SC200 Power Amplifier: Correct Bias Resistor Value, BC556B/BC546B Gain Impact, F2 Fuse

Hi I have built the SC200 power amplifier and have the following issues and question about the circuit.
1.)  The circuit diagram shows a 150 ohm resistor in the bias circuit but on the parts list lists this as 120 ohm and the circuit baord also shows this as 120 ohms. Which is correct, the circuit diagram or the circuit board/part list.2.) The low power transistors I have used are BC556B and BC546B and have about twice the gain of the basis versions. These were the only ones available at RAPID Electronics. They did not stock the basic version ie BC556 and BC546.  3.) I put power to the amplifier initially using a current limited supply with the bias set to its lowest value. I measured the output voltage of the amp which was about 5mV, this seems to wonder around a little and dropped to 2.5mV. I then put the amplifier on a higher current power supply and adjusted the bias so that the voltage between test points as follows. TP3 and amplifier output = between 4-5mV TP5 and amplifier output =  between 4-5mVThe Amplifier seemed fine at first. The voltage between the amplifier and ground  was still about 5mV.I then shorted the amplifier input to ground so I could measure the amplifier output noise.After this the amplifier blew Fuse F2 and burnt 220ohm resistor which is connected to the base of Q12.See the attached circuit snippet.I will recheck all the components and make sure all the flux had been removed from the board tonight.
Do I need change the transistors from BC556B and BC546B to the basics version??, are they causing a problem with the amplifier ie a stability problem.Also did shorting the input to ground cause a problem.I have not installed the ferrite bead.
Can anyone help, has anyone else built this amplifier and had issues.

I think the main problem I had  was due to a driver resistor not mounted flat and bolted  to the heat sink bracket properly. I have remounted the transistors, replaced an output transistor which blew and replaced the driver transistors and also made sure they are mounted flat and secured properly. The amplifier now operates and the bias circuit seems to adjust the bias current to the output transistors correctly as the temperature increases. I have yet to inject a Sin wave into the Amp which I will do tonight and look at the output on my scope.Hopefully I have fixed the issue.

Hi,A quick answer, no need to change the BC556B transistors as the change in current gain is not an issue. Despite the 5mV output offset, shorting the amplifier output to ground is not a good idea, since 5mV/0ohms still is a lot of current. This is why commercial amplifiers have protection circuits which test for output short circuits etc.
So, i expect that the shorting of the output was the cause of the blown fuse etc., and i assume you will test each component to see if they have failed etc.
As per your second post - you seem to have changed those components that needed changing. If the design notes state bolt the driver resistor to the heatsink, and you did not do this, then this may be an issue. I still expect that shorting the amplifier output caused excessive current, which therefore caused catastrophic failure of the relevant components. The resistor in the diagram, does not state a specific power rating, so i would assume 1/4watt is sufficient. Therefore, it is unlikely that this resistor needed to be attached to a heatsink under normal operating conditions.
Hope this helps.Regards,Richard.

 

Hi Richard you misread my post, I shorted the
INPUT to Ground not the output.

As I said in my previous post I think I have
sorted the problem out replaced the failed transistors and re seated all the transistors making sure they are all
flat and bolted down securely.

 

My other question refers to a resistor values
that is shown on the circuit diagram as 120 ohms which is connected to

The bias trim pot. This is shown as 150 ohms on
the PCB. Which is correct.

 

I am current testing the amplifier and will be
measuring it with SCOPE and TRURTA software on my PC.

I will let you know what I find and if the
measured distortion and noise is similar to the those quoted in the Article.

 

Regards

 

Heath

Richard with reference to a resistor values that is shown on the circuit diagram as 120 ohms which is connected to the bias trim pot. This is shown as 150 ohms on the PCB. Can you tell me which value is the correct one??????.

RegardsHeath

Hi Heath,Apologies - i should be more careful in reading. Shorting the input to ground should not cause the failure - as you already have a 12kohm shorting the input transistor. This then leads to the question, as to why the transistors failed and the resistor burned. If you connected a pre-amplifier that has a very low output impedance, which is near 0ohms, then the amplifier should still be stable.
For the 120ohm/150ohm, the circuit diagram shows 150ohms. I checked elsewhere, the circuit diagram is on the web in its entirety, and it states 150ohms. http://transmactech.blogspot.co.uk/2017/09/sc200-audio-amplifier-complete-project.html
The difference should not matter - since it is in series with the 1kohm trim pot. This means that the trim maximum is either 1150 ohms, or 1120 ohms. Either value is acceptable, but the 150 ohm resistor will provide slightly more range. Difference is 3% for the range - so not an issue.
From the full circuit diagram, there is VR2 - offset adjust - have you set this up as required ?
Regards,Richard.

Hi Richard the problem was a classic case of thermal runaway I think. At least one bias transistors was not mounted flat and securely bolted to the heat sink bracket. This is why the 220 ohms resistor to the base ofbias transistor Q12 burnt out and why an output transistor blew.
RegardsHeath