Thanks for the explanation Roy.tubes4hifi wrote:the PH16 and PH16-X use different power transformers, so the value of R27 is usually either 1K or 2.2K, depending.
R27 is both an RC filter and a voltage drop for excess B+, so it's value is tailored depending on users AC input and power tranformers to get the correct voltage to the B+ regulator.
+3
Dale Stevens
Peter W.
mijohn
7 posters
PH16X Hum/Buzz
mijohn- Posts : 119
Join date : 2013-06-19
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
I am sorry - but this explanation strikes me as disingenuous. Why?
North American Utility Power Standards are, generally, 120 VAC, 60 hz, +/- 5%, and have been so for many years. Sure, there are variations by region and time of year. But, there is the tariff, and that tariff is pretty reliable.
It would make sense that any piece of equipment made these days would be sufficiently resilient so as to manage the North American Standard right from the git-go.
North American Utility Power Standards are, generally, 120 VAC, 60 hz, +/- 5%, and have been so for many years. Sure, there are variations by region and time of year. But, there is the tariff, and that tariff is pretty reliable.
It would make sense that any piece of equipment made these days would be sufficiently resilient so as to manage the North American Standard right from the git-go.
mijohn- Posts : 119
Join date : 2013-06-19
Peter, I was thanking Roy for his explanation to my questions about R27 in post No.23, but you have a good point about it's value being tailored to suit input voltage.
The AC voltage at my home can vary up to about 10 volts over the course of a day depending on network load, but my PH16 doesn't seem to mind.
The AC voltage at my home can vary up to about 10 volts over the course of a day depending on network load, but my PH16 doesn't seem to mind.
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
equipment can be designed to run most efficiently, or be designed to run under any circumstances.
My choice is not to generate excessive heat internally in my equipment by running it within nominal limitations.
A simple resistor change can eliminate several huge heatsinks.
My choice is not to generate excessive heat internally in my equipment by running it within nominal limitations.
A simple resistor change can eliminate several huge heatsinks.
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
tubes4hifi wrote:equipment can be designed to run most efficiently, or be designed to run under any circumstances.
My choice is not to generate excessive heat internally in my equipment by running it within nominal limitations.
A simple resistor change can eliminate several huge heatsinks.
Let's parse this just a bit:
For me, that resistor change would be simple, yes.
However, I would likely want to switch in more than one value - so that our main house (118V pretty much all the time) and our summer house (121V to 123V) would be equally friendly.
Not everyone has steady voltage such that a one-value change would serve all conditions.
Not everyone knows one end of a soldering iron from another.
Not everyone wants to ship equipment multiple times.
It would seem to me that this situation is crying out for an SMPS - one that could handle pretty much anything from say.... 90 V to 240 V? The cost of the supply would, in part, be balanced by the savings on the iron. Sure, there are those that recoil in horror at the thought of an SMPS - but, as with most adherents to audio myths - without reason.
Thoughts?
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
SMPS, not really related to the original thread, but also as far as I know an SMPS does not have a fixed output voltage,
it just has a wide range of input voltage
no offense, but I often find many forum participants making comments either unrelated to the thread, or about equipment that you have no knowledge of.
Now if you have a PH16-X and you are intimately familiar with it's design, then maybe you could offer something useful here.
It's a simple matter that a PH16-X usually has a 230vac output, and a standard PH16 has a 200vac output,
the difference simply requiring a change of one resistor in the power supply.
Either will work perfectly fine within 115-125vac input once they are set up correctly, and that's the answer to the original question.
it just has a wide range of input voltage
no offense, but I often find many forum participants making comments either unrelated to the thread, or about equipment that you have no knowledge of.
Now if you have a PH16-X and you are intimately familiar with it's design, then maybe you could offer something useful here.
It's a simple matter that a PH16-X usually has a 230vac output, and a standard PH16 has a 200vac output,
the difference simply requiring a change of one resistor in the power supply.
Either will work perfectly fine within 115-125vac input once they are set up correctly, and that's the answer to the original question.
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
SMPS, not really related to the original thread, but also as far as I know an SMPS does not have a fixed output voltage,
An SMPS may be designed to do both. Typical output regulation will be from 0.05% to 0.5% off the shelf. Additional controls may be added to get that to as low as 0.002% with a nominal input voltage of 90 VAC to 240 VAC, 50 or 60 hz. Units are available with up to four (4) outputs typically between 0.5 & 5+ Amps.
