Amplifier Input Impedance


Why are so many high-power solid state amps designed with such low input impedances. Doesn't that really low input impedance limit the range of pre amps that can be used? Are there technical reasons why designers make these impedances so low? Why not design your muscle amp with a really high input impedance so it will potentially work well with all pre amps?
stickman451
I think the obvious answer is that the SS amp designer would prefer that you also buy their SS preamp. Yes, SS amps usually have lower input impedances than their tube brethren, but SS preamps usually have much lower output impedances than their tube counterparts. The SS amp designer does not really want you to use a tube preamp with their amp.
Low input impedance is an advantage regarding noise, and a disadvantage regarding flexiblilty...
But, a good designer could in fact design a powerful SS amp with a high input impedance if pushed...
Stickman451, you are correct as is Jmcgrogan2. Its easy enough to design a solid state amplifier with a high impedance input (100K); they could even make it switchable if they wanted.

Although our preamps are vacuum tube, they can drive even 5K with ease (they are designed to drive 600 ohms). But this is unusual with tube preamps so for the most part its likely that this will continue to be part of the equipment matching conversation for the foreseeable future.
Recently I was in the hunt for a really good set of 'muscle amps' to use on my Magnepan 20.7's and it was a little frustrating to see just how many big-name powerful SS anps have atrociously low input impedance.
Stickman451,
it's easy to levy the blame on the low impedance of the power amplifier but your Hovland HP-100 is also to blame. It's output impedance is 2500 Ohms i.e. 2.5KOhms. That's really high for a tube amp (but maybe normal??). I've seen an ARC Ref3 have a 600 Ohm output impedance, my CAT SL1 has a 100 Ohm output impedance, several Audio-Note (UK) preamps are xformer coupled to have low output impedance. SO, I know that it's very possible to have a low output impedance tube pre - the HP-100 is not one of them tho'.

Obviously you want to keep the HP-100 so it might be worth exploring a buffer stage between your pre & yet-to-be-purchased solid-state power amp?

Recently I was in the hunt for a really good set of 'muscle amps' to use on my Magnepan 20.7's and it was a little frustrating to see just how many big-name powerful SS anps have atrociously low input impedance.
can you please let the forum know which solid-state power amps you researched? And, how "atrociously low" their input impedance was?
Thanks.
The pro guys seem to handle this issue better: standard impedances.
I should keep my mouth shut because I'm ignorant, but I have read that it's hard to design a SS amp with high input impedence and still get enough power or maybe rate the amp with higher power. I know Levenson has cheated so to speak on their imput impedence ratings which are sometimes only half as high as they claim.
I have to try again. I have the same complaint on low impedence SS amps and have spent years trying to match preamp to amp. I think that the lower the input impedence, the more gain the amp has which in turn degrades sound quality. For better SQ you want low gain in amp and pre. So if company X wants to make a 200w amp, it is easier accomplished by making the amp low impedence at the input. But I don't have the technical knowledge to prove this or discuss it.

And I haven't had to deal with this issue since I got my hk990 integrated. It has a variable gain setting for the active pre which doesn't address the issue of amplifier input impedence directly (and hk doesn't say what the amp's input impedence is and hopefully hk has matched pre and amp), but it does provide a tool that affects impedence in regard to the speakers so that gain can be adjusted according to the speaker demands. If you had speakers that were 16 ohms which you could get with autoformers, a SS amp would have a lower power output.

But again I have the same complaint. Amps get gain and hence power depending on their input impedence. I'm sure someone can explain this better.
It is true that the Hovland has a pretty nasty output impedance which is just as unforgivable as exceedingly low input impedance monster amps. My point is only that this hobby hs quite enough challenges in producing great musical sound in the home, so why do some designers complicate matters with silly impedance design.

For example, the Bryston "Flagship" 28B SST2 mono blocks spec at 15,000 ohms. The Aragon Iriduim, same input impedance. These are just two examples (not trying to beat-up on these companys); there are many many other examples.
Most solid state amps are designed to work with typical all SS pre-amps/systems, not higher out impedance tube pre-amps.

