I like this sollution, best of both worlds.
What kind of pickup does the Martin have?
Not all acoustic pickups like an input cap... Combined with guitar cable and input cap you can get a resonant circuit. You get the typical high whistle like
feedback on acoustic guitars.
Use the coupling cap between gain stage and follower as hpf.
Depending on your guitar/pickup 80-120Hz should be fine.
As for bias, stay close to centre biassing, just a little to the hot side, if you go too hot, you get "electric sound", if you go too cold you favour odd order distortion and you don't want too much of that.
What kind of amp are you driving? Is 18-20 dB really too much?
Also, a little suggestion for the power supply of the circuit.
The cathode follower needs its power to be as clean and stable as possible.
But the gain stage for a guitar amp (no difference between acoustic or electric here)
likes it's voltage a bit "dancing" on the dynamic content you provide while playing.
An RC circuit with a time constant of 100 to 300 ms can do that.
So for the gain stage: from PS to 4k7 to 10µF (to ground) to anode resistor of the gain stage.
This will keep your guitar "alive"...
--> voltage will drop, so you have to re-calculate bias.
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Thanks OneEyed! OK I will drop the bias resistor back to 1.5K and behave myself! I will give Merlinb's solution a trip around LTSpice as soon as I get the time and report back.
The pickup is a K&K Western Mini (a very, very good pickup system by the way, very natural sounding!) Here is the link...
Pure Western - K&K Sound
I have this system installed in a Martin J-41 and I run this via the TL071 preamp, into a little Tapco mixer's Line In socket and on to the pA. The Line In has the following spec
Maximum Input Levels:
Line Input: +30 dBu,
Gain @ +10 dB
Input Impedance:
Line Input: 20 kΩ, balanced
This gives a very good sound. I reckon the line in can take the gain but I try to avoid running a "Over Hot" control on the Line Input for gigs, as my experience is someone, at some time, will try to turn it up with a heavy hand and take the speakers out with the resulting guitar feedback! I like to run the gain at just past 12 oclock position if at all possible.
Very interesting on the Power Supply suggestion. I will keep this in mind for later (and no doubt perster you with questions!)
Cheers
Ray
The pickup is a K&K Western Mini (a very, very good pickup system by the way, very natural sounding!) Here is the link...
Pure Western - K&K Sound
I have this system installed in a Martin J-41 and I run this via the TL071 preamp, into a little Tapco mixer's Line In socket and on to the pA. The Line In has the following spec
Maximum Input Levels:
Line Input: +30 dBu,
Gain @ +10 dB
Input Impedance:
Line Input: 20 kΩ, balanced
This gives a very good sound. I reckon the line in can take the gain but I try to avoid running a "Over Hot" control on the Line Input for gigs, as my experience is someone, at some time, will try to turn it up with a heavy hand and take the speakers out with the resulting guitar feedback! I like to run the gain at just past 12 oclock position if at all possible.
Very interesting on the Power Supply suggestion. I will keep this in mind for later (and no doubt perster you with questions!)
Cheers
Ray
sounds like this in an old Seagull: http://users.telenet.be/kastaar/Shredlov/Sredlov - Here comes the rain again.mp3Nevermind the sloppy recording/mixing (too much monitoring
)
Not sure if it was answered but a "tone stack" is added components for bass, treble and perhaps mid controls. This would reduce overall gain under most settings and provided EQ, assuming you don't have that elsewhere in the system. It would not change the input nor output impedance since it's placed between the 2 stages. And a "rumble" stopper/switch (HPF) is a good idea for acoustic setups as mentioned. Stage monitors can often cause rumble (low frequency) feedback thru an acoustic.
Hi Wavebourn
Yes indeed, for recording I always use microphones and never use a pickup of any kind. However if you have ever tried using condendser mics at gigs then you know they can be a nightmare and tend to pickup everything else really well except the guitar and at best you have to keep pretty still! The best of both worlds is a pickup taking care of the bulk of amplification and then blend in a little microphone to fill it out and take the edge of the pick-up. This works well as any mics are then only operating at low gain.
This is why I need to use a preamp to work the Line In on my mini mixer as I only have two mic channels, one for vocals and the other for mic’ing the guitar with the pickup.