An SMPS may be designed to do both. Typical output regulation will be from 0.05% to 0.5% off the shelf. Additional controls may be added to get that to as low as 0.002% with a nominal input voltage of 90 VAC to 240 VAC, 50 or 60 hz. Units are available with up to four (4) outputs typically between 0.5 & 5+ Amps.
thermionicvinyl- Posts : 32
Join date : 2018-06-10
Hi everyone,
Just wanted to update you on my progress. My resistor was delivered today (no electronics shop here so I had to order it) and I switched R27 for a 1K. Unfortunately the hum did not go away but my D3 voltage is now above 20VDC. These are the voltage measurements I took:
D3: 218.8V, 178.1V
D3: 218.1V, 177.5V
B+1: 176.6V, 176.1V
B+2: 168.1V, 167.5V
Just wanted to update you on my progress. My resistor was delivered today (no electronics shop here so I had to order it) and I switched R27 for a 1K. Unfortunately the hum did not go away but my D3 voltage is now above 20VDC. These are the voltage measurements I took:
D3: 218.8V, 178.1V
D3: 218.1V, 177.5V
B+1: 176.6V, 176.1V
B+2: 168.1V, 167.5V
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
thermionicvinyl wrote:Hi everyone,
Just wanted to update you on my progress. My resistor was delivered today (no electronics shop here so I had to order it) and I switched R27 for a 1K. Unfortunately the hum did not go away but my D3 voltage is now above 20VDC. These are the voltage measurements I took:
D3: 218.8V, 178.1V
D3: 218.1V, 177.5V
B+1: 176.6V, 176.1V
B+2: 168.1V, 167.5V
Somehow, I did not think it was the resistor.
Roy?
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
For the record, I sat on my fingers for ten (10) days before asking this of the OP:
Have you tried isolating the cartridge from the mains ground by sending the TT ground to one of the phono jack headshells? Meaning NO OTHER GROUND from the TT to the Pre except through the headshell?
Have you tried isolating the cartridge from the mains ground by sending the TT ground to one of the phono jack headshells? Meaning NO OTHER GROUND from the TT to the Pre except through the headshell?
rjpjnk- Posts : 262
Join date : 2018-07-18
Peter W. wrote:For the record, I sat on my fingers for ten (10) days before asking this of the OP:
Have you tried isolating the cartridge from the mains ground by sending the TT ground to one of the phono jack headshells? Meaning NO OTHER GROUND from the TT to the Pre except through the headshell?
You did a very good job waiting
But are you talking about something different than this from Post 3?thermionicvinyl wrote:I tried disconnecting the phono input and the hum is still present. I also tried it with and without the ground wire connected.
I think OP tried to rule out the TT or its wires including ground as a source of the hum. However, I wonder if he grounded the phono input jacks at the back of the preamp when the wires were disconnected. That is important.
Last edited by rjpjnk on Wed Jun 19, 2019 3:13 pm; edited 1 time in total
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
I tried disconnecting the phono input and the hum is still present. An open ground via the head amp is an opportunity for hum.
I also tried it with and without the ground wire connected. With and without the ground connected to what? And how?
Snark Warning: The English Language is a powerful tool. In a technical discussion, being clear, specific and accurate is critical to conveying information and towards reaching an optimal solution.
I also tried it with and without the ground wire connected. With and without the ground connected to what? And how?
Snark Warning: The English Language is a powerful tool. In a technical discussion, being clear, specific and accurate is critical to conveying information and towards reaching an optimal solution.
rjpjnk- Posts : 262
Join date : 2018-07-18
Also... Was the ground wire disconnected at the same time the phono inputs were disconnected?
That's one I was wondering.
Definitely a lot of room for confusion in English.
That's one I was wondering.
Definitely a lot of room for confusion in English.
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
nothing new here since I found out that the power supply ground wasn't connected to earth ground thru the IEC AC input filter.
That should make a huge difference!
That should make a huge difference!
thermionicvinyl- Posts : 32
Join date : 2018-06-10
Here’s a quick update for everyone:
I make sure there was a good connection between all of the grounding points, and checked with the continuity setting on my digital multimeter. I also checked the power cable just in case the ground pin had gotten disconnected somehow. After that the hum persisted.
