Technical Question - How Much Amplifying Does a Pre-Amp Actually Do?

[Benedict_Arnold]

I know the output from a phone cartridge is miniscule compared to a line level output from a CD player, cassette deck (younger readers please ask your parents) or whatever, 2 to 5 millivolts (I think) for a phono cartridge compared to 2.5 volts (I think) for line level sources (i.e. a 1:1000 difference).

But how much amplification does a pre-amp actually do? If there aren't any tone controls, is a pre-amp just a selector switch and a volume knob?

Please enlighten.

 

[Benedict_Arnold]

Actually, testically, I suppose a pre-amp really doesn't give much of an output at all, not in amps terms, anyway. It just controls the voltage across the bases of the transistors in the power amp, or whatever the equivalent is for tube amps?

(If I remember my O-level physics, applying a voltage across the base of a transistor allows a current to pass from the emitter to the collector and varying the voltage varies the current?)

 

[davedotco]

Can be whatever the design calls for, it can even be negative.

Pre CD, line sources, tuners, cassettes etc would have quite modest output, 300-500mv typically.

At that time the (separate) pre-amp would accept this level, increase the gain by 6-10dB and output around 1 volt to drive the power amp. In the USA pre-amp outputs were usually higher, primarily because of the 'crossover' between hi-fi and pro power amps.

The advent of CD lifted input levels to a nominal 2volts, Red book standard, though some manufacturers allowed it to creep up a little. So the output of a CD player is sufficient to dive most power amps directly and to full output. So a 'purist' integrated amplifier might consist of a power amp with, say, an input sensitivity of 750mv to 1 volt and a volume control that actually attenuates the signal to normal levels, ie 'negative gain'.

This type of design was briefly popular but gave the designer little scope to 'voice' his products, most amplifiers now have way to much gain which is why they get so loud at modest volume settings and sound 'exciting'.

 

[Benedict_Arnold]

Am I right, then, in thinking the "output" from a state-of-the-art pre-amp is 0 to 2.5 volts, and the "output" of the power am is 0 to 5 volts? Yes, I know it's all current as far as genuine "output" goes, but oyme a mekanikual injunear, not a sparkie...

 

[davedotco]

I just explained that, what did you not understand?

It accepts an input voltage and delivers an output voltage sufficient to drive the power amplifier(s) it is designed to partner. There are no standards in hifi, most pre-amps drive most power amps because there is usually so much gain that pretty much any pairings work, after a fashion.

The 'after a fashion' bit means that you need to read the threads regarding input sensitivity, volume control settings, overload and the rest. Start here....

http://www.whathifi.com/forum/hi-fi/how-loud-versus-how-far-you-turn-amplifier-volume-control

 

[Benedict_Arnold]

I just explained that, what did you not understand?

I towld ewe oim a mekanikual injunear not an elektrikul wun

 

[Benedict_Arnold]

I'm also interested in figuring out if I can use off the shelf resistors to increase the impedance of 2 x 8 ohm speakers wired in parallel back up to 8 ohms. I need to use 2 speakers for my home theatre centre channel - one each side of a stud that is Murphy's Law exactly in the wrong place. If I wire them in series I get 8 + 8 = 16 ohms, half the current I would into 8 ohms and thus half the power output. If I wire them in parallel without ballast resistors I get 1/(1/8 + 1/8) = 4 ohms, which isn't too bad, but which might drop dangerously low for an AV receiver. Now I know there's all sorts of reactances and reluctances to take into account, but in principle if I put a 4 ohm resistor in series with the two 8 ohm speakers in parallel I get back up to 8 ohms or near enough.

If the speakers are being "fed" 150 watts at 5 volts peak, that's 4.33 amps, which seems like quite a lot to fit through a 30p resistor from Maplin....

 

[splasher]

I could be wrong, but I'm sure most modern AV receivers include over-current protection. Most aren't designed to deliver their full quoted power on all channels at the same time. Given that you need to balance the channels on an AV setup to get a good result, I think adding a resistor would just make your amp compensate by increasing the level to the centre making it just as likely to go over current as a low impedance. Unless you have a reveiver that categorically won't handle 4 ohms and has no level of protection, I'd just go for it.

Whether two centre speakers is a good idea is another kettle of fish of which I've seen plenty written but never tried myself.

 

[lindsayt]

Benedict:

P=VI

P=(V*V)/R

Sqrt(P*R)=V

Therefore if your amp can provide 150 watts into 8 ohm speakers it is feeding them about 35 volts and about 4 amps.

