Playing in the Sandbox

[sogo]

Hello?

[mcandmar]

Would you consider trying this? Nope

Running below the stock bias setting of 200mv increases distortion rapidly, and going higher will reduce distortion, hence the 200mv measured across the source resistors was the chosen sweet spot for the circuit in terms of performance, and heat dissipation per device.  From memory i ran mine slightly hotter at 225mv which is ~450ma per device for 50c on the heatsinks as i really didn't care about the heat dissipation.  Have a read of the 2nd post from 6L6 in this thread for more info and measurements http://www.diyaudio.com/forums/pass-labs/234355-guide-building-pass-f4-amplifier.html

[sogo]

Have a read of the 2nd post from 6L6 in this thread for more info and measurements http://www.diyaudio.com/forums/pass-labs/234355-guide-building-pass-f4-amplifier.html

Yes, the Impasse http://www.audioxpress.com/article/The-ImPasse-Preamplifier
and also the Aikido are probably the two most popular DIY tube preamps that F4 users go for http://glass-ware.stores.yahoo.net/listheam.html

But what’s the best preamp that you’ve used? What did it do for the F4’s performance-particularly its soundstage? Is it now much deeper?

And wider, both horizontally and vertically?

Did the F4’s tonality and imaging also benefit from this new preamp?

Which make and model preamp?

And its output voltage?

Was it also driving those KEF Cadenza speakers?


http://

[mcandmar]

The F4 doesn't really have a sound of its own, it just sounds like whatever you put in front of it.

I built my own preamp using 4P1L tubes similar to Moglias design here http://www.bartola.co.uk/valves/2014/04/21/4p1l-dht-preamp-siberian-gen3-finished/ using Rod Coleman regulators in a filament bias configuration, but instead of an output transformer i used a Bottlehead style constant current source and capacitor coupled output.  I've not heard it myself but i would expect the Bottlehead Smash +smashup upgrade would sound very similar.  The 4P1L is a lovely tube for preamp stages, the only downside is it can be microphonic.

The Bottlehead Quickie + PJCCS for $120 would really surprise you too, amazing performance for the price.  On the other end of the spectrum i would love to try the Bottlehead 300B BeePre kit, maybe some day..

I have also experimented with many other tubes layouts, some push pull configurations, and even some solid state designs but i always come back to single ended DHT tubes, they just have a sweet and delicate sound that i love.  Experiment a little and see what works for you, different speakers and tastes will prefer different things.

[sogo]

as design here http://www.bartola.co.uk/valves/2014/04/21/4p1l-dht-preamp-siberian-gen3-finished/ The 4P1L is a lovely tube for preamp stages, the only downside is it can be microphonic....... I have also experimented with many other tubes layouts, some push pull configurations, and even some solid state designs but i always come back to single ended DHT tubes, they just have a sweet and delicate sound that i love.  Experiment a little and see what works for you, different speakers and tastes will prefer different things.

Please excuse the length, but these things are hardly ever simple, and everything counts. Thanks for an informative helpful and candid reply, and for confirming what all users have said about the F4; ultra transparent to whatever preamp is driving it.

And I take it that to date the F4 has been your sole, if not frequently used output stage? And that you’ve fed it with two or more preamps that you’ve built over the years?

Incidentally, you may or may not be interested in reviews, but here’s probably the best known published one of the F4. http://www.6moons.com/audioreviews/firstwatt6/f4.html

Though your speakers may be much different than Srajan Ebaen’s, it would be interesting to learn where your opinions or experience with the F4 may agree or differ.

In his reply to my inquiry about preamp recommendations, Srajan seemed to agree with you, or at least suspect, that single-ended DHT preamps might be ideal candidates to drive the F4.

“Given your speaker's sensitivity and desired SPL, you don't need additional voltage gain so using the F4 as an amp follower is unnecessary. Now it's down to a preamp…… To my mind, the F4 would be an ideal candidate to experiment with a DHT preamp which, admittedly, are rare beasts. Basically, the sound of the F4 will be what precedes it. I would go after an active circuit not because you need the gain but because the F4, if I remember correctly, has a very relaxed character. If preceded by a passive to rely exclusively on source output voltage (sufficient in your case), I'd expect too mellow a sonic presentation.”