I think its that simple.

With ongoing popularity of tube gear in high end audio, some lines are designed with higher input imepdance to work well with tube pre-amps as well. But it is a niche market so still not as common as would be the case otherwise.

Impedance matching is important for optimal sound quality,but many might not notice otherwise or care. Plus practically its really only a significant issue when a higher out impedance tube pre-amp is involved.
How much power are you looking for ?

Good Listening

Peter
02-10-14: Cobra2
Low input impedance is an advantage regarding noise...
I might be missing something here but in an audio power amp we are considering only voltage gain & not power gain. So, how does input impedance factor into noise when we are considering only voltage gain (in an audio power amp)?

02-10-14: Arnettpartners
I should keep my mouth shut because I'm ignorant, but I have read that it's hard to design a SS amp with high input impedence and still get enough power or ...
again, I could be missing something - how does input impedance figure into the voltage gain of an audio power amp? If we were talking power gain then I could see input impedance figuring into the overall (power) gain calculation.

02-10-14: Atmasphere
Stickman451, you are correct as is Jmcgrogan2. Its easy enough to design a solid state amplifier with a high impedance input (100K);...
so, what sets the input impedance of an audio power amp?
My understanding was that an audio power amp's input impedance could be increased by using global negative feedback (which is a no-no 'coz of all the other issues this brings with it) & by selecting the type of semiconductor device used for audio power amplification. If the designer uses BJTs the input impedance is going to be lower than if the designer used a MOSFET or even a JFET. What are the ways to make it arbitrarily high in a s.s. amp?
In tube amps this might be different as the designer can put a resistor of arbitrary large value in series with the tube input. Is that why you say that the amp input impedance could be arbitrarily high, Ralph/Atma-sphere??

How come Almarg hasn't spotted this thread as yet?? ;-)
02-11-14: Bombaywalla
How come Almarg hasn't spotted this thread as yet?? ;-)

Even the great and powerful Al needs a little vacation time every now and then. ;)
02-11-14: Bombaywalla
How come Almarg hasn't spotted this thread as yet?? ;-)

02-11-14: Jmcgrogan2
Even the great and powerful Al needs a little vacation time every now and then. ;)
:-). Thanks, guys. No vacation time needed, though, as I'm always on vacation (i.e., happily retired).

Actually, I had spotted the thread. But having never designed an audio power amplifier, I can't speak knowledgeably about what the tradeoffs would be if a solid state one were designed with a high input impedance. Certainly it's readily doable, but I don't have a good feel for what the inevitable tradeoffs would be.

In addition to those tradeoffs, of course, as John and others have noted I don't doubt that in many cases a significant factor is a lack of motivation to provide compatibility with tube preamps.

Best regards,
-- Al
Input impedance has nothing to do with gain.

You can have a high input impedance on a solid state amp by using FET devices at the input.

Large solid state amps with a low input impedance often do have a lot of gain as they are designed for use with loudspeakers that need a lot of power. You need gain for that- 30 db might barely be enough. Conversely if you are driving a horn speaker you don't need nearly so much gain so many SETs don't have all that much- sometimes 15 db is plenty.

So there is a rough correlation but it is not based on input impedance => gain.
OK. So are not some loudspeakers designed with very low impedences so that they will audition better because they are louder? (and I understand that this is not the same as sensitivity.) Does that not relate to input impedence in amplifiers?

I'm not arguing with an audio engineer--just asking?
If I remember correctly, some years ago Mr. Wilson in an article in the Absolute Sound reported that there were some advantages to using low impedance DC coupled components in audio.
http://www.audioholics.com/audio-amplif

The above site agrees with some above comments from Bombaywalla and others. To summarize, according to the source, high input impedence in SS causes high voltage gain which in turn causes noise. It can also cause bandwidth to decrease. And finally it can introduce DC offset.

My head is still spinning from trying to absorb what I read in 15 websites which destroyed my misconceptions.
But apparently high input impedences can cause more harm than good.
OK. So are not some loudspeakers designed with very low impedences so that they will audition better because they are louder? (and I understand that this is not the same as sensitivity.) Does that not relate to input impedence in amplifiers?

