Acoustic guitar amplifiers

[G._Hoffman]

I've been thinking, for the last few days, about possibly making an acoustic guitar amplifier.  What I've sort of got in mind is something based on an Ampeg B-15N (simple preamp, Braxandall tone controls, quite clean overall), with maybe some tweaks (shifting the eq section to a more usable range for guitar, something to tame quacky piezo pickups, etc.).
I am a bit concerned about the input impedance, as passive piezo pickups provide a very weak signal, and I'd like a preamp to be optional.

But the thing I really want to figure out is possibly bi-amping the power amp, and having a separate woofer/tweater.  What are some examples of good bi-amped tube power amps which might be a decent starting point.

[passaloutre]

Any of the Fender AB763 amps with reverb could be considered bi-amps, as the reverb driver constitutes a small single ended power amp, even with substantial high-pass filtering in this case.

I don’t know what kind of power is required to drive your tweeters, but that might be a place to start.

[jjasilli]

Do you literally mean bi-amp, or just 1 amp into a 2-way speaker cab with passive crossover?

[G._Hoffman]

Do you literally mean bi-amp, or just 1 amp into a 2-way speaker cab with passive crossover?

Biamp.  I'm kind of thinking of going with a 6v6 single ended power amp for the upper mids and highs (which take less power to drive, and which could benefit from the way single ended amps compress), and a push-pull 6L6 power amp for the lows and low mids.  It's those really harsh upper harmonics that really kill the sound of piezo pickups.  Having the opportunity to tame them down seems very useful.

[jjasilli]

Checkout the Bogen CH A20.  It may be pretty close to what you want for the main amp.


Meanwhile I suggest you try something like a champ or similar hi fi amp into a tweeter along with a regular amp into a guitar speaker.  Maybe use an active crossover to feed the 2 amps.  This may ne a good feasibility test.

[G._Hoffman]

Checkout the Bogen CH A20.  It may be pretty close to what you want for the main amp.

Thanks!

Meanwhile I suggest you try something like a champ or similar hi fi amp into a tweeter along with a regular amp into a guitar speaker.  Maybe use an active crossover to feed the 2 amps.  This may ne a good feasibility test.

Yup, that's exactly the idea.  I just am not familiar enough with crossovers yet to really know what that part will look like.

[jjasilli]

This has been making my head explode.  I think the technology is closer to the hi-fi world than guitar amps (except for the input impedance).  So I think you have to work backwards, starting with speaker (driver) selection.  I.e., I think speaker selection is critical.


For the tweeter:  Your upper frequency is, say, 10,000Hz to cover extended guitar string harmonics.  To avoid distortion, you need a tweeter that can cover up to 12,000 -15,000Hz, to avoid maxing it out. 


Then, how low can the tweeter comfortably go?  That's your crossover point with the main speaker.  The main speaker needs to comfortably cover from say 80Hz up to the crossover point. 


Now , back to the input.  You need a hi-impedance input as discussed in this or your companion thread. Re the acoustic guitar PU: the input gain stage will act as a buffer / voltage amplifier.  Now you have choices:


1. Use an effects loop out or line out from the main amp > outboard active crossover such as a Behringer rack unit which will split the signal for the tweeter, and for the main speaker. 
          Lo Freq feed from active crossover unit > Effects Loop/Line In on main amp > main speaker
          Hi Freq feed > SE amp > tweeter


2.  Split the signal internally in the main amp after the input stage:
           One half goes downstream in the main amp > internal lo-pass filter > power amp section > main speaker
           Other half to Line Out > SE amp with internal hi-pass filter > power amp section > tweeter

[G._Hoffman]

Oh, definitely #2.  Not sure if I need the filters to be actively isolated (two buffer stages in parallel going to the crossover filters), or if I could just run them both directly off of a single cathode follower.
Like so....

[j-b-c]

A magnetic pickup is an inductive load, while a piezoelectric pickup presents a capacitive load. In a perfect world you would match a specific pre-amplifier to each.

It is a lucky coincidence that magnetic pickups overcome the capacitance of the guitar cable.

[jjasilli]

@ G._H:  You made my headache return.    Your block diagram fails to account for the very hi-Z input that a piezzo PU wants to see.  That's why my concepts have the guitar signal 1st going into the hi-Z input, which is designed into the main amp; then split the signal. 