Sadly, the finicky drivers I’ll be using surely won’t tolerate that 4P1L tube’s monophonic behaviors. Indeed, Gary Dahl, who co-designed the speakers, placed the crossover boards on top of the sealed midwoofers boxes rather than inside them to prevent cap microphonics.

But if a DHT single-ended preamp using another tube (?) is the ideal sonic signature to be expressed through the F4, then precisely what kind of speakers should or should not be used?

Here’s what’s become a truly hellish predicament, as I wade through countless commercially built and DIY line stages. As I had explained to Srajan Ebaen, I am cloning Gary Dahl’s Azurahorn 425/Radian 745 Neo Be/GPA Altec 416 system. http://speaker system The speakers were the result of one of several previous Gary
Dahl and Lynn Olson collaborations http://www.nutshellhifi.com/

Gary’s speakers are at least 94db sensitive, my room is 16 ft x 14 x 8 and I’d only need the F4 to drive the midwoofers down to 70Hz at ~ 72db, since I’d want 70Hz to be equally loud to my ears as a pure tone level at 1Khz is at ~ 50 phon, as per the (2003 revised) Fletcher-Munson curve.
https://en.wikipedia.org/wiki/Fletcher%E2%80%93Munson_curves
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/eqloud.html
https://www.osha.gov/dts/osta/otm/noise/health_effects/physics.html

Gary put each Altec 416 in a sealed box to deliberately cut off the drivers below 70Hz, to minimize IM distortion. My pair of Rythmik 12" sealed powered subs take over below that. So as the F4 has nothing to drive below 70Hz, I may only need ~ 8V input for enough power from the F4.

Gary also added a passive EQ circuit to the passive crossover, which tremendously flattens the Radian driver’s otherwise ragged response, and further extends it, albeit out to only 13kHz. See crossover and EQ here
http://www.diyaudio.com/forums/multi-way/100392-beyond-ariel-1295.html
 
From the beginning, Gary used a pair of Lynn’s 300B push-pull mono integrated amps that Gary cloned himself from Lynn’s design
http://www.nutshellhifi.com/IT-Triode-Amp.gif  The diagram does show the 300B
tubes to be directly heated, yes?

For a quite some time Gary was very pleased with the overall sound. But when he recently completed his build of his friend Gary Pimm’s unique integrated amp he was amazed at the improvement in the Radian drivers’ response performance.

Though both amps use a transformer output and of very similar quality (Lundahl) and also used Pimm’s constant current biasing, they believe that, unlike Lynn’s 300B amps, it is the absence of interstage transformer coupling in the Pimm amp that to at least some degree accounts for its “clean and strong control” of the equalized Radian compression drivers. Said Gary, “The tube amp’s IT-coupled input stage is quite good but not nearly as clean and strong as the Pimm amp’s input stage.”

Indeed, one would-be Amity amp builder also said that the 300B amp (at least PP types), while able to deliver beautifully rich and even 3D sound quality are not ideal for horn speakers:  ”……..I don't agree with Lynn Olson is putting 300B push pull amp and mate with very sensitive horn speaker. It sounds a bit edging on several different horn systems (from Western Electric type to Homebrew electromagnetic system). Maybe his Avangarde behave differently. However, I insists a good 300B / KR300XLS in SE type circuitry after so many years of work on DHT push pull.”  http://www.audioasylum.com/cgi/vt.mpl?f=tubediy&m=201099&VT=T

As for the Pimm amp, while details are currently unavailable, it has balanced and unbalanced inputs. I believe is has only two stages and that the driver stage may be cascoded JFETs. And the output stage may be a MOSFET-based Pentode circuit. It does have that one transformer at the output and the entire amp is solid-state.
 
Question: What, if any, benefits would a Pentode and transformer output stage, made from MOSFETs (?) have over the F4 follower amp’s MOSFET output?

Question: While sound quality is the primary concern, I am curious: Assuming the above description is correct, if the all solid-state Pimm amp is push-pull, biased Class A and can deliver at least 25 wpc, will the amp run about as hot as the F4?

