With respect to mastodon, I think the op-amp circuit is intended to supply a balanced output rather than as a tuner amp. there are two separate questions in the post. 
But I agree on both counts. Tapping off for the tuner from the input is fine, but you'd (OP) be better off with a higher input impedance so that your bass is driving a more "comfortable" load. If the load impedance is much lower than the output impedance of your bass, you'll end up with a thin sound.
Likewise with the balanced output. Go for a higher input impedance at the op amp. 470K upwards would be my choice, but you can always tweak the values to suit.
But I agree on both counts. Tapping off for the tuner from the input is fine, but you'd (OP) be better off with a higher input impedance so that your bass is driving a more "comfortable" load. If the load impedance is much lower than the output impedance of your bass, you'll end up with a thin sound.
Likewise with the balanced output. Go for a higher input impedance at the op amp. 470K upwards would be my choice, but you can always tweak the values to suit.
Here is what I have so far. Any problems you think?
The tuner output was tapped off the node between the 68k and the 1M resistor.
An externally hosted image should be here but it was not working when we last tested it.
The tuner output was tapped off the node between the 68k and the 1M resistor.
Last edited:
With your unbalanced to balanced section... Why not just split the input so it feeds both op amps, rather than feeding the output of the non-inverting stage to the input of the inverting stage? Something like this:
An externally hosted image should be here but it was not working when we last tested it.
Should all work. For the input stage, yes you've got an active bass, but wouldn't it be nice if you could plug just any old bass guitar into it? After all, accidents do happen. In which case your 330k input impedance (1meg in parallel with 470kΩ) is a bit low. For a nice, clean snap on basses I prefer at least a meg, preferably 4M7. Then the 68k in series with the input. It's adding noise when driven from the lo z active instrument (yeah, who's going to hear it with an instrument input? Straight perfectionism) and unless you're struck by lightning, I can't see it being much needed as protection.
I'd be interested in seeing what you intend as an EQ; that's a very high impedance point you've put it at. If you're intending to plug an external equaliser, buffer it. Cathode follower or, since you're going through op amps anyway for the balanced send, an op amp buffer.
The picture of your output jack is wrong. I've no doubt you'd get the wiring right, but rather than a three pole RTS jack, wired balanced (consider using an XLR so you can wire in a ground lift switch) you've put the negative output onto the switch contact of a normalling two pole jack.
470k resistors round the inverting OP amp give more noise than necessary, and while I prefer Dragondreams' balanced driver (if you get a short on the positive output – not impossible with jacks – you still have some signal coming out of the other polarity, and it goes on working, after a fashion) running the 1 meg pot (far too high for my taste, but I assume you had them in stock?) into 33k makes for a very strange law on your volume, and reduced maximum gain. I'd have used 220k or 100k pots everywhere.
You've marked 6.3 volts for the heaters, with one end grounded, probably AC. Have you considered running them in series (the 12AX7 is already wired series centre tap) off the IC DC supply, keeping hum away from the sensitive bits of the circuit?
Have you ever measured the level you get out of your instrument? With unamplified basses I've had peaks of over twelve volts direct off the pickups (very dependent on playing style, of course). One of the reasons why tube preamps are so good for them. It just strikes me that your output ICs might be a limiting factor; have you considered a direct tube output, or a transformer?
I'd be interested in seeing what you intend as an EQ; that's a very high impedance point you've put it at. If you're intending to plug an external equaliser, buffer it. Cathode follower or, since you're going through op amps anyway for the balanced send, an op amp buffer.
The picture of your output jack is wrong. I've no doubt you'd get the wiring right, but rather than a three pole RTS jack, wired balanced (consider using an XLR so you can wire in a ground lift switch) you've put the negative output onto the switch contact of a normalling two pole jack.
470k resistors round the inverting OP amp give more noise than necessary, and while I prefer Dragondreams' balanced driver (if you get a short on the positive output – not impossible with jacks – you still have some signal coming out of the other polarity, and it goes on working, after a fashion) running the 1 meg pot (far too high for my taste, but I assume you had them in stock?) into 33k makes for a very strange law on your volume, and reduced maximum gain. I'd have used 220k or 100k pots everywhere.
You've marked 6.3 volts for the heaters, with one end grounded, probably AC. Have you considered running them in series (the 12AX7 is already wired series centre tap) off the IC DC supply, keeping hum away from the sensitive bits of the circuit?
Have you ever measured the level you get out of your instrument? With unamplified basses I've had peaks of over twelve volts direct off the pickups (very dependent on playing style, of course). One of the reasons why tube preamps are so good for them. It just strikes me that your output ICs might be a limiting factor; have you considered a direct tube output, or a transformer?
