ST-70 circuit topology limits its performance

[arnoldc]

"The bottom line is: for an audio amplifier to sound good, it should be designed technically correct. That's why one cannot further improve the sound of dynaco st70 even with audiophile-grade components because of its circuit topology that limits its performance!"

the above quote is what is posted at PinoyDVD. i asked the poster how can he make such a bold claim and i am asking those who know about this, and will admit if this is really the case. i am not an expert on tube amp designs so, anybody care to comment?

[mozilla]

People can argue all they want about circuits 'til their eyes pop out and their faces turn blue.

I can quote several "experts" and here goes one of them:

In the end, I've butchered most of the pieces I own that I've bothered to work on. Exceptions include the Marantz 8B, Marantz 1's, Marantz 10B (actually, I'm not big on modding tuners anyways, as I'm not so clever with the RF stuff), QUAD II's & 22, and of course, the Dynaco 70 (and the little ST-35...) and its' mono twin, the Mk.IV.

I looked at what David Hafler did with that front end in the 70, and I think it's quite brilliant. Many other lesser minds obviously disagreed, or their egos were so blown up that they wanted to put their own stamp on the tube world, usually courtesy of Glass Audio, where at least once a year some dork was writing yet another "Destroy a perfectly good Dyna 70 and put my crap circuit in there instead" article...

I've seen the Purist Mod (a mind-numbing, dumbed-down abortion of a circuit, more a low-slew rate inverter in search of an input voltage amplifier and some loop feedback than any sort of complete or well thought-out input circuit), the Sutherland (if I want a bass-ackwards hybrid, I can just buy a used Audio Research Classic 30 and be done with it. Besides, it would sound WAAAAAAAY better, and it would actually have some resale value!), the Curcio (similar in many ways to the Purist, only it has a little more gain so you can put a laughably insignificant amount of loop feedback around the finished amplifier. The basis for the horrible Sonic Frontiers SFM-75 amplifier...), the Van Alstine (which I've nicknamed the Von Frankenstein, and for good reason. Here's a guy who REALLY doesn't understand how amplifiers work!), and the best of 'em, the Audio Research "ST70-C-3", in which you stick in the dreadful and overcomplicated input circuit from a 1950's Radio & TV News project ("Build the Junior "Golden Ear" amplifier!") which is the basis for all the early (and sonically horrible) ARC "D" series of utterly undesirable but nicely overbuilt crud amplifiers.

That the ARC mod is the best one (and you have to switch to the mucho inferior 6L6 as the output valve, because you've added so many input valves that it would otherwise overload the filament windings if you tried to stay with the EL34's, never mind keeping the 5AR4 rectifier tube!), is to me, a frank testimony to the dearth of intelligence & hearing acuity amongst the modifiers (never mind the commercial amplifier builders, of which some of these modifiers are or were...).

I took the 70 design apart piece by piece...

What does this resistor do? Why this value? What if I took this out? Or added this? Or moved this over here?

And I had no delusions when I did things that made the amplifier sound or work worse, regardless of how badly I otherwise wanted it to.

Again, get too egotistical with this stuff, and the only one you end up impressing is yourself, because you are so deluded!

So I listened, and when I had to, I measured. And when I finished, I measured again, just to make sure...

And the conclusions I came to with the Dyna 70 were surprising.

For one, unlike most know-nothings & tube god wannabees, I discovered that the pentode of the 7199 is probably the best sounding small-signal pentode on the planet! I tried using EF86's in the 70 long before Uncle Ned decided to plop a pair onto a new input board for the 70. Like, 15 years before, I'd say! The EF86, shock! horror! as good a pentode as it is supposed to be, DIDN'T COME CLOSE, SONICALLY! And not just the horrid Russian ones that are all you can find today, but the nice NOS Siemens & Mullards I had didn't measure up, either!
The 7199 is a truly underrated device, and I've tried the Slob-Tek ones, and as bad as they are, they're still at least as good if not still better than any NOS EF86 with a nice NOS 12AU7/ECC82 combo...

[ichabod]

Arnold, I think what the guy said is a fact and true to any well designed amp. All amps will have its own limits in performance based on the price point they've been pegged at.

Choice of materials or parts used will make it perform as well as it is expected. But I know that you've been doing parts comparison vs. sound improvements and may have at one time concluded that more expensive parts may not make the amp all that sonically spectacular. In other words, using premium or botique parts may not after all make an iota of difference sonically. This could well apply to the design concept and materials/parts selection for the price point of the ST 70 when it was first introduced in the audio market. It's one thing to make the amp more reliable by changing the ailing parts, and quite another to say that one can improve on the limits imposed not by the parts but by its own design. How'd one think Halfler et. al. made good money of the ST 70 saved by its own sound?

The point in all this which I think the poster may be trying to say is the fact that it's hard to improve on an already good sounding product such as the ST 70. I remember one reviewer of this venerable amp saying that he has heard most all mods on the dynaco and have not heard that it has dramatically improved on the good sound he has heard thus far of this amp sonically. But each amp has its own limitations, and it would be foolhardy for one to make enormous improvements claims through mods without necessarily re-inventing the Hafler-Laurent design considerably.

From Joe Rosen's perspective, there's hardly any improvement unless or until the amp has been entirely re-invented.

Just my take.

[arnoldc]

ichabod, that's a good one.

[ichabod]

I am in perfect accord with you.

Tubes are the way to go whatever pp, se, 45s, 2A3, 300s. There's just so much fun here in the listening. Even ordered, no hardlike ss odd harmonics, just the music as it is, and the ST 70 carries that valve tune as any other vintage sound.

My ears told me right a long time before when I first dabbled into audio with fishers, scotts, dynaco, and a little of Quad, Radford, and those big horn guys.

Keep it coming! Guys like you are a diamond in the rough!