Hiss Issue with miniDSP HTx and Hypex Amps - Gain Mismatch or Defective Unit?

[rzrike]

I’m troubleshooting a hiss issue and could use some insight. I replaced a Denon AVR with a miniDSP HTx (balanced outs) feeding two Hypex amps: a four-channel Hypex NC502MP and an NCx500OEM monoblock. Speakers are Philharmonic BMRs (86.5 dB), listening distance around 5-6 ft.

I get a noticeable hiss (not hum) from all speakers, worse on the NC502MP. It’s constant regardless of HTx volume, present on all outputs even with no inputs connected, and stops only when the HTx is fully muted. RCA outs (using RCA-to-XLR cable) reduce the hiss by about 30%, but I bought this thing for the balanced outs. The HTx is 2 Vrms out of RCA and 4 Vrms out of TRS (balanced).

I am pretty certain that it is not a ground loop or any other kind of noise. I’ve gone through the full gamut of tests (many different cables, different outlets, power conditioner, etc).

I compared it with a Schiit Modius + Topping A90 Discrete (4 Vrms balanced) going into the same Hypex amps: hiss matches the HTx when driven at full output (100/100 volume), but is inaudible below 70/100 volume. I believe the Topping A90 uses analog attenuation for its volume control while the HTx uses only digital volume control.

Here is what I’ve been told by the retailers:

• The Hypex retailer says the 26.5 dB gain is standard but optimized for 2 Vrms inputs, not 4 Vrms. They believe the amps are just amplifying the HTx’s inherent noise floor and suggested trying -5 or -10 dB XLR attenuators or upgrading the NCx500OEM buffer to a lower-gain version.
• The HTx retailer suspects a defective unit since they believe the noise floor shown in Device Console (-117 dBFS, is this a real number?) shouldn’t be audible even after 26.5 dB of gain. They suggested a DOA warranty return.

Here’s what I’m thinking currently:
The HTx’s analog noise floor seems like it could be totally normal but is likely being boosted by the high amp gain and 4 Vrms output, making it audible at close range. The RCA test supports this, and my Modius+A90 test seems to confirm it’s a gain mismatch issue, not a defective amp or miniDSP.

Does this sound like a gain mismatch between 4 Vrms sources and 2 Vrms-sensitive amps? Or something else?

Would balanced XLR attenuators (-5 or -10 dB) be the right fix, and does anyone have any recommendations for specific models (I’m looking at the Sescom ones)?

Any chance the HTx noise floor is actually higher than spec and should be returned?

Any better ways to handle this besides attenuators (passive preamp, buffer change, etc.)?

 

[win]

I'm not sure how helpful this is
but I run a flex htx at full volume (volume is attenuated upstream) with very similar sensitivity speakers and have never heard any hiss. If I'm listening to something with very low overall volume (e.g. older classical recordings) my volume might be close to maxed out, and still I get 0 hiss.

 

[jmdesignz2]

I’ve run my Buckeye mono block NCX500s at the highest gain setting and lowest with 4v and higher preamps and no hiss

Im thinking it’s your HTX

I have Philharmonic BMR towers

 

[win]

Minidsp is very responsive with support inquiries. They probably have a test procedure you could run through to test it.

 

[rzrike]

I'm not sure how helpful this is
but I run a flex htx at full volume (volume is attenuated upstream) with very similar sensitivity speakers and have never heard any hiss. If I'm listening to something with very low overall volume (e.g. older classical recordings) my volume might be close to maxed out, and still I get 0 hiss.

What are you attenuating upstream with? Something with analog volume control (like my Topping A90 Discrete) or the HTx with digital volume control?

I've gotten in touch with miniDSP, but I'm waiting on a response to my last email. Would be unfortunate having to send it to China with how crazy customs is at the moment; hopefully they have some understanding of the issue.

 

[win]

I have a tube amp (schitt Freya) that I typically just run in passive mode. I use that, although I also run a rme adi-2 that I could use instead but don't have any issues using the analog attenuator.

I like having the option of tube amplification for some very older, quiet classical recordings.

 

[rzrike]

I’ve run my Buckeye mono block NCX500s at the highest gain setting and lowest with 4v and higher preamps and no hiss

Im thinking it’s your HTX

I have Philharmonic BMR towers

Nice to see a Philharmonic fan!

What preamps are you using? Do they have analog volume attenuation or digital volume control?

