The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    VTA ST70 low gain board with high efficiency speakers

    Share

    tyort1

    Posts : 2
    Join date : 2011-02-26

    VTA ST70 low gain board with high efficiency speakers

    Post by tyort1 on Sat Feb 26, 2011 4:20 pm

    Hello,
    I have a low gain (12au7) which I converted from a high gain 12at7 board a while back. I recently built some new speakers and they are MUCH more efficient than my last speakers. Sensitivity is about 105db. So, now I hear noise (sounds like tube rush).

    Just to see a bit where it's coming from, I removed my 12au7 tubes and turned on the amp - very quiet! Nice.

    So I guess my question is - how do I get the VTA 12au7 ST70 to be more quiet with these high efficiency speakers?

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA ST70 low gain board with high efficiency speakers

    Post by Bob Latino on Sat Feb 26, 2011 6:18 pm

    Hi,

    105 dB speakers are very high efficiency speakers and as you know the higher the efficiency of you speakers the more apparent any amp or preamp noise will be. You can try two things to lower noise a little ...

    1. Have your 12AU7 tubes screened for low noise. Upscale Audio and a few other vendors will screen tubes (at extra cost of course) for lower noise. The center 12AU7 (voltage amplifier for both channels) is the one most likely to cause noise.

    2. Drop the values of R7 and R8 (the two feedback resistors) from 7.5K to 3.5K. This will increase the amount of feedback but will also drop the gain of the amp somewhat and lower the noise level a little.

    Bob

    Kegger

    Posts : 5
    Join date : 2009-11-16

    Re: VTA ST70 low gain board with high efficiency speakers

    Post by Kegger on Sat Feb 26, 2011 6:23 pm

    Gain is generally where the noise will come from on an amp with high sens speakers.

    But you say you already did the low gain conversion, if you did ALL of it then there
    isn't much left but use maybe 12bh7 or the jj ecc99 tubes, one to check is there is
    a bypass cap on the voltage gain tubes cathode on the high gain board, sometimes
    when people convert to the low gain board they didn't remove it, check to see that
    you have and if not remove it then, you'll drop a bit more gain and may fix your issue.
    (c9 and c10 I believe are the bypass caps on the voltage amps cathode)

    Otherwise supply capacitance can help, I tend to run between 220-470uf after the choke.

    Also feedback can help, raising the feedback(lowering the resistor value) to an extent can
    lower the noise floor, I just wouldn't go to low there.

    tyort1

    Posts : 2
    Join date : 2011-02-26

    Re: VTA ST70 low gain board with high efficiency speakers

    Post by tyort1 on Sat Feb 26, 2011 6:58 pm

    Wow, VERY fast responses! Thanks Smile

    I did remove C9 and C10, so that's not an issue. And I'm actually using some low noise Brimar CV5042 (12BH7 equivalents) already.

    I will give the lower R7 and R8 values a shot. In fact, I'm already in process of ordering new resistors as it is, since I had tried to "upgrade" my board with a bunch of carbon film resistors, which are supposed to be "warm" sounding (Kiwame and Takman REX to be specific), but they turned noisy after about a year of use Sad So, I'm going back metal film resistors right now.

    Sponsored content

    Re: VTA ST70 low gain board with high efficiency speakers

    Post by Sponsored content Today at 4:37 pm


      Current date/time is Sat Oct 01, 2016 4:37 pm