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#1
Posted to rec.audio.high-end
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Active vs. passive pre-amp
Out of curiosity, I hooked up my DAC directly to my amp just to hear
what it would sound like without going through my preamp. The DAC is fed by a Squeezebox Touch, so I can use it's digital volume control for attenuation. I was rather shocked how much more vibrant and involving the sound was. It does make me wonder what the benefit of all that electronics in a pre-amp really is. So I'd like to try going passive with something like a stepped attenuator. Is there anything I should know about the downside of going passive? It does sound like there might by some loss of bass, though I'd like to narrow this down with whatever measurements I can do with my voltmeter and SPL meter. My typical "serious" listening level is about -20dB (relative to unity gain). A low level recording might require the volume to be raised to -15dB or so. So I don't think I'll have any problems with having enough gain. Equipment, all connected with XLR cables, is as follows: DAC: Cambridge Audio Azur 840C. Output impedance: 50 Ohm. This seems to have a pretty beefy power supply for a CDP/DAC. Pre-amp: BAT 3iX, Output impedance: 1000 Ohm I also have BAT's VK-42SE solid state amp on loan, but it still seems too smooth and laid back for my speakers, which are already laid back. Amp: BAT VK-250 150W @ 8 Ohm Input impedance: 100K Ohm. Should be any easy load? Gain: 26 dB Speakers: Vandersteen Quatros, not very efficient but an easy load for the amp. |
#2
Posted to rec.audio.high-end
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Active vs. passive pre-amp
"Dave Cook" wrote in message
... Out of curiosity, I hooked up my DAC directly to my amp just to hear what it would sound like without going through my preamp. The DAC is fed by a Squeezebox Touch, so I can use it's digital volume control for attenuation. I was rather shocked how much more vibrant and involving the sound was. It does make me wonder what the benefit of all that electronics in a pre-amp really is. So I'd like to try going passive with something like a stepped attenuator. Is there anything I should know about the downside of going passive? **Given the low output impedance of your DAC, your findings are hardly surprising. The high output impedance of your preamp was likely causing the problems you cited. It does sound like there might by some loss of bass, though I'd like to narrow this down with whatever measurements I can do with my voltmeter and SPL meter. **If you keep the passive attenuator as low as practicable impedance-wise, you should enjoy excellent results. I'd start with a 1,000 Ohm pot and see what happens. My typical "serious" listening level is about -20dB (relative to unity gain). A low level recording might require the volume to be raised to -15dB or so. So I don't think I'll have any problems with having enough gain. Equipment, all connected with XLR cables, is as follows: DAC: Cambridge Audio Azur 840C. Output impedance: 50 Ohm. This seems to have a pretty beefy power supply for a CDP/DAC. Pre-amp: BAT 3iX, Output impedance: 1000 Ohm I also have BAT's VK-42SE solid state amp on loan, but it still seems too smooth and laid back for my speakers, which are already laid back. Amp: BAT VK-250 150W @ 8 Ohm Input impedance: 100K Ohm. Should be any easy load? Gain: 26 dB Speakers: Vandersteen Quatros, not very efficient but an easy load for the amp. ..**I suggest you search out an active preamp which exhibits a nice low output impedance. IME, such preamps can be blameless, sound-wise and equal a passive attenuator in terms of distortion. -- Trevor Wilson www.rageaudio.com.au |
#3
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On Sun, 17 Oct 2010 09:11:21 -0700, Dave Cook wrote
(in article ): Out of curiosity, I hooked up my DAC directly to my amp just to hear what it would sound like without going through my preamp. The DAC is fed by a Squeezebox Touch, so I can use it's digital volume control for attenuation. I was rather shocked how much more vibrant and involving the sound was. It does make me wonder what the benefit of all that electronics in a pre-amp really is. So I'd like to try going passive with something like a stepped attenuator. Yes, This is something that most audiophiles all seem to try at some point. I once built a preamp that had one transistor (as a buffer) and a hand-full of passive parts per channel after trying the bypass test you just did. This was before digital though. My source components were an Otari MX5050 half/track, 15ips tape deck (which I still have), a stand -alone phono preamp (don't remember the brand), and a Scott 4310 AM/FM Stereo tuner (which I WISH I still had). When I replaced my (expensive, for the