dynaco st-120 question....

Discussion in 'Solid State' started by vinylisfinal, Dec 8, 2008.

  1. vinylisfinal

    vinylisfinal AK Subscriber Subscriber

    i know, i know.. why an st-120? well its not my main system and when ya run it through a mcintosh c-28 it sounds pretty damn awesome... my question is, i thought the st-120 could be run bridged? i cannot remember how to hook it up in bridged mode? i want to run two units for stereo.. any help is appreciated.. thanks, david:thmbsp:
  2. EchoWars

    EchoWars Working a Deal in Denmark Subscriber

    The ST-120 runs on a regulated supply...I can't imagine it being able to deal with the rigors of bridging.
  3. whsh93a

    whsh93a Banned

    A properly built and operating ST-120 is a very nice sounding amp. I use one to power my subwoofer.

    And - funny you should mention bridging. The ST-120 is not designed to be bridged. It's bigger brother - the 150 is, though. Since I own about a zillion ST-120, ST-80 and 150's I thought I'd play with the 120 to get it to bridge and use it to run my subwoofer.

    ANY stereo amp can be bridged. The basic process is to invert the input of one amp, feed a common signal into both, and then take the output off of both amp's "hot" sides.

    I don't think he 120's regulated supply should be an issue because the 120 is capacitively coupled. That means the caps will dump their voltage into the bridged outputs - sort of like a voltage doubler circuit.

    The most elegant way is to locate the point where the negative feedback is fed back into the input of one amp and use that portion to invert the amp's signal.

    The other, much easier way, is to use a dual audio op-amp set to unity gain. Feed the input signal into both op-amps. One op-amp then inverts the signal and the other does not. Then, feed each seperate op-amp's output into the two seperate stereo amp's inputs.

    Done correctly, the final power output is much more than twice the combined power of each amp (P= E squared/R

    Finally, I have not gotton around to modifiying the 120 yet to do that. Some other project always gets in the way and it gets bumped aside (because it's not going to take much effort and it's working OK as it is now, etc.). But, there is no reason it shouldn't work for the 120 just like any other amp. (Murphy's Law therefor predicts that it probably will be a LOT harder!)


  4. justw

    justw Well-Known Member

    Since the ST-120 has is a wanna-be high frequency oscillator, I wouldn't try it. Though they sound good I recall that some tended to break out into oscillation for no good reason at all.
    (that's from memory and I could be wrong)
  5. onplane

    onplane What! No Wake???

    Help me, DH, understand your comment above. Why should it make any difference if the amp is capacitively coupled or direct coupled. Seems to me the result is pretty much the same either way.

    Further, since the 120 is a semi-regulated supply I think EchoWars has a legitimate point. I would bet the stock bridge rectifier would not handle the increase in current. Naturally, I mean the increase in current if you attempt to use that additional power. Next, you'll have more current going through the output transistors (again assuming you are actually using this additional power) and I wonder whether they will handle it.

    OK, DH, but at what IM and THD??

    I mean we know that no amp is prefect, but here we have two amps working in tandem. Won't we be multiplying both the IM and THD?

    I haven't worked out the math, but my gut is distortion will grow in the same proportion as the power output grows. That is, it's not linear!

  6. whsh93a

    whsh93a Banned

    For a direct coupled amp, there is no way for the output power to be stored. In a cap coupled amp, the capacitors must first be charged then uncharged. If connected in bridge mode, essentially they act in series and theoretically double the voltage available.

    The powers supply may be an issue. But I think the issue is not so much with the rectifier but with the current limiting scheme. It may be necessary to take out the current limiting resistors and the diode in the feedback loop.

    Since the amps are bridged, the two transformers are working in parallel - so power capability is not too much of an issue.

    The peak ratings of the transistors may be a long term problem, but I doubt if they would be damaged. More of a problem may be the peak rating of the output capacitors. However, since each amp is essentially handling only 1/2 of the combined peak voltage, it is not likely.

    The distortion would be a product of unbalanced currents between the two amps. Assuming that the two amps have the same gain, etc. distortion will not increase much, if at all.

    One final note - the 120's have no problems with high frequency oscillations - at least no more than any other amp of that era. That reputation came about when people tried to do the TIP upgrade and left out the bypass capacitors needed to prevent the <new> oscillations that could happen because of various issues with increased bandwidth, etc..

    Finally, we also use a 120 for a PA amp driving two ohm loads in my wife's band. HERE

    We sometimes use a smaller ST-80 for the monitor speakers or even for the PA in smaller venues. This amp - this ACTUAL amp - is very cool in that it was also used as a PA for Perry Como and Cab Calloway.

  7. onplane

    onplane What! No Wake???

    Sure there is, DH! Power is stored in the large caps in the power supply. It's actually easier to see what happens in a bridged dc coupled amp. Let's assume we have rails of +43 and -43 volts.

    Unbridged the maximum voltage applied to a speaker would be ... what? +40 volts and ground = 40 volts.

    When bridged we'd have +40 volts and -40 volts = 80 volts.

    Problem is we have not changed the load, but have double both the voltage and current and all that current must flow through the output transistors.

    Unless the heat sinks were designed with this in mind, I'll bet there would be a problem. I mean there is just going to be too much heat.

    I really don't think those coupling caps do anything but insulate the speakers from DC.

    Well, any messing with the feedback scheme and distortion will go to hell in a hand basket.

    What two transformers?? I can only see one!! I thought you were talking about bridging the two amps within a single ST-120.

    Bingo!! That's just the problem. There is no chance in hell, that the two amps will have EXACTLY the same gain at all frequencies. Just look at the tolerance of the components.

