Tube Pre-amp (1 Viewer)

[Brett DiMichele]

Not sure what my Dampening factor is on the monoblocks but
I do know the THD@ 1Watt is .07

Hmm I wonder what the Dampening factor is.. It's probably
low...

 

[Chu Gai]

For the 20's that you've got we have:
Output impedance 8 Ohm
ergo DF = 8/8 = 1

there'll be a 'wee bit' of spectral contouring

 

[Brett DiMichele]

Wow you make it seem so easy...

Too bad I can't count! lol

 

[Michael R Price]

Wow that's a high output impedance. I see that specification, but maybe it refers to the recommended output load instead? The specs seem to be written by a foreign person, or translated... "frequency responsible... power request." I thought tube amps were usually more around 1 ohm?

The Zen V4 amp I have has a damping factor of 40 into 8 ohms (0.2 ohm output impedance). It has noticeably weaker bass than my other amp (damping factor of 200, I think)... but I don't think damping factor is the only reason. Pass says his amplifiers compromise damping factor for other concerns, yet they apparently still have pretty good bass. I wonder why?

 

[Brett DiMichele]

Who knows.. That's why Nelson's the Genius and we are the
Peons!

Everyone has thier place

Hey while we are speaking of tubes.. Who wants to buy the
tubes out of my AV-20's that are DEAD allready... *Shrugs*
Perhaps I had them overbiased.. Perhaps it's just that they
are junk Chineese Tubes.. Shoulda been Ruskies in there!

 

[Lee Scoggins]

Here are the specs on my amps. I don't follow the specs too much though as any metric is inadequate for describing sound. I test my midbass by listening to acoustic jazz recordings I have from the studio.

http://www.audioresearch.com/vt100new.html

 

[Brett DiMichele]

Lee,

Don't take this wrong (I am saying this Tongue In Cheek)


Imagine if car sales wern't based on specs..

[Customer] How many horsepower does this baby make?

[Sales Troll] We don't go by specs.. This car goes VROOOM
other cars go vroom.....

 

[Michael R Price]

Lee, that is a very impressive amplifier. Note that damping factor and distortion into lower than 8-ohm loads, however, are not specified. Not as much of a problem with efficient speaker, though.

Brett, sorry to hear you blew the tubes... the more reasonable explanation is that they were overbiased and couldn't take the heat. Depending on their type and manufacturer the tubes could have a power dissipation rating that would dictate the highest point you could bias them. I wouldn't expect clipping/high levels to damage an amplifier, especially a Class A amplifier. If that was true, from trying to drive my 281s full range, my Zen would be dead ten times over by now.

 

[Chu Gai]

Well I must've missed it earlier on Lee's amp.
The DF is invariably specified @8 ohms so we've got...

DF=11=8/(output impedance)=0.73

So if we consider that DF has not so much to do with control of the woofers but more to do with imparting an equalization to the output signal that both mirrors and is dependant on the speaker's impedance curve, it's easy to see the technical reasons why Lee states his tube amp 'gets the mid bass' right. Further the greater the output impedance or the smaller the value of the speaker's impedance, the greater the 'equalization'.

Now what's bad about this, at least as I see it, is that if Lee had a similar speaker that had an impedance curve (see the Stereophile link above) that had a maximum that was dispaced to the right, his tube amp would now not be getting the mid bass right and instead would be giving some 'sizzle' or whatever you recording engineers call these things.

Now you might not feel that a metric describes the sound you're getting Lee, but in effect what you're doing is equalizing your music. Maybe it'd be better to apply that mid-bass boost during the recording so the 99% of the population that runs solid state and regular speakers can enjoy a better recording .

Michael, I've no idea what speakers you've got but you yourself can get a rough idea of what sort of equalization it is that you're imparting. Given that many speakers have rising impedances down low and in the upper frequencies, in a very general sense you might be imparting a bit of boost in those areas.

So next time you see your little kid playing with your 1/3 octave equalizer on your solid state system and you see the levers are up and down in a random pattern, don't get upset. They're just trying to get a tube sound out of your system.

My good friend Brett needs to do a better job of practicing safe-valving

 

[Brett DiMichele]

Sure pick on the fat kid...

Hey so I burnt some tubes... I had planned on upgrading
anyways.. I know I had them overbiased but these amps won't
let you overbias that much. The grid plates were never red
the only thing that got hot was the heater. I dunno.. It's
my first real tube amps so it's a learning process.

Now I can go with a set of Quad Matched KT66's or 6L6's for
the same rough price (figure about $80.00) I just need to
figure out what I want.. I may just stick with the 6L6's
and go with Electro-Harmonix.

 

[Michael R Price]

Chu, I have Adire Kit281s, they're vented so the impedance has ~6 ohm peaks on either side of the 2.8 ohm minumum at 25Hz. The impedance rises to a big peak (20+ ohms) at the 2200Hz crossover, then peaks again somewhere in the treble. In other words, a pain-in-the-butt load; the little Zen doesn't like it, and I get obvious amounts of 2nd order distortion at >85db levels. But at really low volumes that amplifier, well, just sounded better (and if it's because of the distortion... whatever). That's why the Zen is sitting on my dresser doing nothing, and there are ???-watt mondo monoblocks behind each speaker. Once I get around to building an active crossover that doesn't entirely screw up the Kit281's frequency response, I'll bi-amp and the Zen will drive the 92db, 8 ohm tweeters directly which should be no problem.

