Impedance/Voltage mismatch? Balanced pre pro with non-balanced power amp

[roachy]

I've recently come into possession of a StormAudio ISP processor. Really impressed (and a bit overwhelmed) with the level of customisation available and the sound out of the box is very nice.

I'm yet to run a Dirac calibration to see what it can do compared to my previous Anthem AVM60 with ARC.

 

I've hit a small snag. I have a fully active front end (Seaton Cat 8s) which have balanced inputs. I'm currently running a JBL Synthesis SDA-7200 (purchased here from Marc) for my surround and Atmos channels. It only has RCA line level inputs.

 

My previous pre-pros have had balanced outputs, but also single ended outputs which i have used to hook up to power amp (JBL now, previously Elektra Theatre 6).

 

The StormAudio only has balanced outputs. I originally purchased some balanced to rca adapters to hook up. Upon first power up, i now hear an audible hiss from my surround/atmos speakers, loud enough from my listening position to be annoying.

 

I have since purchased some "unbalancing" cables which have pin 2 (hot) to the RCA tip and pins 1&3 to the sleeve as per StormAudio owners manual recommendations. Still have the hiss.

 

My thoughts (which may be way off) is there may be and impedance or voltage mismatch between the pre pro and the power amp. I understand the StormAudio may put out as much as 8v.

 

If this is the case, would something like this on each channel ( https://www.swamp.net.au/sescom-aud-xlr-rca-1-channel-xlr-to-rca-balanced-to-unbalanced-audio-converter ) be an idea? 

 

Otherwise would my best bet to offload the JBL Synthesis amp and get one with balanced inputs (Something like an Elektra HD2)?

 

Side note, my front end hooked up via XLR are dead quiet.

 

Thanks

Edited April 22, 2021 by roachy
Typo

[betty boop]

2 hours ago, roachy said:

 

I have since purchased some "unbalancing" cables which have pin 2 (hot) to the RCA tip and pins 1&3 to the sleeve as per StormAudio owners manual recommendations. Still have the hiss.

 

My thoughts (which may be way off) is there may be and impedance or voltage mismatch between the pre pro and the power amp. I understand the StormAudio may put out as much as 8v.

 

If this is the case, would something like this on each channel ( https://www.swamp.net.au/sescom-aud-xlr-rca-1-channel-xlr-to-rca-balanced-to-unbalanced-audio-converter ) be an idea? 

hmmm converting XLR- RCA or other around can be fraught with issues...personally... given the Storm has XLR only... I would be going back to them to understand what is best means they suggest to hook up RCA..... you wouldn't be robinson crusoe here wouldn't think  

 

and yes if something like 8V thats rather high for RCA too, theres a possibility to overload ! ... so rather than just doing things with pins....definitely need a convertor of some sort... usually these conversions are done by build in electronics of the amps/processors .

 

not sure why the storm is XLR only and synthisis amp RCA only ! definite pickle ! 

 

 

[roachy]

Sorry didn't mean to say I did anything with pins. Just the configuration to the xlr to rca cable I purchased. 

[roachy]

[betty boop]

id be more concerned with the 8V...considering typically with RCA you usually have like 2V

 

unfortunately just doing a cable with pin outs (which is what i meant above) isnt going to do anything about the voltage ?

[stereo coffee]

17 hours ago, betty boop said:

id be more concerned with the 8V...considering typically with RCA you usually have like 2V

 

unfortunately just doing a cable with pin outs (which is what i meant above) isnt going to do anything about the voltage ?

Consumer Line Level is nominal  0.310mv RMS, its peak amplitude is 0.447mv  and its Peak to Peak is 0.894mv. 

 

In @roachy circumstance firstly establish what level is resident in the Storm audio product. Likely is you may need to simply use resistance to attenuate its level to be matching to consumer line level. The article below has a calculator providing  all the needed info about levels.

 

A calculator for how much resistance too is here  https://www.digikey.com/en/resources/online-conversion-calculators

 

[roachy]

Thanks @stereo coffee. This is all new to me a good learning experience. 

Some measurements from Audio Science review

 

[bob_m_54]

1 hour ago, stereo coffee said:

Consumer Line Level is nominal  0.310mv RMS, its peak amplitude is 0.447mv  and its Peak to Peak is 0.894mv. 

 

In @roachy circumstance firstly establish what level is resident in the Storm audio product. Likely is you may need to simply use resistance to attenuate its level to be matching to consumer line level. The article below has a calculator providing  all the needed info about levels.

