[sceleratus]

I just ordered the Mogami Neglex 2549 balanced cables.
They look great.  Thanks
 
Edit.
I think I'll wait to make any changes until the cables arrive.  Then I'll have my ducks in order and implement possible modifications.
 
 

 

[sceleratus]

What about using Copper Braid Jackets internally? 

 

[sceleratus]

Quote:

OK, looked at your amp picture a few pages back, you do have the input transformer, so the wiring should work (to the amp board).  I'd need to see how you wired the pot, and any additional resistors you added to handle 4-channels with a 2-channel pot.  It's doable, but it's not as common.
 
Some resistors in line, configuration B below.  Ideally, the pot would come after the input transformers for the amp.  You could point to point wire it, if it's easy to remove the transformer, or cut the trace and feed it in and out from the solder pads.
 

 
 
This is a good exercise  for me.  It makes me try to reconcile the board traces to the schematic and a diagram like yours.  Which I am only partially able to do.  It's hard to follow the top traces and the bottom traces and match them to the schematic.  I probably need the board in my hand to do that.  I found a photo I took of the driver board before the IPT's were installed.   The traces for C3A/B and C8A/B are visible.  Both are DNP on the schematic.  Obviously since they are under the IPT's
 
I revisited the DACT site and looked at the CT2 and the CT2 Balanced.  The CT2 is configured exactly like the Alps.  I fave no Idea how to integrate the balance model.
 
EDIT:  Why remove the IPT''s?
 

 

 

[holland]

Leave the input transformers alone for now.  You can feed into it without much of a problem.
 
Remove the pot entirely, and wire the XLR directly to the amp board.  Control the volume with your laptop to make sure there's no problem.  There should be none.
 
I've never wired a pot this way, just read about it.  All the circuits I've seen have an input transformer before the pot, probably because wiring it the way indicated in configuration B changes the load resistance seen to the source.  Anyway, try it without, put 2 47.5K resistors in line, and wire the pot as indicated in the diag.
 
Here's another diagram from the monolith balanced amp, this one is without the input transformer in place.
 

 
Here's the same amp with the input transformer.
 

 

[holland]

Quote:

 
 
I fave no Idea how to integrate the balance model.

 
It is configuration A.  Balanced signals have 4 incoming signals, related to ground.  Single ended (RCA) has 2 incoming signals, related to ground.  RCA has no negative signal, only ground.  In the balanced 4-gang pot, when you make one click, it reduces the incoming signal on + and - for left and right channels the same amount.  You could replicate that by using 2 stereo volume controls.  One for left, and one for right channel.  For the left channel, as an example, you would use each gang in the pot for the + and the - signal, with the wiper being a voltage divider to ground for each gang.  That is configuration A.
 
The amp you have is not differential, so wiring in an XLR is somewhat pointless.  The input transformer converts the signal to single ended for the amp topology.  The gungnir does the same in it's output stage.  DACs are differential in nature.
 
If you put the pot after the input transformer, then you would have familiar wirings, dealing only with 2 signal channels, Left and Right (again there is no negative only "positive" signals which is the combined signal).
 
Configuration B (or the schematics I posted from the Monolith amp) controls balanced signals with a stereo pot.  It alters the impedance seen by the source, however.  It is no longer a static 10K, but will vary by wiper position.  The inline resistors limit it to something manageable.  This is, in effect, what is known as the shunt mod (if you didn't have a negative signal).
 
I hope this helps.  It's a bit confusing.
 
Morgan Jones has this in his books too.  I can see if I can dig out the text and photocopy it, but it might be a few days or weeks before I get to it.

 

[sceleratus]

Thanks Holland,
I have more to digest.
 
I have the Morgan Jones book so you don't have to go through that exercise.
 
I am not going to do anything until mid week when the cables come in from eBay.... prob later I think they are in MN.
Truth be told with all my swapping and grinding output jacks the only think I was left with for the XLR inputs was a set of Neutrik Male ports.
So the homemade XLR cables are the nonexistent F/F.   Thus I ordered a set of Neutrik Female jacks.
 
I will look through all the input you have provided and see if I can understand it.   I did make some progress with the schematic.
 
I thought about bypassing the volume pot entirely and using the source software control, but I got sad that the focal point of the enclosure, the volume knob, would be useless.
Form over function.
 
Thanks again,  I will do some studying.

 

[tomchr]

Yeah, work's been kicking my bee-hind lately. Such is life in the tech industry, though.
 
I bet you've created a ground loop, hence, get some 60 Hz hum. You can't 'mod' your way out of this. Fix the ground loop.
 
The DG300B Amp was designed as a power amp, hence, the assumption was made that any volume control was handled in a preamp of sorts. With a single ended input (think RCA), it's trivial to add a volume control. With differential inputs (XLR), it becomes more tricky. You can design a differential volume control (see below) but now you need a 4-gang pot with audio/logarithmic taper for a stereo volume control. These are available.
 
