SS amp and vintage speakers-- how to emulate tube amp?

[DaveD]

I received some advice regarding voicing of tube-era speakers, such as AR and KLH models from the '50s and '60s, and their less than optimal compatibility with direct coupled solid state amplifiers. The difference in the output characteristics between tube amplifiers and direct coupled solid state ones makes the vintage speakers sound recessed when used with the solid state amplifiers.

The suggested technique to emulate tube amplifiers is to use either a 1 ohm, ten watt or .5 ohm 5 watt wire wound resistor in series when connecting the speakers. This should improve the sound by reducing the recession of the upper midrange and highs and by opening up the sound and fattening the bass somewhat. The choice should be made according to which resistor sounds better.

Would a .5 ohm 10 watt be better to try than the 5 watt one? Does the wattage of these resistors have any effect on the amount of power that can be delivered to the speakers, or the potential to compress dynamics?

Can forum members confirm this technique or offer an alternative?

[soundminded]

>I received some advice regarding voicing of tube-era

>speakers, such as AR and KLH models from the '50s and '60s,

>and their less than optimal compatibility with direct coupled

>solid state amplifiers. The difference in the output

>characteristics between tube amplifiers and direct coupled

>solid state ones makes the vintage speakers sound recessed

>when used with the solid state amplifiers.

>

>The suggested technique to emulate tube amplifiers is to use

>either a 1 ohm, ten watt or .5 ohm 5 watt wire wound resistor

>in series when connecting the speakers. This should improve

>the sound by reducing the recession of the upper midrange and

>highs and by opening up the sound and fattening the bass

>somewhat. The choice should be made according to which

>resistor sounds better.

>

>Would a .5 ohm 10 watt be better to try than the 5 watt one?

>Does the wattage of these resistors have any effect on the

>amount of power that can be delivered to the speakers, or the

>potential to compress dynamics?

>

>Can forum members confirm this technique or offer an

>alternative?

While I'm not sure I'd agree with the way you characterize tube and transistor amplifiers, you are not the first person to ask this question. Bob Carver said he could make any amplifier sound like any other amplifier given 24 hours by changing the transfer function (technospeak for frequency response) and I think he could...within its power rating. There's no magic to this, it's pretty straighforward stuff IMO. Adding a resistor in series with a SS amplifier output emulates the fact that the overwhelming majority of tube amplifiers have very low damping factors (high output impedence) even with an impedence matching transformer needed due to tubes characteristically high plate impedence compared to the very low output impedence of common emitter/collector output circuits of most SS amps. This means tube amps have less control over the woofer cone motion and exaggerate any tendency it has to less than ideally damped resonance. Minor differences in frequency response play an important audible role IMO and the common practice of testing amplifiers at 1 watt output with a resistive load does not tell the whole story about this. To measure the real world FR, you have to connect the amplifier to the speaker it will drive. The speaker's impedence becomes a factor. The speaker wire is only a factor if its electrical parameters are significant compared to the speaker's such as very long runs of wire or deliberately constructed to have high inductance or capacitance. Low values such as short runs of #16 AWG lamp cord usually have no audible effect.

The best way to adjust the FR to match the tube amplifier is with a graphic equalizer connected to the SS amp, both amps connected to identical speakers, and a null circuit to measure the difference in outputs between them. A differential amplifier whose output is connected to a spectrum analyzer is a good tool. Adjust the equalizer and gain of the SS amp until there is a complete null across the audio spectrum with a white noise or sweep generator input to both amplifers simultaneously and they should sound exactly the same. An alternative which might be risky is to ground the amp outputs together and connect the speaker between the two hot legs. Then adjust the gain of the SS amp and equalizer for a null. (This won't work if one amplifier inverts phase and the other doesn't.) You can't however compensate for non linear distortion differences. The good news is that this is usually inaudible even for tube amplfiers whose non linear distortion is invariably over 1% or even 2%. Crossover notch distortion in a properly operating SS amplifier is also well below audibility. What accounts for the differences? IMO, it's mostly the FR distortion caused by the tube amp's output transformer. Hysterisis and eddy current losses in the core tend to reduce high frequency output and inductive coupling reduces extreme low bass while the low damping factor exaggerates other bass frequencies. Tube amps therefore often sound to me to have more prominent midrange, muddier, louder, and less extended bass, and less high frequencies than SS amps. Perhaps this is useful in taming the shrillness of many modern so called audiophile loudspeakers which inexperienced shoppers find initially attractive in stores but which quickly become tiresome at home. That is not the best way to fix the problem as I see it. BTW, my personal preference is for SS amps.

