Hello!
I want to show you my latest project.
This time it is not a tube amp.
This is the first time I worked with transistors. In this case, the amp uses a silicon type in the pre- and the driver-stage (BC170A and BC328-25). The power-stage uses germanium type (AD161/162).
Where the idea come from?
I am collecting old tube radios and old transistor Radios. One day I bought a radio at the flea market. At home I tried it, the IF-stage did not work. There was a damage at a special kind of coil, so it was not possible to repair it.
Somehow I got the idea to convert the AF-stage into an amplifier.
So I cut away the AF-Stage from the rest of the circuit board and began to work. I wanted to get a complete linear amp, so I removed all the parts of the sound control. As you can see, the amp has now three stages. In the original it has four stages. With four stages the amp was to sensitive, so I removed the fourth stage. Then I changed some of the capacitors to get a lower and higher frequency range and a linear frequency response. I added three potentiometers to adjust the middle-voltage, the bias of the power-stage and the feedback.
The tests with the oscilloscope and the sine-generator were so positive, that I checked the amp with my speakers. I was very surprised about the sound, so I mounted the circuit board on my “amp-testing-board”. I built a power-supply. The only negative was the “plop” when switching it on. One of the reasons for that was the simple couple capacitor / boostrap circuit. As you can see in the first circuit, the collector current of the driver transistor goes to earth using the loudspeaker.
I do not want any DC-component to my speakers. So I re-developed the boostrap-circuit to a DC-free variant, as you can see in the second circuit.
The final test were so positive that I decided to mount the parts into a housing. I also added a 5 sec. delay for the speakers using a LM741 and a relay.
The output power, measured at 1 kHz, is 6 Watt sine at 8 Ohm an 7,5 Watt sine at 4 Ohm load. More than enough for my 92 dB speakers.
How is the sound? Very fine and smooth, wide and deep much more like a tube-amp than a transistor.
Kind regards, Frank
I want to show you my latest project.
This time it is not a tube amp.
This is the first time I worked with transistors. In this case, the amp uses a silicon type in the pre- and the driver-stage (BC170A and BC328-25). The power-stage uses germanium type (AD161/162).
Where the idea come from?
I am collecting old tube radios and old transistor Radios. One day I bought a radio at the flea market. At home I tried it, the IF-stage did not work. There was a damage at a special kind of coil, so it was not possible to repair it.
Somehow I got the idea to convert the AF-stage into an amplifier.
So I cut away the AF-Stage from the rest of the circuit board and began to work. I wanted to get a complete linear amp, so I removed all the parts of the sound control. As you can see, the amp has now three stages. In the original it has four stages. With four stages the amp was to sensitive, so I removed the fourth stage. Then I changed some of the capacitors to get a lower and higher frequency range and a linear frequency response. I added three potentiometers to adjust the middle-voltage, the bias of the power-stage and the feedback.
The tests with the oscilloscope and the sine-generator were so positive, that I checked the amp with my speakers. I was very surprised about the sound, so I mounted the circuit board on my “amp-testing-board”. I built a power-supply. The only negative was the “plop” when switching it on. One of the reasons for that was the simple couple capacitor / boostrap circuit. As you can see in the first circuit, the collector current of the driver transistor goes to earth using the loudspeaker.
I do not want any DC-component to my speakers. So I re-developed the boostrap-circuit to a DC-free variant, as you can see in the second circuit.
The final test were so positive that I decided to mount the parts into a housing. I also added a 5 sec. delay for the speakers using a LM741 and a relay.
The output power, measured at 1 kHz, is 6 Watt sine at 8 Ohm an 7,5 Watt sine at 4 Ohm load. More than enough for my 92 dB speakers.
How is the sound? Very fine and smooth, wide and deep much more like a tube-amp than a transistor.
Kind regards, Frank
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Please have a look at R174. This is a thermistor.
I think it is not necessary to give each heatsink of a channel of the amp an own thermistor.
If the middle voltage is adjusted correct both transistors have equal voltage and current so they get equal hot.
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