? About Low Wattage Class A Amps

Systems

I've been looking into some of the lower wattage class A amps lately but have several questions about them. I'm most baffled about the volume level with these 8 wpc amps. Most all of my tube amps are rated between 50 & 70 wpc. I often find that the "usable" power falls a little short of the volume I may want to CRANK it up to. I see at lower volumes that the sound is warm, clear and clean for the most part and I like this. Here are a few questions I'd like to ask. Any input would be greatly appreciated.

1. What should I look for in a purchase of a low watt class A amp?

2. Should I pay attention to speaker selection and what direction should I go?

3. Your thoughts and concerns with these type amps.

I hate to sound like a dumb a$$, but thats what I am when it comes to this subject.

Thanks in advance,
Chuck.

madmax

The main things to be concerned about are speaker efficiency and heat. Class A is running full all the time which uses a lot of energy and produces a lot of heat. An 8 watt amp can sound like a 500 watt amp if the speaker is efficient enough. Once you get into high efficiency speakers you have to start worrying about the queitness of the amp considering hiss and hum. Just thinking of getting into it myself.
madmax

Face

Start looking for efficient speakers.

Systems

And a quality efficient speaker would be................................................................
BTW...I'm not opposed to buying older yet great quality.

Face

I'm a fan of Tannoy and currently own 9 of their speakers. Usually the larger the driver, the more efficient. Do some research though, sometimes a high current amp is recommended for some of the larger drivers. This is because some of the larger cones, such as the HPD-385's, are heavy and need a high current amp to control it.
Look into Klipsch and Altec also.

Systems

My $.02
As others have mentioned speakers with high sensitivity are needed and the speaker needs has to present an easy load (8 ohms-ish).These low powered single ended tube amps are essentially distortion generators with high amounts of 2nd order THD.While this can give them a pleasing sound with the right speaker,it is far from accurate.

Yashu

Why would they distort more than, say, a typical push-pull or whatever type of design when you are not overloading them?

I would think that as long as you are not overloading the amp, it's not going to distort any more than any other would. It should distort less, considering there is far less in the signal path, and the amp circuit is very straightforward. Multi-output tubes would be more distorted because there is no way to perfectly match tubes, at least this is what common sense leads me to believe. SET amps should be fine given the proper speakers. That is the hard part... finding speakers that can fill your room on a few watts.

Systems

Yashu wrote: »

Why would they distort more than, say, a typical push-pull or whatever type of design when you are not overloading them?

A push pull output stage will reduce(cancel) distortion.Also many of these low power single ended designs have low levels of negative feedback which can if applied reduce distortion.

Yashu

As I understand it, negative feedback actually increases several types of distortion while reducing only harmonic distortion. Harmonic distortion is not the only type.

SET amps seem to have the advantage on most types of distortion, with the con being harmonic distortion, but on orders that the brain finds pleasant. That is the amp... I don't like them because there just aren't that many speakers around that can use them well enough. Horns will amplify the shortcomings of more than just the amp.

TroyD

Look for a speaker at 90db 1w/1m with a reasonable impedance and you will be ok as long as you aren't looking to play at rock concert levels.


BDT

mhardy6647

If you have, for example, a pair of Klipschorns, you can get 107 dB at 1 meter (two 104 dB/W/m) speakers, stereo, with a 1 watt (more correctly, 2.83 VAC into 8 ohms) input.

Yashu

Yes... but be very very careful about what amp and source you use. For something like that, I suggest all tubes and vinyl. I can't imagine how bright those things would be on SS and modern upsampling CDPs.

mhardy6647

you are correct, sir.

ben62670

I like big powerful amps that run in class A for the first 10 or so watts before going A/B. Like the big Adcom's. GFA-565, GFA-585, GFA-5802, and GFA-5800. They have the class A power for low level listening, AND the power to keep going. I am not knocking nice class A's, but you can have your cake, and eat it too. To me it doesn't make a lot of sense to spend a bunch on efficient speakers that may compromise your taste, or wallet. I like the old monitors/SDA's that play nice with 10 watts, and can handle many more.

Ben

Systems

I'm wondering how a pair of older Monitor 10's would sound? I've got a load of old Altec's, Jensen's & EV's somewhere and some horns and crossovers. I think I'm going to find some time to dig this stuff out and try and put together something just for fun. I knew I should hang onto that stuff for some reason.

Face

Monitor 10's would not do well with only a couple of watts.

Yashu

hehe, probably not. Sealed enclosure with large passive radiator... definitely needs some power there.

TroyD

There is a guy on here who runs a pair of RTA12's in an 8wpc tube amp.

I've run my Monitor 7a's on 15wpc.

If you are trying to play at rock concert levels, no, it doesn't work but at reasonable listening levels it's just fine. But, what would we know.

