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Is a preamplifier really necessary?


Flevoman

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6 minutes ago, Steve. said:

You don’t NEED one….. but once you have a truly great one,  I doubt you would go back to passive…..

 
Maybe. maybe not.

 

I once had an Audio Research SP9 II, an Art Audio VPL, a Lamm LL2 and a CJ 17LS all in house at the same time. All were very good, but every one was beaten by a Sonic Euphoria AVC (autoformer volume control). So if your system is passive friendly it can take quite a bit of $$ to beat a competent passive device.

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17 hours ago, captainbeefheart said:

 

As Edgar said we aren't always driving a resistive load.

 

I believe you are talking about as you turn traditional volume pot following R2/(R1+R2) which defines the voltage ratio output to input, as you further push the wiper down the resistance where R2 is much smaller than R1, you have now additionally increased the output impedance of the source before the volume pot. Say for a 10k pot, you could easily have 8,000 extra resistance added to your source no matter if it had a lower than 1 ohm output impedance it's now 8,000 ohms. When driving a reactive load like say the cable capacitance plus the input capacitance of the amplifier, you now can possibly shift the -3db corner frequency down lower into audible territory. For 8k ohms you would need 1nF of load capacitance to get below 20kH.

 

 

What this means is a passive preamp could create a situation where volume setting will effect frequency response. What you would do with a passive preamp is either keep the cabling short, or use an active buffer after it to create a low output impedance with enough current to not slew rate limit your largest peak amplitude and highest frequency of interest into worst case scenario load capacitance. It doesn't take much current for 2.8v peak, uA's even, but less than 100 ohms is ideal to easily drive 99% of the situations.

Basically, if I understand it correctly, you need either one or two different functions in a classic analogue preamp. The first function is the preamplification to a level that the power amp needs. This was more necessary in the past than it is today (except for phono preamp). On the contrary, today the output voltage of many sources is sometimes as high or higher than what the power amp needs for full power. For this reason, we sometimes don't know that the preamp nowadays often downregulates the signal and doesn't amplify it additionally.

 

I have an extreme example, a Leak Stereo 20 with 0.125V input sensitivity which is similar to a guitar amp. You can connect a "passive" control, because you have to regulate down in any case. But I have a very simple Croft Supermicro pre amp. Glen Croft designed it with the Leak Stereo 20 in mind. It has only one cathode follower stage and so the input signal is slightly attenuated rather than amplified. The Leak Stereo 20 has an input impedance of 1 megohm. So you might think that a passive pre amp is enough, but it sounds much better with the Croft pre amp with cathode follower, which is an impedance converter. I can only say for my ears that the leak amp without Croft cathode follower is more than loud enough but it sounds sterile and without meat with less timing and punch. I may be interpreting distortion but that's how it is for me.

 

This brings me again to the second function of a classic analogue preamp...precisely the impedance conversion. The impedance becomes too high due to the potentiometer, so the reduction of the impedance follows actively after the control in many analogue pre amps regardless of using tubes or solid state.

 

I'm willing to be convinced by your assumptions and calculations, but so far I've never heard a stereo system that sounded better without a preamp.

I will try it with my current very simple system: Bluesound Node (which has an adjustable output) to Quad 34 pre amp and Quad 306 power amp. According to the manufacturer's forum, the Node has an output impedance of 650 ohms, which is about the same as the Quad 34. The fact that the Quad 306 has a very high input sensitivity of 0.35 V should not be a problem for the output of the Node. So I will try it to link the Node directly to the 306. I will report back.

 

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1 hour ago, KT88 said:

Basically, if I understand it correctly, you need either one or two different functions in a classic analogue preamp. The first function is the preamplification to a level that the power amp needs. This was more necessary in the past than it is today (except for phono preamp). On the contrary, today the output voltage of many sources is sometimes as high or higher than what the power amp needs for full power. For this reason, we sometimes don't know that the preamp nowadays often downregulates the signal and doesn't amplify it additionally.

 

I have an extreme example, a Leak Stereo 20 with 0.125V input sensitivity which is similar to a guitar amp. You can connect a "passive" control, because you have to regulate down in any case. But I have a very simple Croft Supermicro pre amp. Glen Croft designed it with the Leak Stereo 20 in mind. It has only one cathode follower stage and so the input signal is slightly attenuated rather than amplified. The Leak Stereo 20 has an input impedance of 1 megohm. So you might think that a passive pre amp is enough, but it sounds much better with the Croft pre amp with cathode follower, which is an impedance converter. I can only say for my ears that the leak amp without Croft cathode follower is more than loud enough but it sounds sterile and without meat with less timing and punch. I may be interpreting distortion but that's how it is for me.

 

This brings me again to the second function of a classic analogue preamp...precisely the impedance conversion. The impedance becomes too high due to the potentiometer, so the reduction of the impedance follows actively after the control in many analogue pre amps regardless of using tubes or solid state.

 

I'm willing to be convinced by your assumptions and calculations, but so far I've never heard a stereo system that sounded better without a preamp.

I will try it with my current very simple system: Bluesound Node (which has an adjustable output) to Quad 34 pre amp and Quad 306 power amp. According to the manufacturer's forum, the Node has an output impedance of 650 ohms, which is about the same as the Quad 34. The fact that the Quad 306 has a very high input sensitivity of 0.35 V should not be a problem for the output of the Node.
I will report back.

 

 

Yes you understand correctly, but I can't say which one will "sound" better to any one individual because that's a subjective preference, all I am saying is theoretically the lower amount of active stages the less the signal can be changed by noise and distortion. We did a test where we wanted to see how many unity gain buffers in a row we could add together before we heard a difference. This wasn't a blind test so take it with a grain of salt but most of us felt that after 5-6 buffers were put back to back we could notice a difference. It was due to all the electronics in the studio no matter how simple has a bazzillion opamps in the signal path even as just unity gain buffers.

