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    ST70s as Mono Blocks

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    Bugs

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    ST70s as Mono Blocks

    Post by Bugs on Sat Jul 11, 2009 1:30 am

    I think I read here that the mono switch setup that Dynaco used on the ST70 had a pretty limited application.

    Has anyone ever set two ST70s up as mono blocks, how did or would it work? Question

    Thanks,
    Bugs

    Bob Latino
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    Re: ST70s as Mono Blocks

    Post by Bob Latino on Sat Jul 11, 2009 7:04 am

    Bugs,

    It is probably better to use two ST-70's and to biamp with two ST-70's if your speakers have a separate input for the woofer and tweeter sections. (Run one channel to the woofer and the other to the tweeter). If you run the ST-70 in the mono mode you don't just flip the switch on the front to mono. When you flip that switch you now just have two channels carrying the same signal. To truely run the ST-70 in mono you have to run a wire from the 16 ohm tap on one output transformer to the other 16 ohm tap (if you are using 8 ohm speakers) OR from the 8 ohm tap to the other 8 ohm tap (if you are using 4 ohm speakers). By doing this you have the two output transformers wired together. The only problem with this is that no two output transformers are exactly alike and any differences between the two transformers will show up as negative interactions and usually somewhat higher distortion ratings. The ST-70 usually will sound better in the stereo mode than the mono mode. IMHO if you want to get the 70 watts from an ST-70 into one speaker then do the vertical biamp procedure mentioned above. Of course, if your speakers only have one input then the vertical biamp won't work for you.

    Bob

    Bugs

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    Re: ST70s as Mono Blocks

    Post by Bugs on Sat Jul 11, 2009 11:18 am

    Thanks Bob,

    I do have one set of 4 ohm speakers I can biamp. Would anything be gained by building a pasive filter set at the woofer XO frequency to send the high frequencies to one amp and the bass to the other amp? I know there are electric XOs for doing this, but they are a bit spendy.

    Bugs

    GP49

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    Re: ST70s as Mono Blocks

    Post by GP49 on Sat Jul 11, 2009 12:06 pm

    Since the input impedance of the ST-70 is known, it would be relatively easy to install a RC (resistance-capacitance) network at the input to gently roll off either low or high frequencies. Implementing such a high- or low- pass filter would involve some math to calculate the proper values. In effect you would be building something like a loudspeaker crossover network, with values of the parts selected for the input impedance of the ST-70 rather than the 4 or 8 ohms of a typical speaker driver. But the rolloff may not be steep enough to use with your tweeter, especially if your biamp setup would bypass the tweeter's own high-pass filter.

    The formulae for calculating the necessary values would be in a basic electronics text. Which is to say, I'm not going to do them for you (makes my head spin when I try, but I generally muddle through).

    Bugs

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    Re: ST70s as Mono Blocks

    Post by Bugs on Sat Jul 11, 2009 1:59 pm

    So, it sounds like I would need to build a two-way crossover calculated for the impedance values of the amp?

    Would the factory loudspeaker crossover be left in place to act as a final filter? I would assume that they should be left in the circuit.

    The speakers I would be using are a pair of Boston Acoustic A400s that I'm currently refreshing the crossovers with poly caps and new resitors.

    I recently did this with a pair of Boaston A200s. The old electrolitic caps were 29 years old and were in dire need of replacing. The refresh tightened up the base and really brought the mids and highs back to life.

    Bob Latino
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    Re: ST70s as Mono Blocks

    Post by Bob Latino on Sat Jul 11, 2009 3:59 pm

    Bugs wrote:Thanks Bob,

    I do have one set of 4 ohm speakers I can biamp. Would anything be gained by building a pasive filter set at the woofer XO frequency to send the high frequencies to one amp and the bass to the other amp? I know there are electric XOs for doing this, but they are a bit spendy.

    Bugs

    Bugs,

    I don't feel that sending the high frequencies to one ST-70 and the bass to another ST-70 is the best way to do it ...