I went ahead and tried what Peter W. suggested and connected a ground wire from the lug of the phono preamp to the outer head shell of one of the RCA cables on the turntable. The moment I connected the wire the hum actually increased in volume.
I doubt it’s a ground loop partly because I hooked up the phono pre to my DAC and I was able to record some of the hum. There’s a large fundamental at 60Hz and many many harmonics above it. I’m not sure what that could indicate.
I make sure there was a good connection between all of the grounding points, and checked with the continuity setting on my digital multimeter. I also checked the power cable just in case the ground pin had gotten disconnected somehow. After that the hum persisted.
I went ahead and tried what Peter W. suggested and connected a ground wire from the lug of the phono preamp to the outer head shell of one of the RCA cables on the turntable. The moment I connected the wire the hum actually increased in volume.
I doubt it’s a ground loop partly because I hooked up the phono pre to my DAC and I was able to record some of the hum. There’s a large fundamental at 60Hz and many many harmonics above it. I’m not sure what that could indicate.
rjpjnk- Posts : 262
Join date : 2018-07-18
thermionicvinyl ,
I would first want to rule out any possibility that the noise is from the turntable or it's cables. This can be done by disconnecting all wires between the TT and preamp and then shorting the inputs to the preamp at the back of the preamp. You will need a pair of shorted RCA plugs to do this. What I did in my case was to cut the ends off an old RCA cable about 1 inch long and short the center connector to the shield. This completely removes the TT from the equation. Then we can focus on the preamp and power amp.
It is good you measured the frequency of the noise. You mention 60Hz is dominant and you also see all harmonics of 60. This suggests that the primary noise source is indeed 60Hz line noise rather than 120Hz power supply ripple.
Did you see my thread on this topic a few weeks back?
https://dynacotubeaudio.forumotion.com/t4079-how-much-additional-noise-is-normal-when-adding-a-phono-stage-to-a-preamp
Here is the spectrum of the noise (hum) I measured in my system. Sounds a lot like what you describe. At present I believe this sort of noise (60Hz and all harmonics) is inevitable in any home system powered by 60Hz AC, but I am still exploring this hypothesis. So while I don't think the spikes in this spectrum will ever go away even with the best design, it is important that they be at a low enough level so as not to be a nuisance during listening. Even though this spectrum looks terribly noisy, the actual hum to my ears is rather minor.
If you read through the thread you will see that I later determined that approximately 3dB of this noise was due to excessive loading of the filament power supply. I have plans to upgrade this supply to a beefier transformer in the future. But as I mentioned the hum level is not bad enough that I am in any hurry.
Here is the spectrum. Phono inputs are shorted. Preamp volume is at 0dB gain (i.e., output level = input level). Audio spectrum was measured at woofer after ST-70 amplifier. Notice there are spikes at 180Hz, 300Hz, 420Hz, and also other multiples of 60 that are not also multiples of 120? These harmonics couldn't get there if 120Hz was the fundamental frequency. So 60Hz is the culprit.
The power amp by itself does not produce these spikes, so somewhere in the preamp 60Hz is getting into the signal path.
Maybe this is similar to what you are experiencing?
I would first want to rule out any possibility that the noise is from the turntable or it's cables. This can be done by disconnecting all wires between the TT and preamp and then shorting the inputs to the preamp at the back of the preamp. You will need a pair of shorted RCA plugs to do this. What I did in my case was to cut the ends off an old RCA cable about 1 inch long and short the center connector to the shield. This completely removes the TT from the equation. Then we can focus on the preamp and power amp.
It is good you measured the frequency of the noise. You mention 60Hz is dominant and you also see all harmonics of 60. This suggests that the primary noise source is indeed 60Hz line noise rather than 120Hz power supply ripple.
Did you see my thread on this topic a few weeks back?
https://dynacotubeaudio.forumotion.com/t4079-how-much-additional-noise-is-normal-when-adding-a-phono-stage-to-a-preamp
Here is the spectrum of the noise (hum) I measured in my system. Sounds a lot like what you describe. At present I believe this sort of noise (60Hz and all harmonics) is inevitable in any home system powered by 60Hz AC, but I am still exploring this hypothesis. So while I don't think the spikes in this spectrum will ever go away even with the best design, it is important that they be at a low enough level so as not to be a nuisance during listening. Even though this spectrum looks terribly noisy, the actual hum to my ears is rather minor.