I thought for a while it had to be the fault of the HTx, but then I did the test with the Schiit Modius + Topping A90 Discrete (completely unconnected to the HTx). The hiss was present at 70-100% volume (and the subjectively the same level as the HTx's hiss when at 100%). So I can't see how it is exclusively a problem with the HTx if it is also present with the Modius.

But on the other hand, my explanation (and VTV's explanation) implies that the NC502MP is not really suited for 4Vrms which is peculiar given it has grounded inputs and 4Vrms is pretty standard for grounded inputs. You'd think more people would run into this issue.

On Buckeye’s page about the NCx500, they say:
“You have a choice of two different gain stages during checkout (both are the same price and both sound identical):

• Hypex gain stage, which renders our Low/Med/High toggle switch inactive (gain cannot be changed). This configuration is best for use with an AV Receiver and/or when using other Hypex amplifiers in your setup.
• Buckeye gain stage, allowing for Low/Med/High gain settings via toggle switch on the rear. This configuration is best for use in setups with a source/DAC capable of higher pre-out voltages.

The gain stage can be switched via internal jumpers by the customer if future setups dictate the need to change.”

Most likely the low gain setting would eliminate the hiss. Unfortunately, my NCx500 has the Hypex standard gain stage with no gain control. And anyway, it's really the NC502MP that is the problem child within this set-up with more noticeable hiss and there is no version of it with gain control.

If you were to set the Buckeye’s gain to high, set any analog volume attenuation to 100% volume (since HTx is digital volume control only), do you hear a hiss at all at a close listening position in a quiet room? That basically simulates my situation (if your DAC has a similar noise floor to the HTx or Schiit Modius).

 

[tmtomh]

I'd say it's the HTx. I would do the DOA warranty return with the HTx retailer just to rule out a defective unit.

 

[pderousse]

I have a HT 88 with 4 Hypex NC400, and I had a hiss issue too. I found that minidsp customer service always wanted me to troubleshoot mismatched gain. It was not the issue,the components are highly compatible. After months of frustration, I realized after swapping cables that I had a bad monoprice XLR cable. It had actually been soldered incorrectly. I would urge you to simplify the signal path as you would to diagnose a ground loop and assume nothing is wholesome. Hypex amps also do burn up (I’ve replaced 2 over the years). Good luck!

 

[rzrike]

I'd say it's the HTx. I would do the DOA warranty return with the HTx retailer just to rule out a defective unit.

I’m just not understanding how it could be the HTx if the Modius + A90 Discrete (at 100% volume) causes the same hiss… unless there’s also something wrong with my Schiit DAC…

I’d feel kind of bad if I send the HTx back, and they just confirm it’s within spec and turn it back around (although it was the retailer who suggested the DOA warranty return). But questioning if I’ve got a lemon might just linger in my mind forever if I just hold onto it.

 

[rzrike]

I have a HT 88 with 4 Hypex NC400, and I had a hiss issue too. I found that minidsp customer service always wanted me to troubleshoot mismatched gain. It was not the issue,the components are highly compatible. After months of frustration, I realized after swapping cables that I had a bad monoprice XLR cable. It had actually been soldered incorrectly. I would urge you to simplify the signal path as you would to diagnose a ground loop and assume nothing is wholesome. Hypex amps also do burn up (I’ve replaced 2 over the years). Good luck!

That’s interesting because miniDSP tech support has so far pretty much ignored my suggestions of it being a gain mismatch issue. I am waiting on them to respond to my email about the Modius + A90 Discrete test, though.

I think I’ve exhausted all the tests I can do when it comes to ground loops. I’ve tried multiple reputable cables (Canare, Cable Matters, Benchmark, and World’s Best Cables). Lots of different outlet configurations, multiple circuits in my house. Every other device unplugged, no inputs plugged in, Furman power conditioner, HumX, thrown the kitchen sink at it. It also sounds different than a ground loop. I’d describe it as a “hiss,” not a “buzz” or a “hum.” Sounds like it is coming out of mainly the tweeter (my BMRs are so directional that the hiss gets cut in half if I put my ears 15 degrees off from the tweeter).

 

[pderousse]

Sounds very familiar. Why do you not suspect the amp?

 

[bungle]

Sounds like it is coming out of mainly the tweeter (my BMRs are so directional that the hiss gets cut in half if I put my ears 15 degrees off from the tweeter).

Does your speaker/amp have adjustable sensitivity? How far from speaker the hiss is audible?

 

[rzrike]

Sounds very familiar. Why do you not suspect the amp?

