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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Amplification stages vs. preamps vs. the 'Gain Cell'
Amplification stages vs. preamps vs. the 'Gain Cell'
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Old 13th March 2019, 07:49 AM   #1
Vortex is offline Vortex  Hungary
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Default Amplification stages vs. preamps vs. the 'Gain Cell'

Dear all, can you please enlighten me, what is the truth behind (external) preamps in context with number of amp. stages and gain cell like volume control ?

To my understanding there are basically passive and active preamp types.

- Passive is used to attenuate input signal and let the signal pass towards the amplifier(s), it also might have an input selector but more or less that's it. For a monoblock setup it's mostly an external preamp, for integrated amplifiers, the preamp is built into the same amplifier chassis, alongside with the gain and output stages.

- Active can have active component too and raise signal level over standard (CD player-like) line-out voltage level (these outputs are called Pre-Out, right ? )

Volume control:
- for passive preamps, it's attenuation actually and lower pot settings (for normal or less room listening levels) have lower SNR than higher pot settings (although when implemented well, unnoticable)
- for active preamps, volume control can still be passive (attenuation) OR an active solution, like a gain cell, where an inverting opamp circuitry is taking care of attenuation/amplification and the volume pot itself is only changing the opamp's gain, this way achieving a similar volume-control effect.

Now I'd like to have some questions based on all these:

Conventional Class A/AB amplifiers (monoblocks too) usually have 3 amplification stages: input stage, driver stage, output stage.

- Does an external active preamp mean a 4th amplification stage in the whole gain chain ? (Or better said, it's actually the 1st stage when we count from source towards the output stage).

- Does an additional active circuitry, like the gain-cell, really achieve better sound (whatever this means) while introducing possible additional errors to the signal, than a single good quality attenuator and staying with 3 amplification stages ?

- Conventional Line-In-s on monoblocks or integrated amps have limits regarding input signal voltage. This is normal. When an active preamp's Pre-Out signal is somewhat stronger, than a standard Line-Out, what is it used for ? Are there amps where they have a Line-In and a Pre-In connector too ? I assume Pre-In is fed by an active preamp's Pre-Out and the final amp's Pre-In is bypassing its normal input stage (to avoid overdrive) so its Pre-In is connected to the driver stage ? Kind of an integrated-preamp bypass.

- When I have a HT Processor with Pre-Outs (alongside with Line-Outs), I assume I can use these Pre-Outs to feed some external amps via their Pre-Ins, again bypassing their own input stages, right ? If I used Line-Outs, I would have to use the final amps' Line-Ins, right ?

- My father used to build quite a lot of tube and transistor amps couple of years ago. He told me, due to SNR considerations, the best to use volume control (attenuation) is before the output stage. I haven't asked him further about this but I don't see such implementation as a common practice at all.. why ? Comfort, complexity, other reasons ? For separates I can understand, but for integrated amps ?
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Old 13th March 2019, 07:01 PM   #2
TBTL is offline TBTL  Germany
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Originally Posted by Vortex View Post
- My father used to build quite a lot of tube and transistor amps couple of years ago. He told me, due to SNR considerations, the best to use volume control (attenuation) is before the output stage. I haven't asked him further about this but I don't see such implementation as a common practice at all.. why ? Comfort, complexity, other reasons ? For separates I can understand, but for integrated amps ?
For best SNR the signal must be as strong as possible in all stages. In other words, the signal must be strong in as many stages as possible. That is achieved when the volume control is downstream as far as possible, which usually means right before the power amplifier. It is common practice in integrated amplifiers.

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Old 13th March 2019, 09:44 PM   #3
xrk971 is offline xrk971  United States
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Amplification stages vs. preamps vs. the 'Gain Cell'
Some power amps are 0dB gain and will rely on the preamp to basically serve as the 3-stages of a typical Class AB amp. This kind of a preamplifier will have some challenging requirements of needing to swing as high as the rails of the power amp. For a 25w amp that is 40Vpp and 50w amp, it is 57Vpp for 8ohm loads.
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Old 14th March 2019, 12:49 AM   #4
PRR is offline PRR  United States
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The phrase "passive pre-amp" makes no sense. A pre-amp amplifies, which needs active parts. A passive device does not amplify.

