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SDB777 13th March 2010 07:09 PM

How many watts are really needed?
My audio system is ready for a overhual!

I am wanting to go the direction of a tube amp/pre-amp. And I'd like to make both of these myself.

Exactly how many watts do I need to make some Polk Audio Monitor 40 active? I don't need ear drum splitting SPL, I like quality.

Suggestions on kits would be very welcome?!?!? Please?

Scott (tired of my Onkyo) B

bigwill 13th March 2010 09:27 PM

An extremely general guideline, assuming averagely efficient speakers (90dB) and modest room sizes,

5W: Fine for casual listening, you'll get frustrated if you want to crank it, but most of the time you'll be actually within this power range. Single ended amplifiers and teeny PP amps live around here.

15W: In a medium sized room this is actually loud enough for most cases. Sometimes you'll run into the limit with 'spirited' listening, but much above this power range for any period of time is very fatiguing and bad for your ears. Big single ended and small PP amps live here.

30 - 50W: A good power output to aim for in an all-round amp, it will drive most speakers satisfactorily with plenty of headroom and can be easily obtained without exotic valves and circuitry. This is mostly the domain of medium sized PP amps.

100W+: Not generally needed unless you have very power hungry speakers. In valve amps it isn't usually worth the added cost of bigger valves / transformers. Big PP amps with KT88s etc live here.

Richard Ellis 13th March 2010 09:34 PM

1 Attachment(s)
Your Polks are rated at 89 Db worth of sensitivity...kinda low. This limits you to your choices of tube amplification as most tube amps are low-power (2-30) Watts. There are of course higher powered tube amps but the price seems to increase exponentially.
How big is your room and are you aware of how many watts you are running now?...a 'comfortable', maximum listening level?
Can you upgrade to a so-called tube friendly(95Db+) speaker set?
Perhaps a common, numerous circuit is a single-ended 6SN7 300B amp is only going to run about Eight watts.

Eli Duttman 13th March 2010 10:14 PM

Paul Joppa has provided us with a useful rule of thumb. The rule states than in a "typical" listening space, an amp/speaker combo should be capable of producing 102 dB. SPL peaks at a 1 M. distance. Your Polks are spec'd as being 8 Ω and 89 dB. sensitive. Joppa's Rule tells us you need upwards of 16 WPC. I did not see any info. about impedance dips and the impedance minimum. Tube amps lack the damping factor typically found in "sand" products. So, it pays (IMO/IME) to derate and treat the Polks as 4 Ω/86 dB. sensitive. That brings you to the 35 WPC that a "clone" of the Dyna ST70 yields. Please observed that my "math" leads to the same conclusion Will arrived at.

IIRC, more than 1 vendor offers a "clone" ST70 kit. I'm familiar with Triode Electronics' version. You want the driver board that takes 2X EF86 and an ECC99, as the tube complement.

I suggest you speak to Jim McShane about the necessary tubes. Disturbing signs about quality have emerged recently regarding the production of the SED (=C=) plant in St. Petersburg, Russia. I'm inclined to think that EH "fat bottle" 6CA7s might be your best bet in the O/P positions. Only JJ makes the ECC99. So, that's settled. EH offers a decent EF86 and that gets my nod, unless you are willing to pay 2X for "reissue" TungSol EF806s.

What sort of control center are you considering? The 470 KOhm ST70 I/P impedance makes a 50 KOhm passive control center feasible, in combination with short low capacitance interconnect cables.

boywonder 13th March 2010 11:04 PM

I'm running a handful of tube amps (all PP) through a pair of Polk SDA-1C's (90db) in a fairly large room, and my listening position is about 8' from the speakers.

My tube amps range in power from about 11W (baby huey-PP EL84) to about 70W (Conrad Johnson Premier 11a PP 6550).

I recently connected a voltmeter across one of the speaker's terminals and cranked up some tunes to my (fairly healthy) listening level; the meter averaged around 6 watts.

Is this a reasonable test to determine average power needs? If it is, it may be worthwhile to conduct the test before springing for a new amp, as it's simple and takes all of 5 minutes to accomplish. I understand that it does not address transients and that the AC voltmeter is optimized for 60 hz.......