Why don't you just try wiring your centre speakers in series? And see what it sounds like once you've rebalanced the volume settings? Surely from your A/V receivers point of view it's just the same as coping with speakers with lower efficiency?

 

[andyjm]

Can be whatever the design calls for, it can even be negative.

Pre CD, line sources, tuners, cassettes etc would have quite modest output, 300-500mv typically.

At that time the (separate) pre-amp would accept this level, increase the gain by 6-10dB and output around 1 volt to drive the power amp. In the USA pre-amp outputs were usually higher, primarily because of the 'crossover' between hi-fi and pro power amps.

The advent of CD lifted input levels to a nominal 2volts, Red book standard, though some manufacturers allowed it to creep up a little. So the output of a CD player is sufficient to dive most power amps directly and to full output. So a 'purist' integrated amplifier might consist of a power amp with, say, an input sensitivity of 750mv to 1 volt and a volume control that actually attenuates the signal to normal levels, ie 'negative gain'.

'Line level' is a mess. Lots of the standards date back to telephone engineering, and were adopted piecemeal as audio developed. dBm, dBu, dBV, VU, dBfs - it is all over the place.

Analogue equipment generally has headroom, digital systems do not. The redbook standard for CDs specifies that for dBfs (fs= full scale, the very loudest a CD can go) a CD player should generate a signal of 2v RMS. This sounds a lot more than the analogue standards, but in reality, the assumption was that the analogue systems would actually peak at much higher levels than the standard. CD players cannot.

If you assume reasonable headroom on an analogue signal, then the 300mV to 500mV 'standard' signal could easily be peaking somewhere near 2V - not really that different to a CD.

 

[davedotco]

Can be whatever the design calls for, it can even be negative.

Pre CD, line sources, tuners, cassettes etc would have quite modest output, 300-500mv typically.

At that time the (separate) pre-amp would accept this level, increase the gain by 6-10dB and output around 1 volt to drive the power amp. In the USA pre-amp outputs were usually higher, primarily because of the 'crossover' between hi-fi and pro power amps.

The advent of CD lifted input levels to a nominal 2volts, Red book standard, though some manufacturers allowed it to creep up a little. So the output of a CD player is sufficient to dive most power amps directly and to full output. So a 'purist' integrated amplifier might consist of a power amp with, say, an input sensitivity of 750mv to 1 volt and a volume control that actually attenuates the signal to normal levels, ie 'negative gain'.

'Line level' is a mess. Lots of the standards date back to telephone engineering, and were adopted piecemeal as audio developed. dBm, dBu, dBV, VU, dBfs - it is all over the place.

Analogue equipment generally has headroom, digital systems do not. The redbook standard for CDs specifies that for dBfs (fs= full scale, the very loudest a CD can go) a CD player should generate a signal of 2v RMS. This sounds a lot more than the analogue standards, but in reality, the assumption was that the analogue systems would actually peak at much higher levels than the standard. CD players cannot.

If you assume reasonable headroom on an analogue signal, then the 300mV to 500mV 'standard' signal could easily be peaking somewhere near 2V - not really that different to a CD.

I understand what you are getting at, but the final paragraph does not relate to my experience.

CD players are subjectively much louder than the kind of older analogue sources I was discussing, some modern analogue sources may have upped their output to be more 'compatible', not sure as it is not some thing I have checked out. Perhaps it is simply that digital recordings are, generally, pushed higher up in the available dynamic range than analogue recordings?

Line level standards are a mess but are of little concern to hi-fi, there is so much gain in an amplifier as to make matching irrelevant in all but a handful of situations.

 

[Benedict_Arnold]

Right.

I've done my sums properly (ish) now. All based on linear assumptions about impedance, of course.

If I have a single 8 ohm speaker producing (or perhaps consuming would be a better expression) of power, it's going to draw 4.33 amps at 34.64 volts.

If I have two 8 ohm speakers in series, the impedance goes up to 16 ohms. With the same peak voltage of 34.64 volts (assuming that's all the amplifier can muster), the speakers will draw 2.165 amps (half of 4.33 amps for twice the impedance). And each speaker will produce. sorry, consume, 2.165 x 2.165 x 8 = 37.5 watts, giving a totla consumption of 75 watts, or roughly half the power of just one speaker, not two. To get 150 watts total (75 watts per speaker) you need 3.06 amps, meaning the voltage drop across each speaker would be nearly 24.5 volts for a total across the amp of 49 volts, which I'm not sure many amps could muster.