The amp isn’t above suspicion. But I’m inclined to think the hiss is generated upstream because the hiss increases/decreases with analog voltage attenuation via the Topping A90 Discrete volume control. If the hiss originated within the amp, I would assume it would stay at the same intensity no matter what the preamp is set at.

The retailer has told me about a test to supposedly rule out the amp (“shorting pins 2 and 3 to pin 1 (ground) at the inputs of the amp”). That freaks me out a little bit since I’ve never done it before, but I probably should have done it before starting this thread.

 

[rzrike]

Does your speaker/amp have adjustable sensitivity? How far from speaker the hiss is audible?

Speakers are passive and the two amps have a set gain stage (26.5dB gain). The NCx500 can have a gain stage installed with low/medium/high settings (the low setting would be 7dB lower than the current configuration, 19dB), but I’d have to pay for that to be installed. The four-channel NC502MP cannot have a gain stage with different levels installed; it’s stuck at 26.5dB.

Hypothetically I could adjust the sensitivity by using a voltage attenuator (which I was inquiring about in my original post). I don’t know exactly what models if any have been tested to not affect any sonic properties other than the voltage.

The hiss is audible from around 5-6’ away in a quiet room (as long as my ears are lined up with the BMR’s tweeter). That’s just my normal listening position; I haven’t moved back farther to see how far I can hear it. Also, I’ve been doing my tests with a single speaker, so I assume the hiss would be louder (cumulatively) with all the speakers hissing together.

 

[MaxwellsEq]

In order to isolate noise, you need to work backwards from the speakers to the source.

You should unplug the feeds to the power amplifiers and then you must short their inputs, otherwise you will hear the noise from the high input resistances. If no noise, then it's not the power amplifiers. Then continue the process with upstream sources.

 

[pderousse]

In order to isolate noise, you need to work backwards from the speakers to the source.

You should unplug the feeds to the power amplifiers and then you must short their inputs, otherwise you will hear the noise from the high input resistances. If no noise, then it's not the power amplifiers. Then continue the process with upstream sources.

Do that. It is what I meant, but Maxwell Esq put it better than I did.

 

[rzrike]

I answered miniDSP’s questions about a week ago, and they haven’t responded (was waiting to see what they said before posting here again).

In order to isolate noise, you need to work backwards from the speakers to the source.

You should unplug the feeds to the power amplifiers and then you must short their inputs, otherwise you will hear the noise from the high input resistances. If no noise, then it's not the power amplifiers. Then continue the process with upstream sources.

Do that. It is what I meant, but Maxwell Esq put it better than I did.

I would say I am fairly confident this is not a ground loop.

I did a shorted-input test on the Hypex amp a few days ago. I made a shorting plug by connecting XLR pins 2 and 3 together (leaving pin 1 unconnected) and plugged it into the amp input. With this in place, the speaker was completely silent, no hiss at all. Confirming that the amplifier itself is quiet, and that the hiss originates upstream.

With my other test that I mentioned previously, the hiss was present with the Schiit Modius + Topping A90 Discrete (not connected to the HTx). The hiss changes with intensity when the analog volume control is used on the A90 Discrete. If the hiss originated in the amp, the volume of the hiss wouldn’t be controllable by a device upstream.

I currently don’t see the issue being anything other than 1) a gain mismatch between the HTx and Hypex amps, or 2) a defective HTx unit. Though I’m of course open to anyone’s suggestions.

 

[TimoJ]

Test what happens to the hiss when you connect 10k resistors from pin 2 to pin 1 and from pin 3 to pin 1. Do this to an XLR audio cable connected between the HTx and the amp.

 

[rzrike]

Sent miniDSP another email, and I guess they’re just ignoring me now for some reason. Kind of terrible tech support—ask me questions, I answer them with lots of detail, and then they ghost me. Or maybe something is happening behind the scenes with their company, but either way, they were of no help at all.

The retailer is sending me a warranty return authorization. Not sure if we’re talking about a return and refund or return and wait around for a repair (contingent on it actually being out of spec or not functioning correctly).

I still think it’s probably something to do with the gain. I bought a Sescom -5dB XLR attenuator. It cut down on the hiss, but not enough to be inaudible at my listening position (I guess I would need the -10dB). But almost everyone, the retailer, people on this forum, other people I’ve asked for advice, are insisting I return the HTx.

Test what happens to the hiss when you connect 10k resistors from pin 2 to pin 1 and from pin 3 to pin 1. Do this to an XLR audio cable connected between the HTx and the amp.

Bought some 10k resistors from Amazon; hopefully I can try them before shipping off the HTx.