 

[Chu Gai]

I seem to recall that Kef's had a fairly flat impedance curve at one time. Whether that's still true, or whether my memory in general has failed me on that recollection, well I just don't recall. Sounding more like Bill C I think!

Just in a very general kind of way Michael, utilizing a relatively high impedance source with those speakers would be similar to creating a 'smiley face' equalization with some peaks tossed in the middle. Not saying you won't like it or even prefer it to something that's flat. Just stating that that's how these things shake out when you look at the numbers and all. BTW, a design that was generally higher overall in impedance...8...hell 16 ohms, would minimize interactions with tube amps.

Good luck on the kit project!

$80 for tubes!!! I tell you, I'd be pretty pissed off if I bought a $500 microwave and wound up having to replace the tube in that! You might want to check around the pro-audio places Brett and possibly save some money.

 

[Michael R Price]

Chu,

I knew the output impedance had some effect on frequency response, but isn't it small since the maximum voltage reduction is only to 2.8/(0.2+2.8) = 93% or -0.6db? +/- 0.3db of amplifier effect isn't so bad considering my speakers are something like +/- 3.0db in the first place, plus room effects.

It's not like the Kit281s are the kind of speakers I want to own. I'd prefer a dipole line array or some big horn deal, but you know how it goes. I'm just a kid...

I was sort of waiting for Pass to publish an article about a discrete (JFET) active crossover maybe with PCBs available, because the general opinion is that opamps hurt the sound a bit (not that I'd know). Since I'm in no hurry to get to that and it's not like I have a ton of time for this stuff anyway, I can wait a couple months for that to come around (and also waiting for Adire to supply me with the driver-on-baffle response curves so I know what filters to use). Barring that I'll just wire something up, or buy an opamp based kit from Rod Elliott. Brett, do you plan to bi-amp actively?

Also I decided my amps and speakers are fine, but my semi-good/semi-pathetic CD player and the 50 cent not-intended-to-be-permanent wires are not.

Heh, whatever happened to the original topic? Tube preamps?

 

[Chu Gai]

Yeah, it got hijacked! Do you really think Pass could pick his amp out if he didn't know what it was? Check out this link for a couple of people's effort along those lines.

 

[Yogi]

The output impedance on my proceed amp is 0.05 ohms from 20-20KHz implying a dampening factor of 160 yet its bass is more taut and defined than most other amps whos dampening factors are greater than 800 (for example all of Rotels and Parasounds and ATIs etc.) So just the dampening factor doesnt tell the whole story, IMHO. Its just one of those specs that I dont pay much attention to.

Also I notice that everyone talks about the flat frequency response of SS Vs tube devices but all we are talking about is the amplitude of the response. Thats only half the story. How about the phase response of these devices. Do they have prefect phase response all through the freq spectrum? Why doesnt anyone talk about that? I know the phase affects things like imaging and soundstaging. So what about phase response? I dont know the answer myself so would like someone to shed some light on that.

 

[Michael R Price]

The phase response is usually pretty flat, since there are no rolloffs near the audio band for the most part. Speakers screw up phase response a lot, anyway.

 

[Chu Gai]

Well the DF for most well designed amps is really a non-issue when it comes to the control of the woofer as that's really more dominated by the units suspension and of course the inherant resistance in the voice coils.
Far as phase goes it starts getting screwed up or altered during the recording, the mixing and all that. I'll toss one back at you Yogi. What amount of phase error is audible?

 

[Yogi]

I have no idea what amount of phase error is audible or even if its audible in the conventional sense. I would think phase distortions would affect the placement of instruments across the soundstage and so it would be hard to measure/hear the differences between various tube and SS gear. What could be a reason that people find tube gear to sound more real? I know its the second order distortion that adds to the richness of the music but what about better 3-dimensionality of tube gear. What could be causing that? Phase accuracy/distortions? or something else? I dont know. You tell me. Right back at ya Chu. Well I guess we'll have to get back to basic human hearing and how we hear. Too far beyond the scope of my understanding:frowning:

In any case I luv my tube preamp hooked to my Proceed amp. Makes the music sound more live as if the artists are preforming right there in front of me. I am amazed everytime I hear it. Although I have put in more premium components ($200+) than the orignal cost of the preamp ($180) but I am amazed at what component quality can achieve in terms of musical realism. I have changed all the Caps to higher voltage Auricaps. Changed the Power supply caps to Blackgate. Changed all wiring to pure Teflon coated silver wiring. All the resistors to Vishay 1% metal film. shielded the transformer with Mu-metal (grounded). Sheilded the whole chassis with Cu tape. The result: pure unadulterated music that brings the performance to my living room. My choice is tube front end with SS amplification. Its a killer combination IMHO. The best of both worlds.

Over and out.

 

[Michael R Price]

Yogi,

Now, what if you built the preamp entirely from scratch? What if you tried a solid state preamp with such attention to detail? What if your amplifier was that good, too? How about the transport and DAC? So many questions! I can't answer any of them. Until I try all those things myself, that is. At present I have messed with amplifiers, wires, and speaker placement (and these only to a limited extent without going to really high-end stuff) and that's it. At least I have no shortage of other things to play around with, given time and money that I sort of don't have. Boy, this is fun.

 

[Darrel McBane]

Yogi, I was trying to make the same point earlier. My tube Audible Illusion Mod 3A preamp and SS Rotel 1090 stereo amp gives me the best of both worlds. I love the sound that a tube preamp brings to music. And the power of a SS amp adds all the punch in the mid to lower bass that I need. Now I'm just adding the right cables to brings things together and add a little more balance in my system.