a Volumite to control the output of the DAC.
acl0056  
#4 Posted : Tuesday, November 16, 2010 8:00:55 PM(UTC)
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But can the attenuator go between the digital board and the line stage board?
Russ White  
#5 Posted : Tuesday, November 16, 2010 8:54:30 PM(UTC)
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Much better than that, it works purely in the digital realm.
acl0056  
#6 Posted : Tuesday, November 16, 2010 9:51:26 PM(UTC)
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I'm inquiring for my mastering studio. I can already control the volume digitally from the computer. But, I believe that every DAC that I've ever plugged directly into my amp causes a loud popping noise when I switch sampling rates. It is basically the sound of my amp trying to send the speaker coils across the room, and it is quite a concern. That's why I've always used a passive, buffered "preamp" in my signal chain, to attenuate the analogue signal. But, it is unbalanced. I would like to build a DAC with a balanced output that I could run, safely, straight into my amp, and control the volume of it and the sub, which is on the separate SE output, simultaneously.
Russ White  
#7 Posted : Tuesday, November 16, 2010 10:07:53 PM(UTC)
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You won't have any trouble switching sample rates with the Buffalo II. I do it regularly, and I have mine connected directly to my amps. I would still recommend the volumite. At least give it a shot. I think you will save yourself a lot of hassle.

Edited by user Tuesday, November 16, 2010 10:10:09 PM(UTC)  | Reason: Not specified

acl0056  
#8 Posted : Tuesday, November 16, 2010 11:25:43 PM(UTC)
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Well, actually I am looking to use this with a different DAC board. It is a piece of gear that hooks up via firewire and has great conversion, but it has a transformer I/V stage that colors the output, making it less suitable for mastering. So I'm hoping someone can answer my specific question. Can I use a stepped attenuator to control the volume of the output for both the balanced and the SE of the Legato?

This brings up another question. Are there no DIY firewire DACs?
Russ White  
#9 Posted : Wednesday, November 17, 2010 4:55:28 AM(UTC)
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I would not try that. Yet it may indeed be possible. If you look at the schematic you should be able to work it out.

You may need to attenuate the balanced and SE signals separately.

What chip does your DAC use? It may not be a good match for the Legato in the first place.

Edited by user Wednesday, November 17, 2010 6:43:41 AM(UTC)  | Reason: Not specified

acl0056  
#10 Posted : Wednesday, November 17, 2010 6:53:58 AM(UTC)
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It's a Texas Instrument PCM1794.

So what if it went from the Legato's balanced outputs to the stepped attenuator, and then of some kind of buffering circuit, and then a separate Ballsie Lite board to run the sub?

I can't seem to find the output impedance of the I/V stage.
Russ White  
#11 Posted : Wednesday, November 17, 2010 7:11:10 AM(UTC)
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Ok that DAC should work fine with Legato but you will want to change the resistor values for the CCS and the I/V.

The output impedance will depend on if you use the buffer or not. If you use the buffer its around 1ohm. If you don't it will be just a bit less than the value of the I/V resistor.

You could cut a couple of traces and put the DACT between the balanced output and the BAL/SE conversion part of the legato. That should work fine especially if you use the buffers.

Just remember that the input impedance of the BAL/SE stage is pretty low. 4.7K

Edited by user Wednesday, November 17, 2010 7:12:45 AM(UTC)  | Reason: Not specified

acl0056  
#12 Posted : Wednesday, November 17, 2010 2:45:07 PM(UTC)
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Great! But, to clarify...

Are you saying that, everything aside, in order to use the DAC with the I/V stage, I have to change resistor values?

Or, are you saying that if I change the resistor values, I may not need to use buffers?

Thanks!
Russ White  
#13 Posted : Wednesday, November 17, 2010 4:24:31 PM(UTC)
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I think you are likely going to need the buffers for your hypothetical setup.

You will need to also change the CCS and I/V resistors because of the lower output from the PCM1794.

I would use something like 352R for I/V and 750R for the CCS.

You will want version 1 of the legato if you want to inject the DACT between the balanced and SE stages because your likely to need the buffer.

I don't have one to try, I am only giving you hypothetical advice. You will need to check the schematic yourself to determine if it will work.

You would get *much* better performance out of the BufII + Legato2 and Volumite.

Edited by user Wednesday, November 17, 2010 4:25:17 PM(UTC)  | Reason: Not specified

acl0056  
#14 Posted : Wednesday, November 17, 2010 9:55:21 PM(UTC)
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I may do that in the future. But, in the meantime, how do I order version 1 of the Legato kit (and does that version work with LCBPS kit)? And just so I don't mess anything up, could you give me the resistor numbers for those as they are on the schematic?

Thanks for the help!

