Troubleshooting your Symasym

[Sheldon]

Originally posted by anatech
Is it possible you are picking up RF noise? Your speaker lead is another input terminal and may have high impedance at RF frequencies.

You can use an RF LC filter on your speaker outputs, or possibly the input pair needs some additional filtering.

This is when a 'scope is very handy to have around.

I know next to nothing about RF, but just the behavior suggests that I'm picking up something in the RF region. It appears that the woofer frame acts as an antenna. I have a scope, and can drag it down to the listening room, but the problem doesn't show up most of the time. As noted, I never hear it with the woofer frame AC connected to the speaker common. Just touching the frame, or even coming within a couple of inches with my hand or head cuts off the interference, if it is present. I suspect that the scope probe would likely do the same.

Originally posted by AndrewT
The symasym has a full Thiele network on the output.
Have you fitted that?

Yes, built to spec, with boards from an early group buy.

Sheldon

[AndrewT]

Tannoy have introduced a 5th terminal on some of their speaker range.
This 5th terminal is connected to the driver chassis and gets taken to the amp ground terminal.

[DoubleG@MDG]

hi again.

I still have problem building symasym, but i have very interesting observations.

output stage: toshiba 2sc5200/2sa1943
drivers: toshiba 2sc5171/2sa1930
this configuration gave me hum and noise but stabilizes when the bias is more then 13mv on one resistor. when i drop the voltage less then 13mv noise comes back and the bias it self gets unstable on minus side i have -10mv but on plus side 30mv but thats oscillations i think. when i set it to stable bias above 13mv the sound is very nice the bass middle and hi all normal no oscillations.
but when i start the amplifier there is a thump and whistle for 1-2sec

output stage: toshiba 2sc5200/2sa1943
drivers: toshiba 2SA1837/2SC4793
the same thing as above

output stage: toshiba 2sc5200/2sa1943
drivers: toshiba MJE15031/MJE15030
Every thing is silent on all bias settings from 0 to 40mv
and theres no noise when turning it on. amplifier sounds normal

all this transistors all original legit
the biggest difference between this transistor is i thing capacitance that what regulates the speed

2SC4793 100Mhz
2sc5171 200Mhz
MJE15030 only 20Mhz big capacitance

with 2SA1837/2SC4793 theres very nice sound much more interesting colorized then with MJE15031/MJE15030.
so i would like to use them but how to stabilize them?

or i have some different problem then just drivers ?

pleas help!!!!

[DoubleG@MDG]

and foto of my symasym:

[formula 22]

Hi
In order to reduce the gain of 27dB [Av = (R29/R30) +1] - see schematic, and the quality amplifiers that i should lose nothing?
Now gain is: Av = (22 / 0.499) +1 = 45.1, or Av = 33.1dB!
Attention: If the gain is too high because symasym is driven from preamp, R30 can be increased from 499ohms to 1k, but in this case R16/19 (22 or 33 ohms, not on schematic, REs to Q1/2) are required to keep feedback at same level and...
This somehow does not understand?
thanks

[AndrewT]

Rduce the closed loop gain and you end up increasing the feedback.

The Symsym is tuned to be stable at the stadard gain setting of +33.1dB

Move away from that standard gain and in effect you have built a different amplifier.