time) Crown IC-150 preamp with it I too was amazed at the sound **. That started me thinking about the number of active stages in a piece and how they affect the sound. Eventually, I went to all-tube amplification because the tube circuits are generally simpler than solid-state circuits. Is there anything I should know about the downside of going passive? It does sound like there might by some loss of bass, though I'd like to narrow this down with whatever measurements I can do with my voltmeter and SPL meter. Passive preamps can work and well, but you have to have to be careful. Some source components will have higher output impedances than others and their sound can end-up being compromised by the variable-load placed upon them by the volume control. That's why I advocate a "semi" passive approach where an emitter follower (in bipolar transistors) or a common-drain (in FET circuits) precedes the volume control which is just before the output (an op-amp like the National LM49710 can be used too, but this makes the circuit more complex). My typical "serious" listening level is about -20dB (relative to unity gain). A low level recording might require the volume to be raised to -15dB or so. So I don't think I'll have any problems with having enough gain. Equipment, all connected with XLR cables, is as follows: DAC: Cambridge Audio Azur 840C. Output impedance: 50 Ohm. This seems to have a pretty beefy power supply for a CDP/DAC. Pre-amp: BAT 3iX, Output impedance: 1000 Ohm I also have BAT's VK-42SE solid state amp on loan, but it still seems too smooth and laid back for my speakers, which are already laid back. Amp: BAT VK-250 150W @ 8 Ohm Input impedance: 100K Ohm. Should be any easy load? Gain: 26 dB Speakers: Vandersteen Quatros, not very efficient but an easy load for the amp. Don't know, of course, how handy you are around a soldering iron but what I would do, if I were you, is to go to: http://www.dact.com/html/passive_preamp.html And try their passive attenuator. Make it simple, just one input, a volume control, and an output and plug each of your sources into it one at a time and see how it works. If none of your source components is loaded-down by the volume pot, then you can safely go buy (or build) yourself a switchable version. You might also take a look at: http://www.stereophile.com/solidpreamps/54/index.html Some of Corey Greenberg's points about passive vs, buffered control stages are well taken (if you can stand his too-cute-by-a-mile writing style), although his design puts the volume potentiometer BEFORE the buffer stage which seems to be elementary purpose defeating to me. If you decide to build Greenberg's circuit, keep in mind that this article was written almost 20 years ago. I'd replace the spec'd PMI BUF-03AJ milspec op-amp with the aforementioned LM47910N from National. It should drop right-in with no circuit modification required. Just make sure that when you wire it, you change the pinout to suit the Natty part's mini-dip configuration. PMI isn't even in business anymore and I don't think the BUF-03AJ is available. Good luck. ** Makes one wonder. If all amps and preamps have NO sound of their own, as I have seen many people here advocate, then how come there is such a difference between NO active preamp and any active one? |
#4
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On Oct 17, 2:52=A0pm, "Trevor Wilson" wrote:
**Given the low output impedance of your DAC, your findings are hardly surprising. The high output impedance of your preamp was likely causing t= he problems you cited. Well, the pre-amp and amp are both BAT gear, so I assume the sound is what BAT was aiming for. Also the 100k Ohm input impedance of the amp seems like it should be an easy load for the pre (few amps, mostly tube amps, have a higher input impedance.) The bass is excellent, but the pre has an overall dark sound that doesn't work well with the already laid back Vandies. .**I suggest you search out an active preamp which exhibits a nice low output impedance. IME, such preamps can be blameless, sound-wise and equa= l a passive attenuator in terms of distortion. The BAT SS pre that I have on loan has an output impedance of 200 Ohm, which still seems a little too "smooth". Maybe I'll try some semi-pro gear like Bryston. Dave Cook |
#5
Posted to rec.audio.high-end
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Active vs. passive pre-amp
"Dave Cook" wrote in message
