Looking for a pre-amp or voltage attenuator

[rzrike]

Some of you might have seen my previous post about using a miniDSP HTx with Hypex amps. I am as certain as it gets that it’s a gain mismatch issue now (a month of testing everything to do with ground loops, and I’ve basically had it confirmed by VTV and miniDSP: the NC502MP’s 26.5dB gain at 4V boosts the HTx’s noise floor into audibility at my listening distance).

My question now is what options I have for attenuating the voltage. I bought a Sescom -10dB XLR Attenuator and a Pearstone Switchable Attenuator (-10dB, -20dB, -30dB). The -10dB attenuation seems to be enough to get rid of the hiss (maybe -12dB is ideal). However, I have no idea what the attenuators are doing to other sonic properties. Have any balanced voltage attenuators or passive analog pre-amps been tested?

There’s also the possibility of getting legit analog pre-amps. A multichannel pre-amp that isn’t a fortune doesn’t really exist, so I would likely be getting more than one stereo pre-amp. It needs to have analog volume attenuation (not digital) and nothing else—it doesn’t need any other inputs or outputs other than XLR.

I have a Topping A90 Discrete which works well as a pre-amp, so that has two channels covered. It’s overkill and too expensive to buy a second unit. What would you all recommend? Is there a cheaper Topping or other brand headamp that has a good balanced pre-amp built in? There’s the Fosi ZP3, though it has the annoying issue of not having a level readout, that might not be an issue for me since I’ll keep it at the same volume at all times.

 

[LTig]

If 10 dB attenuation are enough to get rid of audible hiss then stick with it. Put the attenuator at the input of the power amp to get rid of any influence on frequency response (which may occur when the attenuator is at the output of the HTx and the connection cable has large capacitance due to its length). There is no other influence on sonics.

 

[antcollinet]

I am as certain as it gets that it’s a gain mismatch issue now (a month of testing everything to do with ground loops, and I’ve basically had it confirmed by VTV and miniDSP: the NC502MP’s 26.5dB gain at 4V boosts the HTx’s noise floor into audibility at my listening distance).

SNR 127 db from the HTx. At 4V output with 26.5dB of amp gain, that is 0.0000378V of amplified noise at the output of the amp. This is 97dB below the 2.83V that speaker senstivity is measured at.

If your speakers have sensitivity of 90dB (quite high) then the noise SPL out of the speakers at 1m is about -7dbA. -13dbA at a more typical 2m listening distance.


My back of an envelope calculation suggests that this is unlikely to be audible - especially in a typical room with around 30dB of ambient noise.

On the other hand the noise rating of the 502mp amp is 0.000047V, higher than the amplified noise of the HTX.

It is highly likely that the noise source causing the problem is somthing other than the HTx. Most likely IMO ground loop related. Though by using an attenuator, you will reduce that also if you don't need the amp gain.

 

[Sokel]

SNR 127 db from the HTx. At 4V output with 26.5dB of amp gain, that is 0.0000378V of amplified noise at the output of the amp. This is 97dB below the 2.83V that speaker senstivity is measured at.

If your speakers have sensitivity of 90dB (quite high) then the noise SPL out of the speakers at 1m is about -7dbA. -13dbA at a more typical 2m listening distance.


My back of an envelope calculation suggests that this is unlikely to be audible - especially in a typical room with around 30dB of ambient noise.

On the other hand the noise rating of the 502mp amp is 0.000047V, higher than the amplified noise of the HTX.

It is highly likely that the noise source causing the problem is somthing other than the HTx. Most likely IMO ground loop related. Though by using an attenuator, you will reduce that also if you don't need the amp gain.

Yep.
26dB gain is already on the low side traditionally speaking and at 90dB sens speakers everything should be grave silent.
I mean it's the norm for decades and is extremely rare to be audible, even with older stuff were SNR was way worst.

I would search one piece of gear at a time. I would also suspect strange gain settings at miniDSP, bad filter structure and gains that can up quantization noise as much as 30dB as seen here, etc.
All, the above without the chance of, for example filling dips (you shouldn't) applying positive gains, etc.