[stan461]

The trick is to get the whole system to sound right. Synergy?

I have tried a number of tube amps and listened to many more tube amps. I prefer the sound of SS when setup properly. The best systems I have heard have been SS.

About 5 years ago, I was attending a meeting of an audio club which I beloged to. The meeting was being held at a audio store with a few listening rooms. We were listening to a tube amp and swapping out some speakers and there was the usual bickering whether push-pull was better than single-ended. So I wandered of to another listening room where there was a pair of Proac speakers, a monster Krell amp and a preamp that I really don't recall the name of. So I sat down and listened. And I was blown away. Here I was, all alone, listening to "Also Sprach Zarathustra" in all its glory. Right then I realized that SS could really be grand and didn't necessarily have to be harsh or grainy. I wanted to call the others in but my exitement over an SS amp would probably have gotten me expelled from the club.

Also, the room is as big a factor (probably the biggest) as any other component. And it's hard and expensive to get the room right.

[DaveD]

Soundminded,

Your answer is thorough and also beyond what I would want to do. The advice I got was specifically directed toward emulating the source impedance of a tube amplifier using the resistor with a transistor amplifier. This difference in impedance that the speaker "sees" was said to cause the recessive sound of the vintage speakers (voiced with a tube amplifier) when used with solid state amplifiers.

Do you agree that the generalized difference in source impedance between the two types of amplifiers could have the effects that I described in their use with a vintage speaker system? Would you expect that using one of the resistors might improve the sound in the ways that I described in treble and midrange? If so, I will take a trip to the Rat Shack and buy some reisitors and try them out.

Since the transistor amplifier will surely have a higher damping factor and better control of the bass than the tube amplifier, could a woofer that was voiced for tubes have mechanical properties that would be detrimental when used with a higher damping factor SS amplifier? Would using a resistor in connecting the speaker reduce the damping factor of the SS amplifier so that it more closely matches that of the tube amplifier and, potentially, the mechanical properties of the woofer?

Or, do you believe that the point is moot, as a general statement, about the difference between the two amplifier types when used with tube-era speakers?

[Guest palomar]

I agree with Stan in that synergy is really the key. I can relate two examples, with my personal opinion (strictly personal, as I had no measurements, or live reference against which to truly judge).

I have a pair of KLH 6's that I've run with a Dynaco Stereo 70 tube amp, and a Dynaco Stereo 150 solid state amp. Even though the 6's would have been voiced with tube amps, I preferred their sound with the Stereo 150. I thought that the bass was tighter, the mids were a little less forward, and overall the system didn't sound quite as sweet. (I found with the tube amp it sounded too sweet.)

I also had a pair of EPI 100s that I ran with both amps. I'm all but certain that the EPIs would have been voiced with solid state amps, given that they came out in the early 70's. (Someone feel free to correct me if I'm wrong - I have no personal knowledge of how they were voiced.) In this case, I preferred them with the tube amp, mainly because I found that the more forward midrange of the tube amp better complimented the EPI's more recessed midrange.

I guess my bottom line is that for me, I wouldn't worry about what the speakers may have been voiced with. It's what they ultimately sound like in your system (with all of your other components, room, recordings, and so on).

[soundminded]

>Soundminded,

>

>Your answer is thorough and also beyond what I would want to

>do. The advice I got was specifically directed toward

>emulating the source impedance of a tube amplifier using the

>resistor with a transistor amplifier. This difference in

>impedance that the speaker "sees" was said to cause

>the recessive sound of the vintage speakers (voiced with a

>tube amplifier) when used with solid state amplifiers.

>

>Do you agree that the generalized difference in source

>impedance between the two types of amplifiers could have the

>effects that I described in their use with a vintage speaker

>system? Would you expect that using one of the resistors

>might improve the sound in the ways that I described in treble

>and midrange? If so, I will take a trip to the Rat Shack and

>buy some reisitors and try them out.

>

>Since the transistor amplifier will surely have a higher

>damping factor and better control of the bass than the tube

>amplifier, could a woofer that was voiced for tubes have

>mechanical properties that would be detrimental when used with

>a higher damping factor SS amplifier? Would using a resistor

>in connecting the speaker reduce the damping factor of the SS

>amplifier so that it more closely matches that of the tube

>amplifier and, potentially, the mechanical properties of the

>woofer?

