I just fixed a bass amp I got cheap in a garage sale. I'm trying to see what tweaks I can do. Can somebody explain what C1 & C2 are doing as the input signal is going to the first opamp stage? The value .01uF looks low for a bass signal. Is this some kind of high pass filter? I looked at other bass amp designs and they have a much larger cap like 2.2uF going to the first opamp stage
Yes, it is indeed a high pass filter, C1 and C2 in series along with R19 give the rolloff frequency of 67 Hz, which isn't that low. The op amp configuration even flattens that a bit. The circuit probably adds the lowest of frequencies later, especially if it has some distortion or clipping afterwards. If you want more bass in the preamp, you can replace C1 and C2 with 22nF caps, which would yield a cutoff frequency of approximately 33Hz.
EDIT: C1 is also a coupling capacitor, it prevents any DC from going through your bass guitar, but since the circuit has dual power supply, it is redundant. So instead of replacing both caps, you might want to try just shorting C1, since it would have the same effect.
Hope his helps
Cheers
Thanks for the info. The rest of the circuit goes to an equalizer with the lowest pot being 63 Hz so thats probably why they made the cutoff at 67 Hz. I replaced C1 & C2 with 22nf and there was indeed more bass coming through the speaker. So you're saying I can replace C1 with a jumper and keep C2 at 22nf? Here is the rest of the circuit before the equalizer
Well, you could, but if you've already replaced the caps, there wouldn't really be any point and/or audible difference as far as bass is concerned.
If you are just looking to do some tweaks, I'd try using some LEDs as clipping diodes at the IC2A output, but maybe put the diodes in series with a capacitor, which would limit the clipping to upper frequencies.
I think I'll leave it as is. don't wanna push my luck. swapping those caps for 22nf really improved the bass
curiosity won't leave me alone so I continued tweaking xP It's a 100% opamp design and the only opamp is the NJM4558. So I replaced all 5 opamps with sockets and popped in NE5532's. Wow the sound is more clearer and the hiss at high volumes went away. For sh*ts and giggles I popped in some LM4562's and it was a noisy mess. more than likely they were oscillating. I was wondering are there more modern chips I can replace the NE5532's with? I was going to try some NJM2068's next but its old like the 4558
made everything OPA2134's and the sound is incredible. Its the sound I've been looking for 8)