 

[DVDdoug]

However, I have no idea what the attenuators are doing to other sonic properties.

It's just resistors so it should be fine. There should be no noise,* no distortion, and flat frequency response.

Probably the only thing that would show-up in measurements would be the variation/tolerance of the attenuation. I assume they don't give you a spec but 1% resistors are cheap so that's what they probably use. (1% is less than 0.1dB.)



* Resistors do generate thermal noise but so does everything else, it's VERY low level (depending on resistance and temperature) and active electronics are worse. But most line-level active circuits don't generate any audible noise.

 

[dualazmak]

I believe we definitely do not need powerful amplifiers for highly efficient tweeters and super-tweeters, but we do need low/small power high S/N low-distortion excellent amplifiers for them.

Even Greg Timbers uses "reasonable and budget" Class-A Pioneer Elite A-20 integrated-amplifier for compression drivers (super tweeters) in his extraordinary expensive multichannel stereo system with JBL Everest DD67000 which he himself designed and developed (ref. here #435 on my project thread).

It is important, therefore, in multi-SP-driver multi-amplifier audio system, selecting amplifiers in the policy of "right-person-in-right-place";
not only the utilization of excellent pre-amplifier but also excellent small-power (and affordable) class-A/class-AB and/or tube integrated-amplifiers would fit in your perspective, I assume.

Just for your reference, I use four excellent integrated amplifiers in my PC-DSP-based multi-SP-driver multi-amplifier fully active audio setup along with the above my thoughts; please find the latest system setup in my post #931 and #1.009 on my project thread.

In my post #931, I wrote as follows under the below spoiler cover.

Spoiler

Here in this post, please let me emphasize again about the pros and merits of relative gain (i.e. tone) control not only in digital domain but also in analog domain using pre-amplifiers or integrated-amplifiers (in my setup). I recently wrote again in my post #56 on a remote thread like these;
Yes, as for safe and flexible tone controls (or I can say "relative gain controls among the multiple SP drivers"), my stance (policy) at least, is that we are encouraged to utilize the "best combination" of "DSP configuration in digital domain" and "analog domain tone controls using HiFi-grade preamplifiers and/or integrated amplifiers".

We need to note (and to respect for) that analog domain tone controls (relative gain controls among the multiple SP drivers) give no effect nor influence at all on the upstream DSP configuration (XO/EQ/Gain/Phase/Polarity/Group-Delay). I believe that this is a great merit of flexible tone controls in analog domain. We know well, on the other hand, in case if we would like to do the "tone/gain controls" only within DSP configurations, such DSP gain controls always affect more-or-less on "XO" "EQ" "phase" and "delay" of the DSP settings which will leads you to possible endless DSP tuning spirals every time; within DSP configurations, XO EQ Gain Phase and Delay are always not independent with each other, but they are always interdependent/on-interaction.

Just for your possible reference, my DSP-based multichannel multi-SP-driver multi-amplifier active system has flexible and safe analog level on-the-fly relative gain controls (in addition to upstream on-the-fly DSP gain controls) for L&R subwoofers, woofers, midrange-squawkers, tweeters, and super-tweeters, all independently and remotely. My post here shows you a typical example case for such safe and flexible on-the-fly analog-level tone controls. This my post (as well as this post) would be also of your interest.

Of course, I know well that I (we) can also perform such relative gain control using DAC8PRO’s 8-channel output gain controllers. I do not like, however, to change the DAC8PRO’s output levels frequently on-the-fly (while listening to music) due to safety and inconvenience concerns; I like to keep DAC8PRO’s analog out gain level always at constant -4 dB which should remain to be usually “untouchable” in my case.

One of the very unique aspects/features of my multichannel audio rig is that I fully utilize four HiFi-grade “integrated amplifiers” plus L&R active subwoofers, each of them have its own gain (volume) controller for safe and flexible relative gain (tone) control in analog domain even on-the-fly i.e. while listening to music.