If you have 2 x 8 ohm speakers in parallel with no ballast resistor, and apply the same 34.64 volts, you get 150 Watts per speaker, a total of 300 watts. BUT your current demand goes up to a whopping 8.66 amps. Again, I'm not sure most amplifiers would handle this. To get 150 watts total (75 watts per speaker) you need 3.06 amps per speaker again, giving a total of 6.12 amps and only 24.48 volts to drive the speakers to that level. so you have to drop (34.64-24.48) = 10.16 volts across a ballast resistor. 10.16 volts and 6.12 amps means a 1.6 (ish) ohm resistor.

Answer? Trim the center channels down a bit, or if you're a bit Mutton Jeff like me, leave 'em alone and enjoy the clearer voices, remembering the decibel curve is logarythmic not linear.

Glad I sorted myself out on that one.

 

[Benedict_Arnold]

Incidentally, it just occurred to me that this is why you should use the higher power amplifier of a two-power amplifier to drive the treble, not the bass, in a bi-amp setup.

The impedance of a speaker typically halves above 5 kHz certainly by 10 kHz Let's not bandy about specifics, let's just talk general, relative, values.

In simple maths, if the high frequency impedance of the speaker is 2R and the impedance of the treble is R (i.e the impedance halves at high frequencies) then the current needed to produce the same power (and I'm assuming the same decibels as a result) from the bass is the square root of (P/2R) or 0.717 times the square root of (P/R). The same sums for the treble yield that the current required is the simple square root of (P/R). Thus the bass only requires 72% of the current needed by the treble to produce the same watts. Hence, in theory, for the same wattage from the same speaker, the treble uses more power than the bass.

Asbestos underwear going on now....

 

[davedotco]

Incidentally, it just occurred to me that this is why you should use the higher power amplifier of a two-power amplifier to drive the treble, not the bass, in a bi-amp setup.

The impedance of a speaker typically halves above 5 kHz, so to get the same wattage you need In simple maths, if the impedance of the bass is 2R and the impedance of the treble is R, then the current needed to produce the same power (and I'm assuming the same decibels) from the bass is the square root of (P/2R) or 0.717 times the square root of (P/R). The same sums for the treble yield that the current required is the simple square root of (P/R). Thus the bass only requires 72% of the current needed by the treble to produce the same watts. Hence, in theory, for the same wattage from the same speaker, the treble uses more power than the bass.

Asbestos underwear going on now....

You will need that too.

In a bi-amp system both amplifiers are being driven full range. The lowest powered one will clip first and that is the maximum power of the bi-amped system.

I'll say that again, the maximum power of a passively bi-amped system is the maximum power of the smaller amplifier.

The rest of your post is garbage.

 

[Benedict_Arnold]

Incidentally, it just occurred to me that this is why you should use the higher power amplifier of a two-power amplifier to drive the treble, not the bass, in a bi-amp setup.

The impedance of a speaker typically halves above 5 kHz, so to get the same wattage you need In simple maths, if the impedance of the bass is 2R and the impedance of the treble is R, then the current needed to produce the same power (and I'm assuming the same decibels) from the bass is the square root of (P/2R) or 0.717 times the square root of (P/R). The same sums for the treble yield that the current required is the simple square root of (P/R). Thus the bass only requires 72% of the current needed by the treble to produce the same watts. Hence, in theory, for the same wattage from the same speaker, the treble uses more power than the bass.

Asbestos underwear going on now....

You will need that too.

In a bi-amp system both amplifiers are being driven full range. The lowest powered one will clip first and that is the maximum power of the bi-amped system.

I'll say that again, the maximum power of a passively bi-amped system is the maximum power of the smaller amplifier.

The rest of your post is garbage.

Wrong.

If I have a 5 gazillion watt Motorhead PA system hooked up to one side and a 5 watt tube amp hooked up to the other, which are you going to hear?

Yes, the lower powered amp will clip first. But since the two amps are independent, one clipping first will not cause the second to, any more than, say, a Ford Mundane-Oh running out of steam at 120 mph while the Ferarri in the other lane zooms off at 200.

 

[davedotco]

Wrong.

If I have a 5 gazillion watt Motorhead PA system hooked up to one side and a 5 watt tube amp hooked up to the other, which are you going to hear?