 

My calculations were for a 10k pot, but it could easily be 25k or even 50k. I have checked out a lot of systems and some passive preamps I have opened up were very high value pots. They figured since the vintage amplifier had an input impedance of 470k or whatever a 100k passive preamp was fine, yes it is for just resistive loads but add in 200pF of cable capacitance plus the 80k ohms of resistance now in series with the source it only takes 100pF to get into audible treble bleed territory. A buffer clearly showed a huge improvement with sound quality.

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On 3/13/2022 at 6:54 PM, captainbeefheart said:

 

Yes you understand correctly, but I can't say which one will "sound" better to any one individual because that's a subjective preference, all I am saying is theoretically the lower amount of active stages the less the signal can be changed by noise and distortion. We did a test where we wanted to see how many unity gain buffers in a row we could add together before we heard a difference. This wasn't a blind test so take it with a grain of salt but most of us felt that after 5-6 buffers were put back to back we could notice a difference. It was due to all the electronics in the studio no matter how simple has a bazzillion opamps in the signal path even as just unity gain buffers.

 

My calculations were for a 10k pot, but it could easily be 25k or even 50k. I have checked out a lot of systems and some passive preamps I have opened up were very high value pots. They figured since the vintage amplifier had an input impedance of 470k or whatever a 100k passive preamp was fine, yes it is for just resistive loads but add in 200pF of cable capacitance plus the 80k ohms of resistance now in series with the source it only takes 100pF to get into audible treble bleed territory. A buffer clearly showed a huge improvement with sound quality.

 

I tried it out, the Bluesound Node connected directly to the Quad 306. The technical data of the output and input impedance should speak for a suitable connection.
I listened to string quartets, piano music, symphonic music and Herbie Hancock's "Butterfly" from the 1970s, as well as some internet radio from stations whose sound fingerprint I know well.
Without the Quad 34 pre amp it sounds more open, direct and immediate. But the sound impression is not really right. The mids sound a bit boomy with some instruments and the bass is weaker, loud enough but without slam. Maybe it is measurably better without pre amp but I don't like the sound. Even though the pre amp takes away some of the definition, it sounds somehow "right" with it.

I can't and don't want to generalise my listening impressions, just my experience. It was such a difference that I was really relieved when the pre amp was back in the chain, a feeling of well-being.

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8 minutes ago, KT88 said:

 

I tried it out, the Bluesound Node connected directly to the Quad 306. The technical data of the output and input impedance should speak for a suitable connection.
I listened to string quartets, piano music, symphonic music and Herbie Hancock's "Butterfly" from the 1970s, as well as some internet radio from stations whose sound fingerprint I know well.
Without the Quad 34 pre amp it sounds more open, direct and immediate. But the sound impression is not really right. The mids sound a bit boomy with some instruments and the bass is weaker, loud enough but without slam. Maybe it is measurably better without pre amp but I don't like the sound. Even though the pre amp takes away some of the definition, it sounds somehow "right" with it.

I can't and don't want to generalise my listening impressions, just my experience. It was such a difference that I was really relieved when the pre amp was back in the chain, a feeling of well-being.

 

After all this is entertainment and the only thing that matters is what you like. When discussing technical facts I don't want them to reflect as something that should be a rule. E.g. preamps are bad. It's just what we measure and know to be true so we can quantify the physics of what's happening. This doesn't relate to what anyone likes, as that's subjective and one's own journey.

 

To be clear, over the years I can almost say with conviction many times people like non-perfect playback systems. Many people seem to prefer small amounts of low order harmonic distortion. Same is true for somewhat limited bandwidth, mainly rolled off treble as it can be fatiguing to the ear. Many amps I have heard people claim as "great bass" when measured has extremely high distortion at low frequencies (think tube amps with small output transformers). It's really odd, many of us just don't like perfection. I have read theories to why this is which I have somewhat touched upon how sound waves propagate in nature etc.   Very fascinating stuff

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39 minutes ago, captainbeefheart said:

 

After all this is entertainment and the only thing that matters is what you like. When discussing technical facts I don't want them to reflect as something that should be a rule. E.g. preamps are bad. It's just what we measure and know to be true so we can quantify the physics of what's happening. This doesn't relate to what anyone likes, as that's subjective and one's own journey.

 

To be clear, over the years I can almost say with conviction many times people like non-perfect playback systems. Many people seem to prefer small amounts of low order harmonic distortion. Same is true for somewhat limited bandwidth, mainly rolled off treble as it can be fatiguing to the ear. Many amps I have heard people claim as "great bass" when measured has extremely high distortion at low frequencies (think tube amps with small output transformers). It's really odd, many of us just don't like perfection. I have read theories to why this is which I have somewhat touched upon how sound waves propagate in nature etc.   Very fascinating stuff

 

I heard from an old buddy who works in marketing for a German company that makes PA systems, DB Audio, which is very successful or even in the forefront worldwide that they are experimenting with distortions that are artificially added to make the PA sound of a rock concert much nicer and richer and fuller.

 

https://www.dbaudio.com/global/en/

 

I worked as a stagehand in the 80s while I was studying. That was the golden era of PA sound for me. Everything was analogue, the best PA systems were from Turbo Sound or Clair Brothers. I heard all the big stars of that time who performed in North Rhine-Westphalia, from Prince to Michael Jackson to Toto and Deep Purple, from U2 to Lionel Ritchie. The sound was super good. When I go to concerts today, it often sounds antiseptic and sterile…digital. With too much compressor because of hearing protection (in principle a good thing) but it sounds like a living room hi-fi system in large. And it sounds terrible compared to the old days.

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