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Less Than 300mV Out: Sign of a Poor Quality Preamp?
Knowzy
post Oct 18 2010, 22:58
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According to Wikipedia, line level is 300mV at 10 kohms.

I have yet to come across a preamp in a USB turntable that comes close to that. Most decent to good standalone preamps seem to send out 300mV.

When manufacturers send out 150 or 200mV, are they playing it conservative in case the record is hot or the cartridge has a stronger-than-average voltage? Or is the pre-amp just so bad that you're better off amplifying the signal the rest of the way in your standard amplifier?


USB Turntable Pre-out Voltages:
  • Stanton T.92 USB: 36.8 - 73.5 mV
  • AT-LP120-USB: 150mV, 36dB gain
  • Pro-ject Debut III USB: 160mV, 32dB gain
  • AT-PL120: 200mV, 36dB gain
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saratoga
post Oct 18 2010, 23:26
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QUOTE (Knowzy @ Oct 18 2010, 17:58) *
I have yet to come across a preamp in a USB turntable that comes close to that. Most decent to good standalone preamps seem to send out 300mV.


For consumer electronics the line level is usually all over the place because the exact level doesn't really matter.
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DVDdoug
post Oct 19 2010, 00:43
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I never worried too much about output level, because it always seemed adequate. I did worry about noise & signal-to-noise ratio. A higher-output cartridge helps... assuming the high-output cartridge otherwise sounds good... because it boosts the signal without boosting the noise. (You could usually hear hiss and/or hum with the volume up.)

If you're using the USB connection, the important thing is the calibration betweeen the preamp and the ADC. The actual analog signal level shouldn't matter as long as the ADC is getting enough signal to get to (nearly) 0dBFS (digital) on the peaks.

QUOTE
USB Turntable Pre-out Voltages:

Stanton T.92 USB: 36.8 - 73.5 mV
AT-LP120-USB: 150mV, 36dB gain
Pro-ject Debut III USB: 160mV, 32dB gain
AT-PL120: 200mV, 36dB gain
How is that measured? You'd need a calibration/test record. I'm not sure if there ever was an agreed-upon 0dB calibration standard for LPs... I'm sure somebody here knows... wink.gif

I seem to have a vague (bad?) memory that a typical phono preamp has a gain of 40dB @ 1kHz. And, cartridge output is usually based-on mechanical movement rather than "0dB" on a test record. (The specs for the Shure M97 say: "4mV RMS @ 5cm/sec.")

This post has been edited by DVDdoug: Oct 19 2010, 00:45
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Arnold B. Kruege...
post Oct 19 2010, 01:15
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QUOTE (Knowzy @ Oct 18 2010, 17:58) *
According to Wikipedia, line level is 300mV at 10 kohms.


Hmm, 300 mv sounds to me like "-10." . I don't think there is a standard for "line level" in general. There are standards for two flavors of line level called "-10" and "+4".

QUOTE
I have yet to come across a preamp in a USB turntable that comes close to that. Most decent to good standalone preamps seem to send out 300mV.


I wouldn't get too bent out of shape by that, since the line level inputs of a lot of hifi gear can be driven to full output by about 100 mV, 200 mV at the most.

QUOTE
When manufacturers send out 150 or 200mV, are they playing it conservative in case the record is hot or the cartridge has a stronger-than-average voltage? Or is the pre-amp just so bad that you're better off amplifying the signal the rest of the way in your standard amplifier?


The real world is so approximate and non-standard that getting too precise with one's requirements is a bad idea.

Just remember that 150 mv and 300 mv are just 6 dB about, and it takes 10 dB to create the perception that something is twice as loud, all other things being equal which they rarely are.

Most consumer home audio gear has enough reserve gain that they will drive their amplifiers to full output with inputs on the order of 100 mv. So 150 and 200 and 300 mV only differ in terms of how much you will end up turning things down.

QUOTE
USB Turntable Pre-out Voltages:
  • Stanton T.92 USB: 36.8 - 73.5 mV
  • AT-LP120-USB: 150mV, 36dB gain
  • Pro-ject Debut III USB: 160mV, 32dB gain
  • AT-PL120: 200mV, 36dB gain


OK, I see no problem.

Next! ;-)
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pdq
post Oct 19 2010, 01:38
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QUOTE (Arnold B. Krueger @ Oct 18 2010, 20:15) *
Just remember that 150 mv and 300 mv are just 6 dB about, and it takes 10 dB to create the perception that something is twice as loud, all other things being equal which they rarely are.

OK, now you are confusing me. I thought that 2x voltage = 4x power = 12dB?
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Knowzy
post Oct 19 2010, 01:51
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QUOTE (DVDdoug @ Oct 18 2010, 16:43) *
If you're using the USB connection, the important thing is the calibration betweeen the preamp and the ADC. The actual analog signal level shouldn't matter as long as the ADC is getting enough signal to get to (nearly) 0dBFS (digital) on the peaks.

This really gets to the heart of why I'm asking. The last two USB turntables I've tested digitize at a very low volume.

I was curious about the extent to which a weak signal from the preamp contributes to the problem.

From what I'm hearing, the gain between the preamp output and the analog side of the ADC is solely responsible for the low volume as long as the voltage coming out of the preamp is adequate (> 100mV).

Of course, that gain is fixed on just about every USB turntable on the market. You get results like this using the on-board ADC and there's nothing you can do about it, at least in the analog domain.