BDT

Yashu

Monitor 7s I can see on a 15w tube amp... I can see that sounding good. The tens are like two 7s in one big enclosure, and SET amps are much lower than 15w, so I am not sure it would work out so well.

A good push/pull tube poweramp that gives you around 65w, that can drive plenty enough speakers to build a great system. I have been looking at a tube amp that is about that, with a switch to a class A mode that is a fraction of that... That would be a great way to really test lower power, but still have the power you know that you need.

TroyD

No, there not. While I don't have the specs off hand, they aren't all that different. The efficiency and impedance curves are also similar...so, no, you aren't right.

A push/pull amp's power ratings depend on the circuits/tubes involved..it's not inherent to the design. I've owned a few. My current tube amps are ~25wpc and I've run some pretty big speakers on them and never run out of gas.

Again, experience sometimes has it's advantages.

BDT

Yashu

I know Troy... I wasn't trying to say all amps are like X. I was giving an example of a good place to start with tubes... have some power if you need it and ultralinear mode when you want to play with pure class A.

I am not sure you are right about the 10s though... it's double the active drivers and almost double the cabinet space. The tens clearly are asking for more power. I am sure there is some crossover work to keep the impedance from being cut an half, but I would say the 7s and the 5s would play better on lower power amps than the 10s. I can't see any way around that. Besides... don't the 7s image better anyway?

mhardy6647

10s are more efficient than the 7s. My own experience with 7s: they work reasonably well with a push-pull EL84 amp (ca. 14 wpc, an EICO HF-81 in my case) but did not sound good at all with a single-ended 2A3 amp (3.5 watt Bottlehead Paramour monoblocks, in my case).

EDIT: remember that the 10s are nominal 6 ohm speakers. Try them on both the 4 ohm and 8 ohm taps of a tube amp if possible; hard to say which'll sound better.

The early Polk Monitors were not designed for tube amps, and like many (but NOT all) speakers designed when solid state amplification was the norm, they really don't benefit much from tubes, IM (albeit somewhat limited) E and IMO.

TroyD

I've owned the 10's, I've owned the 7's. I've run them both on low powered amps (but not SET's). My experience is different from what you are saying.

So, you can 'think' something is true but I 'know' it not to be.

BDT

mhardy6647

Fair enough. What is your experience?

How satisfactory the pairing will be will also depend on type of music and volume level desired, of course. Just to clarify: I thought the 7s sounded OK but not life-changing on the HF-81. Conversely, a pair of ads L-710s I have were fabulous on the HF-81.

I have a pair of 10s now (original series) but haven't tried 'em on any tube amps at the house. Something like a push-pull 7591/7868 amp (e.g., Scott LK-72, Fisher 400/500 series receiver, ca. 25 wpc) would be well worth a try for a 7 or a 10... but I haven't actually tried it.

EDIT: oops you werent' talking to me, were you :-P sorry.

TroyD

Sorry about that, I was replying to Yashu.

Everyone's mileage will very, my point being that wattage isn't 'really' the issue.

BDT

mhardy6647

Yeah, I'm in space...

my point being that wattage isn't 'really' the issue

Absolutely true.

Yashu

Yes, I agree with that. The only way to know is to try.

TroyD

:rolleyes:

Yashu

It's true and you know it Troy

I hear so many angles when it comes to tubes it isn't funny... some say tube watts are more robust than SS watts, some say the overdrive is what makes them sound so good, some say that watts are watts... some say it's the manufacturers that are misleading us by printing false measurements. Really... the only way to know is to try.

I know that, in my experience, double the drivers and the load is heavier, but twice the cabinet space and a properly tuned PR could offset that. You are dealing with surface area movement per watt, is really what it comes down to. If the 10s can move more air with less power then they are more sensitive. I do believe that there is a happy medium, and once over that, you are ending up in the low sensitivity range.

I had some speakers a while back, they were very sensitive yet huge, no horns or anything, just big 12" woofer and such. They were my first real set of speakers, they were from 1980. My amp was low power as far as SS goes, yet I could not turn the volume up past 12 o clock... now, fast forward to today, I still have that old amp, and I can turn the volume all the way around to full and not reach the DB levels with my bookshelves and small towers that I was able to get from those big vintage monsters.

Air displacement per "watt". If the 10s can displace more air per watt than others then that is reasonable... I am just saying that there will be a point where the 10s will stop scaling at a point smaller speakers wouldn't on fleapower amps.

I don't know... I still don't think we should even be considering <100db speakers for these little SET amps. It is different in the nearfield where you are closer to the speaker, but inverse square law applies to farfield.

TroyD

I agree the only way to 'know' is to try. That's my whole point, I have and you haven't.
You can suppose all you want but if you don't have any experience with it, at a certain point, it's just hot air.

A simple 'huh, I wouldn't have thought that. Perhaps I may be mistaken' would do nicely.

BDT

Davidpsalt

agreed