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.
Amplifier design parameters required for a good match for high (16+) ohm speakers?
Though I have a reasonable understanding of, or at least am satisfied with, the requirements for an amplifier asked to drive low impedance speakers, I am wondering about specific design parameters that might need be considered when mating an amplifier to high impedance ones. Asked another way: Are there specs that one should seek out when searching for an amplifier to be mated to such speakers?
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Forgot to add that the manufactures rated sensitivity was based on 1 watt, not 2.83V.@mesch In that case 'sensitivity' is not the right word- 'efficiency' is. IME monostrapping the ST-70 is easy and works quite well. |
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. |
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. |
The Model One should be very nice with the ST-70. They rate the sensitivity at 92 dB if memory serves; if that is actually sensitivity, the speaker is 95dB 1 watt/1 meter, which will mean the ST-70 has adequate power. If the amp has not been used much, it will need some break-in time during which is will become more lucid. By mono mode, do you mean monostrapped? |
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 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.If it came though here, then we replaced the filter caps. The ones installed allow the 5AR4 to be used, but do have a bit more capacity than stock. We also replaced the bias supply filter caps and rectifiers and probably the coupling capacitors too- the amp was set up to be reliable. |
Your welcome and I hope some of it’s useful(I try). btw: I found this power cable will fit the ST-70, with a rubber grommet and no need to further open the chassis’ original hole(after removing the strain relief, of course). I crossed and closed(without crushing) a couple cable ties around the cord inside, to prevent it’s escape(kinda Mickey Mouse). A snap to hard-wire, internally. It made an audible improvement, though I went cheap on that one(an experiment, at that point). No doubt, there are better cords now, just as small. https://www.zuaudio.com/classic-cables/birth-power-cable There’s an inexpensive/NOS one, currently on eBay: https://www.ebay.com/itm/Zu-Audio-Birth-power-cord-2m-6-5-ft-Brand-New-Factory-sealed/113672669785?h... Happy listening! |
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! |
If you can solder and read simple schematics, improving the ST-70 is cake. There’s a wealth of info available. Just restrict yourself to those best reviewed, far as advice(ie: Ralph). Joe Curcio gave me some valuable tips, on how to increase my Joules, without adding/changing anything outside the chassis. Regarding that wire swap, you might want to save this: http://lib.store.yahoo.net/lib/thetubestore/6U8A-7199.pdf Don’t know if Joe still offers free advice, but: http://www.curcioaudio.com/dynadr_3.htm#DYNACO%20STEREO%2070%20POWER%20AMPLIFIER
Was that a Heathkit Mod #IG 5282, in a blue plastic case? Wasn’t my first build, but it wound up working for years, testing speaker drivers in my shoppe.
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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! |
If I still owned an ST-70, I’d be forced to purchase four of these(Did I mention, "salty"?): https://www.tubedepot.com/products/el34-mullard-double-o-getter-welded-plates I might be disposed to try the NOS, 1970s, Siemens version, toward the end of this page(and keeping a matched pair in each channel, if a matched quad couldn’t happen): http://www.audiotubes.com/el34.htm That page contains a lot of good info, if you’re going to shop EL34 types. How old are the Conns? Assuming you enjoy experimentation, here’s an easy/inexpensive way to enable you to try various NOS 6U8 type tubes(cheaper and more plentiful than the 7199), in your amp: https://tubedepot.com/products/6u8-to-7199-adapter Should you find a 6U8 tube, the sound of which pleases your tastes(Tung-Sol made some nice ones, in the 60s), it’s simple to move a couple leads on the tube sockets, obviating the adapter. https://tubeworldexpress.com/collections/preamp/6u8a Again- if the power supply capacitance is still stock(or aged), it’s still inadequate(by modern standards). Have fun and happy listening! |
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. |
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 always found that the ST-70 played better bass if it was possible to use the 16 ohm tap. This is assuming of course that the speaker was at least 16 ohms in the bass region! The power rating of the amp actually goes up a watt or two as the output transformer is more efficient when used on a 16 ohm load. |
Does your ST-70 still incorporate the 7199? If so- not much available, as far as tube rolling, now days. The EL34/6CA7 has never been known for a lot of dynamics, regarding bottom end. Sylvanias or Mullards, are what I had always kept in mine. They were much more plentiful(and- way less salty too), back then. No experience with what’s being manufactured now, so- can’t comment on their bottom-end performance. What rectifier are you using? The Mullard(Blackburn plant)GZ34, is what I always recommend. Not that it offers any more juice, but- that it sounds so nice and lasts so long. I tried SS rectification(didn’t like it). You might contact Ralph and ask about beefing up your power supply(capacitance, etc), if that wasn’t already part of his refurb work. It’s been my experience, upgrading power supplies has always resulted in improved reproduction/greater dynamic impact, across the board. |
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. |
Your Dynaco ST-70 is a tube amp. While SS amps reduce output, as speaker system impedance rises, the typical tubed amp does not(given a correct impedance match/output connection). You might find this article enlightening. Especially, the comments immediately below graph 7, regarding output and distortion figures, into various loads. https://www.stereophile.com/content/dynaco-stereo-70-ii-power-amplifier-measurements Use your 16 Ohm tap, for best results. |
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. |
As long as the amplifier has enough power that it won’t be clipping into the 16 ohm load, it should work just fine, unless there is some other mis-match going on. I have made speakers that are user-configurable as either a 4 ohm load or a 16 ohm load. As long as the amps weren’t pushed into clipping, to the best of my knowledge all of my customers who tried it both ways preferred the 16 ohm configuration. I have been told by multiple amplifier designers that distortion is lower into a high impedance load. At least one amplifier designer, Ralph Karsten of Atma-Sphere, recommends high impedance speakers particularly for his lower-powered OTL (output transformer-less) amps, which unlike solid state amps actually make MORE power into a 16-ohm load. If you haven’t already, I suggest taking a look at his amps. (Disclaimer: I’m a dealer for Ralph, and have been making speakers with his amps specifically in mind for 14 years.) Duke |
Most SS amp will output half the wattage @16ohm than @8ohm, you
need to check if a amp has enough output power to drive the 16ohm speaker to
desired
loudness/SPL. https://www.crownaudio.com/how-much-amplifier-power |