I assume that when the inputs are shorted, the AC at the output of a preamp is due to circuit noise, transformer radiating, etc. Some things are easy to fix, others more difficult or impossible (e.g. due to circuit design or pcb layout).
I wondered how many mV AC is acceptable at the output of a preamp when the input is shorted. I realise 0 is best, but how much is what you should aim for in the real world?
I wondered how many mV AC is acceptable at the output of a preamp when the input is shorted. I realise 0 is best, but how much is what you should aim for in the real world?
No, I have not listened to the amp yet. This afternoon I was testing a new amp and a new i/o board with a dmm. The test setup was not ideal, and I will do more extensive tests tomorow. I was just wondering what kind of level is acceptable. I read a number of threads hoping I would find a reference but could not find a mention.
I just measured my RA-88a mixer with 3 silent inputs connected - FM radio, MM cartridge, CD player. All were turned up to normal listening level. On the 2.5 VAC scale of my VOM the mixer output is reading zero. The smallest division on that scale is .05 Vac. The needle did not move even a tenth of that. So that is below 5 mvac. The analog VOM will read 20 hz to 1 mhz anyway. The measurement was in parallel with the 200k parallel 68 pf input impedance of my power amp.
I spent a year modifying to eliminate hum & hiss from that mixer. There are parts I could put in to reduce it more, but now I don't see the point. There are worse systems in my house, like the Samsung flat TV that started buzzing this month.
I spent a year modifying to eliminate hum & hiss from that mixer. There are parts I could put in to reduce it more, but now I don't see the point. There are worse systems in my house, like the Samsung flat TV that started buzzing this month.
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Thanks indianjo. I have not seen values above 20mV AC (mainly a few mV with my fluke 179, no frequency detected btw) during my quick tests, and I am not sure if you would have picked that up with your dmm. (Edit: sorry, your vom rather than dmm)
I have a gut feeling says that after 20-30dB amplification by a poweramp this could be annoying.
I have a gut feeling says that after 20-30dB amplification by a poweramp this could be annoying.
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I only put a 100k resistor at the load. It will eventually drive a poweramp but this is diy so I have 4 or so to choose from haha.
Fluke 179 doesn't pick up anything above 7000 hz if I remember correctly. I've chased down 1 mhz op amp oscillation previously with the simpson 266 XLPM VOM. You can estimate the frequency by changing the series input capacitor.
I just tried the power amp with and without the mixer powered up. No change in output noise, which is mostly hum. The power amp output with the speaker unplugged and the mixer on is 28 mvac. Work to be done in the amp. But the audible hum is masked by the fan noise. *****y heat sinks on a dynaco ST120, I run the fans all the time, masked by the organ in front of it.
I just tried the power amp with and without the mixer powered up. No change in output noise, which is mostly hum. The power amp output with the speaker unplugged and the mixer on is 28 mvac. Work to be done in the amp. But the audible hum is masked by the fan noise. *****y heat sinks on a dynaco ST120, I run the fans all the time, masked by the organ in front of it.
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Thanks indianjo. I will have a look tomorrow and will see what I can measure with a Rigol scope. My (cheap) Owon signal generator is playing up, which is no help :-(
All my preamps with inputs shorted should measure no higher than 0.1mV ac rms with my Fluke 101. At 0.4mV or more, I can hear hiss/hum. My best preamps measure 0.0mV on Fluke 101. Some DMM's have a different bandwidth and pick up RF noise, but I find the model 101, the most representative of the audio band hiss/hum noise. So basically, under 100uV rms is acceptable.
Say preamp "loud" output is 1 Volt. And say that produces 100dB SPL in the room.
Ambient sound in a living room may be near 20dB SPL. 80dB lower. 80dB is 10,000:1 of voltage. So electric signal at 0.1mV is similar to ambient room sound.
We paid good bits for CD-quality 16-bit 96dB DR audio. On that scale we demand stray noise >96dB or >63,000:1 lower than peak signal. <0.016mV. (This is a very tough spec to meet.)