Chris; I have a fully regulated 6.3VDC supply for the heaters as well as a +/- 15VDC regulated opamp supply.
I have no knowledge of how to use a direct tube output or how to size up a transformer. I would enjoy the best output I can get since I would like to use this for recording and with my bass rig.
The opamp output was simply the best thing I could come up with considering my knowledge is very limited on audio.
My bass information:
C 130.81 Hz
G 98.00 Hz
D 73.42 Hz
A 55.00 Hz
E 41.20 Hz
B 30.87 Hz *Lowest fundamental frequency.
C: 523.25 Hz *Highest fundamental frequency. Highest fret on highest string.
C 1.26mVpp
G 800mVpp
D 760mVpp
A 840mVpp
E 920mVpp
B 1.52Vpp *Hard fingerpick.
Cable resistance: 300mOhms sleeve, 555mOhms tip (including meter leads).
Output impedance of bass: about 2KOhms (I used a voltage divider and adjusted it until the voltage was halved. That's the best way I could figure it out.)
I couldn't figure out how to make a pad that would allow different instruments, but I DO want to be able to use this with other instruments as well.
Thanks!
I have no knowledge of how to use a direct tube output or how to size up a transformer. I would enjoy the best output I can get since I would like to use this for recording and with my bass rig.
The opamp output was simply the best thing I could come up with considering my knowledge is very limited on audio.
My bass information:
C 130.81 Hz
G 98.00 Hz
D 73.42 Hz
A 55.00 Hz
E 41.20 Hz
B 30.87 Hz *Lowest fundamental frequency.
C: 523.25 Hz *Highest fundamental frequency. Highest fret on highest string.
C 1.26mVpp
G 800mVpp
D 760mVpp
A 840mVpp
E 920mVpp
B 1.52Vpp *Hard fingerpick.
Cable resistance: 300mOhms sleeve, 555mOhms tip (including meter leads).
Output impedance of bass: about 2KOhms (I used a voltage divider and adjusted it until the voltage was halved. That's the best way I could figure it out.)
I couldn't figure out how to make a pad that would allow different instruments, but I DO want to be able to use this with other instruments as well.
Thanks!
Full circuit including preamp, balancing circuit, and EQ.
An externally hosted image should be here but it was not working when we last tested it.
I wouldn't worry about trying to match the impedance of your bass to the input impedance of the amp. Just give the amp a fairly high input impedance and your bass and other instruments will drive it.
I record with my own guitar amps, but I don't use a balanced output. Most mixers have a line-level input (the Behringer MX2004A I have here certainly does) so I just run the amp into that at line-level.
Even with the amps that have a power stage (EL84 and 6N1P), I've simply added another socket and fed that from the speaker output through a 150K resistor. The results are noise-free, even when I've got the gain cranked on the pre-amp stages. The speaker output socket also has a switched "dummy load" (a 25 watt 7ohm resistor) so that I can disconnect the speaker if I want to record without annoying the neighbours.
Example recorded with one of the amps.
I think what I'm trying to say is that you could simply plug the line-level output of your pre-amp direct into a mixer for recording, or into your power-amp for performing. Or use a DI box between the pre-amp and the power amp with a feed out to the mixer. It would make your pre-amp simpler...
Last edited:
I'm just a studio sound engineer with occasional outside PA/live recording gigs; I haven't built preamps since the early seventies. So I'm more into practical details, like grounding and connectivity. All right, I hate (as a purist, not for practical reasons) the signal running through all those integrated circuit sections for the equaliser, when you've started it off tube, but agree that a flexible tube equaliser would be ridiculous both financially and in terms of power consumption. (and I find the equaliser a bit odd, but that's normal. instrument equalisers are always a bit strange to someone who's used to symmetrical console designs.)
Don't forget output impedance is not the same thing as current drive, but you can always drive a lower impedance into a higher one (well, unless you're many kilometres from your amp, in which case we switch to telephone technology, and matched source/cable/input. But that's seriously improbable.) So the noise from the 4M7 input I like using on bass is shorted out by the lower impedance of the instrument.
I'd buffer the tuner output off the anode of the first tube; a touch of harmonic distortion can't worry a tuner much (and you're always checking single notes, so intermod is irrelevant). I like the balanced out, by the way, but it needs a ground lift. As Dragondreams observes, you could put a DI box straight off the output, and get all your matching and isolation switches in the bargain, but If you're going for the all in one solution you've planned, put protection capacitors in series with the outputs, so you can plug into consoles with 48 volt phantom without worrying if the FOH engineer's remembered to cut the power… A direct, unbalanced tube output is dead easy, you just buffer off your pot wiper with a cathode follower, to bring the impedance down.