Also, my listening room is very "live" so that may also impact the power needed.

Bob, W7ETA 13th March 2010 11:17 PM

May be the easiest to build will be a re-build of PP El84.

I run a pair of Paradigm Titans, 89db, 6-8 ohms, with a Scott 222B and cannot turn the volume past 12 O'clock--its just too loud!

Right now, the output tubes I use cost more than the integrated amp; the 12AX7s I use in the tone control and phono input cost more than the unit. The rectifier tube, about half of what I paid for the unit. New caps, new ss rectifier, new resistors.

Its been a FUN project with rewarding results.

Best from Tucson

Captn Dave 13th March 2010 11:55 PM

I too am a Polkie. I run a pair of SDA-2Bs with a wide variety of tube amps.

I had a McIntosh MC-240 in here to repair and once finished, it won the penultimate performer prize, second only to a Conrad Johnson. The MC 240 is a 6L6GC/KT66 push pull and it had all the power you need for performance listening levels and had the kind of bass that makes you look around to make sure the bass player hasn't snuck up behind you.

I also have several 7591 push pulls (The Fisher and Scott). I love their sound and they do very nicely up to performance levels although they do run a little thin in the lower registers when played at performance levels. Otherwise they do very well.

The EL34 push pull (like the ST70) would be another good choice for you. They have a little more power than the 7591 but less that the 6L6GC; they too can play at performance levels with the SDA-2Bs.

My EL84s push pull amps are fine for "normal listening levels" and have great mid-range detail that I like. You will rarely **** off your neighbors with an EL84 PP but at 10 watts they have plenty of power for normal listening levels. Ditto for the 6V6 push pulls; they are very similar in power. These are very inexpensive to build. One of my favorites I built on a chassis from a Magnavox console I got for $25. Good vintage transformers can be had on fleabay pretty cheap.

I also have a single ended EL84 and I only tried that once. You will need more than 5 watts for these inefficient speakers. Push Pull is your domain.

That ought to muddy the water.

Listens2tubes 14th March 2010 12:39 AM

Head over to and check out building a pair of Dynakit Mark IV amps. They will work very well with your Polks.

Loren42 14th March 2010 01:14 AM

Your speakers are 89 db/W/m. That isn't too bad.

You could do with some lower wattage amps, but the reason for having horse power isn't the need for high volume, it is for dynamic range.

Normal listening doesn't require high wattage, but the transients in the music can generate a lot SPL and if the amp doesn't have the reserve available, you get clipping for an instant and you lose that dynamic range.

More wattage is better if you can afford to have it on tap. I would guestimate a minimum of 25 WPC and more is better, if possible.

Every 3 dB of volume increase requires 2 times the wattage to generate. Also, 50% of your music power is below 500 Hz. So good bass demands power on tap.

If you are looking for a kit, the Dyna kist that are available are a very good value and there are a huge number of support groups as well. So if you want to modify them in the future you have wiggle room to grow.

If you want something as an investment that will gain value over time, the vintage McIntosh tube amps are a no-lose investment. Last year I had a chance to pick up two MC-40 monoblocks locally for under $1000. Wish I had bought them because it is like sitting on gold.

tomchr 14th March 2010 01:32 AM

It all depends on what you want. If you want the sound of a single ended triode amp, you live with the fact that you only have maybe 10 W available -- unless you go crazy with a few kV on the plate and some huge transmitter tubes, then you can get more like 200 W. But that is not a beginner project. Many professionals would shy away from it also.

A single ended amp like the Tubelab SimpleSE or Tubelab SE (see the tubelab forum under vendors or Tubelab Home) would be a reasonable beginner project assuming you know how to solder and work with high voltages (3-400 V) without killing yourself or anybody else.

I'm running with a small single-ended amp that delivers a bit over 4 W before it runs out of steam. This with 87 dB speakers, 2.5 m from the speakers to the listening position, and a 20-ish m^2 listening room that's fairly live. It's loud enough for casual listening and loud enough that OSHA would require me to wear earplugs if I was exposed to that SPL at work, but not loud enough to rock the house off the foundation.


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