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Preamplifiers v power amplifiers

otto88

Member
2007-12-05 10:39 am
and return in "sound quality" versus project time and cost

Of course both should be good, but which is harder to do really well?
Like any audio component, either can be stuffed by poor design or execution.

Requirements differing: switching, boosting low level signal, volume control and preferably remote, vs having enough power and current into the speakers’ load.

In the last month I've come across two comments elsewhere from experienced tube amp makers that it's harder to make a good preamplifier than a good power amplifier.

If the speakers used are an *easy load: moderate flattish ohms, not needing lots of power: a power amplifier's task isn’t that hard
The #1 difference that I see - they work at different signal levels, with different needs for gain. A pre can/ should be just one stage; a power amplifier needs 2 or 3 stages.
So while a power amp's job is harder, it'll be more complex, and esp if used full range, needs good OPTs.

For us folks who like tube amplifiers (I esp prefer SE), it’s certainly a lot cheaper to implement that in a preamplifier . .

But is it any easier or harder to get/ tweak a great result?
 
I currently prefer integrated amps, with external phono pre-amp as required.
The reason is I think because of the efficiency, low cost and lack of wires and connections. And one less box.

In my amp for instance I have a volume pot and an 6N2P (12AX7 like). This does all the pre-amping I need in one hit, job done.

I just think the line level to output transformer only needs three stages myself - so my digital setup is DAC chip -> input tube -> driver tube -> output tube -> transformer.
 
There is little need for a preamp for digital sources, as their output is usually sufficient to drive a power amplifier. People sometimes add a line stage, then need an attenuator too - completely daft! The combination simply adds noise and distortion.

Phono preamp has problems, just different problems from a power amp. It is not helpful to compare apples and oranges.
 
Certainly don't need a preamp in todays world if driving a high impedance load, i.e. amplifier. They are not any harder to build than an amplifier imho, just a different set of parameters requireing different precautions.

My12b4 linestage (apparently I am daft!) only has 5db gain. I need a little bit of its gain for my phono stage, but usually don't need (acctually attenute) it for my HTPC or CD player. Given its good drive capabilities, I prefer it over a straight attenuator for my amps since I had rather long cables to the monoblocks.

I will agree that the 12b4 adds to the noise floor over my amps noise floor so that is a correct statement from DF96. It also adds some 2nd order distortion.
 

otto88

Member
2007-12-05 10:39 am
Globulator

Yes if your system is unlikely to change, an integrated saves chassis work (and a liitle space)

And I agree that in an all tube system, line level to speaker output potentially and ideally uses only 3 stages



I’m not after gain, but apart from switching & level control - I want some tube amp 2 & 3 HD.
So in my case while part way into a 15 watt tube power amp project, it occurred to me that it might have been simpler and cheaper to get my watts from SS :idea: and my harmonics from a tube pre . .
 
Globulator

Yes if your system is unlikely to change, an integrated saves chassis work (and a little space)
:
it occurred to me that it might have been simpler and cheaper to get my watts from SS :idea: and my harmonics from a tube pre . .

If it changes I'll just swap in another integrated amp. After all the preamp in an integrated amp is a single tube and a pot (as the basic level). I prefer a soldered 2" wire to a socket - plug - wire - plug - socket that people seem to imagine sounds better ;)

As for SS providing the power I think this is a mistake. The critical interface is amp/speaker - and a tube amp in my view provides a much better match. In addition most SS designs are dreadful - being low voltage low linearity designs with huge numbers of devices in the signal path to distort the sound, which is then squashed down in a feedback loop. It is in my view far easier to design a decent tube amp than a decent SS amp because you are dealing with fewer and more linear devices.

For instance the input stage of a SS amp is usually a BJT sitting at 40V, so a 1V signal uses 2.5% of an already non-linear range. Contrast with a triode sitting at 200V so a 1V signal uses 0.5% of an linear range.

SS however would be fine for sub-woofers, very well suited.
 

Candisa

Member
2009-05-23 7:48 am
At first, I would like to apologize if I say stupid things since I'm new to tube amps. I never built one and I even haven't used one before, but we are planning to buy/build a tube power-amp in the near future to power the mids and tweeters of our coming-up 4-ways project.

According to what I've read about tube-amps, the power-amp section has the greatest influence on the sound and that's why most tube enthusiasts prefer an integrated tube-amp or a tube-power-amp above "hybrid" amps (tube-pre-amp combined with SS-power-amp).

