Troubleshooting your Symasym

[Sheldon]

Thanks Mike,

All the ground connections beyond the board are solid and have been rechecked. Time to look at the board. Checked R2 as well. I was wondering if I wasn't getting an oscillation somewhere, but no unusual heating.

Sheldon

 

[DoubleG@MDG]

noise and distortions

HI.

I have a lot of problem witch my symasym, first of all my configuration:

output stage: toshiba 2sc5200/2sa1943
drivers: toshiba 2sc5171/2sa1930
they should be all original.

power +- 40V, pcb has pretty much the same layout as on project page, c14 5pF.

All other transistors and parts same as in original project.

I don't know is that a big difference I use bd139-16 for bias witch is the version of a hi hfe about 230.

Now the problem.
when bias is set to 0mv(not by setting pot to like 800ohm but it must be set to max 0mV witch is 1Kohm on pot) everything is ok. their is no noise or distortions.
But when i set it even as less as 1mv it starts to make noise (white noise).
when i set the bias on right side about 12mv on the left i have 50mv
so i think the noise came from left side the + one.

all transistor seem fine, I've measured hfe for etch of them and seem ok. but that don't guaranty that they don't make noise.

what i should check first?

the amplifier is mounted on open air, scrubbed to the radiator.
their is no source or even a volume potentiometer attach.

the solider points are ideal I've check them all 2 times, all elements legs are as short as possible.

 

[clem_o]

Which version / PCB layout are you using? 5_3?

Seems you may have stability (HF oscillation) issues. Changing the original MJE15030/31 drivers to the toshiba pair may have such consequences.

Try playing around with C14, as well as C7 values.

Cheers!

 

[anatech]

Hi Sheldon,
To add to everyone else's excellent points, have you made sure that that heat sink is well grounded? This can be very important.

-Chris

 

[clem_o]

Good point Chris!

I remember we had a little CD player that simply refused to play any disks when the top cover (casing) was open, over at my brother's house. We found a multi-KW radio station not far away enough....

 

[DoubleG@MDG]

I found that removing c6 completely stops the noise, what is C6 used for ?

And i use 5.3 version of pcb.

 

[clem_o]

C6 is used to speed up the turn-off of the driver transistors. The amp should be useable without it, but will have higher HF distortion.

You may want to consider using the originally-specified drivers and retain C6. Or perhaps playing with R8 and R7 values (to 33 or 47 ohms?)... Chris or MikeB should be able to give a more definitive answer!!

 

[Sheldon]

have you made sure that that heat sink is well grounded?.

Yes, it's integral to the chassis. It's hard to track the problem, because it's intermittent. It only happens on the speaker that is farthest from the amps (though cables are same length for both speakers). It seems the common is picking up noise. I can eliminate the symptoms completely by connecting the frame of either of the two woofers to the common speaker terminal. I don't need a DC connection. I can put a film cap in series with the connection, and that works fine too.

Sheldon

 

[anatech]

Hi Sheldon,
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.

-Chris

 

[AndrewT]

The symasym has a full Thiele network on the output.
Have you fitted that?
The Thiele network acts as both an HF load to keep the amp stable and as an HF filter to attenuate interference getting back to the inverting input.

 

[Sheldon]

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.

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.

 

[bluevas]

dear guys,

Are there any japanese transistors to substitute 2N5401 in this symasym amplifier? or maybe with mpsa42/92 series?

Best regards,

JOsh