[bg4533]

I am currently building a high end Gilmore Dynamic using a Dynahi PSU that would also function as a preamp. My source is unbalanced and I was planning on the amp being unbalanced. This will probably be my last big project for a while so I want to be happy with it.

The PSU has plenty of headroom to handle the extra amp and there is enough room in the case. I already have an extra Dynalo board and all the parts to build it.

AMBs post gave me the idea of doing this. Are there any disadvantages to running balanced output like this? I am not concerned with added cost since I already have the parts except the XLR stuff. I currently use HD650s and was planning on building a nice cable anyway.

It looks like only 2 extra resistors need to be added or am I missing something? Are the resistor numbers AMB mentioned still correct for the group buy Dynalo board? I want the preamp section to be unbalanced. Can I just take the output from the noninverting side and ground and just ignore the inverting side of the amp? I already have a nice 3pdt toggle I planned on using and already drilled a hole for. I won't be able to switch everything with it. Would it be a problem to just switch ground and noninverting? The inverting channel would always be connected to the headphones. I am mainly concerned with headphone performance, I can sacrafice a bit on preamp performance.

I have never used XLRs before and want only 1 plug. Would a 5 pin XLR for headphone out be a good way to go? I would like to be able to build an adaptor to plug in headphones with a shared ground. Does the ring of the XLR connect to anything? (if so I would only need 4 pins)

Thanks and sorry for the long post.

 

[Garbz]

Well yes, build it balanced. Since i've balanced my main system i've started to do it to my headphone amp too.

However I recomend a different method. Rather then create a balanced amp to run form a single ended cable using the second diagram, why not build it fully balanced with XLR's on the input, and the RCAs on the input should connect to 2xDRV134 or similar balanced line driver. They work perfectly and the improvement in sound far outweighs the additional active part in the signal path.
The reason I sugest this is with a 4pdt switch it will allow you to select RCA / XLR in, retain a fully balanced signal regardless of which is used, and in future should you buy a balanced source like a fine Marrantz CD player you will have a truly balanced headphone amp.

When building the HD650 cable remember that the quality of the cable means a LOT less in a balanced system. But overkill is still the way to go

Not quite sure what you are trying to accomplish with the switch but yes if you want an unbalanced signal simply float the -ve and connect as per normal. Although you will loose 6dB if you do it this way so take that into consideration when setting the gain level to make full use of your pot. I had to actively decrease the level of my system via resistors after balancing it because the output of my cdplayer + balancing meant that I only ever used about 1/4 of my pot swing.

Why would you need a 5 pin XLR? There's only 4 connections. +/-ve L/R. Ground is not connected to the headphones and the cable doesn't need to be shielded as any noise picked up on the cable will be cancelled off when the signal hits the headphone.

Remember you need a 4 channel pot as well. Don't make the mistake I did.

 

[bg4533]

I thought about the DRV134, but it seems like it would be inferior to the method AMB laid out. Can anyone comment on this?

I will be graduating college in 6 months and hopefully moving away not long after. I won't have the money for a suitable source upgrade for a year probably. By that time I won't mind converting the amp to full balanced and I might want a new amp anyway.

The switch is to switch between headphone out and preamp out. The preamp will be connected to my poweramp. Unbalanced I was planning a gain of 3.2. Balanced I would probably go with 4. The gain of 2 will probably be better for the preamp and 4 perfect for the HD650.

As for the 5 pin XLR I was hoping to output the 4 channels as well as ground. That way I can build an XLR to 1/4" converter. I would likely balance any headphones I regularly use, but it would be nice to be able to test other headphones.

Are you sure about the 4 channel pot? The pot would be before the first amp. Since the inverting amp takes input from the noninverting it seems 2 channels is all I need. If the 4 channel pot is required I probably will skip this as too much will need to change.

 

[dip16amp]

There will be a slight time delay going through the positive amp and then another time delay as that is feed through the negative amp which will result in a phase shift between the two outputs which could be noticable.

The shield connection on the XLR connectors can not be used as a reliable signal ground as it does not use gold or silver contacts that the signal pins have. I use the gold contact 5 pin XLR Neutrik connectors with pin 1 as ground per the AES pinout standard for two balanced channels in one XLR connector.

 

[bg4533]

Unfortunately AMBs diagram doesn't seem to be correct for the group buy Rev C board.

Going from AMBs diagram to the positions of the new board is this correct:
R22=R16
R55=R46
R23=R7
R57=R37

 

[bg4533]

Quote:

Originally Posted by dip16amp There will be a slight time delay going through the positive amp and then another time delay as that is feed through the negative amp which will result in a phase shift between the two outputs which could be noticable.

