Amplifier design parameters required for a good match for high (16+) ohm speakers?

Thanks for the responses. 

I am using a Dynaco ST-70 (35wpc) into Contrast Model One As3 speakers having a rating of 16ohms and 92dB sensitivity.  I believe the sensitivity rating might be a little high, even considering that 92dB equates to 89dB with an 8ohm speaker. I have no knowledge about the impedance across the frequency range, however the manufacturer states that it does not vary much.

I guess then, when considering an amplifiers rating into an 8ohm load, one should half that rating into a 16ohm load as a way of assessing that amplifiers ability to provide enough power for a particular circumstance.  

I might add that Ralph of Atmosphere refurbished my ST-70. 



 

Rodman, Thanks much for your input and the link. I can't believe I did not find this review myself. It seems that the input impedance of this amp is not the reason I didn't find it as enjoyable using a Schiit SYS passive pre as with my Jolida active pre. Bass was lacking with the SYS.

I have always know that he tube rectifier in the original ST70 was a weak link. Though I would not want to replace with SS rectification. This was why I spent so long looking for a high impedance speaker to use with it. I was also looking to find one with greater than 92dB sensitivity. I have been using the 16 ohm taps with the Contrasts. I have used the 8 ohm taps with other 8ohm speakers, however approached use with them with caution as they all drop below 4ohms in the bass region. I never considered these speakers ideal for this amp.

Rodman, my ST-70 has the 5AR4 rectifier tube made by GE, and the 7199 driver tubes made by Conn in USA. Currently using JJ EL34 output tubes. I know the JJ EL34 could be replaced with a smoother tube. Any particular Mullards you might recommend?

Ralph, I am using the 16ohm output with the Contrasts, which are rated at 16ohms. I am not sure about actual impedance within the bass frequencies. 

Rodman, Thanks very much for the additional info. 

Yes, I will likely shop for a different set of tubes and check out the Mullards offered by Tubedepot. I don't know when the Conn 7199 tubes were manufactured or how many hours are on them. I bought them when I received it back from Ralph.  If needing to replace them I would likely switch to the 6U8 tubes after removing the two leads. The power supply caps have been replaced, however I will have to call Ralph to see if capacitance has been increased. 

I believe my ST-70 was manufactured in 1969. The first numbers of the serial number is 69 and I know it came from that period. I was in storage for most of it's years and is in excellent condition. Ralph said it was in the best condition of amy he has seen for that time. It was factory manufactured, not put together as a kit. I own a original owners/instruction manual for the kit version so following it, I could likely replace the power supply caps myself if I feel the need. I am not an electronics buff, however did build a single generator as a kit when in high school electronics class. Worked from the start. Kit was much as the ST-70 one, just follow directions step by step. Seems the craft is in making a good solder joint. 

Getting a little long winded today, am having fun!

Rodman, Thanks for the additional info!  I have taken a brief look at the additional links. I intend to peruse all the links provided again and possibly set up an upgrade project with my ST-70. Won't happen in the near future however, other things on to-do list. 

I don't remember the signal generator model. Built it fall of 1967. 

Thanks again, you have been most helpful!

Ralph, I haven't put many hours on my ST-70 since I had you refurbish it in spring of 2016. However I now fell in position to do so. Took a long time for me to find a complimentary set of  speakers I could afford.  I have never any doubt that my ST-70 would prove reliable. As an aside, how do the St-70s fair when used in mono mode? 

Speakers are Contrast Model One AS3 Ref. 

Ralph,
I find the paring of the Contrasts with the ST-70 to be quite adequate in my 12'x15' acoustically treated room. Bass has improved with playing time. The Contrast are certainly revealing such that for my tastes best mated to warmish SS or tube amplification. It was their 16 ohm impedance that drew me toward the purchase. 

Regarding the mono mode question: I am asking about your experience using two ST-70s as mono amps. I believe this is strapping.  Based on the owners manual this can be done.  

Forgot to add that the manufactures rated sensitivity was based on 1 watt, not 2.83V.  This is what seems consistent with the loudness I obtain when comparing to other 8 ohm speakers having 88-89dB sensitivity at 2.83V.  My assessment is based on comparison with equal volume setting on different preamps, integrated amps. Not a true measurement. Again, I was more concerned about the impedance factor than the sensitivity as I have a small listening room and listen in a near-field environment.

Thanks Ralph! As it turns out, I may become an owner of another ST-70. Same vintage as the one I had you refurbish in spring of 2016. I will be giving you a call regarding doing same work on it should ownership happen.