 

If you want a differential input, but don't want to spend on a 4-gang pot, your best option (aside from building a differential preamp) is to insert a volume control at the junction between R7 and R8. Like this:
 
 

My favorite XLR interconnects are the ones by GLS Audio. I love the multicolored cables. It makes hookup much easier when you're dealing with a fully differential, biamped system with active crossovers... I don't bother making my own interconnects. It's less expensive to let someone else do it. But then, I'm also not a believer of megabuck interconnect cables either. 
 
Re. DIY Valve Amp Beginner Thread: One of the first posts (if not the first post) needs to be on high-voltage safety.
 
~Tom

 

[sceleratus]

Quote:

 
It is configuration A.  Balanced signals have 4 incoming signals, related to ground.  Single ended (RCA) has 2 incoming signals, related to ground.  RCA has no negative signal, only ground.  In the balanced 4-gang pot, when you make one click, it reduces the incoming signal on + and - for left and right channels the same amount.  You could replicate that by using 2 stereo volume controls.  One for left, and one for right channel.  For the left channel, as an example, you would use each gang in the pot for the + and the - signal, with the wiper being a voltage divider to ground for each gang.  That is configuration A.
 
The amp you have is not differential, so wiring in an XLR is somewhat pointless.  The input transformer converts the signal to single ended for the amp topology.  The gungnir does the same in it's output stage.  DACs are differential in nature.
 
If you put the pot after the input transformer, then you would have familiar wirings, dealing only with 2 signal channels, Left and Right (again there is no negative only "positive" signals which is the combined signal).
 
Configuration B (or the schematics I posted from the Monolith amp) controls balanced signals with a stereo pot.  It alters the impedance seen by the source, however.  It is no longer a static 10K, but will vary by wiper position.  The inline resistors limit it to something manageable.  This is, in effect, what is known as the shunt mod (if you didn't have a negative signal).
 
I hope this helps.  It's a bit confusing.
 
Morgan Jones has this in his books too.  I can see if I can dig out the text and photocopy it, but it might be a few days or weeks before I get to it.

Getting pieces of this.
 
Differential?
The Gungnir is differential.
If I am connected prior to the input trans, I'm not differential.  As you said, it's pointless.  Is that because you need differential to differential..... bear in mind, I need to understand "differential".
Would it become differential if the pot was inserted after the input transformers?
 
If not I should control the volume from the source and abandon the volume POT.  This scenario is almost noiseless, snow or hum.  If I do this should I revert to "RCA"
Remember the schematic calls out XLR input wiring as the first choice.

 

[sceleratus]

Here's a DACT audio 4 gang pot.
I think it's the CT-2 Balanced.
If it is the best solution and fixes the problem I will probably go in that direction.  I'm at the 2 yard line.
 
I'm going to do more testing but it's very interesting that when the 10K Alps POT is pegged wide open and the source is nearly off it sounds magnificent.
If I pause the track I'm sure it's 80% quieter....
 
I will test again and report.

 

[sceleratus]

I have read about ground loops before while trying to help others.
Cutting the ground pin is NOT attractive.
 
 
I have a Furman AC215-A
 
I thought that would eliminate any potential ground loop.

 

[sceleratus]

This is a cool ground loop document from Jensen transformers published on MIT.edu
http://web.mit.edu/~jhawk/tmp/p/EST016_Ground_Loops_handout.pdf

 

[sceleratus]

Hum issue
The DG 300B and The Gungnir are plugged into a Furman AC-215A Conditioner with ground contamination circuitry.
 
No hum was present when the input cables/interface was RCA.  That's a red flag that I did something wrong.
 
Since the design utilizes Jensen input transformers how can ground loop hum "jump" this?
 
Yet there is noise associated with mains power.
 
I performed the following tests.  
All tests had HP's plugged in.
1 = minimum setting on volume pot
10 =max setting on volume pot
 
1)  Nothing connected to the amp inputs:  1 = loud snow 10 = less snow  I believe the snow was less with RCA's but I don't think the volume pot had any effect.
 
2) Plug the homemade XLR cables into the amp only. 1 = loud snow  10 = some snow slight hum
 
3) Plug XLR's into both amp and Gungnir (powered off)  1= Low snow  10 = same amount
 
4) Same as #3 except the Gungnir is powered on. 1= Med. Hum plus snow 10 = Very small hum / snow
 
5) If the Gungnir is plugged into a different outlet on the same circuit the Hum is very loud at 1.  Perhaps because the amp is in the Furman and the Gungnir is straight in an outlet.
 
Thinking outloud:
 
Again.  Wouldn't the IPT's isolate ground noise?
Again. I didn't have this problem with RCA's.  Perhaps I flipped input terminal pins 1 & 3 at the driver board.  The were jumped with RCA.
 