If you read through the thread you will see that I later determined that approximately 3dB of this noise was due to excessive loading of the filament power supply. I have plans to upgrade this supply to a beefier transformer in the future. But as I mentioned the hum level is not bad enough that I am in any hurry.
Here is the spectrum. Phono inputs are shorted. Preamp volume is at 0dB gain (i.e., output level = input level). Audio spectrum was measured at woofer after ST-70 amplifier. Notice there are spikes at 180Hz, 300Hz, 420Hz, and also other multiples of 60 that are not also multiples of 120? These harmonics couldn't get there if 120Hz was the fundamental frequency. So 60Hz is the culprit.
The power amp by itself does not produce these spikes, so somewhere in the preamp 60Hz is getting into the signal path.
Maybe this is similar to what you are experiencing?
thermionicvinyl- Posts : 32
Join date : 2018-06-10
rjpjnk wrote:thermionicvinyl ,
I would first want to rule out any possibility that the noise is from the turntable or it's cables. This can be done by disconnecting all wires between the TT and preamp and then shorting the inputs to the preamp at the back of the preamp. You will need a pair of shorted RCA plugs to do this. What I did in my case was to cut the ends off an old RCA cable about 1 inch long and short the center connector to the shield. This completely removes the TT from the equation. Then we can focus on the preamp and power amp.
It is good you measured the frequency of the noise. You mention 60Hz is dominant and you also see all harmonics of 60. This suggests that the primary noise source is indeed 60Hz line noise rather than 120Hz power supply ripple.
Did you see my thread on this topic a few weeks back?
https://dynacotubeaudio.forumotion.com/t4079-how-much-additional-noise-is-normal-when-adding-a-phono-stage-to-a-preamp
Here is the spectrum of the noise (hum) I measured in my system. Sounds a lot like what you describe. At present I believe this sort of noise (60Hz and all harmonics) is inevitable in any home system powered by 60Hz AC, but I am still exploring this hypothesis. So while I don't think the spikes in this spectrum will ever go away even with the best design, it is important that they be at a low enough level so as not to be a nuisance during listening. Even though this spectrum looks terribly noisy, the actual hum to my ears is rather minor.
If you read through the thread you will see that I later determined that approximately 3dB of this noise was due to excessive loading of the filament power supply. I have plans to upgrade this supply to a beefier transformer in the future. But as I mentioned the hum level is not bad enough that I am in any hurry.
Here is the spectrum. Phono inputs are shorted. Preamp volume is at 0dB gain (i.e., output level = input level). Audio spectrum was measured at woofer after ST-70 amplifier. Notice there are spikes at 180Hz, 300Hz, 420Hz, and also other multiples of 60 that are not also multiples of 120? These harmonics couldn't get there if 120Hz was the fundamental frequency. So 60Hz is the culprit.
The power amp by itself does not produce these spikes, so somewhere in the preamp 60Hz is getting into the signal path.
Maybe this is similar to what you are experiencing?
Just tried recording some of the hum with the inputs shorted and TT disconnected exactly as you describe it. Here is an FFT of signal coming from the outputs:
Looks like a large spike at 60Hz with almost no noise at 120Hz but a another spike at 180Hz (three times the fundamental.) I see many other harmonics cascading across the rest of the frequency band as well. What could all of the this indicate?
Guest- Guest
Here is the spectrum of the noise (hum) I measured in my system. Sounds a lot like what you describe. At present I believe this sort of noise (60Hz and all harmonics) is inevitable in any home system powered by 60Hz AC, but I am still exploring this hypothesis
I don't subscribe to that thought. And the reason I say that with confidence is that this is not an issue I experience.
What is perhaps inevitable is that with any high gain preamp, such as a phono preamp, some background noise will always be there, BUT, this should only become audible when the volume is set to max whilst standing very close to a speaker. It should NEVER be audible during normal listening sessions and this includes head phones!
For me, there is no excuse for hum in tube audio gear.
It starts with correct pcb design and layout, using large ground planes on both sides of the pcb, separating the ground planes of the power supply and audio circuit, ONLY connected via a jumper link, 'lifting' B+/audio ground from household ground, chassis layout and proper power transformer positioning and neat and tidy wiring, keeping all wires as practically short as possible and twisting them.
