Quantcast
Can someone sanity check me? - Gearspace.com
The No.1 Website for Pro Audio
Can someone sanity check me?
Old 25th September 2012
  #1
Gear Guru
 
2 Reviews written
🎧 10 years
Can someone sanity check me?

I have a pre-amp I built. I was going to sell it to someone, so I sent him a recording of the self-noise of the box. This is a vintage style, tube based pre-amp, so it's not supposed to be uber-quiet, but he said it was excessively hummy. I don't hear it that much, so I was just wanting to get a sanity check.

http://www.charmedquark.com/Web2/Tmp...Noise_0920.Wav

This is a 24/48 WAV with 15 seconds each of minimum gain, mid-gain, and maximum gain. It's a mono track exported to mono. There was no attenuation of the signal on the way in. It was exported the same format as recorded, so no dither was applied.

So, in theory, if you set up your speakers to standard monitoring levels, and you import this track and play it, that's what you'd hear from the device on a track that you recorded and left the fader at unity during playback. So this is what it should sound like relative to other content assuming you didn't lower the track at playback.

It is mono of course, so if you import it you may want to pan it hard left or right to avoid any pan law adjustments of the signal. I'm seeing about -70, -65, and -60dBFS on the meters at the three boost levels, which should be pretty low level, I would think.

If you can really easily hear hum at normal monitor levels, then maybe I have something seriously wrong with my monitoring setup here. I don't think so, but I just wanted to get a sanity check. Would you consider this unacceptable self-noise? Assuming tracking levels of -10dBFS it would be from 50 to 60dBFS below signal peaks. Of course if you lower the signal thereafter to achieve some particular metered level, then the noise would go down with it as well.
Old 25th September 2012
  #2
Deleted 4205102
Guest
Can't hear nothin...
Old 25th September 2012
  #3
Gear Guru
 
2 Reviews written
🎧 10 years
Anyone else?
Old 25th September 2012
  #4
Lives for gear
 
KRStudio's Avatar
 
🎧 10 years
I hear 60 cycle hum, and yes it's a little much especially if recording a quiet source.
Old 25th September 2012
  #5
Gear Guru
 
2 Reviews written
🎧 10 years
If it really is 60 cycle hum, then I could see why I wouldn't hear it since these speakers won't go nearly that low, nor would my headphones really get down that low either. However, he said he heard significant hum on his computer speakers, which I doubt go even as low as the small monitors I have here. So that couldn't be what he was hearing.

I guess I'll have to use a spectrum analyzer plugin to test for any low frequency hum, but I doubt it 'splains what he was hearing.
Old 25th September 2012
  #6
Gear Nut
 
🎧 5 years
60Hz harmonic distortion from hitting the 60Hz into a cheap pc computer speaker?

Will have a listen for myself tonight if I remember!
Old 25th September 2012
  #7
Lives for gear
 
KRStudio's Avatar
 
🎧 10 years
I take back my earlier post. Digital input to my Barefoots are silent, dead silent. It's the computer anolog crap and/or sound card making the noise. I am going to bet your friend made the same rookie mistake I did!
Old 25th September 2012
  #8
Gear Addict
 
disinfor's Avatar
 
🎧 10 years
I could only hear the "hum" at your maximum gain and my headphones cranked. That self-noise would disappear on any normal listening level.
Old 25th September 2012
  #9
Gear Guru
 
2 Reviews written
🎧 10 years
Thanks! Anyone else? It would be nice to have a couple more sanity checks before I go back to him, so that I don't waste his time (and my shipping money as well of course.)
Old 25th September 2012
  #10
Gear Guru
 
theblue1's Avatar
 
🎧 15 years
Quote:
Originally Posted by Dean Roddey ➑️
I have a pre-amp I built. I was going to sell it to someone, so I sent him a recording of the self-noise of the box. This is a vintage style, tube based pre-amp, so it's not supposed to be uber-quiet, but he said it was excessively hummy. I don't hear it that much, so I was just wanting to get a sanity check.

http://www.charmedquark.com/Web2/Tmp...Noise_0920.Wav

This is a 24/48 WAV with 15 seconds each of minimum gain, mid-gain, and maximum gain. It's a mono track exported to mono. There was no attenuation of the signal on the way in. It was exported the same format as recorded, so no dither was applied.

So, in theory, if you set up your speakers to standard monitoring levels, and you import this track and play it, that's what you'd hear from the device on a track that you recorded and left the fader at unity during playback. So this is what it should sound like relative to other content assuming you didn't lower the track at playback.

It is mono of course, so if you import it you may want to pan it hard left or right to avoid any pan law adjustments of the signal. I'm seeing about -70, -65, and -60dBFS on the meters at the three boost levels, which should be pretty low level, I would think.