I know the cables are OK because I ohm'd out the pins.
 
I can't ignore #5 that must be a ground loop, however in practice I am using the same outlet. #4  The loop occurs if gear is on different outlets one of which has a contaminated ground.  Even so if the ground is contaminated by voltage it can cause problems.
 
I won't know more until I open the amp up next week.

 

[tomchr]

The document by Jensen Transformers that you linked to earlier is actually a pretty good description of what's going on. Differential signaling is also described here: http://en.wikipedia.org/wiki/Differential_signalling
 
Your XLR cable should connect pin 1 to pin 1, 2 to 2, and 3 to 3 in the XLR connectors. Pin 1 may be connected by the shield of the cable. Commonly, this is done with a mic cable that has two conductors plus a braid or foil shield. Pins 2 and 3 use the two conductors. Pin 1 use the shield. There should be no connections between pin 1 and 2,3 anywhere. 
 
Pins 1, 2, and 3 of the XLR connector should go to the appropriate pins on connectors J3 and J7 of the driver board. If you want volume control, you either need to implement a differential volume control and insert it between the XLR connector and the driver board - or you can use the hack I provided and put it between R7, R8.
 
There should be no hum if the XLR is connected directly to the driver board. 
 
If you accidentally flipped the connections between pins 1 and 3 on the input connectors, you'd get hum for sure. Use your ohmmeter to ring it out.
 
~Tom

 

[sceleratus]

Quote:

The document by Jensen Transformers that you linked to earlier is actually a pretty good description of what's going on. Differential signaling is also described here: http://en.wikipedia.org/wiki/Differential_signalling
 
Your XLR cable should connect pin 1 to pin 1, 2 to 2, and 3 to 3 in the XLR connectors. Pin 1 may be connected by the shield of the cable. Commonly, this is done with a mic cable that has two conductors plus a braid or foil shield. Pins 2 and 3 use the two conductors. Pin 1 use the shield. There should be no connections between pin 1 and 2,3 anywhere. 
 
Pins 1, 2, and 3 of the XLR connector should go to the appropriate pins on connectors J3 and J7 of the driver board. If you want volume control, you either need to implement a differential volume control and insert it between the XLR connector and the driver board - or you can use the hack I provided and put it between R7, R8.
 
There should be no hum if the XLR is connected directly to the driver board. 
 
If you accidentally flipped the connections between pins 1 and 3 on the input connectors, you'd get hum for sure. Use your ohmmeter to ring it out.
 
~Tom

Thanks Tom,
 
When I open it u I'll see if I can fit a stepped attenuator in.  I can do it by hook or crook using a flexible shaft.
 
I like the Goldpoint Mini-V out of Sunnyvale CA.
If I choose a 2 deck I have to go back to RCA, correct?
 
A 4 deckk would give me what I wanted to have.
 
I've read that some users don't like the detent clicks.  I don't think it would bother me and I really like the concept.  It's much more precise with [size=small]0.1%[/size][size=small] tolerance, 25ppm, Thin Film, 0805 [/size][size=small]SMD[/size] vs a 5% pot.
 
 
Still the ground loop has me baffled.

 

[tomchr]

Quote:

If I choose a 2 deck I have to go back to RCA, correct?
 
A 4 deckk would give me what I wanted to have.
 
I've read that some users don't like the detent clicks.  I don't think it would bother me and I really like the concept.  It's much more precise with [size=small]0.1%[/size][size=small] tolerance, 25ppm, Thin Film, 0805 [/size][size=small]SMD[/size] vs a 5% pot.

 
An attenuator is basically a potentiometer with steps instead of linear travel. The number of sections needed for a volume control doesn't change just because it's labeled "attenuator" rather than "potentiometer". Hence, for a differential volume control, you'd need an attenuator with four decks.
 
I used to use an attenuator, but it drove me nuts. If you look at most attenuators, you'll see that to get a reasonable range on a 24-position rotary switch, the first couple of steps are *HUGE*. Like 10 dB or so. In my case, I ended up with a setup where step 3 was so low volume that I could barely hear the music, and step 4 was so loud I couldn't concentrate. 
 
In my preamp, I use a PGA2320. -95.5 dB to +31.5 dB in 0.5 dB steps. You won't find that in a rotary attenuator.
 
The absolute tolerance of the potentiometer may be 5 %, but really it's the tracking between channels that matter. This will improve with an attenuator. But if it isn't bothering you now, it clearly isn't an issue for you.
 
An attenuator will not fix the hum. Fix the ground loop before you start thinking about more modifications.
 
You had an amp that had no hum. Then you modified it and got an amp with hum. Something in the conversion went wrong and caused the hum. Trace it down and fix it. Then you can start talking about modifications.
 
~Tom