It takes a lot of thought, including some trial and error, to get all of the above right for the best possible results.
I don't subscribe to that thought. And the reason I say that with confidence is that this is not an issue I experience.
What is perhaps inevitable is that with any high gain preamp, such as a phono preamp, some background noise will always be there, BUT, this should only become audible when the volume is set to max whilst standing very close to a speaker. It should NEVER be audible during normal listening sessions and this includes head phones!
For me, there is no excuse for hum in tube audio gear.
It starts with correct pcb design and layout, using large ground planes on both sides of the pcb, separating the ground planes of the power supply and audio circuit, ONLY connected via a jumper link, 'lifting' B+/audio ground from household ground, chassis layout and proper power transformer positioning and neat and tidy wiring, keeping all wires as practically short as possible and twisting them.
It takes a lot of thought, including some trial and error, to get all of the above right for the best possible results.
rjpjnk- Posts : 262
Join date : 2018-07-18
Understood, but without some absolute numbers to indicate SPL in the room corresponding to the spikes in this noise spectrum there is no way to tell from looking at it if this would be audible during "normal listening sessions", so we are speaking subjectively here.
I can tell you that I personally can hear this noise at mid volume in a quiet room about 1 meter from my speakers. And to my ears it sounds exactly like every other phono amplifier I've every heard when there was no music playing. Whether or not this can be improved upon with specialized design I cannot say, but I am interested to learn more about it.
It is already good enough for me, but of course quieter is always better.
Could you provide a similar measured spectrum from some gear you own for comparison? Of course we cannot compare the total absolute output level, but I would like to see the relative height of the power line spikes compared to the residual noise floor in another system.
I can tell you that I personally can hear this noise at mid volume in a quiet room about 1 meter from my speakers. And to my ears it sounds exactly like every other phono amplifier I've every heard when there was no music playing. Whether or not this can be improved upon with specialized design I cannot say, but I am interested to learn more about it.
It is already good enough for me, but of course quieter is always better.
Could you provide a similar measured spectrum from some gear you own for comparison? Of course we cannot compare the total absolute output level, but I would like to see the relative height of the power line spikes compared to the residual noise floor in another system.
Guest- Guest
Will do once I am back home
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
thermionicvinyl wrote:Here’s a quick update for everyone:
I went ahead and tried what Peter W. suggested and connected a ground wire from the lug of the phono preamp to the outer head shell of one of the RCA cables on the turntable. The moment I connected the wire the hum actually increased in volume.
No. That is not what I suggested. In point of fact, by connecting a mains ground in any way, shape, or form to a TT circuit. should there be any differential whatsoever in that ground to the system Neutral, it will be amplified. Twice.I believe I mentioned that at some point in this discussion.
What I continue to suggest: Take the TURNTABLE GROUND - that is the extra wire from the turntable, often supplied with a little spade lug on the end, and connect THAT to a headshell - either one on either RCA jack from the TURNTABLE to the HEAD AMP. The HEAD AMP should have NO EXTERNAL GROUND to the Pre-Amp - other than what is designed into the line cord. If it is an EIC 3-wire, leave that. If it is an EIC 2-wire, leave that. DO NOT ground the Phone head amp to the Pre-amp with an "extra" wire. FULL STOP.
rjpjnk- Posts : 262
Join date : 2018-07-18
thermionicvinyl wrote:
Just tried recording some of the hum with the inputs shorted and TT disconnected exactly as you describe it. Here is an FFT of signal coming from the outputs:
Looks like a large spike at 60Hz with almost no noise at 120Hz but a another spike at 180Hz (three times the fundamental.) I see many other harmonics cascading across the rest of the frequency band as well. What could all of the this indicate?
Looks to me like you have 60Hz plus every integer multiple of 60Hz with the exception of second harmonic, 120Hz. Very similar to mine other than the missing component at 120. To me, this suggests that every spike on that spectrum is due to 60Hz leaking into the signal path somewhere. I also notice the even harmonics are lower than the odd harmonics in general. Kind of the opposite of "tube amp" harmonic distortion, so maybe this is a clue as to the insertion path? A 60Hz square wave would have all odd harmonics for example, but how could we have clipped 60HZ in the circuit? Just thinking out loud.
