How do you know if an amp can properly accept balanced inputs?

[shagone]

How do you know if an amp can properly accept balanced inputs?

I'm somewhat fammiliar with balanced signals in that the audio signal rides on both the center conductor and the sheild conductor of the signal cables. My BMW 3 series factory head unit has balanced outputs to the factory amp that i would like to send directly to an amp. i know that some amps are capable of accepting this signal directly because they advertise this and all that i would need to do is solder on an RCA connector and plug it in. on the other hand there are some amps that reportadley work just fine with the balanced signal and i had no idea that that was possible. how can i check to verify if a given amp can accept the balanced signal? is it just a matter of how much input voltage it can accept? is there a test i can do with a meter to determine the input setup?

on the E46Fanatics forum we started a list of amps that can accept balanced inputs. can anyone confirm or deny the amps on the list or add to the list below.

JL Audio (all)
Xtant (all)
Alpine PDX (all)
Zapco (most)
JBL (older amps)
SoundStream (most older amps)
McIntosh (all)
HiFonics (some)
Crossfire (some)
Autotek (some)

 

[Perry Babin]

If the RCA shields for the left and right channels are directly connnected (~0 ohms between them -- with no RCA signal cables plugged in), they are NOT fully balanced.

You need to be careful when driving a strong signal (speaker level or preamp level from a source with a ±15v supply) into the input of an amplifier. If the amp is off or shuts down for some reason and the signal drives the input of the front-end op-amps above the power supply input of the op-amp (which is essentially at 0v since the amp has shut down), the front-end op-amps can be damaged.

Some balanced inputs REQUIRE a dedicated ground connection between the signal source and the amp. Without it, the amp may oscillate which could cause damage to the amp or to the speakers. Always monitor the output of all channels of the amp with a scope when experimenting.

 

[vasyachkin]

i'm very skeptical about the whole balanced thing in car audio, i think its all one phat lie.

i have some "balanced" audiocontrol crossovers and it seems all they produce is alternator whine ... total ripoff

the conclusion i came to is that most stuff in car audio is simply garbage and the only way to know for sure is run it and see how bad it blows

i wonder if isolation crossovers would help in my case ?

 

[Perry Babin]

Quality varies widely in car audio. Some of the equipment is garbage and some is absolutely top notch.

Balanced inputs can work as well as (or better than) unbalanced inputs.

The Audio Control components often have multiple grounding schemes set by switches. If they weren't set properly, that could have been the source of the noise.

It's also possible that you had a defective component.

 

[vasyachkin]

i played around with those switches. i also reorganized the ground wires to make them shorter and thicker ... all in all that got the whine down by maybe 20 decibels but with a 150watt/channel amp driving 104db/watt tweeters directly and without padding (active system) the whine is still so loud that i can hear it even wearing my etymotic research earplugs

the only thing i haven't tried yet is isolation transformers. if that aint gonna work either ... its gona be bad

 

[AndrewT]

A good clue to whether it can receive a balanced signal will be an input socket for the hot + cold + chassis ground pins sent by the balanced output of the source.

Usually this is an XLR or TRS socket waiting for the appropriate plug with the appropriate voltages.

 

[shagone]

A good clue to whether it can receive a balanced signal will be an input socket for the hot + cold + chassis ground pins sent by the balanced output of the source.

Usually this is an XLR or TRS socket waiting for the appropriate plug with the appropriate voltages.

yes but this is rare on car audio amps. I have seen some that use standard RCA inputs and have a switch or a jumper to change between standard and balanced input. i also measured a crossfire class D BMF1000D amp and the L and R RCA shields are not connected so this should work with balanced inputs??

 

[AndrewT]

separate RCAs with separated grounds and using the grounds as the signal return cannot be balanced signal lines.

 

[shagone]

separate RCAs with separated grounds and using the grounds as the signal return cannot be balanced signal lines.

i don't think the shields are at ground? could they be floating? i'm still trying to figure this out.

 

[shagone]

i do have a newer crunch amp on my bench now with a mini-din connector for balanced inputs. looks like a keyboard or mouse plug type.

 

[Travis Clarke]

i wonder if isolation crossovers would help in my case ?

Yes I had those AudioControl crossovers you're referring to. They and their EQs are some of the best (along with their White Paper docs).

I have to tell you that 'ground loops' are a pain in the azz but pretty typical, but trial and error (along with star grounding) usually gets the noise out. Of course that assumes the installation is 'up to snuff.'

I only used isolation transformers in extreme cases as an installer and never recommended them for people who care about sound quality (like me).

Since the crossover is a low-current device, if you don't want to hunt down the source of the noise, you can use a DC-DC isolated switching power supply to separate the crossover's power ground instead of the signal ground from the source/vehicle ground for 0 noise.

If you isolated the power ground to the unit, you are isolating the signal ground for most car equipment, but doing it without the cost of affecting the audio which happens with isolation transformers.

But if it was only bass I could see that....

I did this recently to guarantee ZERO NOISE on the +260V car vacuum tube preamps I built. It works-awesome!

(Used a $25 Mean Well DC-DC mini supply)

But I would recommend checking out AudioControl's technical papers one of which covers why 'ground loop' noise happens.

 

[240z4u]

Bumping this one up, my amp shows 15 ohms between the shields when I compare them. This is a Diamond D6 300.4 The manual indicates you can just run speaker level right into the RCA cable jacks, so I assume this is capable of handling balanced connections. Am I off base here?

Thanks! Happy Thanksgiving.

 

[Perry Babin]

If there's only 15 ohms between the shields with no RCAs plugged in, it's unlikely that it will produce clean audio with a balanced signal.

 

[240z4u]

Crap, that's what I figured. Thanks Perry!

 

[240z4u]

Just an update. I called tech support and said that amp does accept balanced inputs without a problem.

Evan

 

[Perry Babin]

With 15 ohms between the shields, it won't work properly. It may work to an extent that it produces audio but that's all it will do. The separation, among other things, will suffer greatly.

 

[240z4u]

Thanks for the headsup Perry. I had pretty much settled on using a processor to convert balanced to unbalanced anyway. Looks best to stay on that path.

Evan