With todays digital sources, I think a pre-amps shouldn't be any more than a source-selector and a volume control. The signal of most todays sources is strong enough to send straight into a power-amp so little to no gain is needed in the pre-amp section.
If you don't need a remote control, you can get away with a passive "pre-amp" and only use a dedicated pre-amp for phono and if you do need a remote control, a SS pre-amp should do the job just fine.

To get the tube-sound you want, get/build a tube-power-amp.

We are planning to get a SS subwoofer-amp a SS integrated stereo-amp with pre-out/main-in connections and a tube power-amp.
We'll use the pre-amp section of the SS integrated stereo-amp and split that up to the SS sub-amp to power the subwoofers, the power-amp section of the SS integrated stereo-amp to power the woofers and the tube power-amp to power the mids and tweeters.
This way, we hope to get the best of both worlds:
- tube sound, since most of the frequency range will go trough the tube-amp, but...
- enough power from the SS amps to shake the power-hungry woofers and subwoofers to very high volume levels at low cost.

Isabelle
 

otto88

Member
2007-12-05 10:39 am
> SS would be fine for sub-woofers, very well suited.

We agree there


> The critical interface is amp/speaker - and a tube amp in my view provides a much better match.

The attraction of SS for the power amp is higher power, which greatly increases the range of quality normal sensitivity speakers (which can also deliver good dynamics in larger space).

By a much better match, do you mean a tube amp's higher Re providing lift c +/- 100 Hz?

> In addition most SS designs are dreadful

May well be, but we can choose a design with less devices and little feedback, ie a good one. :magnify:
 
You need to be aware that "tube sound" has at least two meanings, which people sometimes confuse:
1. the avoidance of the distortions arising from poor SS design (not necessarily all SS design),
2. the inclusion of the distortions arising from poor tube design (not necessarily all tube design).

It can be very difficult to tell which meaning someone has in mind, as it is common to claim meaning 1 while actually pursuing meaning 2.
 

otto88

Member
2007-12-05 10:39 am
I mean something different again, per my post 5:

"tube amp 2 & 3 HD"

Ie a "moderate" level of warmth.

Yes, avoiding higher order distortions arising from poor SS design,

But with the deliberate inclusion of some low level 2 & 3 HD
*possible with tube design
 
Something you can do with triode tubes that you can't do with solid-state:
No global negative feedback in the signal chain -- because triodes can be linear enough without it.

I agree that three gain stages is all that's required (plus whatever you use for vinyl/RIAA).

With simple tube circuits it's also possible to DC-couple stages and use unbypassed cathode resistors or LED's for cathode bias, and get rid of most (if not all) capacitors from the direct signal path.

-=|=-
 
Hello,
While bumming around the WWW 3 or 4 years ago I came across an article discussing the suitability of the 6DJ8 for audio. Turns out at the time of the article there were some Tunsgram variable mu tubes labeled as 6DJ8's I do not remember the name of the imposter tube.
I bought a couple of Tunsgram PCC189’s the same guts as the imposter tube. Looked at the data sheet and picked an idle point where the change in mu was the steepest. IIRC the set up, with bypassed cathode resistor, was about 100 volts on the plate and -3 grid volts and about 4 or 5 ma idle current. I placed a stepped attenuator (12 db per step) before and after this effects line stage. Turn up the input and turn down the output to increase the SET effect. Even turned to full 2nd + 3rd harmonic SET effect. I was surprised that the effect was not in your face, it was definitely tube sound.
BTW,
Never admit that the goal is to add tube sound. It is just an experiment.
DT
All just for fun!
 
With todays digital sources, I think a pre-amps shouldn't be any more than a source-selector and a volume control. The signal of most todays sources is strong enough to send straight into a power-amp so little to no gain is needed in the pre-amp section.

That is true, but the impedances must be compatible, too. In some cases, like e.g. when you have very long signal cables to the power amps, which add significant shunt capacitance, you may be out of luck and the source may not have a sufficiently low output impedance to (dis)charge that capacitance all the way up to 20kHz. In that case, unfortunately a unity-gain preamp (or a relocation of the power amps) would be necessary.

If you don't need a remote control, you can get away with a passive "pre-amp" and only use a dedicated pre-amp for phono and if you do need a remote control, a SS pre-amp should do the job just fine.

There do exist passive pre's that do have a remote control, too. Best of both worlds :)

Kenneth