Yes, the crossover signals need to be isolated.  This is accomplished by my 2 suggestions: either using an outboard active crossover (coming AFTER a hi-Z input); or using internal filters, after splitting the signal, inside each of the two amps.

[G._Hoffman]

A magnetic pickup is an inductive load, while a piezoelectric pickup presents a capacitive load. In a perfect world you would match a specific pre-amplifier to each.

It is a lucky coincidence that magnetic pickups overcome the capacitance of the guitar cable.

You're thinking of passive piezo pickups, which would be intended for the top preamp.

The second input would be INTENDED (users will be users) for buffered (active) pickups or magnetic pickups, which honestly is the majority of acoustic pickups on the market, and both of which work fine with a 500k-1M impedance.  And of course, a lot of magnetic pickups on the market are also active buffered pickups.  The two pickup preamps will have different input stages, reflecting the intended source. 

[G._Hoffman]

@ G._H:  You made my headache return.    Your block diagram fails to account for the very hi-Z input that a piezzo PU wants to see.  That's why my concepts have the guitar signal 1st going into the hi-Z input, which is designed into the main amp; then split the signal. 


Yes, the crossover signals need to be isolated.  This is accomplished by my 2 suggestions: either using an outboard active crossover (coming AFTER a hi-Z input); or using internal filters, after splitting the signal, inside each of the two amps.

We're talking about three separate inputs here - the top one for passive piezo pickups, the middle for active (either type) or magnetic pickups, and a third for lo-z mic inputs (less common, but they are out there).

[DummyLoad]

Probably want to add a tunable notch filter that centers around 100Hz to minimize effect of acoustic feedback due to the high resonance of the guitars body.


--Pete

[jjasilli]

I think you can put all types of acoustic PU's into one very hi-Z input.

[G._Hoffman]

I think you can put all types of acoustic PU's into one very hi-Z input.

Well, the K&K Pure Mini is, according to the manufacturer, not well suited to a very high-z input.  I haven't tried it, though, so not really sure. 
The real trick will be to try them out when I get around to building it.  I kinda go simple/complicated in what I build.  One simplish build, then one complicated.  This would certainly be the most complicated so far, but it is appealing to me in some way.

[j-b-c]

I think you can put all types of acoustic PU's into one very hi-Z input.

No, you can't.  Impedance is a two dimensional vector, with real and imaginary terms.

Z = R + jX

There are two types of the imaginary term X.  Xl and Xc.  Inductive and capacitive reactance.

The X part of Z couples into the miller capacitance that occurs wherever voltage amplification is present.

[jjasilli]

I think you can put all types of acoustic PU's into one very hi-Z input.

No, you can't.  Impedance is a two dimensional vector, with real and imaginary terms.

Not following that. Please explain.


BTW:  My Mesa Boogie V1 Bottle Rocket has a 10M input (grid leak) R, and sounds fine with el guitar.

[j-b-c]

Not following that. Please explain.

There are many impedance tutorials on YouTube.

Search for "impedance imaginary part"

[Merlin]

Not following that. Please explain.

There are many impedance tutorials on YouTube.
Search for "impedance imaginary part"

[j-b-c]

Dear Merlin (and Shooter).  Resistance is a scalar, and Impedance is a vector.  I'm sorry that you find that hard to understand.

[Merlin]

Dear Merlin (and Shooter).  Resistance is a scalar, and Impedance is a vector.  I'm sorry that you find that hard to understand.

That doesn't answer the question. If a hi-Z input is made good enough for piezo pickups, why can't you put all other types of acoustic PU's into it?

[shooter]

 
I had to sit through many many thousands hours of classroom, math'n out electronics only to be released into the wild as a Field Engineer on complex electronics systems, where math was a relegated to obscurity
but I still get my ice from Eli   

[sluckey]

Dear Merlin (and Shooter).  Resistance is a scalar, and Impedance is a vector.  I'm sorry that you find that hard to understand.

Sigh! Still struggling to be helpful or likable. I fear you may never quite fit in.

[jjasilli]

Based on prior postings by G Hoffman, I went to the K&K PU website and related sites.  What I gather is that if a device's input impedance is too high, it it may fail to roll-off enough bass in the guitar signal.