Help me understand preamp gain (and distortion)

[arpod]

So as dive deeper into my audiophile journey I'm realizing that not all preamps work the same. Here's what I understand so far and please correct me if I'm wrong.

Some are passive and some are active. The former put out a line level output while the latter can boost the output beyond line level. Tube/SS buffers help drive long cables. Schiit Saga (I have one of these) has 0db output gain (even though it has tube buffer) but Schiit Freya has up to 14db gain with balanced output and tube buffer engaged. I've been looking into some Cary Audio preamps and from what I've read they have a lot of output gain.

So preamps are help you switch between multiple line level sources and give you a robust line output signal at whatever level of attenuation you want. And power amps accept line level in right?

Okay so I have some audio engineering experience and I know a little bit about gain staging. Enough to know that if you boost the output of a source too much it will distort the next stages input. So it's really important to match the levels at every stage so that you have a clean signal all the way to the speakers.

SO if you boost the signal of your preamp's output past line level into your power amp, aren't you going to get some distortion? And what's all this about some speakers having to low a sensitivity or impedance to be powered by some low powered amps? If you have a passive preamp, won't your amps just play as loud as your speakers will go given their sensitivity without distortion?

My experience is as follows: when using Schiit Saga and some old 14 wpc tube monoblocks, I can turn the volume all the way up and it's loud but listenable and does not distort. Some speakers play louder than others depending on their sensitivity. If I use a more powerful SS amp then at the same volume settings I get more volume. Some recordings I can't even turn up the preamp to max but some recordings that are "softer" like kind of blue or solo piano works or some acoustic singer songwriter stuff I can turn up all the way and still no distortion.

So what do you need to do wrong to get the kind of distortion that will harm your speakers and what am I not understanding?

 

[Blue Shadow]

SO if you boost the signal of your preamp's output past line level into your power amp, aren't you going to get some distortion?

Well preamps have max output levels and amps have minimum input level to produce full power. If you exceed the minimum input level the amp will be putting out all it can and it will cause the amp to distort through clipping.

And what's all this about some speakers having to low a sensitivity or impedance to be powered by some low powered amps?

To low an impedance, some amps do not want to drive a load below 8Ω and many don't want below 4Ω and very few will drive loads below that. The extra current (amperage) needed is too much for the power supply. Read the back of the amp by the speaker terminals for many units.

As for sensitivity, you can run any speaker with any amp but an 85dB sensitivity (1W-1meter) will only play so loud with 10 watts whereas a 102dB speaker will drive you out of the room with sound from the same ten watts.

To make it a bit louder 3dB you need to Double the power!! Try that with the numbers above, 1W, 2W, 4W, 8W, and then no louder cause it was a ten watter and you are into clipping

To make it twice a loud you new TEN times the power, yes TEN times the power.

Amps are available from 1 watt to well over a thousand for a reason.

If you have a passive preamp, won't your amps just play as loud as your speakers will go given their sensitivity without distortion?

Not really, this all depends on the output level from the pre(max is about the same as input level) the minimum input level for full output of the amp 0.7-1.5V is common) and the sensitivity of the speakers (85-105dB is common).

Oh there is another 50 years of experience we haven't gotten to in this post.

Overdrive the amp will hurt speakers.

Others will throw up some more info to fill in some more blanks.

 

[WaynerN]

A passive preamp simply attenuates the signal from the source and passes it along to the amplifier, the volume control being the only device in the path that can alter the signal level. The main problem with passive preamps is that they are at the mercy of the source and not all sources are created equal. The problem is the output voltage and more importantly, the drive current, to get the signal thru the volume control, thru all the wires and jacks and eventually to your amplifier.

an "active" preamp has what is called a times 10 amplifier section in it. It amplifies the signal from the sources, but gives that signal much more current drive to help the signal along to the next component, being your amplifier. The active preamp homogenizes the signals from all of your sources and gives them some push to make your amplifier happy.

If you do any tape recording, you may discover that the signal from a source component to your recorder doesn't pass thru the active preamp board, but goes directly from source to your recorder. I can run mine with the preamp shut off in fact. One other preamp will not allow me to do that, but that is because it has an input buffer circuit in it.

 

[arpod]

Blue Shadow, thank you for the considered response.

Well preamps have max output levels and amps have minimum input level to produce full power. If you exceed the minimum input level the amp will be putting out all it can and it will cause the amp to distort through clipping.

I think that's what is a little bewildering to me. It seems like in professional audio, the output and input levels are very standardized whereas in hifi, there's a lot more variation between different designs. I guess that's why people say system matching is so important and why sometimes, all in one solutions or staying with one manufacturer has it's advantages.