Question: Would know of John Tucker’s designs? Along with Gary Pimm and others, he apparently had co-designed the “Simple 45” http://www.dagogo.com/the-simple-45

John mentioned that it was he who first suggested to Gary Pimm to use constant current source/sink biasing.

He has this expensive preamp http://www.exemplaraudio.com/preamplifier-sel.html
Somewhat strangely, his integrated amp has a 6H6P gain stage and is capacitor coupled to a LM3886 chip output stage.

John also does custom design work.

 In any case, Gary Dahl recently said that Gary Pimm isn’t taking customer orders for this amp, due to output transformer unavailability and/or other reasons.

Therefore, in order to closely approximate the sound that Gary Dahl enjoys from his Radian drivers, it’s obviously essential that the preamp’s input and driver stage must NOT include any interstage transformer coupling. And that it includes constant current biasing?

And from pushpulltriode’s statement above, should we conclude that a 300B SE DHT preamp-or a hybrid or solid-state preamp with a similar sound-would be best for the F4, particularly when driving horns to handle the mids and highs?

The trouble is due to a number of family and career issues, I no longer have time for any further DIY experimenting. I must either purchase a premade commercial line stage (no phono stage needed), or go with a proven design that someone can build for me to drive F4 amp.

Or even an integrated amp-one as close as possible to Gary Pimm’s deign-to drive Gary Dahl’s speakers.
http://www.diyaudio.com/forums/tubes-valves/192284-pcb-gary-pimms-ccs-ss-pentode.html

While budgeting for such a high performance preamp or low power integrated amp is not the problem, spinning my wheels over finding the best and most speaker system compatible design is extremely frustrating and time wasting.

I can’t be sure if I should pursue whichever is the right preamp for the F4 or to try to find some Integrated amp as close to Gary Pimm’s design-or replicating the sound of it-as possible for driving Gary Dahl’s speakers.

Which way to go?

Thanks for your input.


 

[Paul Birkeland]

I'll do my best to answer some of your questions here, but admittedly, I will intentionally also not answer some of them.

But if a DHT single-ended preamp using another tube (?) is the ideal sonic signature to be expressed through the F4, then precisely what kind of speakers should or should not be used?

Use the speakers you like the most that will work with the available power and damping.

Question: What, if any, benefits would a Pentode and transformer output stage, made from MOSFETs (?) have over the F4 follower amp’s MOSFET output?

There are some unusual semantics here.  A mosfet is a mosfet, and a pentode is a pentode, one is a solid state device, and one is a vacuum state device.  Having said that, the characteristic curves of mosfets and pentodes look similar if you ignore the scales on each axis.

Pentodes (when run as pentodes) and mosfets suffer from similar perils when used as voltage amplifiers, where their poor linearity requires feedback to obtain reasonable operation as an audio frequency amplifier.  In a purely Fet follower amplifier, voltage gain is obtained from the preamp, and the fet follower provides current amplification, which circumvents this issue and allows the fet follower to operate acceptably with no feedback.

Mosfet amplifiers with transformer coupled output stages are quite rare for the reasons listed above.

Question: While sound quality is the primary concern, I am curious: Assuming the above description is correct, if the all solid-state Pimm amp is push-pull, biased Class A and can deliver at least 25 wpc, will the amp run about as hot as the F4?

If you have two class A amplifiers that each are putting out approximately 25 WPC into 8 ohms, both installed into identical enclosures, it's reasonable to infer that they will both run at similar (but undoubtedly not identical) temperatures.   

And from pushpulltriode’s statement above, should we conclude that a 300B SE DHT preamp-or a hybrid or solid-state preamp with a similar sound-would be best for the F4, particularly when driving horns to handle the mids and highs?

Your statement above only excludes passive preamps and indirectly heated tube preamps as good matches?  That seems overly broad, and I believe you would benefit enormously from hooking up with some locals in your area who can help you audition some gear. 

[Doc B.]

This whole conversation belongs on some other forum. It really has nothing to do with our stuff.