General Amp Guidance
Aug 8, 2010 at 10:33 AM Thread Starter Post #1 of 4

tank

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Being new to head-fi (thanks in advance for the wallet apologies), I am seeming to have difficulty with quickly eliminating which amps are good for which types of sound systems. I was hoping someone knew of a general knowledge guide they could provide a link for and/or answer a few questions. I'm looking for general rules of thumb.... I know this information can't be set in stone.
 
1. Is there any information off the amp specs that will quickly let me know if the amp is made for driving low impedance headphones? High impedance?
 
2. Are there any specs off an amp that would show it as being a good preamp?
 
3. Is wattage the basic eliminating factor for if an amp will drive speakers? Are there any other factors which would eliminate an amp from being good at driving speakers.
 
Basically, I am trying to find out information that will let me quickly eliminate an amp as a prospect based on my needs now and/or in the future. Currently, I have to base my opinion solely on other people's reviews, which are helpful but inefficient
 
Any other general rules of thumb for amps would also be appreciated. Thanks.
 
 
 
Aug 8, 2010 at 3:35 PM Post #2 of 4

kingtz

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Great questions. I'd like to know, also.
 
Some other questions I'd like to add are:
 
4. Do any of those aforementioned specs or rules of thumb differ between SS and Tube?
 
5. I see power supply come up a lot. What exactly are we looking for?
 
Aug 8, 2010 at 6:21 PM Post #3 of 4

hodgjy

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Quote:
 
1. Is there any information off the amp specs that will quickly let me know if the amp is made for driving low impedance headphones? High impedance?
 
Generally speaking, amps with most flexibility for a variety of headphone impedance are solid state or tube amps with transformer coupling.  Tube amps that are OTL (if it says OTL in the specs, you can pretty much guarantee that high impedance cans will be the best) are generally poor with low impedance phones and are great with high impedance cans.  There really is no "golden" rule for which phones work the best with what amp specs, but most amp makers list a range of phone impedance that they recommend.
 
2. Are there any specs off an amp that would show it as being a good preamp?
 
If it says "preamp" in its specs and has RCA line-level out jacks in the back.  No RCA line-level outs, it's not a preamp.  It's a bad idea to use the headphone jack as a line out to a power amp.  The only real way to tell if a preamp sounds good is to try it.
 
3. Is wattage the basic eliminating factor for if an amp will drive speakers? Are there any other factors which would eliminate an amp from being good at driving speakers.
 
Wattage is one of the most misunderstood concepts with respect to driving speakers and loudness.  Speaker sensitivity is equally important.  A 10 watt per channel amp can be deafeningly loud with speakers with a 103 db SPL and may sound better than a 200 watt per channel amp driving 84 db SPL speakers. 
 
Also, many amp makers artificially boost their watt specs to entice customers. 
 

 
Aug 8, 2010 at 7:42 PM Post #4 of 4

Uncle Erik

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One of the best specs for determining whether an amp will work for headphones is the output impedance. Unfortunately, most won't tell you what it is. I see several manufacturers claiming that their OTLs "work" with low impedance headphones. This is generally not the case. If the output impedance is higher than the headphone impedance, things get hinky. It usually shows up with bloated and lousy bass. Some people seem to enjoy the bass bloom, but I think it sounds awful.

Some manufacturers play games with output impedance using negative feedback. That's feeding part of the output signal back into an earlier stage. It does make the output more linear, but too much NFB strangles the sound. Nothing is instantaneous, so the time it takes for the NFB to go back fiddles with the timing. NFB also lowers output impedance, so sometimes it gets turned up much too high in order to make the amp work with lower impedance headphones better, but it also mucks up the sound. Of course the salesman won't tell you that.

Solid state doesn't have any problems with output impedance, it's naturally low. Tubes only have a low output impedance if you put them on output transformers (expensive) or use something like a 6C33C which has a low output impedance.

Power supplies are very important. Solid state, generally, is cheap to power. It usually runs around 12V-24V so you don't need the heavy-duty expensive iron for the high voltage tubes need. Tubes often run at 200V or more, so the bigger iron costs more. With solid state, you can use inexpensive chips and capacitors to turn the low voltage into clean DC. For high voltage, tubes need big caps and, preferably, chokes to really get clean DC. You've probably already figured that those cost a lot more. Clean DC is hugely important because that's what you're listening to. If a power supply lets some AC noise through, that AC noise ends up in your headphones. Cheap tube amps skimp on power supply parts - that's the easiest way to lower costs. If you want to do a tube amp the right way, you can easily spend $500 or more just on parts for the power supply.

In short, it's easier to do solid state the right way on a budget. Solid state might not be as sexy as tubes, but it will deliver excellent sound on a budget. If you want tubes, you either have to buy one of the better ones or build your own.

Construction is also important with tubes. Cheap ones put everything on a PCB. I don't like that because PCBs are generally meant for low power applications, like solid state. Too much heat can damage them and once a PCB is damaged, they are a pain to fix. Lifted or broken traces require jumpers or heavy surgery if they can be fixed at all. Also, I'm not keen on running high voltages on traces. I'd rather put them through high temperature rated wires. This is why I think tube amps should be built point-to-point. Point-to-point costs more, but it can always be repaired. Also, it keeps the circuit paths a lot shorter since you can built the circuit in 3D instead of 2D. There's more labor involved in point-to-point, but you get a higher quality amp.
 

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