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    Dynaco ST70 with Little dot Preamp

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    tomerte

    Posts : 9
    Join date : 2015-04-25

    Dynaco ST70 with Little dot Preamp

    Post by tomerte on Tue Apr 28, 2015 2:37 pm

    Hallo, my name is Thomas, I am new in this forum and I like that I can get a lot of information here concerning my Dynaco.
    I had some tubeamps, 2A3, 300B, EL84 in triode, EL84 in push pull also a Klein&Hummel. But I like the Dynaco ST70 best.
    It is a Dynaco ST70 with VTA board.

    Since some time as preamp I use a Little Dot III. This works well. It is an very tuby sound.
    Now, one tube of the Little Dot makes noise and I put it out. In the time till the new tubes will arrive I installed only a 25k alps poti.
    No preamp only this poti. It is like only a step up attenuator, but it is a poti.

    The sound is not bad. It sounds very clear. The bass is little more, the hights are little less. The soundstage ist bigger, but not so deep.
    I don't know what I like better. With the poti everything is perfect, but I miss some emotions the tubes gave.
    The tube preamp sounded more lifelike - but also more noisy. There was more swing and vibration in the music.
    The poti sounds more dead, but clear and with punch.

    Should I try a step up attenator? Best a 100k attenuator. The poti only has 25k.
    Really positve with the poti is, that I can handle the power better. I can turn the poti at 12 o'clock. The preamp gave the same
    power at 9 o'clock. Yes, I have very sensitive hornspeakers with more than 105db.

    I read that Bob likes his step up transformer and his preamp is not used since the attenator is in action.
    I thought I would change the Little Dot preamp in future and order a SP14 from tubes4hifi. But now I am not so shure anymore.

    What do you think?

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: Dynaco ST70 with Little dot Preamp

    Post by zx on Tue Apr 28, 2015 4:41 pm

    Here the little Dot 3.................for about $350....looks cool

    https://www.youtube.com/watch?v=i_MKXR9tQNM




    Thanks for the site Bob....................

    tomerte

    Posts : 9
    Join date : 2015-04-25

    Re: Dynaco ST70 with Little dot Preamp

    Post by tomerte on Wed Apr 29, 2015 2:30 am

    Yes, this is the Little Dot I have. It sounds good.
    But in the moment I must change the tubes, they are out.
     Waiting for the new tubes I was experimenting with the alps poti.

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: Dynaco ST70 with Little dot Preamp

    Post by j4570 on Wed Apr 29, 2015 10:46 am

    I run my ST70 with Bob's attenuator, with no preamp. Works great. If the attentuator is all the way up (or is it down....?), it's a straight pass though, so you can still use it with a preamp.

    Though I recently bought a SP14 board, for a more long term project.

    tomerte

    Posts : 9
    Join date : 2015-04-25

    Re: Dynaco ST70 with Little dot Preamp

    Post by tomerte on Wed Apr 29, 2015 11:43 am

    Hey, lucky man.
    Than you can report about the sonical differences between the SP14 and the step attenuator.
    Yesterday i liked the preamp more, today I like the attenuator. Everything has its pros and cons.
    If I go with attenuator it will be a diy thing. I still have the switch, need the resistors for a ladder style
    attenuator. Future will come.

    Bob Latino
    Admin

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

    Re: Dynaco ST70 with Little dot Preamp

    Post by Bob Latino on Wed Apr 29, 2015 11:47 am

    j4570 wrote:I run my ST70 with Bob's attenuator, with no preamp.  Works great.  If the attentuator is all the way up (or is it down....?), it's a straight pass though, so you can still use it with a preamp.

    Though I recently bought a SP14 board, for a more long term project.

    If the VTA stepped attenuator is all the way to the right (complete clockwise rotation) then the attenuator is out of the circuit and the input jacks connect directly to the driver circuit without going through the attenuator. This is probably the best way to use the attenuator if you have a preamp.