If you can really easily hear hum at normal monitor levels, then maybe I have something seriously wrong with my monitoring setup here. I don't think so, but I just wanted to get a sanity check. Would you consider this unacceptable self-noise? Assuming tracking levels of -10dBFS it would be from 50 to 60dBFS below signal peaks. Of course if you lower the signal thereafter to achieve some particular metered level, then the noise would go down with it as well.
Well, you're nutty as a fruitcake... but it has nothing to do with the noise from your preamp. Or lack thereof. heh


I saw an RMS average for the clip of ~ -68 dB and a maximum sample level of something like -56 dB. Playing back at the level I'd been listening to (moderately loud), I couldn't hear anything over the ambient noise in my (non-isolated but reasonably quiet) room.


I know I don't have to tell you that hum is often due to conditions of the circuits across all interconnected devices and potentially the building wiring as well, which is what makes it often so vexing to pin down ground loop hum issues. If you're not getting these noise problems in your testing in your circumstances, you might try the rig in a couple of other physical situations.

I strongly suspect your workshop and studio are pretty nicely set up, wiring is per best practice, and may well be a best case scenario for keeping hum issues minimal.

Perhaps there is a 'weakness' in your design that, in a less-than-ideal power/induction environment, makes it more susceptible to external conditions than desired.
Old 25th September 2012 | Show parent
  #11
Gear Guru
 
2 Reviews written
🎧 10 years
Quote:
Originally Posted by theblue1 ➑️
I strongly suspect your workshop and studio are pretty nicely set up, wiring is per best practice, and may well be a best case scenario for keeping hum issues minimal.

Perhaps there is a 'weakness' in your design that, in a less-than-ideal power/induction environment, makes it more susceptible to external conditions than desired.
That couldn't be it in this case, since he's listening to a WAV I recorded here. So he's evaluating the self-noise as recorded in my setup. It's potentially possible it could be a bit higher there in his system, but not too likely I think. This is a Drip REDD47, not something I hacked together myself (which no one would buy I'm sure, except as a gag gift.)
Old 25th September 2012
  #12
Gear Guru
 
Brent Hahn's Avatar
 
1 Review written
🎧 15 years
I bet I'm the only one so far who has listened all the way to the end of the sample.
Old 25th September 2012
  #13
Lives for gear
 
Lotus 7's Avatar
Ran it (3) times and timed it!

Listening through a new iMac feeding a set of Shure SRH-940s with the iMac gain all the way up:

Silent when track is stopped

Start: Clearly audible 60, 120 and 180 Hz hum with slight wideband "hiss" at low level.

16.5 seconds; Louder hum and "hiss" gets louder (5dB?)

30.0 seconds: Louder again (both hum and hiss).

It's clearly audible on all three steps, but they are also very much below the average listening level at this (high) gain setting.

Can't hear it at all through the iMac internal speakers in a quiet room (40 dB SPL), but that's probably because they're 30dB down at 120 Hz.

With good monitors and appropriate playback gain (-10dBFS peaks hitting 90 to 95 dB SPL) the noise should just be audible in a typical room. (95 dB SPL - 50 dB = 45 dB SPL), although Mssrs. Fletcher and Munson may have something to say about that.
Old 25th September 2012
  #14
Lives for gear
 
KRStudio's Avatar
 
🎧 10 years
I listened all the way both times. In the studio it was damn near silent. My computer is where I heard noise.
Old 25th September 2012 | Show parent
  #15
Gear Guru
 
theblue1's Avatar
 
🎧 15 years
Quote:
Originally Posted by Dean Roddey ➑️
That couldn't be it in this case, since he's listening to a WAV I recorded here. So he's evaluating the self-noise as recorded in my setup. It's potentially possible it could be a bit higher there in his system, but not too likely I think. This is a Drip REDD47, not something I hacked together myself (which no one would buy I'm sure, except as a gag gift.)
Ah... I didn't get that. Sorry.
Old 25th September 2012
  #16
Lives for gear
 
Lotus 7's Avatar
Acceptable or not clearly depends on the application. If your potential buyer is recording a guitar amp playing at 103 dB SPL, he/she has no concern, but if the source is a string quartet playing in a very quiet hall or a good studio, with modern low-noise mics then it's way too high.

