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    ST-70 MONO, STRAPPED. Old question, asked again-- looking for new answer

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    Nycjimp

    Posts : 3
    Join date : 2014-10-05

    ST-70 MONO, STRAPPED. Old question, asked again-- looking for new answer

    Post by Nycjimp on Sun Oct 05, 2014 8:31 am

    I'm an owner of two beautiful sounding fully refurbished including triode mods, etc. I know what the manual says. I know what the experts have said regarding my question. I've read all ( I think) threads on this topic.

    I run two st-70s, strapped in mono mode with BOTH inputs (using a y splitter/combiner) and they sound fantastic. There is no doubt to my ears this IS the best the old ST-70s can sound.

    Please don't just cite others if you don't have a technical answer WHY this might be the case. This fortuitous discovery in the very least should be spread throughout the Dynaco community-- I'd like some logical explanation to back up the magical sound from mono ST-70s.

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST-70 MONO, STRAPPED. Old question, asked again-- looking for new answer

    Post by peterh on Sun Oct 05, 2014 8:44 am

    If the outputs feeds separate speakers, thats fine. But if the outputs are interconnected then
    any unbalance in the two amps will cause distorsion since the "weaker channel" will act
    as loading and the "stronger channel" will be bogged down by the combination av speaker load and
    a weaker partner.

    If the outputs must be tightly coupled, i think the minimum would be common feedback, but
    i still don't see how it could balance dissimular tubes.

    One exception would be a bridge-coupled pair, then one of the pairs needs to have a phase inverter in front.


    Nycjimp

    Posts : 3
    Join date : 2014-10-05

    Re: ST-70 MONO, STRAPPED. Old question, asked again-- looking for new answer

    Post by Nycjimp on Sun Oct 05, 2014 10:03 am

    In theory it should sound worse...I get that. It is, however, substantially better sounding in mono, using both inputs.

    I'm looking for some scientific explanation that the opposite of what you'd expect to happen, actually happens.

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 MONO, STRAPPED. Old question, asked again-- looking for new answer

    Post by Bob Latino on Sun Oct 05, 2014 10:40 am

    Nycjimp wrote:In theory it should sound worse...I get that. It is, however,  substantially better sounding in mono, using both inputs.

    I'm looking for some scientific explanation that the opposite of what you'd expect to happen, actually happens.

    Do you have a wire connecting the two output transformers on each ST-70 in your mono set up ?

    Bob

    Nycjimp

    Posts : 3
    Join date : 2014-10-05

    Re: ST-70 MONO, STRAPPED. Old question, asked again-- looking for new answer

    Post by Nycjimp on Mon Oct 06, 2014 11:09 am

    Yes, 16 ohm taps are strapped. (And common)

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 MONO, STRAPPED. Old question, asked again-- looking for new answer

    Post by Bob Latino on Mon Oct 06, 2014 11:52 am

    Each to his own opinion I guess .. I am wondering whether the fact that you have more POWER now using two amps vs. one amp and you now have more about 3 dB more headroom, could be a factor in your hearing better sound using the amps in the mono mode?

    I have always found that vertical biamping your speakers using separate channels on an ST-70 type amp sounds best to me. In a case like this you would remove the straps between the woofer and tweeter sections of your speakers (provided they have separate inputs for both woofer and tweeter sections). You then run one channel of an ST-70 on the woofer section and the other ST-70 channel of the same amp to the tweeter section. You then do the same thing with your other ST-70 on the other stereo channel.

    When you run an ST-70 amp in mono with 16 ohm speaker taps strapped, the feedback lines for the two channels are combined and this usually results in the sound quality going down. If, however, the mono mode works for you in your music system, then keep using it ..

    Bob

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Need more basic information on "strapped", "bridged" and such, please

    Post by Aljaheejus on Sat Oct 11, 2014 12:27 am

    I'm closing in on a single two-way speaker project, and would like to use (modify) a stock-circuit ST70 for monaurol reproduction for bass guitar and general instrument amplification.  The goal on the speaker is a full-range bass guitar speaker that can also be used for a modern polyphonic keyboard with an array of "instruments" or even to mic a cajon or conga.  

    I can picture a "Y" to utilize R and L channel inputs, but I would appreciate a diagram on how to use both outputs to power an 8 Ohm cabinet.  (Eminence Legend CB15, crossover model=PX21k6, and PSD:2002S-8 tweeter

    Thanks in advance!

    Guest
    Guest

    Re: ST-70 MONO, STRAPPED. Old question, asked again-- looking for new answer

    Post by Guest on Sat Oct 11, 2014 10:12 am

    I think that perhaps if one is really set on mono, and of course depending on the output you're after, look at used MKIII mono blocks. Even if it needs some rework, you'll end up with a ground up designed mono amp giving you quite a good output.
    It would also be relatively easy to install a volume control on a MKIII.

    Aljaheejus

    Posts : 37
    Join date : 2010-01-03

    Re: ST-70 MONO, STRAPPED. Old question, asked again-- looking for new answer

    Post by Aljaheejus on Sat Oct 11, 2014 2:31 pm

    Thanks.  Yes, I understand I would be making it do something it is not designed (well) to do.  A MKIII or dedicated mono build may be in the future.   It's just that I have an ST70 and will try it even with just one channel for a while.  

    Also, I want to understand how to bridge the outputs for an 8 ohm cabinet.   Hoping someone can explain.  I've read the manual and think it makes sense, but without a diagram I'm not sure.

    Do I need to reinstall the "mono" switch per the original schematics?

    Do the two yellow leads (16 ohm), one from each channel get used as + and - for the speaker?

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