    If you have a preamp, you can also use the attenuator as a "master volume control" for the amp. Some preamps have too much gain and the amp is too loud in the first 1/4 turn of the preamps volume control. If this happens with your preamp, you should set the attenuator at maybe 1 to 3 o'clock. Set like this, it will now require much more input from your preamp to achieve full volume with the amp. You will have effectively increased the "range" of your preamp's volume control. You will now have to turn the preamp's volume control up to maybe 1 or 2 o'clock to get a lot of volume out of the amp.

    Bob

    tomerte

    Posts : 9
    Join date : 2015-04-25

    Re: Dynaco ST70 with Little dot Preamp

    Post by tomerte on Wed Apr 29, 2015 12:17 pm

    Hallo Bob.
    Yes, that would be a good way. You also told, that it is possible to lower the resisor of the feedback
    for less volume.

    But for me now, I am researching the soundquality of step attenuator vs. tubepreamp. Here I am not shure what I like more. These are different characters in sound. I like them both.
    But I must say that I compare now a solo alps 25k poti vs. a tubepreamp.
    Perhaps a real step attenuator would change the situation.

    j4570

    Posts : 141
    Join date : 2010-08-30

    Re: Dynaco ST70 with Little dot Preamp

    Post by j4570 on Wed Apr 29, 2015 1:10 pm

    It's the VTA steppped attenuator that I have. Bob's description is right on and much better than mine.

    I have used my ST70 " with the attenuator out of the circuit" but only a couple times. I don't have a quality preamp to hook up to the ST-70, so that's why I don't use one.

    And if you wait on my review of the SP14, you'll be waiting probably a year or more. I just bought the board, I don't have any other parts yet.

    tomerte

    Posts : 9
    Join date : 2015-04-25

    Re: Dynaco ST70 with Little dot Preamp

    Post by tomerte on Fri May 01, 2015 6:20 am

    Hallo Bob,
    what is better for lowering the power of the amp
    1. a step attenuator, or
    2. to give more negtaive feedback (3500 Ohm resistor)?

    the step attenuator ist direct in the signal chain,
    the negative feedback indirect.

    What will have more influence on the soundquality?

    Bob Latino
    Admin

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

    Re: Dynaco ST70 with Little dot Preamp

    Post by Bob Latino on Fri May 01, 2015 8:06 am

    tomerte wrote:Hallo Bob,
    what is better for lowering the power of the amp
    1. a step attenuator, or
    2. to give more negtaive feedback (3500 Ohm resistor)?

    the step attenuator ist direct in the signal chain,
    the negative feedback indirect.

    What will have more influence on the soundquality?

    If you change the feedback resistor you change the "character" of the amp. In other words you actually change the way the amp sounds. You are correct in that lowering the value of the feedback resistors (R7 and R8) will drop the gain of the amp but below about 3000 ohms, the amp will take on a "harder" more solid state type sound. Changing the feedback resistor is not a good way to change the gain of the amp.

    If you use the stepped attenuator, all you are doing is inserting a series of SMD (surface mount device) resistors on the two channels between the input jacks and the driver board input. The SMD resistors are balanced within 1% from channel to channel. This is a better way to control gain than altering the feedback resistor.

    Bob

    tomerte

    Posts : 9
    Join date : 2015-04-25

    Re: Dynaco ST70 with Little dot Preamp

    Post by tomerte on Fri May 01, 2015 9:01 am

    OK, now I have the problem that I only can turn the preamp knob til 9 o'clock.
    To implement a stepped attenuator is it a solution for comfort (to be able to turn to 12 o'clock)
    or is it also better for the signalchain to give more power from the preamp to the ST70.
    You know the ST70 is very sensitive, ba adding more power from the preamp I also will ad
    more distortion?!
    If it is only for comfort, than I could live with it. But if it would be a benefit for the signal it would
    be a good reason to put a steeped attenator in.

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