In this perspective, my posts #438 and #643 should also give you better understandings. Furthermore, my posts #317(remote thread), #313(remote thread) would be also of your reference and interest.

 

[rzrike]

It's just resistors so it should be fine. There should be no noise,* no distortion, and flat frequency response.

Probably the only thing that would show-up in measurements would be the variation/tolerance of the attenuation. I assume they don't give you a spec but 1% resistors are cheap so that's what they probably use. (1% is less than 0.1dB.)



* Resistors do generate thermal noise but so does everything else, it's VERY low level (depending on resistance and temperature) and active electronics are worse. But most line-level active circuits don't generate any audible noise.

Most voltage attenuators I’ve seen don’t really state any specs, but the Sescom -10dB one I bought has this on their website:

• Frequency Response Deviation (20Hz-20kHz): <0.1dB
• Signal to Noise Ratio at 0dBFS, A-weighted: 88 dB
• Total Harmonic Distortion at 1kHz, 0dBFS: <0.001%
• Maximum Input Level: >24dB
• Maximum Output Level: >14dB
• Dynamic Range: >112 dB
• Gain: -10 dB
• Common Mode Rejection: 84 dB
• Input Impedance: 2.3k ohms
• Phase Deviation:<.1deg

If 10 dB attenuation are enough to get rid of audible hiss then stick with it. Put the attenuator at the input of the power amp to get rid of any influence on frequency response (which may occur when the attenuator is at the output of the HTx and the connection cable has large capacitance due to its length). There is no other influence on sonics.

Do you think different attenuators don’t make much of a difference for this use case? There’s definitely some variation in price (could just be branding/build quality). Overall, it would be convenient to just buy a handful of attenuators instead of dealing with a two stereo pre-amps with functionality I don’t need.

SNR 127 db from the HTx. At 4V output with 26.5dB of amp gain, that is 0.0000378V of amplified noise at the output of the amp. This is 97dB below the 2.83V that speaker senstivity is measured at.

If your speakers have sensitivity of 90dB (quite high) then the noise SPL out of the speakers at 1m is about -7dbA. -13dbA at a more typical 2m listening distance.


My back of an envelope calculation suggests that this is unlikely to be audible - especially in a typical room with around 30dB of ambient noise.

On the other hand the noise rating of the 502mp amp is 0.000047V, higher than the amplified noise of the HTX.

It is highly likely that the noise source causing the problem is somthing other than the HTx. Most likely IMO ground loop related. Though by using an attenuator, you will reduce that also if you don't need the amp gain.

The math doesn’t really make much sense to me either, but this is just the conclusion I’ve come to. Here’s the thread where I talk about most of the tests I did, but I’ll try to put a summary on here when I get the time.

 

[LTig]

Do you think different attenuators don’t make much of a difference for this use case? There’s definitely some variation in price (could just be branding/build quality).

Any attenuator contains just 2 (unbalanced) or 3 (balanced) resistors. Even 0.1% class resistors cost less than a $, which means less than the connector or adapter. If you want to spend money than on decent connectors like Neutrik.

Overall, it would be convenient to just buy a handful of attenuators instead of dealing with a two stereo pre-amps with functionality I don’t need.

Absolutely yes.

 

[antcollinet]

The math doesn’t really make much sense to me either, but this is just the conclusion I’ve come to. Here’s the thread where I talk about most of the tests I did, but I’ll try to put a summary on here when I get the time.

When you are talking about noise levels in the few 10s of microvolts level (such as that coming from your amp due to the HTx noise amplified - or the self noise of the amplifier) it is very difficult to get system noise levels down this low. Bear in mind the noise emitted from the speakers has to be around 40dB (100x) bigger in order for it to be on a level with the ambient noise in a typical room. I would say at least 20dB (10x) bigger before it has even a chance of it being audible at your listening position.

It doesn't really matter though - your attenuator approach will have the same benefit whatever the source of your upstream noise.

(I've looked at. your other thread - I still suspect low level system noise - of a type I personally would completely ignore, once it reaches the point of being inaudible with music playing)