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yoyoy 11th September 2013 11:11 AM


Originally Posted by Rudi_Ratlos (
Yoyoy, I do not know the requirements that a bass guitar amplifier must have.
The SYMASYM is a Class A/B amplifier, and it does not have a pre-amplifier onboard.

About capacitors: there have been some questions about the value of the input capacitor C1.
The job of C1 is to block DC voltage. You can take any value from 3.3F - 10F for C1.

You can of course use your 24VAC - 0 - 24VAC transformer.

Best regards - Rudi

about the caps, what i was asking was what kind of cap is C2 and C3? and how are they different? can you also suggest any pre-amp that i can attach to symasym...


Rudi_Ratlos 11th September 2013 12:36 PM

@Yoyoy: I am using a 100nF WIMA cap as C2 and a Cornell/Dublier 100pF as C3.

I do not use an "active" pre-amplifier.
I am using my VCPre passive pre-amp:

for my SYMASYM.

Best regards - Rudi_Ratlos

yoyoy 14th September 2013 03:14 AM

is there any sub for 2sk170?
we dont have it here?

Rudi_Ratlos 14th September 2013 05:49 AM

Yoyoy, MPSA18 is a good substitute or BC550C (be aware about reversed pinout then).

Best regards - Rudi

RKH 14th September 2013 11:52 PM

Help please
My woes are outlined up above.... But last testing, Bias 24 mV, DC offest within spec. Amp was a massive AM radio receiver. I turned it off quickly to avoid toasting things.

Given that I had plenty of small signal transistors on hand, I decided to replace them all... it is an expedient approach to fixing things. I understand, not very sophisticated nor elegant. Regardless... now that I have all new small signals, the bias comes up solid at 24 mV as before. However, I'm now looking at about 1.2V of DC offset on the output.

Update: after 15 minutes or less, it's now to 0.9 VDC on the output.

That is with the input grounded. I DO have matched to 2SK170s on the input. Where else do I need to 'match' the small signal transistors more closely to reduce the DC offset? Or is there something else I'm missing.

Note, I also replaced the input capacitor with another MKP type.


RKH 15th September 2013 01:24 AM

DC offset
is inching down to 0.4 VDC after 1+ hour.

AndrewT 15th September 2013 03:38 PM

check the currents through the "pairs" of devices.

RKH 15th September 2013 06:51 PM

The Vdc offset settled down to about 170mV, well outside of spec. So ...

I had a bunch of MPSA18 sitting around. I quickly matched a pair for Hfe and installed those (at 180degrees to the sk170).
All seems okay. Vdc is now a fraction of a mV (i.e. mostly zero.) I've set the bias to ~25.5 mV as I have larger heatsinks. I will let it settle for a few hours and see what happens. Then maybe some music.

It helps to have the output connected to the speaker terminals on the chassis... brilliant..

OFFSET is still a couple of hundred mV. So I will try to do as Andrew suggested as well as run the Sim to get voltages around the circuit. I was too anxious in hoping for a quick fix. How fussy is the "C" rating in the BC560C position?


RKH 16th September 2013 12:13 AM

Enough for today
My weekend is mostly over, so I've got to quit for now.

With MPSA18, 25mV bias setting, 300mV of VDC on the output:

Of the measurements I've made, there doesn't seem to be anything extraordinarily out of whack but ...

Emitter currents for the 2n5551 (measured across the 150R resistors)

Current through R12 = (.362V/150R) = 2.41 mA into emitter of Q9 (Q11 on sim)
Sim says: 2.48 mA

Current through R11 = (.343V/150R) = 2.28 mA to emitter of Q8 (Q13 on sim)
Sim says 2.44 mA

Current thru 47K resistor that connects to collector of 2N5551:31.9 uA
Simulation says: 18.2 uA ... That seems to be a problem

Current thru 47K resistor that connects to collector of 2N5401: 19.8 uA
Simulation says: 22.4 uA

Current thru 68R resistor that connects to emitters of 2N5401: 4.8 mA
Simulation says: 5 mA

Do these give a hint as to what might be amiss?


Rudi_Ratlos 16th September 2013 07:58 AM

Ryan, a well-matched SYMASYM shows a DC-Offset of < 5mV. My own one has an offset of about 1mV.
Whenever I read about a SYMASYM - DC-Offset problem in the German DIY-forum and then read the
conclusion, it is one of 3 reasons:

1 - solder error (short)
2 - wrong small transistor used
3 - reversed pinout of small transistor used

Your findings (2N5551) point to this direction as well.

Best regards - Rudi

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