Not really. The fact of the matter is that all amplifiers have less distortion driving higher impedance speakers. Some speakers are lower impedance as some solid state amps will make more power (sound quality is being traded off for sound pressure in this case). Many speaker designers don't realize that though. And this **is** in fact what we are talking about with the term 'sensitivity'. But none of that has anything to do with the input impedance of an amplifier.

www.audioholics

The above site agrees with some above comments from Bombaywalla and others. To summarize, according to the source, high input impedence in SS causes high voltage gain which in turn causes noise. It can also cause bandwidth to decrease. And finally it can introduce DC offset.

This is mostly bogus! Input impedance has 100% nothing to do with gain. A high input impedance does not have to have anything to do with noise either unless the design and execution is shoddy. We get 300KHz with 100Kohm input impedance, clearly the bandwidth thing is problematic. Think about a 6AU6 vacuum tube which is very high impedance but can operate easily at 10MHz. Finally, input impedance has nothing to do at all with DC offsets, and cannot introduce it.

I would stay off that website as it is a source of misinformation, and that is being kind.
Hi Atmasphere, what is your opinion for a impedence match for a 100Kohm solid state amp that I do have, no matter a pre-amp is tube or solid state?, what impedences will work to best effect for both tube and solid state pre-amps?, Thankyou.
^Once again I disagree with some of the again mentioned assertions.
Thank you, Atmasphere. A crash course in amplfier design is clearly not going to work for me.
The input loading resistor to ground cannot effect gain.
But in solid state bi-polar input dc coupled amplifiers, if it's raised too much with an open circuit input will effect the amount of DC offset seen at the speaker terminals, which could be out of reach for any dc servo's to correct. But once something is plugged into the input, (eg preamp dac etc) the output impedance of that device becomes the input impedance of the amp, and all goes back to how it was with a smaller input loading resistor.

(as for what varies gain in an amp, it's the feedback loop and values of resistors used that raise and lower the feedback.)

So in a tube or solid state (with fet input) you can raise the input loading resistor.

But in a solid state with bi-polar input transistors you can also raise it within reason 68k or even 100k, but you should never turn it on without anything pluged into the input. Otherwise you run the risk of lots of dc going to the speakers.

Cheers George
George,

How does noise relate to your above description of input impedence, gain, etc if at all?

Lynne
If you use unshielded interconnects, then yes a high input impedance will be more susceptible to rf noise, but it's very unlikely you will hear anything, unless you live outside a taxi/cab rank, analogue cell tower, AM/FM radio tower or similar.
As we are at the low interference end of the system, it would be a different story if we were looking at the input impedance of say a phono stage or MC cartridge step-up device phono preamp.

Cheers George

Cheers George
Many people here seem to be overlooking the fact that the input to a conventional solid-state amplifier is AC coupled, and while any "termination" resistor after does affect AC input impedance, its primary purpose is to provide input bias current to the input transistor.
Finally, input impedance has nothing to do at all with DC offsets, and cannot introduce it.
Most textbooks on the subject of bipolar differential amplifiers discuss this thoroughly. Since the input bias current of a bipolar transistor varies with temperature, if the DC source impedances of each side (that is, the input side and the feedback side) are different, then the voltages developed as a result of the bias current are different, leading to an offset condition.

If a high value for the input bias resistor is desired, then the designer could raise the impedance of the feedback to match, and this would reduce offset drift, but then the noise would increase as a result of the Johnson noise developed on the feedback resistor ladder. He/she could reduce the standing current in the input pair to reduce the bias current, but this would dramatically reduce the slew rate and input-stage transconductance. (BTW insufficient input-stage current is is the true source of TIM - not global feedback.)

One could use FETs for the input stage to raise input impedance, but they have a lower transconductance than bipolars, and most of the good ones have voltage ratings a bit on the low side. Their higher impedance is offset at higher frequencies by higher capacitance, which can be reduced by cascoding, but this in turn introduces another HF pole in the input stage's response.

