Symasym 5.3 "AAK model" builder's thread

[SteveA]

Originally Posted by AAK

Just to make sure the amps output transistors U3, and U4 need to be attached to a suitable heat sink before testing.

They're all attached to heatsinks. I'll report back as soon as my kitchen pass is renewed. IE, got a little honey (you) do list.
SteveA

[routhun]

To be safe with output transistors, especially with EF output stages I don't install them, instead I solder 100 ohm resistors from base to emitter on PNP and NPN transistors. This completes all the feedback circuits, now I measure the critical voltages like, output offset, VBE of the driver and pre-driver transistors. Once these voltages are good, I remove these resistors and install the power outputs. This saves me the output transistors and even most of the circuit if there are any gross mistakes in the assembly. Hope this helps. I've recently finished my SymAsym, almost 5 years after I bought the PCBs. But this is a wonderful amp, make my speakers sing .

Thanks,

[SteveA]

Originally Posted by AAK

SteveA, lets focus on the five amps that have the 1.4 to 1.7 voltage drop across each (+/-) safety resistor which sounds about right.

Take one of the five amps, remove the safety resistors, and insert the fuses making sure that bias pot R22 is turned fully counter-clockwise before powering up. Then set multimeter to lowest voltage setting, should be in the millivolts, then power up and measure the emitter voltage across U3 and U4. The meter should read near zero. If it's not near zero, stop and let me know what it is. Then adjust R22 clockwise until it reads about 20mv, then check the DC offset across T4 and T5. It should read less than 50mv. If everything checks out Ok, repeat with the remaining amps. Let me know how it goes before we move on.

Al

I measured ~13mV on each emitter when powered up thru fuses. Is that adequately close to zero? The measurement seemed to be decreasing very slowly (ie, initially 15mV down to 13.5mV within a minute or so).
SteveA
Thanks AAK, your help is greatly appreciated!!

[AAK]

Hi SteveA,

Sounds like you measured one emitter at time. You need to measure across both emitters by placing the meter's positive red probe on U3's emitter, and the neg black probe on U4's emitter. Assuming you measured 13mv on each emitter separately measuring across both emitters should give approx. zero. volts. If it is zero volts or even 13mv continue my instructions in Post #709. Make sure to turn the pot slowly, it make several turns before you see the voltage increase. Let me know how it goes.

Al

[SteveA]

Originally Posted by AAK

Hi SteveA,

Sounds like you measured one emitter at time. You need to measure across both emitters by placing the meter's positive red probe on U3's emitter, and the neg black probe on U4's emitter. Assuming you measured 13mv on each emitter separately measuring across both emitters should give approx. zero. volts. If it is zero volts or even 13mv continue my instructions in Post #709. Make sure to turn the pot slowly, it make several turns before you see the voltage increase. Let me know how it goes.

Al

Your are correct. I remeasured all 5 smoke free boards as per your note and all gave 0.0mV. I put fuses in one of the other boards to see what would happen. Powering up thru the bulb tester (the bulb stayed dim this time) and I measured 1.5mV. From your comment above I'll assume that 1.5mV is adequately close to zero also.

I'll get to the next step after the Twins game or sooner if they get blown out.

[SteveA]

I measured voltage across the emitters on the 5 boards with all showing 0.0-0.1 mV. On 4 of the boards I was able to adjust the measurement to ~20mV with the trimpot (offset ranged from 2-26mv). On the last board the voltage stayed at 0.0mV while adjusting the trimpot (measurable voltage occurred at about 5-6 turns on boards 1-4; no change on board 5 after at least 10 turns). The BD139 on this board had an open solder joint on one leg. I replaced the part but same result, no ability to adjust voltage across emitters.

One of the 3 boards which had smoked the safety resistor showed 1.5mV across the emitters. I attempted to adjust to 20mV but within 15 seconds of powering up smoke was coming from the R5, R10 area (681R and 68.1R).

[DJNUBZ]

Originally Posted by AndrewT

these 28.4Vac transformers look about ideal for a single pair Symasym.

35Vac is too high. That would allow ~ 100W into 8r0 and a single pair Symasym output stage would not be reliable at this level of input/output.

Could you tell me what the lowest voltage is I could use with a symasym? How many amps does one board draw at full power? I am guessing about 1-1.5 amps but I could use some reassurance.

[AndrewT]

The speakers demand the current.
The amplifier tries to meet that demand by connecting the PSU to the speaker.

As to minimum PSU voltage, I don't know, I have not tested the amplifier at very low voltages. In fact, I have not checked the stability of the amplifier at any supply voltages. I have accepted the design as a mature amplifier that will work as presented.

[Ryssen]

Anyone know what the dampingfactor of the Symasym is?

[lineup]

Originally Posted by RKH

SymAsym5 - Project

Thanks for the link to schematic