Steely Dan - Peg, Audio-Technica AT-LP120USB



Steely Dan - Peg, Stanton T.92 USB



And the other extreme (notice the clipping at the end):


Steely Dan - Peg, Audio-Technica AT-LP2D-USB


QUOTE (DVDdoug @ Oct 18 2010, 16:43) *
QUOTE (Knowzy @ Oct 18 2010, 17:58) *
USB Turntable Pre-out Voltages...
How is that measured?

I don't know. These numbers come from the manufacturer in their respective owner's manual.


QUOTE (Arnold B. Krueger @ Oct 18 2010, 17:15) *
QUOTE (Knowzy @ Oct 18 2010, 17:58) *
According to Wikipedia, line level is 300mV at 10 kohms.

Hmm, 300 mv sounds to me like "-10." . I don't think there is a standard for "line level" in general. There are standards for two flavors of line level called "-10" and "+4".

Wikipedia mentions -10 and +4dBV. But preamp manufacturers only offer mV in their specs. Oh well, as Saratoga said, line level is all over the place in consumer electronics.


QUOTE (Arnold B. Krueger @ Oct 18 2010, 17:15) *
Most consumer home audio gear has enough reserve gain that they will drive their amplifiers to full output with inputs on the order of 100 mv.

That doesn't bode well for the Stanton: 36.8 - 73.5mV.

This post has been edited by Knowzy: Oct 19 2010, 01:52
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Arnold B. Kruege...
post Oct 19 2010, 01:53
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QUOTE (pdq @ Oct 18 2010, 20:38) *
QUOTE (Arnold B. Krueger @ Oct 18 2010, 20:15) *
Just remember that 150 mv and 300 mv are just 6 dB about, and it takes 10 dB to create the perception that something is twice as loud, all other things being equal which they rarely are.

OK, now you are confusing me. I thought that 2x voltage = 4x power = 12dB?


3 dB is a doubling of power, so 4x power is 6 dB.
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Arnold B. Kruege...
post Oct 19 2010, 13:13
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QUOTE (Knowzy @ Oct 18 2010, 20:51) *
QUOTE (Arnold B. Krueger @ Oct 18 2010, 17:15) *
QUOTE (Knowzy @ Oct 18 2010, 17:58) *
According to Wikipedia, line level is 300mV at 10 kohms.

Hmm, 300 mv sounds to me like "-10." . I don't think there is a standard for "line level" in general. There are standards for two flavors of line level called "-10" and "+4".

Wikipedia mentions -10 and +4dBV. But preamp manufacturers only offer mV in their specs. Oh well, as Saratoga said, line level is all over the place in consumer electronics.


Actually, the Wiki aritcle mentions -10 dBv and +4 dbu. Yes, just to keep you on your toes these two standards have different references! dBv uses 1.00 volt as the reference level, while dBu uses 0 Vu = 0.775 volts as its reference. While there may seem to be 14 dB between them, there's actually only about 11 dB since 0.775 volts is about -3 dBv. And -10 dBv is 316 millivolts, not 300.

QUOTE
QUOTE (Arnold B. Krueger @ Oct 18 2010, 17:15) *
Most consumer home audio gear has enough reserve gain that they will drive their amplifiers to full output with inputs on the order of 100 mv.

That doesn't bode well for the Stanton: 36.8 - 73.5mV.


73.5 mV is only 3 dB below 100 mv, so I wouldn't sweat it that much. Look at it htis way - they are trying to keep you out of clipping! ;-)

This post has been edited by Arnold B. Krueger: Oct 19 2010, 13:14
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Knowzy
post Oct 19 2010, 22:40
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QUOTE (Arnold B. Krueger @ Oct 19 2010, 05:13) *
Look at it htis way - they are trying to keep you out of clipping! ;-)

Too true! I wonder if my reports of clipping on the older AT-LP2D-USB made them run the other direction when they set the pre-ADC gain on their new AT-LP120USB.

While we're on the subject, my Steely Dan sample from the LP120 peaks at -10.8dB in the left channel and -13.3dB in the right channel according to Wave Repair.

CODE
Display: 00:00.000-04:23.035
Selection: 00:00.000-04:23.035
Peak: L=9406(28.71%/-10.8dB); R=7061(21.55%/-13.3dB)
RMS: L=752(2.30%/-32.7dB); R=646(1.97%/-34.0dB)
DC: L=0(0.00%); R=0(0.00%)

Is it safe to say that when manufacturers feed such a weak signal into an A/D converter with an ~-85 S/N, they are throwing away more than an insignificant amount of audio material?
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Knowzy
post Oct 20 2010, 04:24
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QUOTE (Knowzy @ Oct 19 2010, 14:40) *
Is it safe to say that when manufacturers feed such a weak signal into an A/D converter with an ~-85 S/N, they are throwing away more than an insignificant amount of audio material?

I thought of a way to test this and the answer seems to be "No, you won't lose anything significant."

Here's what I did.

The sample in question, Steely Dan's Peg, has a fade out at the end. I took the last 6 seconds from the CD version and normalized it; making the nearly inaudible part of the song loud enough to hear down to the last note.

Then I took that same last 6 seconds from the AT-LP120USB version and made it approximately the same volume.

I think I can hear an additional 4 notes in the CD version. But by the last couple of seconds on the AT version, clicks and pops from the record and noise from the A/D converter drown out the music.

The last 4 notes may be there but simply too faint to hear under the rest of the noise.

Clearly not a scientific comparison but the exercise shows (I think) that even at the low volume, the A/D converter captures the noise floor of the turntable itself. Below that is pretty insignificant.


Samples

CD:

AT-LP120USB:


* I had to introduce some clipping because one pop was well above the music level.
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