Iso-transformer vs Amp with balanced inputs

[camshash]

Hey guys,

I have a hafler 9500 amp with unbalanced inputs, which I have integrated in my system that is using all balanced connections. I am getting a considerable amount of hum through the hafler fed speakers that I have attempted to remedy.

Interface-> Passive monitor controller -> Dyna audio (balanced)
-> Hafler 9500 (unbalanced)

In one of my last threads I learned about isolation transformers and also how to wire a a balanced connection feeding an unbalanced input (combining cold and ground wire on the unbalanced side).
After doing this the hum is not eliminated, rather it's no longer a consistent level. It changes with changes made to volume knob on passive monitor controller.

To maintain the best sound quality, am I better off selling power amp for another with balanced inputs or using an iso-transformer?

Bonus question, hafler 9505's on front panel state "True-Balanced JFET input". What do they mean by "True-Balanced"?

 

[ccaudle]

Dyna audio (balanced)

What is a "Dyna audio?"

Hafler 9500 (unbalanced)

A Hafler 9500 has XLR inputs, why are you using the unbalanced input?

Interface-> Passive monitor controller

Does the audio interface have balanced outputs?

 

[camshash]

Dynaudio active monitors with balanced inputs, and the hafler 9505 has balanced input, 9500 is unbalanced only. Audio interface outputs are balanced

 

[scott2000]

Are you parralel jumping off of the Dynaudio amp to the Hafler? What is your interface?
How are your cables wired from the balanced to unbalanced. Any disconnecting of screen at either end?

 

[ccaudle]

Dynaudio active monitors

OK, so just a typo.
How are you using the Hafler amp, since the Dynaudio are active?

hafler 9505 has balanced input, 9500 is unbalanced only

My mistake, I did not notice that the question about the 9505 front panel was not related to the original question.

Input transformer would probably be the easiest way to add a balanced input to the Hafler. Good quality input transformers aren't inexpensive, it would probably be cheaper to build a small balanced input circuit as a front end to the amplifier. Those Haflers run the input stages from bipolar 24V supplies, so you could tap into that to power your input circuit.

I needed something similar several years ago, so I used a 2134 as unity gain buffers, driving an INA137 for the balanced-to-unbalanced function. The only problem is that INA137 is +/-18V maximum supplies, so you would need to also build up some voltage regulators to drop the +/- 24V supplies in the amp down to about +/-16V to power your input circuit.

 

[camshash]

Sorry my diagram got moved around when posted. Im using a UAD apollo going into baby r.a.m controller, which then goes to both hafler amp and dynaudio monitors. Screen is disconnected on the unbalanced input going into hafler.

 

[scott2000]

Yeah, I had an unbalanced amp for a while and, chasing down ageing caps, changing set up and making sure there were no problems with different output topologies sneaking in while using passive controllers just got old. Would work great then not..etc...

A transformer input amp took care of everything but I'm sure a balanced input would have been just as nice all things considered.

My son now deals with the random noises with the unbalanced amp while gaming...lol

 

[ccaudle]

After doing this the hum is not eliminated, rather it's no longer a consistent level. It changes with changes made to volume knob on passive monitor controller

Forgot to comment on this originally, but a hum that changes with the volume knob indicate that you have induced hum into the audio interface, rather than the amplifier.

Screen is disconnected on the unbalanced input going into hafler

So the hot connection (tip of the TRS) connects to the RCA center pin, and the cold connection (ring of TRS) connects to the RCA shell, and the shield is connected at the sleeve connection of the TRS only, no connection to the Hafler?

 

[camshash]

and the shield is connected at the sleeve connection of the TRS only, no connection to the Hafler?

Correct

 

[scott2000]

Have you tried a TS to rca cable to the Hafler?

 

[abbey road d enfer]

It's never a good idea piling up balanced and unbalanced gear. Just one unbalanced connection ruins the CMRR of all the rest.
Those Hafler amps managed to get some following in studio monitoring, but the unbalanced types should be avoided.
Regarding what's best, transformer or active circuit, transformers are less transparent than electronic debalancing circuits; OTOH they provide excellent CMRR, but they may also deteriorate frequency response, particularly with so-called "passive controllers".

 

[abbey road d enfer]

Bonus question, hafler 9505's on front panel state "True-Balanced JFET input". What do they mean by "True-Balanced"?

Everything is there:

• P9505 data sheet - Hafler​

https://hafler.com › pdf › archive › 9505_datasheet

 

[abbey road d enfer]

Have you tried a TS to rca cable to the Hafler?

A simple cable will never solve a CMRR issue.

 

[scott2000]

A simple cable will never solve a CMRR issue.

Makes sense. Was curious about if using a ts on the output of the Apollo was recommended for feeding unbalanced. Should the ring and sleeve be shorted on the Apollo outputs if using a trs to rca or does it not matter ?

 

[PermO]

Don't short ring to sleeve on outputs, only on inputs.

 

[ccaudle]

Don't short ring to sleeve on outputs, only on inputs.

Depends on what type of output you have. There are four types of output topologies, the older style cross-coupled output circuitry can sometimes have problems if you connect the cold side to ground at the far end instead of right at the output. And non-coupled actively driven outputs should not have the cold side shorted to ground at all. You really have to know what type of output the equipment has, there isn't an easy single answer that covers every situation.

 

[PermO]

I apply this as my basic rule, and if I don't get sound... it's a transformer, so I do need to connect it to ground.

I case of active electronics, shorting an output to ground is never a solution to anything, so best leave it floating, or am I wrong here ?

 

[ccaudle]

I[n] case of active electronics, shorting an output to ground is never a solution to anything

Some output topologies are made to handle that. This is an interesting paper to read:
ThatCorp copy of ATI output circuit paper

 

[camshash]

My son now deals with the random noises with the unbalanced amp while gaming...lol

Funny thing is my father gave me this amp as well

 

[Snatchman]

I had a similar situation with my Hafler P-3000 amp years ago. I was using a mixer that had unbalanced outputs and I stuck an Ebtech Line Level Shifter between the mixer's output ( unbalanced) to the Ebtech's trs input, then trs out to the Hafler's xlr input..Works great...!!!