I got a used Hartke A25 Bass Combo recently and it was in good condition but the pots needed cleaning so i decided to show you what is inside this little combo.
Once i took out the amp i noticed the power stage is actually using discrete transistors which is really really nice. The codes are not so well visible but i think it uses 2sd2058 and 2SB1366.
The opamp used is the JRC 2068 and a JRC 13700 is used for limiter (can be replaced with LM13700)
I also liked that the transformer is well shielded. After I cleaned the potentiometers I tested the unit and sounded great. I took some measurements to see the effect of the equalizer, range and center frequency.
I measured the output of the speaker from 50cm away and the speaker about 50cm from ground.
First i compared CD input with instrument input. Note that CD input is not affected by any of the controls, not even the master volume.
Red trace is the CD input and the other trace is the instrument input
Next i tested the equalizer controls.
The above is the bass control. We can see it is centered around 100Hz and indeed it has 30dB control range. Blue is +15dB setting, Red is -15dB setting.
Midrange seems to be center around 700-800Hz and again the control range is about 30dB. Blue is +15dB setting, Red is -15dB setting.
Treble center frequency looks to be 2kHz. The range is close to 30dB again. Blue is +15dB setting, Red is -15dB setting.
I also measure the direct output of the amp to see the frequency range of the preamp. It is quite linear as you can see below.
I can say it is well built and it has a nice tone. It doesn’t go that low but if placed on the floor it can be quite satisfying. In the future i will look more at the speaker and the enclosure as i think it can be improved a bit.
Thank you for visiting.