    Over the years and listening to a number of ST-70's in a biamp mode as well as in mono > IMHO the best way to implement two ST-70's in one stereo system is to VERTICALLY biamp with one ST-70 on each stereo channel. If the speakers have their own crossovers and dual inputs I see no reason to build another crossover. There are two advantages to vertically biamping as opposed to HORIZONTALLY biamping (one amp on the two woofers and one amp on the two tweeters) at least with Dynaco ST-70 amps.
    1. Vertical biamping allows greater signal isolation between the two amps. You may (as one customer noted to me) a greater sense of stereo separation and/or a larger sound stage.
    2. Vertical biamping allows TWO quad caps (B+ DC power storage) to deal with the power hungry bass notes. If you horizontally biamp only one quad cap now has to deal with the bass from BOTH stereo channels.

    There is one caveat here if you run any biamping without an active crossover. Usually the crossover in most speakers is set up so that the woofer section creates a larger impedance than the tweeter section but the TOTAL inpedance is 8 ohms (or 4 ohms if your speakers are 4 ohm speakers). When you separate them, the woofer ALONE may have 7 ohms and the tweeter 1 ohm. In a case like this you can run an 8 ohm non inductive resistor across the tweeter terminals to present the proper impedance for the amp. If you don't, and run one channel of the amp (the one driving the tweeter) hard into a 1 ohm impedance, you could take out an output tube or worse yet an output transformer. Output transformers do not like being driven into a virtual dead short (an impedance of 1 ohm or less).

    Bob

    GP49

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    Re: ST70s as Mono Blocks

    Post by GP49 on Sat Jul 11, 2009 10:31 pm

    "In a case like this you can run an 8 ohm non inductive resistor across the tweeter terminals to present the proper impedance for the amp."

    BUT: If the 1Ω tweeter is run with an 8Ω resistor across its terminals, i.e. in parallel, the overall impedance will be LESS than 1Ω.

    Bob Latino
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    Re: ST70s as Mono Blocks

    Post by Bob Latino on Sun Jul 12, 2009 6:30 am

    GP49 wrote:"In a case like this you can run an 8 ohm non inductive resistor across the tweeter terminals to present the proper impedance for the amp."

    BUT: If the 1Ω tweeter is run with an 8Ω resistor across its terminals, i.e. in parallel, the overall impedance will be LESS than 1Ω.

    GP,

    You are of course correct .. The resistor goes in SERIES not parallel .. I don't know why I said parallel ? old age ?? - lol

    Bob

    Bugs

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    Re: ST70s as Mono Blocks

    Post by Bugs on Sun Jul 12, 2009 11:12 am

    Bob Latino wrote:Over the years and listening to a number of ST-70's in a biamp mode as well as in mono > IMHO the best way to implement two ST-70's in one stereo system is to VERTICALLY biamp with one ST-70 on each stereo channel. If the speakers have their own crossovers and dual inputs I see no reason to build another crossover. There are two advantages to vertically biamping as opposed to HORIZONTALLY biamping (one amp on the two woofers and one amp on the two tweeters) at least with Dynaco ST-70 amps.
    1. Vertical biamping allows greater signal isolation between the two amps. You may (as one customer noted to me) a greater sense of stereo separation and/or a larger sound stage.
    2. Vertical biamping allows TWO quad caps (B+ DC power storage) to deal with the power hungry bass notes. If you horizontally biamp only one quad cap now has to deal with the bass from BOTH stereo channels.

    I need to get a cup of coffee this morning (I'm fighting my own battle with old age LOL) and think about this.

    I believe what you are saying is that experience has shown that running one channel through one ST70 (bass and highs) and the other channel through the other ST70 (bass and highs) is the better of the setups.

    How do you set this up off of the preamp? Would you run the right channel cable to one amp and the left channel cable to the other amp? Seems like that should work.

    Then there is the whole impedence thing that will take me at least another couple of cups of coffee to try to wrap my mind around Razz .

    Bob Latino
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    Re: ST70s as Mono Blocks

    Post by Bob Latino on Sun Jul 12, 2009 11:35 am

    Bugs,

    As you have said, run the right channel cable from the preamp to one amp and the left channel cable from the preamp to the other amp and plug that one cable into either RCA input. On a stock ST-70 flip the switch to mono (if you still have the stereo/mono switch in the circuit) - You will now be feeding the same signal into both channels of one ST-70.