For recording small classical ensembles I will never use a mic pre with a noise floor higher than the internal noise of the mic. For that application, the mic's internal electronics and capsule should set the noise floor, not the mic pre.
Old 25th September 2012
  #17
Gear Guru
 
2 Reviews written
🎧 10 years
I'm pretty sure he's doing rock, and mostly interested in these for drum overheads, though he may use them for other things as well. These are vintage tube based pres, so I don't think anyone would expect them to sound like GML gear or anything like that. I was just trying to get a better feel for what he's hearing vs. what I'm hearing.
Old 25th September 2012
  #18
Lives for gear
 
5 Reviews written
🎧 10 years
I'm listening on my computer hooked up to a 60s Scott stereo thru a pair of AR17s. I have some nice 120 cycle hum, probably due to caps. Anyway, I could hear NOTHING over ambient.
Old 25th September 2012 | Show parent
  #19
Gear Guru
 
2 Reviews written
🎧 10 years
Quote:
Originally Posted by drbob1 ➑️
I'm listening on my computer hooked up to a 60s Scott stereo thru a pair of AR17s. I have some nice 120 cycle hum, probably due to caps. Anyway, I could hear NOTHING over ambient.
Just making sure... you mean you know you have some 120Hz in your own monitoring system due to caps, or that you think that the preamp has some 120Hz hum? Presumably the former since it's a pretty old stereo, but I've learned never to assume.
Old 25th September 2012 | Show parent
  #20
Lives for gear
 
Lotus 7's Avatar
Quote:
Originally Posted by Dean Roddey ➑️
I'm pretty sure he's doing rock, and mostly interested in these for drum overheads, though he may use them for other things as well. These are vintage tube based pres, so I don't think anyone would expect them to sound like GML gear or anything like that. I was just trying to get a better feel for what he's hearing vs. what I'm hearing.
If he's planning on using it for recording rock, especially drum overheads, then he should certainly have nothing to complain about, unless he's cranking the drums way up and compressing them by 20 dB.

He's probably listening through a audio chain with plenty of excess gain and does not have that signal chain set so that his peak levels are realistic. Then again, there are those who monitor at 110-115 dB SPLs so a noise floor that's 60-65 dB below that can certainly be audible at the start of a track, even without using any compression.

Maybe he's just fishing for an excuse to not purchase your mic pre. If you convince him the noise level is reasonable, he'll probably complain about the front panel color.

There is definitely audible noise on your sample that one can hear clearly on a clean, high dynamic range system.
Old 25th September 2012 | Show parent
  #21
Gear Guru
 
2 Reviews written
🎧 10 years
Quote:
Originally Posted by Lotus 7 ➑️
Maybe he's just fishing for an excuse to not purchase your mic pre. If you convince him the noise level is reasonable, he'll probably complain about the front panel color.
Oh, no, nothing like that. He genuinely wants it, because he already has another and wants to get a pair (so that he can use them for overheads.) He just doesn't want to waste time and I don't want to waste money and time, so I just want to be sure. And given that this isn't a store bought system, and each one is a little different, it's hard for me to know exactly what the performance of the one he has is, so it's hard for me to know if I'm competitive with that one.

The optimum thing for him to do would be to set up one track armed with the existing REDD47 he has feeding it with nothing plugged in, and load this Wav into another. Then he can compare the two with relative volumes always being the same. However, the other one is in his friend's studio. So, unlike me where I can just whip up SONAR and do a quick test, he has to find unused studio time and go over there to do it. So initially he just did a listen at home.
Old 25th September 2012 | Show parent
  #22
Lives for gear
 
Lotus 7's Avatar
Quote:
Originally Posted by Dean Roddey ➑️
Oh, no, nothing like that. He genuinely wants it, because he already has another and wants to get a pair (so that he can use them for overheads.) He just doesn't waste time and I don't want to waste money and time, so I just want to be sure. And given that this isn't a store bought system, and each one is a little different, it's hard for me to know exactly what the performance of the one he has is, so it's hard for me to know if I'm competitive with that one.
Too bad you can't set them up side-by-side and calibrate the gain with a mic level signal generator to really measure the differences, if any. I use a Goldline phantom-powered unit to test mic pres. It puts out a repeatable -37 dBV signal so you know you're comparing the units at a realistic gain setting.

Maybe you can borrow a Goldline mic simulator and record a track of the tone followed by silence (noise baseline). Then send him the very same mic simulator (Priority Mail $4.25) and have him do the same. You will then know "for sure" how your pre compares with the one he already has.
Old 25th September 2012
  #23
Gear Guru
 
2 Reviews written
🎧 10 years
Of course it is possible that the other one has a 12AT7 instead of a 12AX7 or something like that, so the lower apparent noise is just due to lower overall gain, in which case our comparing them both at max gain wouldn't be an apples to apples comparison. So, yeh, something like that would be a useful tool. If it wasn't for the fact that I'm broke I'd buy one.

Of course we could both just load a 1K sine wave into a track and feed it out at a given dBFS to an output that is routed to the pre-amp input, and see what we get back at each gain level. But I'm not too sure he is willing to get into a whole lot of spelunking.