Out of curiosity, I hooked up my DAC directly to my amp just to hear what it would sound like without going through my preamp. The DAC is fed by a Squeezebox Touch, so I can use it's digital volume control for attenuation. There seems to be no reason for addding anything else to the signal path if this is the entire system. I was rather shocked how much more vibrant and involving the sound was. Something is not what it seems here. While the BAT 3ix preamp is not any good technican's idea of a good general-purpose preamp by virtue of its poor performance into low impedance loads (Please see http://www.stereophile.com/tubepream...at/index4.html and frequency response curve when driving a 600 ohm load), it should be OK for driving your BAT VK-250 power amp due to its high input impedance. Of course, it may have subtly failed in some non-obvious way except that it sounds bad. It does make me wonder what the benefit of all that electronics in a pre-amp really is. In this system the BAT 3ix represents something that could possibly go awry when it is completely unecessary as in an utterly simple system like this. So I'd like to try going passive with something like a stepped attenuator. What is wrong with just using the digital attenuator in the DAC? The most likely fault would be excessive noise when you turn its volume down. Can you report on this? There is a lot of audiophile myth and misapprehension about digital volume controls, but a properly designed digital volume control can have many practical and sonic advantages over common analog attenuators. For one thing, channel balance of the digital volume control is likely to be near-perfect, even at low volumes. |
#6
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On Oct 18, 7:12=A0am, "Arny Krueger" wrote:
your BAT VK-250 power amp due to its high input impedance. Of course, it = may have subtly failed in some non-obvious way except that it sounds bad. Yeah, the 3iX is rolled off in the bass going into a 10K Ohm Bryston, but is fine with the 100K Ohm BAT amp. I suspect it's just doing it's job, though: sounding dark and laid back to compensate for "detailed" audiophile gear in the rest of the chain. The BAT VK-42SE, an SS unit, is less dark sounding, but still sounds smooth and laid back. One thing, though, the mid-bass going direct does seem a little soft compared to going thru one of the pre-amps. I've measured output from the amp with sine tones and a voltmeter, and there doesn't seem to be any problem in the bass, though. Not sure what the pre-amps are doing to get a fuller bass sound. What is wrong with just =A0using the digital attenuator in the DAC? The most likely fault would be excessive noise when you turn its volume down. =A0Can you report on this? The noise from the DAC is much lower than from the 3iX, which is already pretty low for a tube unit. It's the difference between having to have my ear right on the tweeter to hear any noise vs. having to be a few inches away to hear it. The Squeezebox volume control is also 24-bit, so I'm not worried about eating into the bottom 16 bits. I never seem to need more than 15-35 dB of attenuation. |
#7
Posted to rec.audio.high-end
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Active vs. passive pre-amp
After trying direct for a few days and trying a couple other preamps
borrowed from my dealer, I find I still prefer the sound of going direct for everything but the mid-bass, where all the active pre-amps have more authority and texture. Going direct is not only cleaner sounding, but instruments sound more natural and it feels more like you are in the room with musicians. The actives also perhaps have a wider soundstage, but I don't find the effect all that natural. Dave Cook |
#8
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On Tue, 19 Oct 2010 18:57:50 -0700, Dave Cook wrote
(in article ): After trying direct for a few days and trying a couple other preamps borrowed from my dealer, I find I still prefer the sound of going direct for everything but the mid-bass, where all the active pre-amps have more authority and texture. Going direct is not only cleaner sounding, but instruments sound more natural and it feels more like you are in the room with musicians. The actives also perhaps have a wider soundstage, but I don't find the effect all that natural. Dave Cook This midbass "authority and texture" of which you speak is LIKELY a loading issue. |
#9
Posted to rec.audio.high-end
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Active vs. passive pre-amp
"Audio Empire" wrote in message
On Tue, 19 Oct 2010 18:57:50 -0700, Dave Cook wrote (in article ): After trying direct for a few days and trying a couple other preamps borrowed from my dealer, I find I still prefer the sound of going direct for everything but the mid-bass, where all the active pre-amps have more authority and texture. Going direct is not only cleaner sounding, but instruments sound more natural and it feels more like you are in the room with musicians. The actives also perhaps have a wider soundstage, but I don't find the effect all that natural. This midbass "authority and texture" of which you speak is LIKELY a loading issue. I'd further speculate that it is due to the active preamps introducing a high pass filter effect in the lower bass. The wider soundstage may be due to the active preamps adding a little channel imbalance. |