>

>Or, do you believe that the point is moot, as a general

>statement, about the difference between the two amplifier

>types when used with tube-era speakers?

The only reason a resistor in series changes the spectral balance of some speakers is because it allows the woofer cone to continue vibrating after the applied voltage returns to zero. It means spurious bass resonances are not as well damped out.

The impedence of an amplifier is a complex number especially for most vacuum tube amplifiers. It is not a fixed number but varies in both phase and magnitude with frequency. The ideal amplifier has a zero output impedence. SS amps come closest to this often having damping factors (speaker impedence divided by amp output impedence) of over 100 at all audio frequencies. Their frequency responses are often ruler flat when measued with an 8 ohm resistor at a 1 watt output level. Tube amplifiers are far more variable and as I said before usually have a much higher output impedence. Their frequency response is also often far enough from flat to be audibly different. To make matters worse, amplifiers which do not use negative feedback can drift all over the place. A vacuum tube non feedback amplifier is especially susceptable because the cathode emission varies with the temperature which in turn varies with the square of the filament current. Small changes in voltage can mean larger changes in filament current and there is no compensating mechanism.

When juciously used with a well practiced ear, a graphic equalizer can usually make an audible improvement in most sound systems.

It's not that I don't know the current mantras and doctrines of audiophiles, I just don't believe much of it. IMO it's real intent is to sell expensive products that don't necessarily offer the consumer any real benefits.

[soundminded]

>The trick is to get the whole system to sound right. Synergy?

I prefer to think of it as engineering. The objective is to produce a sound system which meets the users expectations at the lowest possible cost and doesn't entail endless experimentation wondering if this amplifier is slightly better than that one or if it would be better to change wires around.

Somewhere around the 1980s I think many serious electrical engineers lost interest in this area of electronics having considered the problems at least of amplifiers to have been solved. At best, only marginal improvements at substantially increased cost has been the legacy of the last 25 years while many off the wall products have come and gone. If a slight change in the frequency response of a sound system is needed to correct a problem, the professional audio engineer would instinctively first look to equalization. The audiophile would often go out to buy the first miracle cure presented to him, sometimes regardless of what its price is or whether or not it even makes any logical sense. When he can no longer convince himself that it worked, he hunts down the next one. I think some audiophiles spend more time hunting these down than they do listening to music. On other message boards, you'll see audiophiles who have swapped more equipment in a matter of just few years than most people encounter in a lifetime.

[frankmarsi]

Dear soundminded,

Your words are very agreeable to me as they must be for many.

Take the so called leading magazine for high-end audio, Stereophile for instance. All those reviewers insist they would never, ever even consider using an equalizer, for that will stray for the proverbial ‘straight-wire with gain’ concept. Ironically those guys over there would rather spend upwards of $1,200.+ on a set interconnects and several thousand on speaker cables striving for changes in sound, but would never ever dream of using an equalizer. Why don’t I understand that rational?

Back in the mid eighties I started using tape for copying music for my own enjoyment, continued through the early 90’s. I must admit I would never have made so many enjoyable long playing tapes if I didn’t have an equalizer to assist me in doing so. It is also obvious to me that an equalizer is one’s best friend when setting up a system working with and around a room’s idiosyncrasies. Not to mention compensating for varied recording techniques and certain shortcomings of many sources of music.

Frank Marsi

[dynaco_dan]

>Dear soundminded,

>Your words are very agreeable to me as they must be for many.

>Take the so called leading magazine for high-end audio,

>Stereophile for instance. All those reviewers insist they

>would never, ever even consider using an equalizer, for that

>will stray for the proverbial ‘straight-wire with gain’

>concept. Ironically those guys over there would rather spend

>upwards of $1,200.+ on a set interconnects and several

>thousand on speaker cables striving for changes in sound, but

>would never ever dream of using an equalizer. Why don’t I

>understand that rational?

>Back in the mid eighties I started using tape for copying

>music for my own enjoyment, continued through the early 90’s.

>I must admit I would never have made so many enjoyable long

>playing tapes if I didn’t have an equalizer to assist me in

>doing so. It is also obvious to me that an equalizer is one’s

>best friend when setting up a system working with and around a

>room’s idiosyncrasies. Not to mention compensating for varied

>recording techniques and certain shortcomings of many sources

>of music.

>Frank Marsi

Hi Frank;

I am really going to try stretch my memory on this one.