With low efficiency speakers in an urban room we may not have over 96dB SPL loud and 35dB SPL, say 60dB. This was the usual spec before digital audio. On that scale a 1mV noise re: 1V signal is just acceptable.
So for common DMMs with 1mV resolution: if you can see noise, it's probably more than you want. For real testing you need a low-noise amplifier before the meter (or a fancy ACVM, preferably with filtering options).
Oh: a mV of pure 50/60Hz is less offensive than a mV of 1KHz-10KHz hiss. 100/120+Hz rectifier buzz is very offensive out of proportion to the number. So the ear IS the best guide.
Ambient sound in a living room may be near 20dB SPL. 80dB lower. 80dB is 10,000:1 of voltage. So electric signal at 0.1mV is similar to ambient room sound.
We paid good bits for CD-quality 16-bit 96dB DR audio. On that scale we demand stray noise >96dB or >63,000:1 lower than peak signal. <0.016mV. (This is a very tough spec to meet.)
With low efficiency speakers in an urban room we may not have over 96dB SPL loud and 35dB SPL, say 60dB. This was the usual spec before digital audio. On that scale a 1mV noise re: 1V signal is just acceptable.
So for common DMMs with 1mV resolution: if you can see noise, it's probably more than you want. For real testing you need a low-noise amplifier before the meter (or a fancy ACVM, preferably with filtering options).
Oh: a mV of pure 50/60Hz is less offensive than a mV of 1KHz-10KHz hiss. 100/120+Hz rectifier buzz is very offensive out of proportion to the number. So the ear IS the best guide.
Agree that 0.1mV in the 120Hz or 1kHz to 5kHz is unacceptable as it is clearly audible, whereas 50/60Hz, much less so. On measurement with FFT setup, my preamps typically have a "battery like" flat noise floor at -120dB (with 60Hz a mild a bump at -115dB), and those preamps are definitely silent as in nothing audible when source is off. As an example, here is the Aksa Lender preamp driving 20Vpp into 7kohm load. This is with an AC wall supply consisting of Class 2 12v wall wart powering DC-DC step up to about 50v followed by CLC, cap multiplier and then CRCRC.
At 1Vrms out, I would estimate the noise floor is maybe -105dB or so.
At 1Vrms out, I would estimate the noise floor is maybe -105dB or so.
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Thanks for your replies. As you have rightly pointed out, things are not as simple as "pick a number". I had hoped it was (similar to <50mV as acceptable value for DC offset on a poweramp) but understand things are a bit more complicated.
At the moment I am struggling with my (understanding of my) measuring equipment. I bought some things some time ago but never really spent the time to get know my tools.
Measuring DC voltage and AC ripple to check a PSU is easy. I have used a signal generator and scope to check if an arduino based relay volume control board worked, but that was just to confirm that every volume step actually led to an increase in output 🙂
Measuring noise (in a preamp) is something else. The levels are low and I have to make sure that I do not interpret results as due to noise caused by the amp while it may be due to noise in the setup. Time to hit the manuals!
At the moment I am struggling with my (understanding of my) measuring equipment. I bought some things some time ago but never really spent the time to get know my tools.
Measuring DC voltage and AC ripple to check a PSU is easy. I have used a signal generator and scope to check if an arduino based relay volume control board worked, but that was just to confirm that every volume step actually led to an increase in output 🙂
Measuring noise (in a preamp) is something else. The levels are low and I have to make sure that I do not interpret results as due to noise caused by the amp while it may be due to noise in the setup. Time to hit the manuals!
I use a Focusrite 2i4 or Solo USB sound interface for measurements. To measure noise, you don't even need an exciation, just use REW software in RTA mode to look at noise spectrum. Ground loops and hum or hiss, oscillation (in audio band) will show up easily. You can use almost any decent sound card, even $30 Behringer USB UCA202 will work, albeit noise floor is -110dB vs -130dB relative to 0dB input vs Focusrite. For distortion measurements, use a good DAC and create a 1kHz pure sinewave in high resolution FLAC file.
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