To understand how we made this so confusing, you have to go back in history.

We have phono pickups, microphones, radio tuners. The signal from a needle or a microphone is VERY weak. The *first* ("pre") thing we have to do is boost it up (with active amplifiers). But not so it distorts. Disks and singers vary a lot. Typically we aim for a part-Volt. (Tuners are a lot of gain also.)

If we have multiple sources we have a way to select them.

Sources and tracks vary, and we may want loud or soft. Now that the signal has been pre-amplifed to a part-volt, it is OK to cut it down a little as needed. Volume control.

To get the next step you have to know that for most of audio history there was big money in putting audio on telephone lines. The earliest PA systems often relayed a speech in Washington DC to a crowd in Philadelphia. Later radio networks sent programs all around the USA (and this was most of Bell Telephone's profit in the 1930s). Radio studios moved away from their transmitters and had to carry the signal on a telephone line.

"Line Level" can't be weak or it would drown in stray interference. It can't be too strong, for cost, and so it won't cross-talk into other telephone conversations. Line Level is "about 1 Volt".

It does not make sense to build the several pre-amps to make such a strong signal (risks overload). Instead we follow the volume control with a Line Amp. This is also the place to put tone controls.

Well, all this is too complicated for the average buyer. Makers bundled all these stages together in one box called "Preamplifier".

Power amplifiers typically take Line Level and boost it to the desired output. Loudspeaker power amps often have a gain of "about 20". Large transmitters have much higher gain as needed.

If you look at all the classic hi-fis of the 1950s-1970s they have gain-structure and functional blocks about like this.

But then came the CD. The signal from the laser-eye is very tiny gibberish, so a chip bumps it bigger. And for whatever reason, Philips made the maximum level 2 Volts.

If you look at the classic level diagram, 2V is so big that most of these amplifying stages are not needed. All you need is a passive volume potentiometer to knock the signal *down* to get the desired power amplifier level. So now the "pre-amplifier" is not an amplifier at all.
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Old 14th March 2019, 08:44 AM   #5
Vortex is offline Vortex  Hungary
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PRR, many thanks for your time and great explanation. I've never thought there's so much history behind this seemingly simple topic.

Would you please have a look at this short vid ? (5 mins). YouTube

It's about the same "no pot" philosophy I wanted to know, where to implement.

So if I understand you right, a REAL (active) preamp nowadays is actually equivalent to a monoblock's input stage, right ? Technically speaking.

A CD player's RCA Line-Out (or let's talk about a DAC) maxes at around 2V. It can be (and usually is) connected directly to the integrated amplifier's Line-In which is followed by the input stage, driver stage and output stage.

If we have a different approach, like working with separates, we have:
- passive preamp (which isn't a pre-amp actually, you said), with an input selector switch and a main volume control (attenuator)
- DSP or active 3-way crossover behind that
- 6 monoblocks (in case of 3-way stereo)

Now, individual gains representing the speaker sensitivities can be corrected on the monoblocks themselves (again a pot in the signal path, either letting the whole signal through, or attenuating a bit).

But the main volume control to control system-wide overall volume is still in the "preamp" box, again an attenuator pot.

I'm still curious, if this classic solution is better OR to make an active preamp with precise opamp and modifying volume by modifying its gain. A max pot setting would be then 1:1 amplification (neither amplifies nor attenuates the input signal), min pot setting is then a less-than-one multiplier to the signal so it gets smaller and smaller..

And I'm talking here about the system-wide volume control.
Same question goes for the 6 individual amps:
- for class D the gain control remains an attenuator, due to circuit integration and complexity
- for class A/AB (either tube or solid state), the same kind of opamp circuitry could replace the pots before the input stage, right ? OR even replace the complete input stage with it and have it connected directly to the driver stage .. ?
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Old 14th March 2019, 01:35 PM   #6
sgrossklass is offline sgrossklass  Germany
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Originally Posted by PRR View Post
To get the next step you have to know that for most of audio history there was big money in putting audio on telephone lines. The earliest PA systems often relayed a speech in Washington DC to a crowd in Philadelphia. Later radio networks sent programs all around the USA (and this was most of Bell Telephone's profit in the 1930s). Radio studios moved away from their transmitters and had to carry the signal on a telephone line.