The issue of amp distortion is a little foreign to me as I don't think I've ever heard my speakers distort. When I think of distortion I think of distorting a mic preamp which can be bad or the sound you are going for. Same with distorting the input or output tubes of a guitar amp, it's a sound you can dial in but it never harms the speakers. So it's interesting to me that if you push a hifi tube amp too far you don't just hear a little crunch you could actually blow your speakers. I always thought blowing your speakers came from pushing them to hard volume wise.

The speaker impedance + sensitivity issue is more straightforward and I understand that bit the best. Ask the amp to draw more current than it's designed to and it will distort.

So to sum up: you want to check the specs of your preamp's max output and power amp's min input and make sure they play nice together?

 

[Blue Shadow]

So it's interesting to me that if you push a hifi tube amp too far you don't just hear a little crunch you could actually blow your speakers.

Tubes are a bit different. When overdriven they don't just clip off the tops of the loud peaks the way a solid state amp will. They creep up to the limit and soften the clipping and produce even order harmonics instead of a sharp stop to the volume increase a flat top to the peak and odd order harmonics of a solid state amp. That flat top is actually not music going to the speaker and can hurt the drivers.

 

[arpod]

Tubes are a bit different. When overdriven they don't just clip off the tops of the loud peaks the way a solid state amp will. They creep up to the limit and soften the clipping and produce even order harmonics instead of a sharp stop to the volume increase a flat top to the peak and odd order harmonics of a solid state amp. That flat top is actually not music going to the speaker and can hurt the drivers.

Thank you!! Again, to refer back to my time working with mic preamps and consoles and guitar amps, solid state distortion is definitely more of a square waves type of sound. And when I've pushed my tube amps in my hifi they just seem to soften a bit. I'm using a Schiit Saga which I can run in passive mode and when I use that with my old Eico monoblocks, I can be at full volume and it sounds perfectly clear with no noticeable softening compared to playing at lower volumes. My old Marantz 2230 on the other hand does not sound great when I turn the volume up past around 2 oclock or so.

 

[Blue Shadow]

Yes, your experience is valuable and can further show you how things are getting along.

As to matching, synergy of hi-fi gear it is more stuff that sounds right together instead of working correctly as most gear will work together. Preamps have variable maximum outputs but most have specified outputs that are in the range or the specified sensitivity of the power amps. If the pre does more than the amp needs it will work...provided all inputs to the pre meet its input level requirement for full output. But most line level stuff is in the 2V CD range or 1+V of old tuners and will easily provide the level needed for a preamp.

 

[whoaru99]

...

So to sum up: you want to check the specs of your preamp's max output and power amp's min input and make sure they play nice together?

Sorta, yeah.

The amp's input sensitivity is fairly straightforward. In most cases if the input sensitivity is stated 1.0V, it means it requires 1V input to produce rated output. However, there are some cases I've seen where input sensitivity is given for 1W ouput. In those cases you have a bit more math to do to understand the input required to achieve full rated output (if achieving that is important).

Preamps are more challenging because their output is related to the input level as well as the potential for gain. Then you have rated output, max output, etc., all relative to input sensitivity of the preamp.

Fortunately, the vast majority of preamps have sufficiently low output impedance and sufficiently high gain (arguably too much in some cases) where having enough output to drive an amp to full power or beyond is no problem.

 

[arpod]

Sorta, yeah.

The amp's input sensitivity is fairly straightforward. In most cases if the input sensitivity is stated 1.0V, it means it requires 1V input to produce rated output. However, there are some cases I've seen where input sensitivity is given for 1W ouput. In those cases you have a bit more math to do to understand the input required to achieve full rated output (if achieving that is important).

Preamps are more challenging because their output is related to the input level as well as the potential for gain. Then you have rated output, max output, etc., all relative to input sensitivity of the preamp.

Fortunately, the vast majority of preamps have sufficiently low output impedance and sufficiently high gain (arguably too much in some cases) where having enough output to drive an amp to full power or beyond is no problem.

Ok so it sounds like, somewhat counter-intuitively a preamp with too low an output is more problematic than a preamp with higher output. The power amp performs better when it is fed a nice healthy signal..

 

[whoaru99]

Well, it's not just cut and dried. An amp needs "X" amount of signal to produced full rated output. However, if you don't need full rated output you don't need to provide it "X" amount of signal.

But, for a preamp, yeah; if you are short on output you are short on output. With high gain you can always turn it down but the tradeoff is low resolution/small usable range of the volume control.

 

[rothwellaudio]

The power amp performs better when it is fed a nice healthy signal..

No, the power amp plays louder when its fed a higher signal. When the signal is high enough to produce full power, that's the maximum it will take without distorting.