As regards padding it down, perhaps disconnect the cathode bypass electrolytic on the first section? Nice low impedance point to put a switch. But a tube will give enormous voltages without clipping, so it's probably enough to just turn down your gain pot.
Don't forget output impedance is not the same thing as current drive, but you can always drive a lower impedance into a higher one (well, unless you're many kilometres from your amp, in which case we switch to telephone technology, and matched source/cable/input. But that's seriously improbable.) So the noise from the 4M7 input I like using on bass is shorted out by the lower impedance of the instrument.
I'd buffer the tuner output off the anode of the first tube; a touch of harmonic distortion can't worry a tuner much (and you're always checking single notes, so intermod is irrelevant). I like the balanced out, by the way, but it needs a ground lift. As Dragondreams observes, you could put a DI box straight off the output, and get all your matching and isolation switches in the bargain, but If you're going for the all in one solution you've planned, put protection capacitors in series with the outputs, so you can plug into consoles with 48 volt phantom without worrying if the FOH engineer's remembered to cut the power… A direct, unbalanced tube output is dead easy, you just buffer off your pot wiper with a cathode follower, to bring the impedance down.
As regards padding it down, perhaps disconnect the cathode bypass electrolytic on the first section? Nice low impedance point to put a switch. But a tube will give enormous voltages without clipping, so it's probably enough to just turn down your gain pot.
Here we go!
How do things look? Thanks guys. I'm not short on parts. My lab has a ton of components that I can use.
Are my values looking alright for my input section?
An externally hosted image should be here but it was not working when we last tested it.
How do things look? Thanks guys. I'm not short on parts. My lab has a ton of components that I can use.
Are my values looking alright for my input section?
The first resistor in series with the signal is added to attenuated high frequencies such as radio stations. It can be changed to suit the needs of the user in terms of noise. Lower amounts could increase the radio signal noise (it's not unheard of to have the local station playing through the amp if its omitted) but higher values can induce power supply noise.
68k is a vintage value used on a number of fender and marshalls. Values between 10k and 68k are typical on the vast majority of amps today.
Additionally, I'd say it looks good!
All right, noise from resistors is hardly ever a problem with instruments; there are nearly always other problems far more significant. And if it were a conventional, unamplified bass, it wouldn't make the slightest difference. Mine's got a 6k8, and protection diodes to the rails, but that's FET; tubes are much tougher.
Still, I don't put vintage Marshall amps as a reference for noise performance. Or any amps from that period, really; possibly the Wallace. It's only because of digital recording that we're starting to go hysterical over the background hiss when you solo an instrument, anyway; before the junk from the cymbal mics would cover it up nicely.
Yeah, I do remember taxi reception on a variety if instrument amplification (bacofoiling a Mellotron in New York springs to mind). Perhaps change the 470k first grid resistor for a 2M2? It won't change anything for your active bass, but might give some extra clarity on a Fender. Still, that'd make the pad a changeover switch, rather than a straight single make contact; Hmm, perhaps I can manage a diagram…
No, not very well I can't .
Still, I don't put vintage Marshall amps as a reference for noise performance. Or any amps from that period, really; possibly the Wallace. It's only because of digital recording that we're starting to go hysterical over the background hiss when you solo an instrument, anyway; before the junk from the cymbal mics would cover it up nicely.
Yeah, I do remember taxi reception on a variety if instrument amplification (bacofoiling a Mellotron in New York springs to mind). Perhaps change the 470k first grid resistor for a 2M2? It won't change anything for your active bass, but might give some extra clarity on a Fender. Still, that'd make the pad a changeover switch, rather than a straight single make contact; Hmm, perhaps I can manage a diagram…
No, not very well I can't .
Like this?
An externally hosted image should be here but it was not working when we last tested it.
No; you haven't built, tested and tweaked it yet
+1 to that!
It's looking good though. But one thing I've learned is that looking good on paper and working well in the flesh can be a bit of a dark art with valves...
Be prepared for a few minor frustrations such as buzzing and squealing that can have you wondering why you ever started the project.
Iron the wrinkles out before committing to PCB.
I don't think I ever laid out a PC board for a valve circuit; I moved from using the valve bases themselves (I know, ugh; but everyone was doing it) to tag boards, and only got to sticking black tape onto translucent paper when transistors took over at low levels. Bit difficult here for the op amps.
But I've repaired enough PC valve gear with burnt out carbonised boards to know that 300 volts and close tracks make uneasy neighbours (all right, the problem was usually beer spilt into an amp during a gig, but it didn't make it as easy as a tagboarded version to repair).
But I've repaired enough PC valve gear with burnt out carbonised boards to know that 300 volts and close tracks make uneasy neighbours (all right, the problem was usually beer spilt into an amp during a gig, but it didn't make it as easy as a tagboarded version to repair).
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.