I didn't think of that. So both this method and the DRV134 will have some issues. The question is which is worse. I tend to think the passive route would do less damage, but that is a guess at best

And if I solder the inverting output directly to the headphone out and switch the noninverting I am introducing another small time delay. Maybe they cancel out

 

[amb]

Quote:

Originally Posted by bg4533 I thought about the DRV134, but it seems like it would be inferior to the method AMB laid out. Can anyone comment on this?

Each method has its drawbacks. The method I posted is cheap and simple (requires only two additional resistors), but it has the problem of some phase shift in the negative half as dip16amp mentioned (whether that shift is significant in the audio band depends on the amp, which I think should be minimal in the dynalo). The DRV134 method has less phase shift, but does insert more active circuitry (of dubious sound quality) into the signal path of both halves. Furthermore, since the dynalo pcb doesn't contain space for a DRV134 you'd have to either make another pcb/perfboard or do it p2p, creating more clutter.

Of course the best would be to have fully balanced inputs so this is not an issue at all.

 

[Garbz]

The phase problem is the reason that I pick the DRV route. Basically the chip seems to perform like the best of them. Given the choice between unbalanced and having a DRV134 in the signal path i've actually given up using the input select on my amp and instead reach behind to plug different sources into my line driver to retain the balancing. In my cases the benefits of a balanced signal outweigh the disadvantages.

If you go amb's method there's no easy way to switch the amp to have XLR inputs, using the DRV does give that option.

However if you do that you will need 2 pots or a 4 channel pot if you intend to use the XLRs. The reason being that the putting the pot before the DRV will not have any effect if you decided to use the XLR inputs on your amp. Anyway i'm talking crap. This assumes that you even want balanced connectors on your amp instead of just having a single ended input converted to balanced. I just think the DRV gives more options, and also since phase is everything in balanced connections I will argue that it's superior, but without an A/B test my opinion is worthless.

Regarding the output you could simply use 2 headphone sockets. Each would be the best option for each perticular device.

And yes if you want to switch to a pre-out you can connect the two +signals and ground. Infact you could connect the two -ve signals and gound. Just don't connect out of phase. And in the interest of future upgrades put a balanced output on the back. Don't scrunge the $3 in aditional parts only to find out that down the line you build a balanced amp and forget that your preamp has only got single ended pre-outs.

Meanwhile I'll sit here and wait for rick to jump in and throw a transformer part number to use instead of the DRV ...

 

[bg4533]

Thanks for the help.

I am going to go with AMBs method. If it sounds better than unbalanced I will stick with it. If it doesn't I will try the DRV134.

The Par metals case I have is already fully drilled for an unbalanced amp. I was planning on finishing it tomorrow when the last few parts arrived. I didn't leave enough room to drill bigger holes for XLRs anywhere except the headphone out. I doubt there would be enough room for a quad volume control either. If I ever want to go fully balanced I will need a new case.

The will probably set me back a few weeks now, but I will post pics and impressions when I finish.

 

[bg4533]

Apparently I don't have room for an XLR output unless I get a new faceplace. I do have room for 2 1/4" jacks side by side. This might be even better. I can wire 1 jack fully inverted and the other regular. This way I can plug in 2 regular headphones or 1 pair of balanced.

 

[Edwood]

Quote:

Originally Posted by bg4533 Apparently I don't have room for an XLR output unless I get a new faceplace. I do have room for 2 1/4" jacks side by side. This might be even better. I can wire 1 jack fully inverted and the other regular. This way I can plug in 2 regular headphones or 1 pair of balanced.

One jack with the inverted signals from both left and right? Going to be a weird setup, I guess you can make adapter cables to have other balanced headphones work with it.

So you couldn't get a single 4pin XLR jack to fit?

-Ed

 

[bg4533]

The XLR would be a tight fit and I would have to work around a hole that is already there. The toggles I bought ended up being huge and take up a ton of space behind the face plate.

I am not very concerned with balanced standards as the K1000 is the only headphone I know that comes with a 4 pin connector. I plan on DIYing any balanced headphone cables.

 

[Garbz]

I assume you are trying to get one of these inthere:

Have you considered using the mini connectors:

And i'm guessing these may fit even better, i'll use these:

And regardless which way you choose the balanced should give an improvment over the standard.

 

[bg4533]

Is XLR a better connection than 1/4"?

1/4" actually seems more convenient since I won't need an adaptor to plug in regular headphones.

 

[gsferrari]

XLR connections are better IMO. More sound structurally and the contact points are better...better strain relief.