ST-70 in Mono

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ST-70 in Mono

Postby skidave » Fri Nov 07, 2008 8:28 pm

I had some time tonight to check the power output of my ST-70. (Unit has the stock circuit, rebuilt with new components, a cap board and new tubes E.H. EL34 outputs)

8 ohm tap w/ 8 ohm non inductive load was 38 Watts for each channel. Both channels voltage measurement were very close when calculating power.

I was curious what mono looked like and I read what the manual had to say about using 8 ohm taps in parallel for 4 ohm speakers etc. I tried a combo and here is what I measured:

8 ohm taps in parallel w/ 8 ohm load = 55 watts
4 ohm taps in parallel w/ 8 ohm load = 38 watts

8 ohm taps in parallel w/ 4 ohm load = 72 watts
4 ohm taps in parallel w/ 4 ohm load = 56 watts

I don't have the 16 ohm taps available for speaker connection after installing banana posts, so no measurements were made.

Anybody else measure their ST-70? Comments on my findings? I'd be curious to see if this is repeatable on someone's ST-70...


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Running ST-70's in mono

Postby Bob01605 » Sat Nov 08, 2008 2:52 pm

Over the years I have run ST-70's in mono a few times and was never satisfied with the results with respect to sound quality. At least to my ears the ST-70 always sounded better as a stereo amp. I have a theory as to why but can't really prove the point.

My "theory" > When you run the ST-70 in MONO and run a jumper between the two output transformers you have both output transformers "pulling together" with the same exact signal. Any slight differences now between the two output transformers will cause out of phase information that will degrade the sound quality.

IMHO > The best way to run the 70 watts into one speaker is to BIAMP vertically with one amp on each stereo channel. Disconnect the strap between the tweeter and woofer inputs (hopefully you have a modern speaker set up this way) and run 35 watts into the woofer and 35 watts into the tweeter. Then do the same with another ST-70 on the other stereo channel.

This has always worked out better for me than the alternate method of horizontal biamping in which one amp feeds both woofers and the other amp feeds both tweeters. Set up this way one amp (with it's single quad cap) has to deal with the power hungry low end of the frequency spectrum while the other amp has it easy because the tweeters require less power from the amp. Set up vertically, as explained in the above paragraph, you basically have a setup like two monoblocks and gain the advantage of greater signal isolation which should transfer into a better stereo image with a wider soundstage.

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Postby skidave » Thu Nov 13, 2008 7:24 pm

I was curious about the measured performance. I don't like bridging solid state amps, but I have paralleled my home built single ended MOSFET amp to see performance.

I currently run two solid state amps in a biamp configuration. One amp per Magneplanar; one channel for the bass panel and one channel for the ribbon. The ST-70 holds its own when driving them, but another ST-70 is needed either in biamp config or in mono config. I'm still trying to convience the wife...

I was interested if anyone else has any real measured output specs.


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NFB issues, too...

Postby EWBrown » Fri Nov 14, 2008 7:27 am

In addition to the slight differences between the two OPTs, I would SWAG that the two separate NFB loops are also interacting with less than optimal results.

The other way to use an ST70 in "mono" would be to bridge the two inputs (use teh slide switch, or just jumper the centers together, and feed one) and then leave the outputs separate and use that to drive two speakers, either in the same cab, otr in separate cabs. THis might work best in a bi-amped system, use the ST70s for the woofers, and another separate amp (like an ST35 or equivalent) to handle the midrange and tweeters. This also eliminates the "issues" and losses of dealing with LCR type crossovers.

Just another SWAG, as I haven't actually tried doing this...

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I have Two ST-70 AMPS

Postby METAMUSIC » Wed Nov 26, 2008 2:32 pm

I have Two ST-70 AMPS. I would like to run each amp MONO at 70 watts (one for each speaker). My speakers do not have bi-amp capabilities. Is this possible without a phase inverter?

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Postby burnedfingers » Sun Nov 30, 2008 7:52 pm

What actually are you going to gain? If you double power you will gain 3db at the expense of added distortion. If your really wanting to biamp then forget the mickey mouse internal crossover that is inside the speaker cabinet and go with a professional crossover/time alignment piece/EQ piece. The internal crossovers are going to be wastefull and will not compensate for proper driver alignment /voice coil.

As much as I love tube amplifiers I seriously cannot recommend a Dynaco Stereo 70 to run the low end. Use a SS amplifier for low frequency and the Dynaco for mids and highs.
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