Adding a balanced in to my amp?

robobass · #1 01-07-2008, 04:19 AM

I recently upgraded to the Schertler Pre-IIa. It's got only balanced XLR out. That's a bit snobbish isn't it? I use an old, very beat Hartke head, which serves me well. I use an adapter at the amp input, but that's a bit of a hazard. I have no qualms about drilling another hole in the face and adding an XLR input jack, but I want to know what would be the best way to mate it to the input electronics. My adapter came from Radio Shmuck, so I figure reverse engineering it would provide less than the best possible result. I obviously know nothing about audio electronics, but I'm actually very good at building things from schematics!
Robobass

fdeck · #2 01-07-2008, 07:35 AM

What's the hazard? For such a short cable run, you could just use a XLR to TRS adapter cable (no transformer) and let it run unbalanced.

drurb · #3 01-07-2008, 08:00 AM

I'm with fdeck here. Consider that, even if you drilled a hole and added an XLR connector, you'd have to wire it in a fashion that duplicates what the adapter is doing. This is because the electronics are not actually configured for balanced (differential) amplification.

**Francois Blais** · #4 01-07-2008, 09:23 AM

You can also get an adapter with a transformer for cheap.

Edit: I own an old Whirlwind IMP1 since about 25 years.
see http://www.whirlwindusa.com/dirbox.html for many models.

MR PC · #5 01-07-2008, 12:48 PM

Quote:

Originally Posted by fdeck

What's the hazard? For such a short cable run, you could just use a XLR to TRS adapter cable (no transformer) and let it run unbalanced.

This will work well. I've used this kind of cable to mate direct boxes with piezo friendly electronics to the input of amps that only have 1/4" jacks. No need to drill holes in your amp, just get the XLR to 1/4" TRS cable and you should be in business.

robobass · #6 01-07-2008, 03:05 PM

Well, I tried a plain XLR to 1/4" cable. It didn't give nearly the gain I got with the adapter. Is there something special about this "TRS" cable? or is that the brand? I don't want the adapter sticking out of the amp, as I said. I can't imagine wiring up a transformer inside the amp would be a big deal.
Robobass

robgrow · #7 01-07-2008, 05:46 PM

Quote:

Originally Posted by robobass

Well, I tried a plain XLR to 1/4" cable. It didn't give nearly the gain I got with the adapter. Is there something special about this "TRS" cable? or is that the brand? I don't want the adapter sticking out of the amp, as I said. I can't imagine wiring up a transformer inside the amp would be a big deal.
Robobass

Most mic input transformers have somewhere around a 1:3 to 1:10 turns ratio, which means they have a gain of about 10 to 20 dB. If you go to Jensen Transformers web site: http://www.jensen-transformers.com/apps_sc.html There's lots of information about transformer balanced mic inputs, including how to add one to existing equipment. (They can also explain why you want to use transformer balancing.) If you are going to go to the trouble of adding a xformer balanced input to your amp, do yourself a favor and use a high quality transformer. Jensen is one of the very best, but there are others, such as Cinemag in the US, Sowter in the UK and Lundahl in Sweden.

SB100 · #8 01-07-2008, 08:15 PM

Quote:

Originally Posted by robobass

Well, I tried a plain XLR to 1/4" cable. It didn't give nearly the gain I got with the adapter. Is there something special about this "TRS" cable? or is that the brand? I don't want the adapter sticking out of the amp, as I said. I can't imagine wiring up a transformer inside the amp would be a big deal.
Robobass

It's not a big deal, as robgrow rightly said, lash out on a good quality LoZ balanced to HiZ unbalanced transformer. Make sure the transformer is µm (mu metal) shielded at has 20-20Khz bandwidth and low distortion figures (not all have this).

You can also add 2 extra switches if you want, one for ground lift (SPST) and a DPDT for phase reversal of the hot and cold balanced lines (not really needed but can be useful in special situations).

fdeck · #9 01-07-2008, 08:54 PM

Is the output of the Schertler itself transformer-coupled?

MR PC · #10 01-07-2008, 10:41 PM

Quote:

Originally Posted by robobass

Well, I tried a plain XLR to 1/4" cable. It didn't give nearly the gain I got with the adapter. Is there something special about this "TRS" cable? or is that the brand? I don't want the adapter sticking out of the amp, as I said. I can't imagine wiring up a transformer inside the amp would be a big deal.
Robobass

If you are satisfied with the sound of the mic transformer adapter but don't like the thing sticking out of the front of your rig.....

Well......there are those inline mic transformers with female 1/4" output jacks. That means XLR to transformer to 1/4" cord to amp input. Bingo! You still don't need to punch a hole in your amp chassis. And you WILL need a special chassis punch tool to do that job RIGHT......

robobass · #11 01-09-2008, 02:43 AM

Quote:

Originally Posted by robobass

Is there something special about this "TRS" cable? or is that the brand?
Robobass

Ah! Tip Ring Sleeve! Now I understand. I could make up an XLR to TRS cable and have a balanced input on the amp without drilling a larger chassis hole.
Thanks,
Robobass

drurb · #12 01-09-2008, 08:47 AM

Quote:

Originally Posted by robobass

Ah! Tip Ring Sleeve! Now I understand. I could make up an XLR to TRS cable and have a balanced input on the amp without drilling a larger chassis hole.
Thanks,
Robobass

Just to be clear, you will NOT end up with a balanced input on the amp. You will have converted a balanced configuration to unbalanced and will lose the noise-rejection superiority of the former which, in the case you describe, is probably unnecessary anyway.

Monte · #13 01-09-2008, 01:51 PM

Quote:

Originally Posted by robobass

I recently upgraded to the Schertler Pre-IIa. It's got only balanced XLR out. That's a bit snobbish isn't it? I use an old, very beat Hartke head, which serves me well. I use an adapter at the amp input, but that's a bit of a hazard. I have no qualms about drilling another hole in the face and adding an XLR input jack, but I want to know what would be the best way to mate it to the input electronics. My adapter came from Radio Shmuck, so I figure reverse engineering it would provide less than the best possible result. I obviously know nothing about audio electronics, but I'm actually very good at building things from schematics!
Robobass

It isn't really snobbish, per se. For better or worse (Definitely more expensive!!) Schertler has always touted that their stuff was made to work best with their stuff. The Dyn transducers are XLR in, the Stat is 1/4" so they put in a combo in. Their transducers are advertised as best through a full range system such as a PA or a powered speaker, not a bass amp, which is very true. Thus the need for balanced XLR out.

If you are not using Schertler pickups, the extra expense of the Pre A does not make much sense. I have the opposite problem every time I think I have found a cool preamp or something and it is only 1/4" inputs.