Driving an ST-70 without a preamp

knowledge base for the classic Dynaco ST70

Postby dcriner » Thu Apr 15, 2010 6:17 pm

Geek wrote:I'm sorry, did I misread? I thought the original question was on straight line pre's 0_o
Sorry, I was mixed up in my post. I wasn't even thinking of my ST70 - I was thinking of my Decware amp with volume control and three different souces: CD, tuner, and phono. The first two are line level that go straight into the amp, and the phono has a preamp with RIAA equalization.

I'm using an Onkyo solid state preamp for my ST70. The inputs (CD, tuner, and phono) go through that.
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Postby kevco » Thu Apr 22, 2010 8:44 am

Just for the heck of it I'm going to mention this: My bother was trying to decide whether or not buy an ST70 and wasn't yet prepared to lay out some dough on a preamp as well...As he does most of listening using an Ipod source, I tested my Ipod directly wired to my ST70 with Shannon's driver board installed. I found that the Ipod could drive the ST70 to seriously high volume with no problem at all. I reported this to my bro and he went ahead with his purchase...The only thing is: His amp has a VTA driver board installed using 12AT7's for both VA and drivers...It obviously has less preamp gain and only takes the Ipod's signal to what I would consider to be about 1/3 to 1/2 of it's total output power potential. Basically my amp would work fine without a pre, but his does not.
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Postby mesherm » Thu Apr 22, 2010 4:28 pm

Subsitute a 12BZ7 for the middle 12AT7 in the VTA board and that should increase the VA gain.
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Postby Bob01605 » Sat May 22, 2010 5:06 pm

The main poster's question was "Driving an ST-70 without a preamp"

An ST-70 works very well with a stepped attenuator. Someone mentioned above that a stock ST-70 needs about 1.3 volts for the full 35 watts output. A VTA driver board needs only about .75 volts for full output on an ST-70. I don't know what Shannon's board needs but I am sure it is more sensitive than the stock ST-70. Most CD players put out a pretty much standard (appox.) 2.1 volts - more than enough to drive an ST-70 to full output.

There are stepped attenuators out there now that are small enough to fit into areas that you would not normally think they would fit. I have one now fitted to my personal ST-120 tube amp and it works well. All you do is remove the virtually useless stereo/mono switch on the front of the amp and place the stepped attenuator in its place. If all you have is a single high level signal source like a CD player - then an ST-70 amp may be run without a preamp. You can also use these WITH a preamp as a "master level control" if your preamp is set up so that most of the volume comes on in the first 1/4 turn. By setting the attenuator at 1/2 or 2/3 of its total rotation, you get more range on your preamp's volume control.

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