Yes, the lower powered amp will clip first. But since the two amps are independent, one clipping first will not cause the second to, any more than, say, a Ford Mundane-Oh running out of steam at 120 mph while the Ferarri in the other lane zooms off at 200.

You are approaching Thomson levels of lunacy here, for someone who claims some sort of technical background this is nonsense of the lighest order.

BTW. I have worked with Motorhead on a number of occasions, back in the day......

No Sleep till Hammersmith.....*shok*

 

[Vladimir]

I'll say that again, the maximum power of a passively bi-amped system is the maximum power of the smaller amplifier.

But where do the rest of the watts go????!!!!

 

[davedotco]

I'll say that again, the maximum power of a passively bi-amped system is the maximum power of the smaller amplifier.

But where do the rest of the watts go????!!!!

Down the pughole, counter clockwise in the southern hemisphere.

Think about it.

Clue. What impedance does the amplifier 'see' when driving out of band frequencies into a passive crossover?

 

[Blacksabbath25]

Wrong.

If I have a 5 gazillion watt Motorhead PA system hooked up to one side and a 5 watt tube amp hooked up to the other, which are you going to hear?

Yes, the lower powered amp will clip first. But since the two amps are independent, one clipping first will not cause the second to, any more than, say, a Ford Mundane-Oh running out of steam at 120 mph while the Ferarri in the other lane zooms off at 200.

You are approaching Thomson levels of lunacy here, for someone who claims some sort of technical background this is nonsense of the lighest order.

BTW. I have worked with Motorhead on a number of occasions, back in the day......

No Sleep till Hammersmith.....*shok*

really ! Motörhead were the first ever band I ever see at Cambridge they were the loudest bad I ever see and I have that cd too no sleep till Hammersmith it a great live album

 

[Vladimir]

I'll say that again, the maximum power of a passively bi-amped system is the maximum power of the smaller amplifier.

But where do the rest of the watts go????!!!!

Down the pughole, counter clockwise in the southern hemisphere.

Think about it.

Clue. What impedance does the amplifier 'see' when driving out of band frequencies into a passive crossover?

I have no idea. What impedance does it see?

 

[Benedict_Arnold]

I'll say that again, the maximum power of a passively bi-amped system is the maximum power of the smaller amplifier.

But where do the rest of the watts go????!!!!

Down the pughole, counter clockwise in the southern hemisphere.

Think about it.

Clue. What impedance does the amplifier 'see' when driving out of band frequencies into a passive crossover?

What crossover? If the HF and LF are no longer connected...

Yes, my argument abou the Mundane-Oh and the Ferarri falls flat if the two are tied together by a tow rope, but if they're independent of each other....

 

[Vladimir]

I'll say that again, the maximum power of a passively bi-amped system is the maximum power of the smaller amplifier.

But where do the rest of the watts go????!!!!

Down the pughole, counter clockwise in the southern hemisphere.

Think about it.

Clue. What impedance does the amplifier 'see' when driving out of band frequencies into a passive crossover?

What crossover? If the HF and LF are no longer connected...

Yes, my argument abou the Mundane-Oh and the Ferarri falls flat if the two are tied together by a tow rope, but if they're independent of each other....

But you'll end up plowing the room with bass and no mids and highs that way. Only way to balance everything to listenable is to lower the LF amp. No?

 

[Benedict_Arnold]

If you go to a disco or a rock concert, don't they turn down the treble at high volumes to stop everything sounding too shrill (Rush fans please take note :-0 )?

 

[Benedict_Arnold]

I'll say that again, the maximum power of a passively bi-amped system is the maximum power of the smaller amplifier.

But where do the rest of the watts go????!!!!

Down the pughole, counter clockwise in the southern hemisphere.

Think about it.

Clue. What impedance does the amplifier 'see' when driving out of band frequencies into a passive crossover?

What crossover? If the HF and LF are no longer connected...

Yes, my argument abou the Mundane-Oh and the Ferarri falls flat if the two are tied together by a tow rope, but if they're independent of each other....

But you'll end up plowing the room with bass and no mids and highs that way. Only way to balance everything to listenable is to lower the LF amp. No?

Which goes back to my position that you should use the LOWER paower amp for the bass...

 

[TrevC]

Passive biamping is pointless, and as you were told originally the amplifiers have to match power wise or you are limited to the power output of the lower powered amp. As for the original question, you do only need a pot and a switch at line levels.