The equalizers introduce intermodulation and harmonic distortions to the signals.

The "golden cables", introduce resistance, capacitance and inductance.

I believe it was Altec that had a dual channel 30 or 50 band equalizer for sale a few decades ago.

The dealer would come into your home with test instruments to set up your system.

After your system as setup, the front cover was put on to the equalizer, so that there would be no more tinkering by the owner.

Please, Frank, go to avahifi.com and read Frank Van Alstyne's many years of, "HIFI BASIC" newsletters, and particularily his letter to numerous, "golden cable", manufacturers.

I believe, you will read regarding a cable that Polk produced which destroyed amplifiers, unfortunately the entire story is not told.

My understanding is, that many moons ago, a lot of amplifiers were very unstable or unable to drive a capacitive load.

I believe this is or was the most difficult of loads for amplifiers.

Add speakers that are very much a capacitive load, electrostaics for one and viola, poof goes the amplifiers.

[DaveD]

>

>The only reason a resistor in series changes the spectral

>balance of some speakers is because it allows the woofer cone

>to continue vibrating after the applied voltage returns to

>zero. It means spurious bass resonances are not as well

>damped out.

>

Willing to spend a few dollars at Rat Shack on resistors, I bought two of the 1 ohm, ten watt ones. I will have to wait to get the .47 ohm, 5 watt ones ordered.

Armed with my two chosen test tracks here in my second system at the computer, I listened with and without the resistors connected to my KLH 17's. I listened at a louder volume that usual-- at the back of the room for the bass (because that is where the most of it hides out), and at a typical listening distance for the other aspects of the sound. This is more realistic than my normal use of the system at lower volume in a very near-field setup. Power is provided by an NAD C740 receiver. Test track for bass control is track 10 of The Boston Bass Disc (Boston Acoustics subwoofer demo), Winter Creeper by Paul McCandless. Test track for openness of the midrange, soundstage presentation and midrange/treble balance is track 4 of Mighty Sam McClain's Soul Survivor, New Man In Town (CD layer).

First, I listened for bass control. Yup, no doubt about it, there is less bass control with the resistors installed. After all, one thing that acoustic suspension fans like me want is the bass control and articulation. Less control of the bass could be erroneously interpreted as more bass. So, when I listened to Mighty Sam's track, I DID NOT want to hear any improvment in the other qualities of the the presentation with the resistors installed. But, I have to say that I did hear a more open midrange and more convincing placement of the instruments in the soundstage with the resistors installed. RATS! I could hear no change in the balance of the midrange and treble, but the clearer delineation of the instruments and vocalist might give the impression that things like cymbal crashes and high frequency percussion and sibilants are louder.

Now I look forward to trying the .47 ohm, 5 watt resistors. What I want from them is to get at least enough of the good soundstage without getting much or any of the bad sloppy bass. If this is all delusional, then it is one of the cheapest delusions in the annals of audiophilia nervosa.

[dynaco_dan]

Hi Dave;

Interesting report, non-technical, thank you.

I am looking forward to part 2 and your further comments.

[rrcrain]

Just an odd thought.

How much if any difference would there be between a wire wound resistor and a carbon resistor? I'm wondering if the wire wound resistor might act a bit like a choke at certain frequencies.

[DaveD]

>Hi Dave;

>

>Interesting report, non-technical, thank you.

>

>I am looking forward to part 2 and your further comments.

The basement room that I am doing this in and the setup that I must have for nearfield (which sounds darned good on the 17's without the resistors) is far from ideal for evaluation of normal volume listening. For one thing, the speakers are probably too close to the wall for normal volume listening, giving too much reinforcment to the lower midrange and bass. This would result in a congestive sound.

More than a grain of salt is recommended in reading my comments due to the setup and room.

Still, it is the same setup with and without the resistors.

I have the tweeter control set on increase for all of this, by the way.

In nearfield, I find myself turning the treble control back, so that is as it should be. (I actually want to flip the switches to normal, but if I did this I might be putting too much wear on them.) I turn the bass control up more than a little for nearfield. For the normal volume listening, tone conrtrols are switched out.

[kkantor]

>Just an odd thought.

>

>How much if any difference would there be between a wire wound

>resistor and a carbon resistor? I'm wondering if the wire

>wound resistor might act a bit like a choke at certain

>frequencies.

Sure, at very high frequencies, this is true, although you can buy "non-inductive" wirewounds that have a winding pattern that minimizes the effect. When are are talking about low value resistors, like 100 Ohms, and under, there are so few turns and so low a resistance, that inductance is practically invisible below several MegaHertz.

