Hello all,
I have bought an old subwoofer (JBL 125) that needed a new plate-amp... as I post the schematics, I prefer not to mention its brand and model to avoid waking up sleeping lawyers 🙂
I listen to music at my desktop, with Magnat Quantum Edelstein, and wanted to add this sub to get some more bass, a fuller sound, etc. I connected the sub using the high-level inputs. Because the 2 speakers are really close to me (60cm), I listen to them at low-level when working (I have measured signals on speaker bindings at about 80-120 mV peak-to-peak). My amp is an Elekit tube amp but that doesn't matter I think.
Connected to my sub plate-amp high-level inputs, this signal level never triggers the auto-power on. With my signal generator, I have measured the sub needs about 450mV peak-to-peak to "wake up". That's probably ok for home-cinema, but I'm into music, not bangs and booms.
I have studied the circuit and have achieved to modify it to get triggering at 150 mV, but it's not where I want yet. What I have done is modify R38 value (with a parallel R until I'm happy). This way I rise the U3 amplification factor from 100 to about 280. R28 is now about 9'600 ohms.
I would like to get its sensitivity higher, so the triggering happens at about 80-100 mV. It would need a R28 around 5K or so. What I'm not sure about :
Charles
I have bought an old subwoofer (JBL 125) that needed a new plate-amp... as I post the schematics, I prefer not to mention its brand and model to avoid waking up sleeping lawyers 🙂
I listen to music at my desktop, with Magnat Quantum Edelstein, and wanted to add this sub to get some more bass, a fuller sound, etc. I connected the sub using the high-level inputs. Because the 2 speakers are really close to me (60cm), I listen to them at low-level when working (I have measured signals on speaker bindings at about 80-120 mV peak-to-peak). My amp is an Elekit tube amp but that doesn't matter I think.
Connected to my sub plate-amp high-level inputs, this signal level never triggers the auto-power on. With my signal generator, I have measured the sub needs about 450mV peak-to-peak to "wake up". That's probably ok for home-cinema, but I'm into music, not bangs and booms.
I have studied the circuit and have achieved to modify it to get triggering at 150 mV, but it's not where I want yet. What I have done is modify R38 value (with a parallel R until I'm happy). This way I rise the U3 amplification factor from 100 to about 280. R28 is now about 9'600 ohms.
I would like to get its sensitivity higher, so the triggering happens at about 80-100 mV. It would need a R28 around 5K or so. What I'm not sure about :
- is this the correct approach ?
- do I charge the output of U1 and U1' too much by reducing R38 ? Not sure how to evaluate how much the U3 amp circuit loads its input...
- or is there another approach I have overseen around D3, Q1, etc ?
Charles
Attachments
They used just-enough parts for what they thought was a reasonable trip level. Very close at the desk, your electric level may not be what they expected.
Yes, the no added parts path is smaller R38. Since the '4558 can drive 2K easily, and there is 1k already, R38 "could" be 1k. That makes U3 gain about 2,700. Sensitivity about 0.2mV. '4558 is a 1MHz part so the bandwidth is 0Hz-370Hz.
Some problems:
With phono source, subsonics could keep the amp ON, even if the tonarm lifts (some phono preamps have large subsonic "hiss"). I'd want some bass cut.
A flute solo intro may not put enough signal into the lowpass to trigger the woofer. OK, you don't need it; then the band comes in and the subwoofer thumps into life.
Taken too far, system noise will keep the woofer ON when music is "silent".
But your path may work OK. Make R38 a 10k pot plus maybe a 1uFd cap. Keep turning until it works as desired, or doesn't.
Ah... if R38 is very low it shorts-out the R19 R20 mixer. You need a second mixer. Couple of 1k resistors, jumper, then on to a cap and a 10k pot.
Yes, the no added parts path is smaller R38. Since the '4558 can drive 2K easily, and there is 1k already, R38 "could" be 1k. That makes U3 gain about 2,700. Sensitivity about 0.2mV. '4558 is a 1MHz part so the bandwidth is 0Hz-370Hz.
Some problems:
With phono source, subsonics could keep the amp ON, even if the tonarm lifts (some phono preamps have large subsonic "hiss"). I'd want some bass cut.
A flute solo intro may not put enough signal into the lowpass to trigger the woofer. OK, you don't need it; then the band comes in and the subwoofer thumps into life.
Taken too far, system noise will keep the woofer ON when music is "silent".
But your path may work OK. Make R38 a 10k pot plus maybe a 1uFd cap. Keep turning until it works as desired, or doesn't.
Ah... if R38 is very low it shorts-out the R19 R20 mixer. You need a second mixer. Couple of 1k resistors, jumper, then on to a cap and a 10k pot.
Attachments
I ended up with replacing R38 with a value of 3,3KΩ, without any other change. Never turns it on when I don't want to, but does when playing music at low levels.
There used to be a current spec on the '4558 sheet, but I don't see it on the 2014 re-print?
The "minimum load" is ambiguous. For "perfect" response? For "acceptable" response? Transistor opamps were traditionally rated for 10k load (opposed to 100k for tube opamps) but most will go lower. I have seen chip opamps driving under 100 Ohms, but at low levels.
https://www.ti.com/lit/ds/symlink/rc4558.pdf
Page 7 Fig 7.
It does OK for 2k and up, is falling off fast below 1k. This may not be the THD curve; but it may be close, anyway you don't care here.
Thanks for the info, yes then my op-amp load is no problem. I'm re-learning every day... I was an electronic engineer at the end of my BsC in 1988 but I immediately turned to IT... back to electronics after 32 years, the basics are there but such a lack of experience, it is sometimes painful !
Anyway I learned as well about phono subsonics, I wasn't aware of them. In my config no phono since a very long time, in fact my chain in office is Logitech Squeezebox Touch to Teac UD-503 DAC (toslink) and then an Elekit TU-8200. Subsonics shouldn't be present at all. Since today morning I work with this 2+1 system and the sound is elegant, deep and nice, the way I want for office work.
Anyway I learned as well about phono subsonics, I wasn't aware of them. In my config no phono since a very long time, in fact my chain in office is Logitech Squeezebox Touch to Teac UD-503 DAC (toslink) and then an Elekit TU-8200. Subsonics shouldn't be present at all. Since today morning I work with this 2+1 system and the sound is elegant, deep and nice, the way I want for office work.