"Line Level" can't be weak or it would drown in stray interference. It can't be too strong, for cost, and so it won't cross-talk into other telephone conversations. Line Level is "about 1 Volt".
Oh. I never connected "line level" with telephone lines, but this makes perfect sense.
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Originally Posted by PRR View Post
But then came the CD. The signal from the laser-eye is very tiny gibberish, so a chip bumps it bigger. And for whatever reason, Philips made the maximum level 2 Volts.
The signal from the laser pickup is not only small but digital to boot, and it has to be sync'd up, decoded (including CIRC error correction) and D/A converted.

2 Vrms is 17 dB above the 300 mV nominal consumer line level, which was deemed adequate headroom for an audiophile medium. Early CDs also had corresponding average levels much like existing sources. That content got louder and louder and louder over the years is not Philips' (or Sony's) fault. It is not unusual to see average levels of -10 dBFS or even -7 dBFS these days.

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Old 14th March 2019, 03:41 PM   #7
sgrossklass is offline sgrossklass  Germany
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Originally Posted by Vortex View Post
Would you please have a look at this short vid ? (5 mins). YouTube
Oh, it's this guy...

I think you should start thinking in engineering terms. Dynamic range, levels, noise, distortion, gain. What you have is a classic gain staging problem, governed by Friis' formula and its relatives.

At the power amp, you generally want a noise floor about 110 dB below the voltage corresponding to about 110 dB SPL, so there isn't any audible hiss when nothing is playing. It gets more tricky than you might think if you consider that speakers may vary in sensitivity from below 85 dB SPL / 2.83 V / m (small bookshelves) to over 100 dB (PA / horns). The latter would place correspondingly more stringent requirements on output noise level while potentially only requiring a fraction of maximum output power. (It's even more extreme in headphones.)

Sources in front of a volume control may get away with as little as 70 dB instantaneous, since you may not even want to listen that loud, or if so, a bit of background noise won't matter (the instantaneous dynamic range of our hearing actually isn't all that great).

There are multiple ways of achieving this goal. Which one is chosen will generally depend on what makes the most sense with the technology available.

If you get the levels too low, you may run into trouble with noise (/hum/interference) and may need to jump through some hoops in order to get that down.

If you select the levels too high, you may need inconveniently large power supply voltages, may have to go discrete rather than being able to pick from (inexpensive) high-performance ICs, or may have trouble keeping distortion related to slewing and general nonlinearity down.

There basically is a good bit of leeway there.

Your "inverting amplifier with variable gain" idea is not exactly new - see Musical Fidelity A1 (late '70s I think). It has one big downside - where less than perfect wiper contact will generate rather benign dropouts in a conventional circuit, in this one it creates big gain spikes. The human ear is quite insensitive to short dropouts, but short spikes are plainly audible.

When a large variability in transducer sensitivity has to be accomodated, usually a combination of variable attenuation plus variable (stepped) gain is being used. PGAs tend to incorporate both on-chip.

Many problems in the real world stem from not being able to choose the parameters of all the components at will but rather buying them off the shelf. Your typical hi-fi power amp will have a gain of 26-29 dB, sometimes more, while output noise level with input shorted can be as low as 30-60 ÁVrms. Your typical hi-fi preamp (with volume upfront) will have a gain of 16.5 dB and and output noise level of maybe 4 ÁVrms. That's about 110 ÁVrms of output noise from the preamp alone, or about 12 dB SPL @ 1 m for 100 dB / 2.83 V / m speakers. So if horns are your thing, you may be forced to install a passive attenuator in between to get noise in check, or hunt down a fancy preamp with a 2-stage volume control.

Basically, if you want to end up with a system that works well, you have to consider
* output noise level
* maximum output amplitude
* input level
* input sensitivity
at each junction in the signal chain. As-is, SNR will be limited by the ratio of input sensitivity to the RMS sum of output and equivelent input noise.