You are very fortunate to not have any 120Hz present. I actually wonder how this is possible.
Question1: Could you hear this noise now that the phono inputs were grounded? In other words, did completely removing the TT from the system have any effect on noise level?
Question2: How did you collect this spectrum? You mentioned FFT. You must have some sort of ADC and and FFT application software. What are you using? I would like to get a spectrum analyzer like the kind I use at work for home use but they are extremely expensive. I was wondering if I could make a home brew version for audio frequencies using an ADC probe of some sort and my computer. Maybe just record a digital file and post process in Matlab, but I would prefer to have a real time readout to look at while measuring on the bench. The spectrum I posted was an acoustic spectrum collected using a microphone and REW analysis software.
Peter W.- Posts : 1351
Join date : 2016-08-07
Location : Melrose Park, PA
There is a tiny spike at 120 hz. I suspect that there would be full spike, but it is being cancelled by the rectified chopped 120 hz signal. Which, of course, would exaggerate the audible effect of the 60 hz source.
thermionicvinyl- Posts : 32
Join date : 2018-06-10
Peter W. wrote:thermionicvinyl wrote:Here’s a quick update for everyone:
I went ahead and tried what Peter W. suggested and connected a ground wire from the lug of the phono preamp to the outer head shell of one of the RCA cables on the turntable. The moment I connected the wire the hum actually increased in volume.
No. That is not what I suggested. In point of fact, by connecting a mains ground in any way, shape, or form to a TT circuit. should there be any differential whatsoever in that ground to the system Neutral, it will be amplified. Twice.I believe I mentioned that at some point in this discussion.
What I continue to suggest: Take the TURNTABLE GROUND - that is the extra wire from the turntable, often supplied with a little spade lug on the end, and connect THAT to a headshell - either one on either RCA jack from the TURNTABLE to the HEAD AMP. The HEAD AMP should have NO EXTERNAL GROUND to the Pre-Amp - other than what is designed into the line cord. If it is an EIC 3-wire, leave that. If it is an EIC 2-wire, leave that. DO NOT ground the Phone head amp to the Pre-amp with an "extra" wire. FULL STOP.
It sounds the same as it does when connected to the ground lug of the phono preamp. I have it set up to the best of my understanding.
thermionicvinyl- Posts : 32
Join date : 2018-06-10
rjpjnk wrote:thermionicvinyl wrote:
Just tried recording some of the hum with the inputs shorted and TT disconnected exactly as you describe it. Here is an FFT of signal coming from the outputs:
Looks like a large spike at 60Hz with almost no noise at 120Hz but a another spike at 180Hz (three times the fundamental.) I see many other harmonics cascading across the rest of the frequency band as well. What could all of the this indicate?
Looks to me like you have 60Hz plus every integer multiple of 60Hz with the exception of second harmonic, 120Hz. Very similar to mine other than the missing component at 120. To me, this suggests that every spike on that spectrum is due to 60Hz leaking into the signal path somewhere. I also notice the even harmonics are lower than the odd harmonics in general. Kind of the opposite of "tube amp" harmonic distortion, so maybe this is a clue as to the insertion path? A 60Hz square wave would have all odd harmonics for example, but how could we have clipped 60HZ in the circuit? Just thinking out loud.
You are very fortunate to not have any 120Hz present. I actually wonder how this is possible.
Question1: Could you hear this noise now that the phono inputs were grounded? In other words, did completely removing the TT from the system have any effect on noise level?
Question2: How did you collect this spectrum? You mentioned FFT. You must have some sort of ADC and and FFT application software. What are you using? I would like to get a spectrum analyzer like the kind I use at work for home use but they are extremely expensive. I was wondering if I could make a home brew version for audio frequencies using an ADC probe of some sort and my computer. Maybe just record a digital file and post process in Matlab, but I would prefer to have a real time readout to look at while measuring on the bench. The spectrum I posted was an acoustic spectrum collected using a microphone and REW analysis software.
Yes I could still hear this noise even with the inputs shorted. The noise floor did not change.
I used "Adobe audition" to record and analyze the noise. It's an Asus Xonar sound card connected to a laptop, with which I then record the output via an RCA to 3.5mm jack cable. The recording is then analyzed after the fact. I'm pretty sure if you want RTA you'd have to use REW.