Cheers
Ray
Yes indeed, for recording I always use microphones and never use a pickup of any kind. However if you have ever tried using condendser mics at gigs then you know they can be a nightmare and tend to pickup everything else really well except the guitar and at best you have to keep pretty still! The best of both worlds is a pickup taking care of the bulk of amplification and then blend in a little microphone to fill it out and take the edge of the pick-up. This works well as any mics are then only operating at low gain.
This is why I need to use a preamp to work the Line In on my mini mixer as I only have two mic channels, one for vocals and the other for mic’ing the guitar with the pickup.
Cheers
Ray
Thanks JJman!
I have bass, middle and treble EQ on the mixer strip and I generally prefer to adjust EQ at that stage and so you are right in thinking that it would be best to have the impedance set both ends. I will do away with the input cap as OneEyed suggested as the coupling cap will limit the frequency response. As you said it is the low end, almost sub sonic, that can cause problems, particularly with a Martin J size guitar. Maybe a deep bass cut switch is an idea. I reckon I need to try out the preamp in real world gigs before I opt for any further filtering etc. I am going to sit down and play in LTSpice with the split anode resistance to reduce gain and maybe go back to the ECC83 circuit to see what can be done
Cheers
Ray
I have bass, middle and treble EQ on the mixer strip and I generally prefer to adjust EQ at that stage and so you are right in thinking that it would be best to have the impedance set both ends. I will do away with the input cap as OneEyed suggested as the coupling cap will limit the frequency response. As you said it is the low end, almost sub sonic, that can cause problems, particularly with a Martin J size guitar. Maybe a deep bass cut switch is an idea. I reckon I need to try out the preamp in real world gigs before I opt for any further filtering etc. I am going to sit down and play in LTSpice with the split anode resistance to reduce gain and maybe go back to the ECC83 circuit to see what can be done
Cheers
Ray
Good question, this one: DIY 12AX7 Tube preamp
I really want to build this one though: DIY 6922 / E88CC Tube preamp
Neither of them show the volume control, presumably it's after the output and not shown on the schematics.
That websites rather a laugh though - a valve preamp to give low quality, and he uses silver connecting wires
Acoustic or Electric?
Remember, even with an acoustic (ie minimal distortion) preamp the point of using tubes is to ensure sensible behaviour when we clip.
See Hamm, R. O. (1973). Tubes Versus Transistors-is there an Audible Difference. Journal of the audio engineering society, 21(4), 267-273 (also reprinted in Myles S Rose Guitar Amp Blueprinting Part II which can be found on line)
Unfortunately I've not got the URL for the best article I have on the topic of achieving this.
However having a quick look at the following two amps will give you food for thought.
"Real McTube II" which is a complete pre-amp. You chose the bias settings.
Duncan's Blues 112 is a great example of picking bias settings. Have a close look at the circuit and how the bias (and gain) is adjusted in the various 'modes'.
For acoustic use, swapping to a lower mu tube is also an option.
Remember, even with an acoustic (ie minimal distortion) preamp the point of using tubes is to ensure sensible behaviour when we clip.
See Hamm, R. O. (1973). Tubes Versus Transistors-is there an Audible Difference. Journal of the audio engineering society, 21(4), 267-273 (also reprinted in Myles S Rose Guitar Amp Blueprinting Part II which can be found on line)
Unfortunately I've not got the URL for the best article I have on the topic of achieving this.
However having a quick look at the following two amps will give you food for thought.
"Real McTube II" which is a complete pre-amp. You chose the bias settings.
Duncan's Blues 112 is a great example of picking bias settings. Have a close look at the circuit and how the bias (and gain) is adjusted in the various 'modes'.
For acoustic use, swapping to a lower mu tube is also an option.
Hi Guys
For an acoustic input directly, assuming a raw piezo pickup, and using the circuit from post-1, the input grid-leak resistance must be much higher than 1M if you want to have any bass response at all. 10M would be the minimum acceptable value, with 33M preferred.
There should also be grid-stops added to help block RF instrusion and to protect the grid from overly large signals that might cause rectification.
The gain is easily reduced by lowering the plate resistor value from 220k to 100k or less, as TUT shows.
Have fun
For an acoustic input directly, assuming a raw piezo pickup, and using the circuit from post-1, the input grid-leak resistance must be much higher than 1M if you want to have any bass response at all. 10M would be the minimum acceptable value, with 33M preferred.
There should also be grid-stops added to help block RF instrusion and to protect the grid from overly large signals that might cause rectification.
The gain is easily reduced by lowering the plate resistor value from 220k to 100k or less, as TUT shows.
Have fun
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