Help troubleshooting an equalizer please

diaz028

Member
2007-06-26 7:30 pm
I have put together an equalizer from a schematic, using gyrators.

When I play through it, the sound is very weak and static-y, almost digital sounding. It fizzles away quickly in a distorted mess. If I bypass the EQ, the sound is back and massive. There is also a low end hum with the EQ in. The three frequency controls do have an effect on the output. They all contribute to their corresponding frequency ranges. The low pot accentuates the hum as well.

So far, I have swapped out all the transistors, re-soldered the inputs/outputs, replaces the input/output coupling capacitors, tested all the transistors (diode check), resistances all look good from measuring in circuit. I have combed over the schematic vs layout three times now.

I am running out of ideas other than starting from scratch. Any ideas?
 

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epicyclic

Member
2012-01-06 10:21 am
Glad you have the circuit working now, however by changing to a lower DC current gain transistor you will have compromised the voltage divider formed by R227+R262 and R392 and it will no longer be determining the voltage at the base of Q1 and subsequently the whole of the voltage operating point right up to the output . Q1 and its load will be choosing its own base voltage which could now be -ve 1V and may be the reason you are getting distortion.
 
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