Dynaco ST-70 in mono mode?

I have just bought two ST-70's from eBay and setup both in mono mode with the switches set to MONO. I'm driving a pair of Martin Logan Quest hybrid electrostatic speakers. These speakers sound amazing with the sound stage and pinpoint accuracy and imaging of voice solos. I wanted to try a low cost (relatively speaking ;-) test to see how these speakers work with tube amps. Experiment successful!

The ST-70's work fine at 70W but they don't have the headroom and dynamic range with the Martin Logans as if I had a more efficient bass reflex or acoustic suspension speakers. Martin Logan said these need a minimum of 100W per channel, recommended use of 200w per channel. Am I correct to assume that these in mono mode the ST-70's about 70W?

The Dynaco instructions said to strap the 8 ohm outputs in parallel to achieve the 70W. If I parallel two ST-70's (both in mono mode) in a similar manner by parallel wiring the two amp's 8ohm outputs, will the total power output be the sum of both amps in mono mode, or 140W?

Or would it be better to just buy a higher output tube power amp ($$$$)? Any suggestions?

The Martin Logan's sound fantastic with my Home Theater system (150 watts per channel). I have a 9.2 speaker setup, all Martin Logans. I use a simple AB switch for the front L&R speakers between my HT receiver and the Dyncao tube power amps (PAS-3 & two ST-70's) which I use to listen to my vinyl collection.

1) The rule is you never attach the speaker terminals of two amps together unless you want to destroy your amps. Not only will the speaker see their output, but they will see each other and bad things tend to happen.

2) Running an ST70 in mono has been discussed in other threads. If you search this board and others you should be able to find a few threads on the subject. The general idea is that although possible, it is a less than optimal solution. Biamping is the preferred method if your speakers can be driven that way.

notbananas wrote:
The ST-70's work fine at 70W but they don't have the headroom and dynamic range with the Martin Logans as if I had a more efficient bass reflex or acoustic suspension speakers. Martin Logan said these need a minimum of 100W per channel, recommended use of 200w per channel. Am I correct to assume that these in mono mode the ST-70's about 70W?

No; they don't. More like fifty watts or less, from what I've seen, before distortion goes way high. The negative feedback from the two channels gets mixed, so that instead of correcting for distortion arising in the individual channels, each channel amplifies the other's error signal, increasing distortion. You are better off running ONE Stereo 70 as a stereo amplifier than in the factory's MONO mode. This was one of the errors Dynaco made. Might have arisen from the advertising department.

Search the forum for prior discussions about this topic.

I'm just curious.....has anyone out there ever actually done the 'mono' thing according to Dynaco's instructions......and then run any legitimate tests to see just how well the process actually delivers? I'm not saying that 'bi-amping' isn't a better idea.....but to do that properly, one should have a cross-over ahead of each amp if you're going to run 'bass' with one ST-70 and 'mids & highs' with the other. In the absence of that, there's the issue of running 2 sets of outputs in 'stereo' mode from a pre-amp with only one set of outputs. If you use a 'y' connector on each Rt and Lf output so you can run two ST-70s each in 'stereo' from one set of outputs, your input/output loading is off to some level, not to mention that each amp is running full frequency amplifiction when it only needs to be in the ranges of the speakers it's driving. (thus the need for the outboard cross-over). If I recall, Dynaco published some pretty interesting specs on the ST-70 running in Mono mode. However, I think the difference between 60 watts and 70 watts is only about 2 db. So, if a person wants the 'best' of that world, they might consider the Mk III series amps.......or Mr. Latino's 120s!

wharf-creek wrote:I'm just curious.....has anyone out there ever actually done the 'mono' thing according to Dynaco's instructions......and then run any legitimate tests to see just how well the process actually delivers?

Yes: I have. I was an audio technician, on and off, for years. I ran the tests using conventional bench test equipment: function generator and both harmonic and intermodulation distortion analyzers, and observed both the output waveforms and distorion waveforms on an oscilloscope. Distortion in the strapped-mono Stereo 70 starts off high, even at low output power. In fact the strapped mono Stereo 70 is substantially outperformed on the bench by ONE CHANNEL of a regular Stereo 70 until the single channel clips. Since most of the time an amplifier is not called upon to output more than a few watts, that probably explains why the strapped mono amplifier sounds bad. What little gain in output power the strapped-mono has, is far more than outweighed by its inferior performance, the very great majority of the time.

Conventional bench testing, however, does not reveal how an amplifier behaves on a transient basis. I also suspect, but have not measured, that due to the mixing of the negative feedback signals between its two amplifier channels, the strapped-mono Stereo 70 behaves and recovers from clipping worse than does one channel of a Stereo 70. If this is so, when extreme demands are made, and BOTH strapped-mono AND one-channel amplifiers clip, the strapped-mono will STILL sound worse, even when it doesn't quite clip as early as the one-channel amplifier.

What I do know from listening through real speakers in a good listening room with a listening panel: a PAIR of strapped-mono Stereo 70s is roundly outperformed by a pair of Dynaco Mark II (fifty watts rated, through a pair of EL34 output tubes, the Mark II being selected for the test to eliminate differences between the EL34 and the 6550 tube); and was also surpassed by a SINGLE regular Stereo 70 operating in its Stereo mode. There was NO ambiguity; EVERYONE said he heard those results.