[kkantor]

You could buy a pile of these, too.

http://tinyurl.com/kkw3y.

[kkantor]

>The only reason a resistor in series changes the spectral

>balance of some speakers is because it allows the woofer cone

>to continue vibrating after the applied voltage returns to

>zero. It means spurious bass resonances are not as well

>damped out.

Well, the voltage divider effect from the speaker's impedance curve is a larger issue to my ears.

http://www.stereophile.com/reference/60/

[kkantor]

The resistor will work OK. Just use one with enough Wattage. I'd recommend about 1/5 of your amp's power.

To do it really right, you could run the output of your SS amp through a 1:1 matching transformer!

[soundminded]

>Well, the voltage divider effect from the speaker's impedance

>curve is a larger issue to my ears.

What does that say about using a potentiometer for a volume control for a midrange and tweeter driver?

>http://www.stereophile.com/reference/60

I'm not going to comment on the accuracy of the two models except to say that the point about testing amplifiers is well taken but an additional factor which is left out is the fact that loudspeakers are not passive loads. The reverse emf of a woofer works against the amplifier power supply. Depending on the amplifier and speaker in question, it may be insignificant or highly significant. This is where the extremely conservative power supply will prove its merit even if the cost most amplifier manufacturers demand for one is way out of line with what it's actually worth.

[kkantor]

I'm sorry, I don't follow your thinking on this. As far as I understand, the electrical model behaves EXACTLY like a real speaker, within the limits of the model. The energy stored in the speaker/box system is equivalent to the energy stored in the capacitor/inductor of the model's tuned circuit. Viewed from the terminals, the electrical system shows the same "back-EMF" as the mechanical system.

That's the whole point of this kind of circuit. If there were two sets of connectors on the outside of a sound-proof concrete bunker, there would be no way to tell the electrical model from an actual speaker. Of course, you could tell them apart by examining non-linear behavior such as clipping the speaker. Maybe you could see some hysteresis effects at very small signal levels. Very trivial differences, compared to how similar they would be. What am I missing?

Potentiometers are notoriously terrible for adjusting driver levels. They result in a different response curve at every setting. That's the whole rationale behind the use of an auto-transformer in the AR LST and 10pi. The only reason why potentiometers are even marginally usable at all is that mids and tweets operate well above their resonant frequency, and are generally padded with a fixed resistance. Sure, amplifier damping factor has some sonic impact due to system Q, especially with smaller speakers that have an Fc above 50 Hz. It's just that the audible effect of this is much, much less than tonal changes due to voltage divider issues.

If I continue to procrastinate my real work for the rest of the evening, which seems very likely at this point, I will run you some quick sims to show you how much a few Ohms of source impedance changes the tonal balance of a typical speaker. Not a pretty picture. Mostly a lot of bass boom, due to the impedance rise at the low end.

[soundminded]

>I'm sorry, I don't follow your thinking on this. As far as I

>understand, the electrical model behaves EXACTLY like a real

>speaker, within the limits of the model.

Perhaps I am the one missing something in the model. How can these passive elements simulate the back emf? Shouldn't there be a voltage sourse here which generates a time varying signal, probably an impulse which is dependent on the applied signal?

>The energy stored in

>the speaker/box system is equivalent to the energy stored in

>the capacitor/inductor of the model's tuned circuit. Viewed

>from the terminals, the electrical system shows the same

>"back-EMF" as the mechanical system.

Clearly something for me to consider. Hmmm, this is probably correct.

>

>That's the whole point of this kind of circuit. If there were

>two sets of connectors on the outside of a sound-proof

>concrete bunker, there would be no way to tell the electrical

>model from an actual speaker. Of course, you could tell them

>apart by examining non-linear behavior such as clipping the

>speaker. Maybe you could see some hysteresis effects at very

>small signal levels. Very trivial differences, compared to

>how similar they would be. What am I missing?

Agreed about the hysteresis losses being insignificant.

>

>Potentiometers are notoriously terrible for adjusting driver

>levels. They result in a different response curve at every

>setting. That's the whole rationale behind the use of an

>auto-transformer in the AR LST and 10pi. The only reason why

>potentiometers are even marginally usable at all is that mids

>and tweets operate well above their resonant frequency, and

>are generally padded with a fixed resistance.

