AC Balance Pots On Board

Hi folks,

I have recently completed re-construction of an st-70 using the VTA board. After replacing an output transformer (you were absolutely right Bob...thank you) I have the 1st of 2 amps up and running in my system. Bias adjustment is simple and quick and my redings have been rock solid. The amp sounds fabulous! The only other thing I need to do is adjust the AC balance, but I have no idea how to do it. I remember reading about it somewhere, but cannot find the procedure anywhere. I seem to remember someone saying it was possible to make this adjustment without a scope and would at least get close. I have a friend who has a scope but have not asked him about it yet.

On a side note, I have to say this amp sounds better than I thought it would. It is driving the midrange section of my homebrew speakers which are open baffle line arrays. The midrange lines came out to about 5.5ohms and are running off of the 4ohm taps. The fresh st-70 makes the temporary tweeter amp sound like absolute garbage. A VERY happy customer here!

Thank you very much!!

Thanks very much for this helpful guide.

As I do not have a function generator, I intend to create a test tone CD using a 100Hz tone using Adobe Audition. I have a few options when I do this, and am not sure which to use: Do I need to set it to a sine or square wav? I haven't a clue unfortunately!

I guess I also need to work out at what volume level to create this file with? Any ideas?

Many thanks.

- John


P.S. If anyone else is keen on trying this out using Adobe Audition here's the link which explains it: http://help.adobe.com/en_US/Audition/3.0/help.html?content=WS58a04a822e3e5010548241038980c2c5-7f5e.html

Hi John,

You want to use a 100 Hz sine wave in this test.

If you don't have a function generator with a voltage out readout then you might want to try the test at maybe 25% to 50% volume on your computer. Understand that the dummy speaker loads will heat up in a few minutes of testing.

To all > There is a FREEWARE program that you can run on your computer and output to your ST-70 through the sound card. Download NCH Tone Generator at the link below and run it on your computer. I use connector with a mini stereo plug on the computer end and two RCA connectors to go into the ST-70. The NCH tone generator will output sine, square, sawtooth and other types of waves at different frequencies. Check it out at the link below ..

NCH Tone Generator

Bob

Thanks Bob - and thanks for that link too - nice find!

I'd like to do this on my ST120. The steps are clear and I know I need to work with care under the hood.

Question - For the ST120, is the non inductive 8 ohm resistors 20 watt enough? I know this is a speaker dummy load and I was wondering if I need to go above the 20 Watt as the 120 puts out more Wattage VS an ST70.

I can only get the non inductive 8 ohm resistors in 20 watt as that is all the local Radio Shack has in stock... Otherwise I can order online which is fine.

Thanks, Bob. I was just at this point in my build (meaning it's done!) and had everything for this tweak except for the 0.1uf caps. If it's really not worth the effort, I don't think I'll do it...though then there's the question of why those pots are there in the first place?
The amp is sounding great as is, by the way!
-Brian

I just did this on my amp. Kind of a pain in the rear. My digital multi meter was doing some strange things I think. The Auto feature was a little erratic. Bob is right and this didn't change the sound. Still sounds awesome!, but didn't make any improvement in sound

I think I did this right. The meter reading fluctuates when you are doing this. I got it to a point where if I went clockwise or counter clockwise on the balance pot the reading would go up. The readings on the meter jump around. It isn’t like setting the bias where the reading is a bit more constant (compared to this process). So you kind of second guess a bit when doing this.

Looking back I think I’d have left the balance pot at the 50/50 setting I had it at from the install… Just not mess with it.

I think you just need one .10mfd / 100Volt capacitor for this… I had one end of the capacitor going to the pin 8 on V2 and then the other end of the cap to the digital multi meter (red probe). I then had the black digital multi meter lead going to the pin 8 on V3 (No cap involved). I just mention this because I was wondering if I needed a second cap to run the digital multi meter black (thru the cap) to the pin 8 V3. Let me know if I'm off on this and needed the 2nd cap??

The newbie...