Hello, on my bench now a couple of amplifiers from a small, disappeared Milan factory (this means no schematics) from the Seventies. Design and layout seem only so-so, but the quality of the components is good.
Both are plain-vanilla split-supply Quasis, with discrete differentials at input. One is a bit more powerful than the other, and better cooled, but otherwise identical.
The only controls on the main amps are for idle current.
Unfortunately, they both suffer from DC offset, as high as 140 mV.
I would like to add an offset control, and the simplest example I've found is from the Sanyo DCA 200: making the input transistor base resistor double than the feedback resistor, and adding a 1K VR in series with the input transistor emitter. Other way is as on the Marantz 1040-1070, by varying the DC at the FB transistor base, but it's more work to do.
What do you think ? Of course these are no esoteric amps ...
Thanks in advance !
Both are plain-vanilla split-supply Quasis, with discrete differentials at input. One is a bit more powerful than the other, and better cooled, but otherwise identical.
The only controls on the main amps are for idle current.
Unfortunately, they both suffer from DC offset, as high as 140 mV.
I would like to add an offset control, and the simplest example I've found is from the Sanyo DCA 200: making the input transistor base resistor double than the feedback resistor, and adding a 1K VR in series with the input transistor emitter. Other way is as on the Marantz 1040-1070, by varying the DC at the FB transistor base, but it's more work to do.
What do you think ? Of course these are no esoteric amps ...
Thanks in advance !
Pick any opamp, wire it as an second order integrator and put it’s output to the diff-stage.
We are talking about a vintage stereo amplifier, and adding IC's and providing power supply to them is not feasible
Output offset is affected by the current balance of the input LTP. Even if there are no emitter degeneration resistors for the LTP transistors, there is the option of avoiding any circuit changes and just fitting different examples of the individual transistors used here. Different batches or samples of the same transistor will have a fair spread of Hfe and Vbe so with such cheap sources today, you can afford to mix and match suitable transistors to find a lower offset or perhaps reverse it. Test the transistors before fitting (use a cheap parts test kit) looking for clear differences in Hfe and Vbe in any pair, before fitting. This avoids duplication and wasting time or burning PCB tracks with too much soldering etc. Once you have arrived at < + or - 30 mV offset, there is no point in trying to further gild the lily - all you will satisfy is a numerical ideal.
I recently purchased 100ea x 2SC945/2SA733, 2SC1845/2SA1015 and other similar type low noise transistors which are sold as loose packed, generic KEC and Unisonic products - all for less than $2/100. These types will likely be suitable in the amplifier and there is plenty enough choice and quantity there for this type of experiment. Whilst there are fakes and poor copy transistors of all types available from the major internet sellers, I find that there are also some good, honest and economical deals too.
I recently purchased 100ea x 2SC945/2SA733, 2SC1845/2SA1015 and other similar type low noise transistors which are sold as loose packed, generic KEC and Unisonic products - all for less than $2/100. These types will likely be suitable in the amplifier and there is plenty enough choice and quantity there for this type of experiment. Whilst there are fakes and poor copy transistors of all types available from the major internet sellers, I find that there are also some good, honest and economical deals too.
Well, 140 mV is surely no harm for the woofers, but what concerns me about offset is imbalance in idle current: on a 4 Ohm load, for example, 35 mA go thru the woofer coil. If, without load, the idle current is set to 25 mA , when connecting a load, the idle currents are totally upset, and cause for distortion at low volume.
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