Doesn't the fact that the frequency response of an LCR circuit changes with R suggest a potentiometer is a poor way to adjust the relative loudness of a speaker driver in a multiway speaker system and indicate why an L-pad is a better choice? However, even an L pad assumes a constant impedence matched to the L-pad's impedence, often close but not exactly correct especially as a function of frequency.

>Sure, amplifier

>damping factor has some sonic impact due to system Q,

>especially with smaller speakers that have an Fc above 50 Hz.

>It's just that the audible effect of this is much, much less

>than tonal changes due to voltage divider issues.

Is the impedence of a loudspeaker constant with voice coil displacement? Wouldn't moving the coil with respect to the magnet change its inductance dynamically? How about voice coil heating? Surely the change in temperature affects its DC resistance. These non-linearities suggest to me advantages of multiple amplifiers with active crossovers where frequency response and relative loudness can be adjusted precisely and independently of driver non linearities and where far fewer compromises are necessary over passive crossover multiway systems driven by a single amplifier. This would have been outrageously expensive and extravagent in 1966. In 2006, it seems to me to be a viable option for a well designed high accuracy speaker/amplifier. I'm surprised it isn't used more often so that the speaker designer has far more control over the performance of his product. The notion of buying speakers and amplifiers separately strikes me as an example of "the Chinese Wall syndrome" mentality where the consumer has to supply experimentation as a substitute for manufacturer expertise. Who would ever even have considered way back when, that what was proported as a high fidelity loudspeaker system would need a user supplied outboard subwoofer (except for full range electrostatic systems)?

>If I continue to procrastinate my real work for the rest of

>the evening, which seems very likely at this point, I will run

>you some quick sims to show you how much a few Ohms of source

>impedance changes the tonal balance of a typical speaker. Not

>a pretty picture. Mostly a lot of bass boom, due to the

>impedance rise at the low end.

[kkantor]

Mr. Minded-

I'm going to reply in the Mods forum, so as not to clog up AR with generalized tech discussions.

[Pete B]

I'll just add that there is much confusion behind the term back EMF, I've written about it here:

http://www.diyaudio.com/forums/showthread....=EMF#post554559

http://www.diyaudio.com/forums/showthread....=EMF#post565701

Pete B.

[DaveD]

TECHNICAL RATIONALE FOR THE RESISTOR TECHNIQUE--

While Ken Kantor has wisely taken the technical disucssion to the mods forum, I will report here on my "practical application" listening impressions using the .47 ohm 5 watt resistors which I now have in hand after an odyssey to two Rat Shack stores and two different wrong parts which were ordered for me.

I will also post this message on the mods forum after posting it here.

The fellow who gave me the advice to try the resistors sent me his technical rationale the other day. It is as follows:

My question--

"I have tried the Rat Shack 1 ohm, ten watt wirewound resistors on the 17's, with my NAD C740 receiver. The midrange did open up and soundstage became more dimensional and separated and detailed. Maybe the highs became more transparent and extended, or maybe I only thought so because there was more detail and less congestion. However, the bass became too bloated and uncontrolled. The Shack had to order the .47 ohm, 5 watt resistors that you also recommended trying, and I hope that they will give the midrange improvement without hurting the bass control. Does the damping factor of the particular direct coupled SS amplifier being used determine which resistor will work best? Is there a better choice than the .47 ohm, 5 watt Rat Shack wirewound resistor that I will be trying?"

His answer--

"0.47 is what I wound up with. I think you'll like those. The amplifier DF is really a measure of the amp's internal impedance, but expressed in a back-handed way. The idea of the series R is to emulate the series impedance in the tube amps the 17's were designed to work with. DF = 8 divided by the internal impedance. Tube amps of the 60's ranged from 6 to 18 in DF, which works out to .45 ohms to 1.3 ohms. So that's the range of total series impedance you're looking for. The SS amp you use contributes a small part of that, but typical SS amps have DF from as low as 25 up to 1000. As a rule of thumb, I'd say you would need to worry about the amp's series impedance if it's more than 0.2 ohms or a DF = 40 or less. In any case, there are other things that affect the sound, so you need to try it and see. The math only gets you close for the first try."

[Guest Bret]

http://www.diyspeakers.net/Articles/Richar...NG%20FACTOR.pdf

Thought you might get a kick out of reading this.

[Guest denmarkdrivers]

yeah stan i agree, at least for my price range. tube is costly, exp the sought after tube stuff, you could spend a fortune on monoblock tube amps couldent ya ?