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Double Amping

Discussion in 'Sound Science' started by sunshinereggae, Jul 13, 2014.
  1. SunshineReggae
    I was wondering about how double amping works. I think I understand how things work in relation to distortion; that the second amp will end up amping the first one's distortion etc. So if I were to guess, I'd say that if an amp had audible crosstalk and you doubled amped, you'd make the crosstalk even more audible. Is that right? 
    Also, how does this work in terms of output/input impedance? Is that something that factors into amplification or does a very high input impedance prevent these things from playing a part?
    Also, I was looking into understanding DACs some more, and what I read is that when a DAC has an output stage, it will always amplify the signal to line level. What is meant by 'line level'; what is its significance?
  2. blades
    Very badly.
  3. castleofargh Contributor

    there are many factors to account for. in principle it's nothing much, one signal get in the first amp, gets out with a gain(voltage gain on most audio amps) and into the second to get out with another gain. in some situations, you can get good results by using several op amps instead of trying to get a huge gain from just one. it really depends on a lot of specs and what we're trying to do.
    but to get low SNR from the first amp it would be better to push it close to max volume. while max volume might get more distortion(and maybe some clipping). so there is that decide.
    whatever gets out will indeed be amplified by the second amp. so better have a good margin between sound and noise/disto.
    then there is the problem of the first amp reading a load of a few thousand ohm, so how it deals with that depends on the amp itself. but it should be ok.
    then there is the problem of the second amp not clipping from too much voltage going in. that can very much not be ok.
    I guess all could be determined and with the right amps and right settings it could work ok, but it can also go bad very easily.
    about crosstalk, nope, if all the second amp is doing is amplify left and right, the original left signal and the right mixed into it by the first amp will just both have more gain. so it won't be more audible, both will be louder but separated by the same margin. but of course if the second amp also has a terrible crosstalk it's another matter. but still if I think about it, let's say at some point the signal from one side goes to the other 60db quieter in one amp. then if you do that twice, the last mix up will in fact take the -60db signal from the wrong side to bounce it back with the rest where it should be with another -60db attenuation. so I don't think that matters much as long as both amps have an ok crosstalk (I may be totally wrong here about crosstalk, that's just my own simple logic and it may not be accurate).
    still by principle I'm not a huge fan of double amping.
    Deders, panasonicst60 and koolas like this.
  4. SunshineReggae
    Thanks, appreciate it. Could you clarify this: "then there is the problem of the second amp not clipping from too much voltage going in. that can very much not be ok."? I don't have the knowledge to understand that.
  5. SunshineReggae

    Thanks. Yes, I didn't mean that I was wondering in terms of how feasible it was, or what the pros and cons were. I understand there's never a good reason to double amp if you can avoid it. But, assuming a situation where double amping just happened to be the case, I was wondering what the workings of something like that would be. So if you answer "very badly", I would ask why?
  6. wuwhere Contributor
    Usually the output impedance of an amp is very low but the input impedance is very high.
    alphanumerix1 likes this.
  7. spook76
    It also really depends on your source and your amplifier. With my set up iPod Touch to RSA Lightning I vastly prefer double amping to using an LOD. Use of an LOD locks the output of the DAP (iPod Touch) at close to maximum volume allowing little or no room to adjust the amplifier's volume.

    Using 3.5mm interconnects allows me to turn down the volume output of the DAP to 50% and increase the power on my amp. I completely avoid the possibility of distortion or clipping from the DAP and I gain greater clarity and soundstage/headroom from using the amp (this presupposes a high quality amplifier). My brother was a sound engineer for a regional band years ago and he did the same. His advice was if you have a good quality amp let it carry the load. He would turn down the feed from the band and increase the volume/power of the amps.
    1c3d0g likes this.
  8. SunshineReggae
    Well, don't get me wrong, I'll respect your personal impressions. But I am more interested in the technicality of it. Let's say the first amp has a 50 ohm output impedance and a very high input impedance. How will this affect the second amp?
  9. SilverEars
    I believe all amps have very high input impedance and output impedance magnitude less.  This means that if you have a DAP with significant output impedance such as the AK100MK1 with 22 output impedance, it's a good idea to add an external amp to it if you are using iems as the external amp will act like a buffer reducing the output impedance.  The difference I believe between double amping vs amping a line-out is line-out skips the amp section, which means that that noise and distortion from the additional component(the opamp) was not added.  If let's say you connect the external amp to the headphone out, which includes the amp section, it sould added noise and distortion from the amp, and when this signal is passed to the external amp, depending on the gain, the signal with distortion and the noise will be amped.  Correctly me if I'm wrong, but amping the noise floor of the amp previous would raise the noise floor and subsequently reduce the SNR in the end.  I think there are others that can answer your question better than I.
  10. SunshineReggae
    So the opamp is the output stage in a DAC?
  11. castleofargh Contributor

    well let's take a crappy made up example, a portable amp build mostly for old ipods, so when you feed it around 0.5v, it can output 3v from it. obviously the op amp for that particular amp will be picked to be good with those values(best dynamic, low distortion, whatever) and the power supply will be made to deliver around those values. you won't build something for speakers if it's done for portable stuff.
    so let's say that op amp can deliver a maximum of 6v. it means that if you feed it with anything from 0 to 1volt, it will output the same thing time 6 (0.5v becoming 3v) because that's the set gain and so it will output the music with 6times the voltage. but if the input signal goes up to 2v, the amp will not be able to pass its max of 6v. that's half the desired value.
    so everything in the signal from 1 to 2v will clip.
    it all really depends on the amp and op amp specs. but obviously something done to deal with an ipod might not enjoy receiving a speaker amp signal maxed out as its input signal ^_^.
    I hope the poor example is clear enough.
  12. spook76
    I see Castle answered your question. He knows far more than me and was actually the man who convinced me to buy my current portable amp.
  13. SunshineReggae
    Okay, let's say I use a sound card with a 3.5mm output and i connect an amp to it. The soundcard, next to being a DAC, will also amp the signal, so effectively I am double amping now. Normally, you would say that in order to get the best signal to noise ratio, you maximize volume on the DAC to get output at line level right? But in this case we are double amping, so at what volume would the sound card then need to be? If I maxed the soundcard's volume, I would be also maxing the amplified output from the soundcard, which would probably lead to clipping right?
    It's very confusing to me - I don't quite know how this works.
  14. castleofargh Contributor
    with a soundcard's HO, except potential clipping(nothing tells that there will be any), everything else will probably work better close to the max volume level. for a better SNR and also to avoid using too much digital volume.
    clipping is usually audible so if you hear nothing at max level, you can be pretty confident with say 85 or 90% max volume on the computer.

    but I'd say, go get a cheap usb dac, even a 20$ hifimediy will probably best your soundcard on HO. and never think about double amping again.
  15. Music Alchemist
    Here are some related links to aid you in your research.
    alphanumerix1 likes this.

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