#10
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On 2010-10-20, Audio Empire wrote:
This midbass "authority and texture" of which you speak is LIKELY a loading issue. Going into a 100K Ohm load on the BAT? That's as about a low a load as you can find in an SS amp. The output impedance of the DAC is around 50 Ohm, giving a 200:1 ratio. I should mention for completeness that the Vandersteen Quatros require a high pass filter (6 dB/octave, -3dB@100 Hz) before the amp. The Quatros built-in amps then compensate. This hp filter always needs to be adjusted for the input impedance of the amp. I think it should be transparent to the source, but maybe it's not. I don't know how to measure the input impedance at the hp filter. Dave Cook |
#11
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On 2010-10-20, Arny Krueger wrote:
I'd further speculate that it is due to the active preamps introducing a high pass filter effect in the lower bass. Seems possible. I've checked with a voltmeter and some sine tones, and the BAT VK-42SE (the solid state one) did seem to drop a little in voltage from 200 to 1000 Hz, but I'm not sure I should trust that the voltmeter's VRMS input is accurate at these frequencies. The wider soundstage may be due to the active preamps adding a little channel imbalance. I don't see how that would make the soundstage wider in both directions rather than just shift it. Maybe it was just a volume effect. I'll try more careful level matching. I can't do any AB testing as everything requires cable swaps. Dave Cook |
#12
Posted to rec.audio.high-end
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Active vs. passive pre-amp
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#13
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On Oct 20, 7:21=A0pm, John Stone wrote:
It seems strange that there would be such a difference. Modern high quali= ty preamps should be transparent. Remember I'm dealing with equipment with output impedances in a range of ~ 50 Ohm to 1000 Ohm and with tube gear vs. solid state gear. I do know that when I had the 1000 Ohm BAT 3iX hooked up to my Bryston 3B-SST amp (10K input impedance) I measured a drop in the bass response below 100 Hz of at least 1dB from. the bass response with the 100K Ohm BAT VK-250 amp. The drop relative to the response with the 200K Ohm BAT VK-55 (a tube amp) was more like 2dB below 100 Hz. Are you doing any level matching when you are doing your comparisons? No, but why can't I make one piece sound like the other just by adjusting the volume? I'm trying to set up now for some level matched tests with Room EQ Wizard and my Radio Shack SPL meter. If I'm careful I think that should be able nail down frequency response differences to within 0.1 dB or so. Dave Cook |
#14
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On Oct 20, 7:21=A0pm, John Stone wrote:
It seems strange that there would be such a difference. Modern high quali= ty preamps should be transparent. Are you doing any level matching when you = are doing your comparisons? I did some measurements with Room EQ Wizard. Levels are calibrated with pink noise and log sweep is used for getting frequency response (phase is also recorded, but I'm not sure what to do with that). I think the error here in setting the calibration is at least 0.2 dB, though. I plotted the relative level in dB between each device used vs. going direct from the DAC to the amp. http://www.mediafire.com/?spuun4alsdv5e The devices a Bryston 3iX tube pre-amp with 1000 Ohm output impedance Burson Buffer, which is an active buffering device Gryphon XT solid-state pre-amp with 50 Ohm output impedance (I think I may have messed up the calibration when measuring the Gryphon). I don't know how to do octave smoothing with gnuplot, so you have to eyeball the trends in the graphs. Maybe someone can suggest something free that can do a better job plotting audio data and that can take columnar data from a textfile. However, it seems pretty clear that the BAT 3iX has a rising midrange and a falling treble. I also tried measure the Vrms output from the amp with a voltmeter, but mine does not seem to be accurate over 1kHz or so. Dave Cook |
#15
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On Thu, 21 Oct 2010 21:31:33 -0700, Dave Cook wrote