 

A calculator for how much resistance too is here  https://www.digikey.com/en/resources/online-conversion-calculators

 

0.310V, not mV

ie 310mV

Edited April 23, 2021 by bob_m_54

[BioBrian]

19 hours ago, betty boop said:

converting XLR- RCA or other around can be fraught with issues...

 

I'm hoping someone can explain this too. Seems that by shorting Pins 1 and 3, that half the waveform is lost. Not only that, half of the balanced preamp will have its outputs shorted. What happens down the line to the speakers (only doing the 'out' part of the waves, never going in) is unspeakable. Like Track 27 on the Soundoctor CD - blips showing the cones only moving out (or only in, if wired out of phase).

 

The Elektra amps now have switchable balanced/unbalanced inputs for each channel which is nice, but I'm unsure what happens inside - would hope there's some sort of buffer, unless the amp is fully balanced all through like Luxman, Vitus, etc.

[bob_m_54]

9 minutes ago, BioBrian said:

 

I'm hoping someone can explain this too. Seems that by shorting Pins 1 and 3, that half the waveform is lost. Not only that, half of the balanced preamp will have its outputs shorted. What happens down the line to the speakers (only doing the 'out' part of the waves, never going in) is unspeakable. Like Track 27 on the Soundoctor CD - blips showing the cones only moving out (or only in, if wired out of phase).

 

The Elektra amps now have switchable balanced/unbalanced inputs for each channel which is nice, but I'm unsure what happens inside - would hope there's some sort of buffer, unless the amp is fully balanced all through like Luxman, Vitus, etc.

By shorting pins 1 & 3, what you are doing is making pin 2 referenced to ground, rather than just referenced to pin 1 and floating.

[BioBrian]

6 minutes ago, bob_m_54 said:

By shorting pins 1 & 3, what you are doing is making pin 2 referenced to ground, rather than just referenced to pin 1 and floating.

 

Yes I understand that, but the negative part of the waveform is lost by shorting to ground. Would be like trying to fly an F18 with only one wing?

[bob_m_54]

11 minutes ago, BioBrian said:

 

Yes I understand that, but the negative part of the waveform is lost by shorting to ground. Would be like trying to fly an F18 with only one wing?

Yes, depending on the architecture of the amp, it likely would do that. Depends how the two halves of that channel are referenced to each other, how noticeable and how safe it would be. A transformer setup would be better.

[roachy]

This is all starting to go over my head

I'm guessing my easiest option is to get a power amp with balanced inputs?

[bob_m_54]

Or get something like this:

https://www.cs1.net/products/jensen_transformers/PC-2XR.htm

 

[stereo coffee]

9 hours ago, roachy said:

This is all starting to go over my head

I'm guessing my easiest option is to get a power amp with balanced inputs?

The problem is really stepping outside of normal hifi equipment specification with the storm audio unit.

 

For regular readers we can see ASR encouraging this approach by in this case selecting to measure on this occasion pro level audio gear,  generally in all reviews   they are very good at NOT measuring equipment correct,  with regard to level , time after time you can see them  testing equipment  12.9x above consumer line level, ( thanks @bob_m_54 for the clarification )  and making you think somehow that is perfectly normal....

 

If going down the path of pro audio, then it is a massive departure from consumer audio equipment. You are entering a world doing so that has no financial end, with what to buy next.  In fact you will never get to the end of it, as it steers you away from normality and instead toward trying to change recorded sound.  Without access to the original master tapes - its eternal frustration. 

 

Our pursuit IMO is to enjoy what was recorded with utmost fidelity, but not to try to change what was recorded.

 

Your best friend is to keep your present power amp & following fairly simple conversion to RCA seen in @bob_m_54 post , is to buy a small jiffy box, or if you can use the same box  and using just 4 resistors arrange them as a L pad to attenuate the large mismatch that has been created. 

 

Maybe  see if the Storm audio product can attenuate its output audio and for a real laugh see if the ASR review tells you how to reduce its level to consumer line level   - or as the case may be,  to the level you need. Noting such settings need to be permanent, so be aware of need to make the storm audio unit somehow remember its output setting.

 

[roachy]

Just a quick update. I managed to grab one of these https://www.sonifex.co.uk/redbox/rblu4_ld.shtml second hand on ebay for actually quite cheap.

 

8 channels of active balanced to unbalanced conversion From balanced XLR impedance of 20kΩ unbalanced RCA outputs with an output impedance of <50Ω.

 

It has solved my particular issue. Going by my roughly measured levels looks to have provided roughly 10db attenuation but most importantly the audible hiss is gone.

 

Thanks everyone for your input.