    Unequal gain (over the entire frequency range) = IM distortion!

    Now, would you hear this if used to power a sub-woofer? Probably not, but you sure would if used to power full range, high quality speakers.

    DH, I have to plead ignorance when it comes to PA amps as I have little understanding of the spectral power requirements. For example, is much in the way of low frequencies (like a bass line) going through this amp?

    That is cool!! How did you happen to acquire that amp?

  8. onplane

    onplane What! No Wake???

    vinylisfinal, you never said why you wanted to run "bridged".

    What are you trying to accomplish?

    What speakers do you plan to power?


    PS: You could use the 4 amps within 2 ST-120's to bi-amp your AR's
  9. whsh93a

    whsh93a Banned

    I said: Since the amps are bridged, the two transformers are working in parallel - so power capability is not too much of an issue.

    You said: What two transformers?? I can only see one!! I thought you were talking about bridging the two amps within a single ST-120.

    Here's why: I was talking about st-120s and then wandered off into thinking about my plan to bridge two of my SWTP 207/A monoblock amps.

    On different discussion points.
    I understand totally how a power supply acts in a direct coupled amp.

    Concerning capacitors, I am talking about the charge stored in an output capacitor, not what is stored and used to AC bypass the output transistors in a power supply. The ST-120 does not use a split supply, either.

    I have never seen a bridged amp change much in it's distortion - and I've measured them (like a bridged Crown D-150 or a Dynaco ST-150) on my distortion equipment. There is usually some, but I attribute that to slew rate - which is what causes the main difference in gain between amps operating at high power and voltage levels.
    The diode I am referring to in the 150 is in series with the drive signal for the outputs - it's in the feedback loop for the protection circuitry. It is normally forward biased and therefore is seen as a short to the signal. If the output sees a short, that forward bias is removed and the amp turns off - thus protecting it. It's actually a pretty neat system.
    PA systems usually need a frequency response typically in the range of 50 HZ to about 15KHZ. The PA system I am talking about is for live band performances - not a PA used for, say, motivational speakers. I only mentioned that because for some reason people have the idea that the ST-120 is unstable and unrealiable. I can take a screwdriver and short the speaker outputs of a 120 and no damage will occure. And they still sound great driving speakers like the AR-LSTs at 4 Ohms - my basement system. It even sounds good driving my electrostatic speakers - but I use the 120's for my subs because I had some lying around and just like the SWTP 207/A's even better when teamed with my electrostatics.
    I picked up the ST-80 referenced off of E-Bay and noticed it was set up for PA work (the inputs were changed to reflect normal PA line levels). I e-mailed the seller and asked him where he had used it. Turns out the guy was an older musician with lots of stories and had worked with some of the greats. If he is to be believed - and I think he is - that amp was used a lot by a lot of greats.

  10. onplane

    onplane What! No Wake???

    Well, I think designers of those amps understood the problems associated with bridging and selected components accordingly. It's not at all clear to me that the designers of the ST-120 considered bridging in their design.

    Again, for a sub-woofer the sonic problems are inconsequential compared to the power handling issues of the output transistors and the power supply.

    Wow, DH, you have a set of LST's!! I'm a big AR fan. I have a set of AR-3a's and a set of Teledyne era TSW's.

    If I had a set of LST's and two ST-120's, I would bi-amp those in a minute! I'd further by-pass both the cap bank and the transformer. To me those are engineering "overkill" plus I just hate transformers in the signal path.

    And what to you get for this needless complexity? 6 different audio balance positions! With two amps on each speaker you can get an infinite number of balance positions plus a shorter signal path and ... no transformers!

    Finally, there is a common disease that inflicts all early AR's and that is tweeter output. The foam suspension in those old tweeters has degraded and most believe, limits their sound output. Under a bi-amp it's posible to coax a little more sound out of them and you do this by padding the mid driver.

  11. whsh93a

    whsh93a Banned

    I have done some work on the LSTs. I have rebuilt most of the tweeters and substituted silocone calking compound for the dome suspension. It works very well and actually brightened up the sound a bit. I think that may be because it is more resiliant than the originals and helps compensate for the degraded foam under the domes. I also rewound the voice coils on some of them to improve the flux density - ie. efficiency. That made a nice difference as well. I was a little worried that the small additional mass would reduce the high end but that did not happen. For me, the biggest "problem" the LSTs had was the lack of upper mid to lower high frequency output. That may be why I find electrostatics so appealing and ended up making my own from video tape. I agree about the auto transformer, etc. However, I am not very motivated to do it. I think that the people at AR designed the LST originally to compete with all the other "Studio Monitors" out there - like the JBL L-100 for example. The bells and whistles were added for that reason. That was the era of bells and whistles of which I think Kenwood and Aiwa were tops. Aiwa still has carried on that tradition to today.

    I am quite certain that Dynaco had no intention that the ST-120 ever be bridged. But, that just makes it a more intersting project. I have more than a few of them sitting around so experimentation and possibly torching a couple won't necessarily hurt. Besides, I can repair that model with my eyes closed. I sell a lot of refurbished equipment. That's the only way I can afford this hobby!


  12. vinylisfinal

    vinylisfinal AK Subscriber Subscriber

    i was planning on powering a pair of infinity wtlc towers and a pair of vintage advents or ar-2ax's.. not sure which i will dig up.. thanks for the info.. i just thought i remembered a wiring setup for bridged operation in the original owners manual for the st-120
  13. wbain

    wbain Active Member

    That is very funny.

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