It's all a bit complex but you were basically asking for it. You may want to sketch out what it is that you actually want to do and what components you already have, so that we could apply the general rules to a specific example. There are many way to skin the proverbial cat.
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Old 14th March 2019, 06:03 PM   #8
indianajo is offline indianajo  United States
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Uh, yeah.
Preamps were necessary when LP's mikes & tape heads were the sources.
Now we have Cd's, cellphones, D/A converters putting out about 2 v at 600 to 1000 ohms impedance. Televisions and radios can put out 7 to 20 vac on their speaker lines, but have *****y internal speakers. (And "sound bars" for TV's cost $180 up, which is rediculous IMHO for a device with a 3" woofer).
The term "passive preamp" is used around here to denote a box with source selector switch, volume pot, maybe pre-switch volume pots to even devices up. You can't actually buy such a device in a physical store.
A "passive preamp" has one more function: all those cables connecting the devices to it are out there making a RF antenna and conducting radio modulations, lightning pops and loop hum from the AC wiring in the room to the power amplifier. The grounds in the cables make an AC loop pickup with the safety grounds on the various sources. So some sort of RF & hum filtering is in order on the inputs. The connectors cable to box can make diodes that reduce the radio waves to audible frequencies.
Then the cable from "passive preamp" to the power amplifier can also pick up radio, lightning, hum. The higher power amps have fans, and putting them in a remote location where these can't be heard is desirable. That lengthens the cables.
In many cases, putting an active element in the preamp is desirable to overcome the reduction by the filtering and swamp out the current generated in the cable to the amp by RF pickup and AC loop hum pickup.
I certainly need active drive on my 4 meter cables from control hub to the power amp. Else they pick up KSUG radio. It helps to justify the mixer since I do play LP's, which requires 50x gain and a RIAA filter.
Multiple input amps are made, to eliminate the "preamp", but they are called "mixer amps" and tend to be mono for the building paging market. Stereo mixer amps are sold but tend to come in 12 to 20 input boards, which is a lot of control area to put in your living room. Also they have a lot of devices like effect loops, monitor plus main amplifiers, frequency contouring which run the costs up and don't pertain to home use.
"Preamps" with all the inputs on at the same time are called "mixers". I use one now instead of a preamp, as switching the various inputs around when I change media is a nuisance. Requires walking around the table from the LP to the switch, or the CD to the switch, or the radio to the switch. I leave them all on, and don't pay a huge hiss hum penalty.
Happy shopping, or even better, building your entertainment hub.
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Last edited by indianajo; 14th March 2019 at 06:16 PM.
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Old 14th March 2019, 09:53 PM   #9
Vortex is offline Vortex  Hungary
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Uh Gentlemen, heavy inputs. Respect, what a great forum.
@sgrossklass:
3-way active setup:
- 95dB PA woofer (driven by Class D Hypex UcD2k based monoblock, gain at around 33, attenuated to match the others)
- 96dB PA midranges (driven by Class AB KT150 tube monoblock)
- 98/100dB AMT tweeter (depending on which I'm choosing of the 2 types I need, driven by Class AB Mosfet based monoblock)
- 4-way Linkwitz 4th order.. (1st way is a highpass at ~22Hz to filter out subsonics - either analog-electronic OR via DSP)

6 monoblocks alltogether, all DIY.

Sources:
HTPC with Pink Faun's PCIe I2S Card (PS Audio-like connection via HDMI to a DAC)

DAC: DSP or a Matrix X-Sabre Pro

LP: maybe later

Bluetooth board from Aliexpress.

All in DIY case, all with true balanced (XLR) outputs. The monoblocks will also have XLR inputs only. All AB monoblock based on "fully balanced" principle (pushpull) so I expect low noise levels if designed and built carefully.

That's it so far.

Oh, and maybe also a remote control for volume changing, muting, input selection etc. Some kind of IC based design or even a 256-step relay attenuator, doesn't really matter. Also from aliexpress..
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Old 14th March 2019, 11:13 PM   #10
Vortex is offline Vortex  Hungary
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@indianajo, thanks.
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