A designer could also add a servo to improve offset, but this is far from free, given the fact that it almost always requires lower-voltage supply rails from the rest of the amp. For that matter, he/she could also add an input buffer or balanced input stage . . . but again, there are more tradeoffs.

But the real question is, what kind of source impedance should an amplifier designer reasonably expect to see from the driving source? And in a world where only the wimpiest of preamplifiers have an issue with a 10K-50K load, how much extra cost and design effort is one willing to spend in order to satisfy a few oddball cases?
John (Jmcgrogan2) & Bombaywalla, see what I meant when I said that "I don't have a good feel for what the inevitable tradeoffs would be." :-)

Thanks, Kirk. Good to see you here again.

Best regards,
-- Al
But the real question is, what kind of source impedance should an amplifier designer reasonably expect to see from the driving source? And in a world where only the wimpiest of preamplifiers have an issue with a 10K-50K load, how much extra cost and design effort is one willing to spend in order to satisfy a few oddball cases?

^^ This.
02-13-14: Almarg
John (Jmcgrogan2) & Bombaywalla, see what I meant when I said that "I don't have a good feel for what the inevitable tradeoffs would be." :-)

You're still the man in my book Al! Who wants a silly SS amp anyway....I'll take your VAC amp any day Al.....seriously....I'll take it. LOL! ;)
Audiolabyrinth, if you have an amplifier, tube or solid state, with a 100KOhm input impedance, any preamp tube or solid state will work with it.

The general rule of thumb is that the source have an output impedance that is no more than 1/10th of the value of the load.

But there is a caveat- if there is a coupling capacitor at the output of the preamp, the value of the cap will interact with the output impedance of the preamp- meaning that it may be considerably higher at 20Hz than it is at 1000Hz. For this reason I feel that the output impedance at 20Hz is an important value, as it can effect apparent bass response (FWIW our preamps have the same output impedance at 2Hz, 1KHz, 10KHz and 100KHz).

If the output impedance is higher at 20Hz than 1KHz, you may experience a lack of bass with amplifiers that have a low input impedance.
Thankyou Ralph, You are the man in my book!, I do learn alot from you and Almarg, the both of you gentleman are a asset to us all!, I agree, my 100kohm amp does not lack in bass at all., help me out here, what is 1/10th of 100kohms?
help me out here, what is 1/10th of 100kohms?
Audiolabyrinth
that would be 100KOhms = 100,000 Ohms divided by 10 = 10,000 Ohms or 10KOhms.

The general principle here is that when the output impedance of the preceeding unit is 1/10th the input impedance of the proceeding unit, the output impedance of the preceeding unit has little effect on the (music) signal transfer - you get 90%+ of the (music voltage) signal to transfer & there is (very) little loss.
hope this helps.
This did help, Thankyou Bombaywalla.
I notice on my set up ,
Preamp : Reimyo output impedance is 560 ohm ,
Power amp: AES superamp AE 25 input impedance is 150,000 ohm .
This gives ratio of 267 times . Does it mean I have a total mismatch ? But I find my system sound very sweet , detailed and enjoyable .
I am no expert on such things , so will appreciate some explanation from FM s here .
I notice on my set up ,
Preamp : Reimyo output impedance is 560 ohm ,
Power amp: AES superamp AE 25 input impedance is 150,000 ohm .
This gives ratio of 267 times . Does it mean I have a total mismatch ? But I find my system sound very sweet , detailed and enjoyable .
I am no expert on such things , so will appreciate some explanation from FM s here .
02-23-14: Radni
I notice on my set up ,
Preamp : Reimyo output impedance is 560 ohm ,
Power amp: AES superamp AE 25 input impedance is 150,000 ohm .
This gives ratio of 267 times . Does it mean I have a total mismatch ?

Absolutely not! The 10X rule is a MINIMUM suggested requirement. Some manufacturers, like ARC, recommend a 25X to 30X MINIMUM impedance relationship. Obviously, the higher the ratio, the less likely there will be an impedance mismatch issue. Which should help explain your wonderful results.

But I find my system sound very sweet , detailed and enjoyable .