    If you have the stereo/mono switch bypassed and your preamp has TWO outputs per channel you could run two cables to each amp OR just get TWO "Y" connectors - one for each ST-70 with two male connectors on one end and one female on the other. Plug one Y connector with the two males into each ST-70 and then your interconnect from the preamp into the female of the Y connector.

    Bob

    Bugs

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    Re: ST70s as Mono Blocks

    Post by Bugs on Sun Jul 12, 2009 2:14 pm

    Bob Latino wrote:Bugs,

    If you have the stereo/mono switch bypassed and your preamp has TWO outputs per channel you could run two cables to each amp OR just get TWO "Y" connectors - one for each ST-70 with two male connectors on one end and one female on the other. Plug one Y connector with the two males into each ST-70 and then your interconnect from the preamp into the female of the Y connector.

    Bob

    Okay, now I understand the cable part. So if my speakers have an impedence of 4 ohms and I'm driving one channel per speaker (bass and highs) with one amp, then I should use the 4 ohm speaker tap on the back of the amp?

    If I'm understanding this correctly, the advantages of this setup would be that one quadcap would be powering one bass driver which would keep the amp from working so hard and, I'm guessing, provide more headroom.

    Bugs

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    Re: ST70s as Mono Blocks

    Post by Bugs on Sun Jul 12, 2009 2:21 pm

    And one more clarification.

    After the preamp and amps are set up, for a biamped speaker, one set of speaker taps from the right channel amp would feed the right channel bass and the other set feed the right channel highs and visa versa with the left channel amp.

    Bob Latino
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    Re: ST70s as Mono Blocks

    Post by Bob Latino on Sun Jul 12, 2009 3:28 pm

    Bugs,

    That is correct ..

    Bob

    Bugs

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    Re: ST70s as Mono Blocks

    Post by Bugs on Fri Jul 17, 2009 3:20 pm

    Bob Latino wrote:There is one caveat here if you run any biamping without an active crossover. Usually the crossover in most speakers is set up so that the woofer section creates a larger impedance than the tweeter section but the TOTAL inpedance is 8 ohms (or 4 ohms if your speakers are 4 ohm speakers). When you separate them, the woofer ALONE may have 7 ohms and the tweeter 1 ohm. In a case like this you can run an 8 ohm non inductive resistor across the tweeter terminals to present the proper impedance for the amp. If you don't, and run one channel of the amp (the one driving the tweeter) hard into a 1 ohm impedance, you could take out an output tube or worse yet an output transformer. Output transformers do not like being driven into a virtual dead short (an impedance of 1 ohm or less).Bob

    Does this pertain to vertical biamping? If so, what wattage resistor should be used on the mids and highs for a speaker rated at 4 ohms?

    Bob Latino
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    Re: ST70s as Mono Blocks

    Post by Bob Latino on Fri Jul 17, 2009 5:38 pm

    Bugs,

    There is no real answer to your question because all speaker crossovers are somewhat different. After you separate the strap that goes between the highs and the lows you should measure the resistance across both the lows and the highs. This will only give you a rough idea of the impedance of the lows and the highs because the impedance varys with frequency so the impedance that you measure may be considerably higher or lower. Many (but not all speakers) are set up so the the woofer section has lower resistance and the tweeter section has a much high resistance but together they may be roughly 8 ohms in an 8 ohm speaker. Lets say the woofer section is 8.2 ohms and the tweeter section is 330 ohms. Together they present about 8 ohms impedance to the amp but if you split them the woofer is OK by itself but the tweeter section by itself is not the proper load for the amp. In this case running an 8 ohm resistor across the tweeter terminals will work. In another case you might have to run a resistor in series with the speaker line.

    The crossovers in a speaker system are nearly always PASSIVE crossovers. The BEST thing to do if you biamp is use an ACTIVE crossover. The active crossover's output should present the proper impedance to the amp. Google "active crossover" and you can research how they work.

    Bob

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