(in article ): On Oct 20, 7:21=A0pm, John Stone wrote: It seems strange that there would be such a difference. Modern high quali= ty preamps should be transparent. Are you doing any level matching when you = are doing your comparisons? I did some measurements with Room EQ Wizard. Levels are calibrated with pink noise and log sweep is used for getting frequency response (phase is also recorded, but I'm not sure what to do with that). I think the error here in setting the calibration is at least 0.2 dB, though. I plotted the relative level in dB between each device used vs. going direct from the DAC to the amp. http://www.mediafire.com/?spuun4alsdv5e The devices a Bryston 3iX tube pre-amp with 1000 Ohm output impedance Burson Buffer, which is an active buffering device Gryphon XT solid-state pre-amp with 50 Ohm output impedance (I think I may have messed up the calibration when measuring the Gryphon). I don't know how to do octave smoothing with gnuplot, so you have to eyeball the trends in the graphs. Maybe someone can suggest something free that can do a better job plotting audio data and that can take columnar data from a textfile. However, it seems pretty clear that the BAT 3iX has a rising midrange and a falling treble. I also tried measure the Vrms output from the amp with a voltmeter, but mine does not seem to be accurate over 1kHz or so. Dave Cook Check E-bay for a used HP 400E Audio voltmeter. They're generally available cheap. I bought one last year off of E-bay for $15 sans power cord. |
#16
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On Thu, 21 Oct 2010 21:30:58 -0700, Dave Cook wrote
(in article ): On Oct 20, 7:21=A0pm, John Stone wrote: It seems strange that there would be such a difference. Modern high quali= ty preamps should be transparent. Remember I'm dealing with equipment with output impedances in a range of ~ 50 Ohm to 1000 Ohm and with tube gear vs. solid state gear. I do know that when I had the 1000 Ohm BAT 3iX hooked up to my Bryston 3B-SST amp (10K input impedance) I measured a drop in the bass response below 100 Hz of at least 1dB from. the bass response with the 100K Ohm BAT VK-250 amp. The drop relative to the response with the 200K Ohm BAT VK-55 (a tube amp) was more like 2dB below 100 Hz. Are you doing any level matching when you are doing your comparisons? No, but why can't I make one piece sound like the other just by adjusting the volume? That IS level matching. It must be as close as possible, preferably within 1 dB or less. Even a small level mismatch will make the two sources sound different even if they really aren't different. The problem is that the ear can detect a tiny level difference as a "difference" even if one cannot identify that difference as being a difference in volume. I'm trying to set up now for some level matched tests with Room EQ Wizard and my Radio Shack SPL meter. If I'm careful I think that should be able nail down frequency response differences to within 0.1 dB or so. You'd be better off to use an audio voltmeter across the speaker wires to match source levels using tones on test CDs or an audio oscillator (Behringer sells a cable tester with a built-in 2 tone oscillator that puts out either 400 Hz or 1KHz at "0" Vu, -10 Vu, or -50 Vu. The cable tester is $30 most places and is battery powered. Pretty handy thing to have around). The Radio Shack meter is simply not accurate enough to level match to the degree required for this type of test. You want accurate electrical level parity, not SPL parity. |
#17
Posted to rec.audio.high-end
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Active vs. passive pre-amp
"Dave Cook" wrote in message
On Oct 20, 7:21 pm, John Stone wrote: Are you doing any level matching when you are doing your comparisons? No, but why can't I make one piece sound like the other just by adjusting the volume? I'm trying to set up now for some level matched tests with Room EQ Wizard and my Radio Shack SPL meter. If I'm careful I think that should be able nail down frequency response differences to within 0.1 dB or so. Measurements in the acoustic domain are notoriously unstable. I also tried measure the Vrms output from the amp with a voltmeter, But mine does not seem to be accurate over 1kHz or so. Most general use voltmeters are that way. Their major purpose in life is measuring 50 or 60 Hz power. Just use a 400 Hz test tone! |
#18
Posted to rec.audio.high-end
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Active vs. passive pre-amp
On Sat, 23 Oct 2010 10:36:51 -0700, Arny Krueger wrote
(in article ): "Dave Cook" wrote in message On Oct 20, 7:21 pm, John Stone wrote: Are you doing any level matching when you are doing your comparisons? No, but why can't I make one piece sound like the other just by adjusting the volume? I'm trying to set up now for some level matched tests with Room EQ Wizard and my Radio Shack SPL meter. If I'm careful I think that should be able nail down frequency response differences to within 0.1 dB or so. Measurements in the acoustic domain are notoriously unstable. I also tried measure the Vrms output from the amp with a voltmeter, But mine does not seem to be accurate over 1kHz or so. Most general use voltmeters are that way. Their major purpose in life is measuring 50 or 60 Hz power. Just use a 400 Hz test tone! It's better than acoustic . Besides, absolute accuracy is not important here, anyway. What's important is RELATIVE accuracy. I.E. that the meter read EXACTLY the same for all of the units under test with the same 400 Hz TONE, and that the scale can be sufficiently expanded to get the outputs as close one another as possible. |