Edited by user Wednesday, November 17, 2010 9:59:20 PM(UTC)  | Reason: Not specified

acl0056  
#15 Posted : Saturday, November 20, 2010 8:39:06 AM(UTC)
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Hi, again. So, should I just place my order and specify version 1 in an email?
acl0056  
#16 Posted : Monday, February 28, 2011 11:00:47 PM(UTC)
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Thanks again for all of the help. I sort of changed the plan and simply wish to use the balanced output to go to another unit.

I took my best guess at which resistors to change, but I'm not sure I got it right. I can't get the trimmers to bring the input to 0 V. I think I messed something up. I would really appreciate knowing exactly which resistors to change.

Thanks!

Edited by user Wednesday, March 2, 2011 9:07:31 AM(UTC)  | Reason: Not specified

acl0056  
#17 Posted : Tuesday, March 1, 2011 10:26:15 PM(UTC)
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Finally found this in another thread, after figuring out that the COD uses the same chip.

Russ White wrote:
It will work quite well with COD. You will want to change R1-4 to 357R(or around that) and R20,21,23,24 to around 850R(or close to that).

Also when adjusting the pot for the input bias, set the input bias to 0V(or close to it) and not AVCC/2.

Cheers!
Russ


To whomever it may concern, this advice is obsolete as of version 2 of legato. R20,21,23,24 are now R9-16 and since they are parallel consider the 681Ω value on the Legato2 schematic to be 340.5Ω. So to be clear, this needs to be raised, not lowered. R1-4 are still the same in Legato2.

Edited by user Wednesday, March 2, 2011 7:25:47 AM(UTC)  | Reason: Not specified

rondr  
#18 Posted : Saturday, February 4, 2012 1:02:36 PM(UTC)
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Hi,

My first post here as I am recently discovering this site. I did a search here for "passive attenuator" and this post seems to have some information useful to me. I,m not sure I understand everything discussed in this thread but I do understand some of it.

I'm also interested in adding a buffer to the output of my 48 step balanced passive attenuator preamplifier. It's value is 5k and is connected to the input stage of my power amplifier using short cables. The input side of the attenuator is being fed from the balanced outputs on my DAC. Due to remodeling and having to relocate my equipment, power amplifier, I will have to place the power amplifier roughly 20 or so feet from the attenuator and this creates a problem. So my thoughts are to put a buffer, using short balanced cables, in line between the attenuator and buffer and then on to the power amplifier using the longer run of cables. Would The IVY III Balanced Linestage or The Legato 3.1 Discrete Balanced I/V Stage work well for this application with little or no alterations to their original design?

Thanks,
Ron
Erlend Sæterdal  
#19 Posted : Saturday, February 4, 2012 3:19:30 PM(UTC)
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Russ White wrote:
I think you are likely going to need the buffers for your hypothetical setup.

You will need to also change the CCS and I/V resistors because of the lower output from the PCM1794.

I would use something like 352R for I/V and 750R for the CCS.

You will want version 1 of the legato if you want to inject the DACT between the balanced and SE stages because your likely to need the buffer.

I don't have one to try, I am only giving you hypothetical advice. You will need to check the schematic yourself to determine if it will work.

You would get *much* better performance out of the BufII + Legato2 and Volumite.


I think my EZ dac with PCM 1794 works very well with 360/750 ohm.

Edited by user Saturday, February 4, 2012 3:25:28 PM(UTC)  | Reason: Not specified

acl0056  
#20 Posted : Tuesday, February 28, 2012 7:16:23 AM(UTC)
acl0056

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rondr wrote:
Hi,

My first post here as I am recently discovering this site. I did a search here for "passive attenuator" and this post seems to have some information useful to me. I,m not sure I understand everything discussed in this thread but I do understand some of it.

I'm also interested in adding a buffer to the output of my 48 step balanced passive attenuator preamplifier. It's value is 5k and is connected to the input stage of my power amplifier using short cables. The input side of the attenuator is being fed from the balanced outputs on my DAC. Due to remodeling and having to relocate my equipment, power amplifier, I will have to place the power amplifier roughly 20 or so feet from the attenuator and this creates a problem. So my thoughts are to put a buffer, using short balanced cables, in line between the attenuator and buffer and then on to the power amplifier using the longer run of cables. Would The IVY III Balanced Linestage or The Legato 3.1 Discrete Balanced I/V Stage work well for this application with little or no alterations to their original design?

Thanks,
Ron


Having looked into this a great deal since starting this thread, the term "balanced passive attenuator" doesn't say enough about its configuration. Some attenuation networks maintain constant impedance at their input and output, such as a T-pad. Whatever the configuration, your power amp is likely to buffer the balanced inputs to maintain a high common mode rejection ratio. Otherwise, it at least has some network that will even out the impedance of the hot and cold inputs. There could even be an exception to that, if your amp uses a symmetrical balanced bridged configuration, and doesn't actually have a differential amplification stage (very unlikely). Since your are running a balanced signal, you probably have nothing to worry about.
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