Choosing a Power Amp ST-70 or a pair of MK-III (Or MK-II)

Greetings everyone,

It’s probably obvious but I'm new here. I am hoping to solicit some opinions about the ST-70 and MK-II or III Power Amps. I have a PAS II and two ST-70 power amps. I just picked up the second ST-70 recently. I am running them bridged for the 70watt output to each speaker. I am now wondering if a pair of the Mark III 60 watt amps would sound better/cleaner than the bridged ST-70. I have never had the opportunity to listen to a MK-III so I thought I would pick the “Collective brain” here in hopes that someone or many someone’s had or has both for a comparison. Any advice would be appreciated.

Thanks in advance! Mark

If I were buying monoblocks, I'd buy Bob's M-125's!

Thanks for taking the time to reply. My personal preference for tube equipment is for Vintage unmodified units. Thanks again!

Well, if it's vintage, then you are back to the MK III I guess. While I have not compared bridged ST70's vs. MK III's, every bridgeable amp I ever owned sounded better to me unbridged. I'm interested too if anyone has done an actual comparision.

By "bridged" do you mean the Dynaco-design parallel connection of the inputs, driving both amplifiers simultaneously with identical signals and paralleling the outputs? If so, that is an inferior method that works poorly because...among other reasons...it mixes together the feedback signals that are supposed to correct each channel's "errors" separately; so that it corrects NEITHER channel.

It also does not result in a doubling of output power as one might think, because the power supply of the ST-70 is not sufficient to support that high a combined output. Compared to this, a Mark III or Mark II would be preferable. I actually prefer a conventional single ST-70 with its nominal 35 watts per channel, to two of them connected in this parallel-connected so-called 70-watt hookup. More is lost by this mode of operation than is gained by the added output power.

Does this mean I think Dynaco was wrong to provide this form of mono operation on the ST-70? YES!

A PROPERLY "bridged" operation of a stereo amplifier involves inverting the phase of one channel and taking the output across the two "hot" speaker outputs, with no connections to or between the "common" output terminals. It is most often found on car stereos where it permits a higher output power than would be available from a conventional amplifier on a 12 volt power supply. Some audiophile-grade amplifiers have used it successfully, too. Either way, to get the best results, you need a phase inverter stage for one of the stereo amp's inputs, and it must obviously be of superior quality.

Thank you GP49 for your information on this subject. I have been wondering about this also.

Can you possibly bi-amp your speakers? Use one amp per speaker- one channel for bass, the other treble (or mid & treble). Some speakers have the two sets brought out on the back (and then tied together, which can be taken off), making it easy. That would be the better way to go.
Come to think about it, that's what I should be doing with that other amp...hmmmmmm.

I have never listened to a pair of "mono" ST-70s. I have owned many stock ST-70, several ARC ST-70-C3 and currently own a pair of Mark IIIs that I have rebuilt from the ground up. Driving Quads, Martin-Logan CLS, Magnepans, and NHT "one"... the Mark IIIs easily get my vote. The Mark IIIs produce a more controlled, more extended range. I hear more depth and stereo effect with the Mark IIIs. The combination of the Mark IIIs with the (homely) PAS2 has produced some of the best music I have heard in my home.

j beede wrote:... the Mark IIIs easily get my vote. The Mark IIIs produce a more controlled, more extended range. I hear more depth and stereo effect with the Mark IIIs. The combination of the Mark IIIs with the (homely) PAS2 has produced some of the best music I have heard in my home.

Agreed - I have an ST-70, (Bob's kit), as well as a pair of Mk IIIs fitted with VTA boards... If I were to only have one amp for the rest of my life, I'd go with a pair of Mark IIIs.

-- JJ

Go with MK 111's

I am considering the tubes for hi- fi kit which replaces the 7199 board with a 12AU7 board-Can someone comment on this? Is there a better way to go?

are you kidding? Probably over 1000 people who would tell you there's nothing better.
About 900 people on this forum alone, let alone AudioAsylum and all the others.
But of course there are probably 3 or 4 people somewhere who might claim that something else is better . . .

Of course you could be a little "biased" , If you'll pardon the expression on a tube site....

If you were to consider The MKIII's you won't be happy with them until you use Bob's VTA upgrades. Trust us they really sound wonderful.
Yes they are not original but they are also not for sale.

Scott, . . . .what's not for sale?
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