#19
Posted to rec.audio.high-end
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Active vs. passive pre-amp
"Audio Empire" wrote in message
On Sat, 23 Oct 2010 10:36:51 -0700, Arny Krueger wrote (in article ): "Dave Cook" wrote in message I also tried measure the Vrms output from the amp with a voltmeter, But mine does not seem to be accurate over 1kHz or so. Most general use voltmeters are that way. Their major purpose in life is measuring 50 or 60 Hz power. Just use a 400 Hz test tone! It's better than acoustic. Far better. There really is no such thing as 0.1 dB accuracy or resolution with acoustic measurements. Have someone else walk round the room, or simply wait for the air currents and temperatures to redistribute themselves, and you will measure a different number. Acoustic measurements with pure continuous tones haven't been used by professionals for decades. The first refinement was to start using swept tones, and the second refinement was using spectrum analyzers to measure artificially generated noise. Besides, absolute accuracy is not important here, anyway. What's important is RELATIVE accuracy. I.E. that the meter read EXACTLY the same for all of the units under test with the same 400 Hz TONE, and that the scale can be sufficiently expanded to get the outputs as close one another as possible. Agreed. Matching is of the essence, not absolute frequency response. Most cheap digital voltmeters are already a dB or more down at 1 KHz. But they can generally be used for comparisons up to about 10 KHz or more. The frequency response of DVMs is not always specified. I have two DVMs - a Fluke and a Protek, that are flat within a few tenths of a dB up to 20 KHz. The rest are not really audio instruments, but I could and have used them for level matching. Good frequency response is actually a distraction when you are metering the power line and batteries, which is what most people use DVMs for. |
#20
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Active vs. passive pre-amp
On Mon, 25 Oct 2010 07:09:55 -0700, Arny Krueger wrote
(in article ): "Audio Empire" wrote in message On Sat, 23 Oct 2010 10:36:51 -0700, Arny Krueger wrote (in article ): "Dave Cook" wrote in message I also tried measure the Vrms output from the amp with a voltmeter, But mine does not seem to be accurate over 1kHz or so. Most general use voltmeters are that way. Their major purpose in life is measuring 50 or 60 Hz power. Just use a 400 Hz test tone! It's better than acoustic. Far better. There really is no such thing as 0.1 dB accuracy or resolution with acoustic measurements. Have someone else walk round the room, or simply wait for the air currents and temperatures to redistribute themselves, and you will measure a different number. Acoustic measurements with pure continuous tones haven't been used by professionals for decades. The first refinement was to start using swept tones, and the second refinement was using spectrum analyzers to measure artificially generated noise. Besides, absolute accuracy is not important here, anyway. What's important is RELATIVE accuracy. I.E. that the meter read EXACTLY the same for all of the units under test with the same 400 Hz TONE, and that the scale can be sufficiently expanded to get the outputs as close one another as possible. Agreed. Matching is of the essence, not absolute frequency response. Most cheap digital voltmeters are already a dB or more down at 1 KHz. But they can generally be used for comparisons up to about 10 KHz or more. The frequency response of DVMs is not always specified. I have two DVMs - a Fluke and a Protek, that are flat within a few tenths of a dB up to 20 KHz. The rest are not really audio instruments, but I could and have used them for level matching. Good frequency response is actually a distraction when you are metering the power line and batteries, which is what most people use DVMs for. Yep. But like you say, good frequency response or accuracy are not important here, but repeatability is. Still, in all, I consider my E-Bay purchase of an HP-400E to be about the best $15 I ever spent on audio (and my Behringer cable-tester box with the built-in 400 Hz and 1KHz oscillator was the best $30 I ever spent on an audio device - BTW, If you don't have one, American Musical Supply is selling these puppies fro $21! ) |
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