SymAsym - "The Sequel", AAK's Rev_1.4 PCB Builders Thread

[AAK]

Welcome! This thread is for all those who bought my Rev_1.4 PCB based on Mike Bittner's SymAsym – "The Sequel” circuit design. Of course everybody is welcomed, and if you have something to add by all means feel free to jump in.

Attached is a pdf document with step-by-step instructions to help assemble my PCB.

I've also attached the latest BOM, schematics for the amplifier and PS section and some pictures that should help with the assembly process.

To provide some background for those unfamiliar with MikeB's SymAsym - The Sequel design, here's the link to his thread.

Symasym - the sequel

Here's the thread that I started describing my original Rev_1.3 PCB design based on MikeB's design.

My new SymAsym PCB design Rev_1.3

Note that on the PCB there's a couple labeling mistakes, check page 2 bullets 1&3 of the Assembly Instructions Document for instructions.

I still have a few boards left for those interested.

Regards,

Al

[AAK]

I've attached two real world THD plots of the amp driving an 8 ohm load at 100 watts, and a 4 ohm load at 180 watts at 1khz. In each pic the THD plot of the input signal is on top, and the amplifiers output THD on the bottom.

[andRookie]

I`m waiting for boards an plan to build an amp. I have two transformers. They are both 2x 33Vac/ 600VA. From what I understand using one of them might be enough for a two channel amp?
What C/W - value do I need for the heatsink, each channel, if the psu gives me about 45-46 Vdc?

[coolnose]

Hi everybody,

I omitted including the C24 caps my last order at mousers.

How critical is the voltage rating of C24 as I would have some 470/16 in a drawer ?

TIA,

Max

[AndrewT]

Hi,
in normal operation C24 has virtually no AC voltage across and virtually no DC voltage across it.

In failure mode the cap can see either rail voltage across it.
To prevent damage to the capacitor in failure mode one can add a diode across it's terminals to protect it from reverse bias voltage.
It can also be protected against excessive forward bias voltage by adding a second diode in reverse direction.

Some use two diodes in series in both directions so that at +-1Vpk the diodes do not pass, but they limit the worst case cap voltage to <2Vpk.
These diodes can usually be placed on the bottom side of the PCB and easier if you don't clip off the cap leads until after the diodes are soldered in place.

If the cap is diode protected you can use any voltage from 5V and upwards.

Your 470uF 16V is perfect, just add the inverse parallel diodes.

[coolnose]

Hi Andrew,

thanks a lot for your kind answer.

So, If I get you right it doesn't really matter as the 470/25 as mentionned in the BOM would blow anyway in case of rail voltage, right ?

Maybe you could get back to me per PM as I don't want to pollute this very fresh building thread.

Cheers,

Max

[pchw]

Hi,
is bd139 important to the performance of the amplifier?
I would use any low current, medium gain, medium fT, To126 NPN for the Vbe multiplier.
I would even consider and have used a To92 glued to the output transistor or the heatsink.

R34=499r may be a bit high. You might have to reduce slightly to 390r, or 430r to allow output bias to be set.

I think the 35mV bias voltage is the sum across both emitter resistors, i.e. 17.5mVre.

R36 to it's maximum resistance for initial setting up.
This ensures least or nil output bias current.

The ratio of R35:R34 is ~4:1. I tend to use from 5:1 to 6:1 to ensure sufficient range of adjustability to suit a wide range of different transistor Vbe.
eg if Q17 has Vbe ~0.55V then the maximum bias voltage is ~2.75V
If the output transistors have Vbe ~0.65V then one needs ~2.65V to get a Vre=25mV. That leaves only 100mV spare for tolerances. But this is worst case design, not for a fully specified design. 499r should work, but will nearest E24 value of 510r work?

Rudi also pointed out another variation in
SymAsym - The Sequel, AAK's PCB GB

[AndrewT]

the schematic that Rudi found and posted is wrong. See my later comment.

[poler]

I have what is probably a noob question but do I need LSK170 A, B or C? Thank you.

[AndrewT]

LSK170B is equivalent to 2sk170BL