I did test mine this way in order to make sure that 0dBVU was hitting about right on the way in, to insure I had appropriate output gain relative to meter readings. I was hitting about -16dBFS, which is pretty much where it should be, AFAIK.
Old 26th September 2012
  #24
Gear Head
 
🎧 10 years
Im the guy with the studio the Redd 47 came to

Hello Dean, Im the guy with the studio www.worldclassaudio.com the Redd 47 came to.

First of all, you're not crazy, sounds like you fixed the initial hum problem.

There is hum/noise at about -61 dBm which is right inline with the redd47 ( and other high quality vintage pres i have) unit Greg built for Jos which is almost dead silent, if only those pesky laws of physics would give us dead silence!

There appears to be no problem now.

My monitoring here consists of ATC SCM200 mains (modified with RAAL ribbons and TDM6 mid drivers) and the amazing DEQX crossover system, every driver has linearized freq response, also time, phase and group delay are corrected to the actual listening position, spent over a week yanking drivers out and measuring each driver in outside in "free field" response mounted on 4'x8' pieces of ply wood, then all the measurements were done again at the listening position so the DEQX could separate the wheat from the chaff. Extremely high resolution. my control room is built around the speakers.
Old 26th September 2012
  #25
Lives for gear
 
Unclenny's Avatar
 
🎧 10 years
Glad you guys straightened that out.

I was going to suggest, Dean, that you write into the terms of the sale a good sanity clause, but of course we all know it's true that........

...........
Old 26th September 2012
  #26
Gear Guru
 
2 Reviews written
🎧 10 years
If I had monitors like some of the folks in this thread have, I'd be happy to sit around and listen to hum all the time, ya know? :-)
Old 26th September 2012
  #27
Gear Nut
 
🎧 5 years
Yep, definitely hearing the 60Hz hum here, but only when I crank the gain. Just running through some HS80m's. They don't have any hum in them so it's def the recording.

As some have said, if he's recording rock, prob not an issue. But recording something much softer like orchestral, or maybe even some soft balad singing, it may be problematic.

I like the metallic click noise you get from switching the gain.

Actually, on second listen, I turned the gain to a level I might use when mixing, and it was quite audible. If he's not recording super hot, and especially once some compression is applied, this could potentially be audible in a mix (quiet sections of a rock mix even - maybe just). A bit of noise reduction and eq could probably fix it, but that hum is definitely there. I'd probably be hesitant to make a purchase with that hum. I'd be happy with the noise floor, but I'd want to hear that hum designed out of the system. Any chance it was your set-up, and not the amp?

As with many posts, take mine with a grain of salt. I'm certainly no pro at this.
Old 26th September 2012 | Show parent
  #28
Gear Guru
 
🎧 15 years
Quote:
Originally Posted by Unclenny ➑️
...I was going to suggest, Dean, that you write into the terms of the sale a good sanity clause, but of course we all know it's true that........



Old 27th September 2012
  #29
Lives for gear
 
Unclenny's Avatar
 
🎧 10 years
Old 18th October 2012
  #30
Gear Guru
 
2 Reviews written
🎧 10 years
Just for the record, I've made some more really significant improvements on this unit. How does this sound?

http://www.charmedquark.com/Web2/Tmp...Noise_1016.wav

I've tried a bodacious number of grounding configurations and this is the best so far by far. The primary remaining peak is at 120Hz (2x powerline.) I vastly reduced the 3x hum at 180Hz. 240Hz is about what it was before but it was already pretty low.

Here is a spectrum capture of the first one above vs. this one. Looking at it as a spectrum of noise really helped me understand a lot better what's going on. You can have nothing above -80dBFS at any given frequency, but overall it adds up to 10 or even 20dBFS higher peak level overall. So the overall peak level, which is what I was going by before, isn't so important. It's what reduces the ones that are contributing the most to the problem.

The FFT which these types of frequency analyzers use get less and less accurate at lower frequencies. Those 60Hz multiples points are actually very thin. I can put a razor thin cut at 120Hz and it gets super-quiet. From what I've read a common 120Hz issue is the tube heaters leaking into the signal, or a high voltage ground running parallel to a low voltage signal. But I'm not electronicy enough to investigate those things. Not that it sounds bad now at all. It's pretty dang quite, but it should would be nice to figure out that 120Hz hump.
Attached Images
File Type: png REDDSelfNoise_0920.png (234.6 KB, 284 views) File Type: png REDDSelfNoise_1016.png (226.8 KB, 306 views)
πŸ“ Reply

Similar Threads

Thread / Thread Starter Replies / Views Last Post
replies: 2118 views: 203481
Avatar for TheShadowV
TheShadowV 3 weeks ago
replies: 590 views: 88980
Avatar for Phosphenetre
Phosphenetre 31st January 2021
Topic:
Post Reply

Welcome to the Gearspace Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…

Forum Jump
Forum Jump