Acoustic Guitar Preamp Design

[BTF]

I have been mulling over building a valve acoustic guitar preamp with a circuit layout like the classic 59 Bassman. It must roll off low frequencies from 150 Hz down and yet hopefully fill out the tone of a piezo pickup. Therefore I don't want to thin out the preamp too much. The cathode resistors will be 1.5K, 100K plate resistors and the two primary triodes will be blended into the post-volume triode with a CF tone stack. My actual question:

I can either chose smaller-value cathode bypass caps like a 10uF or 4.7uF (the 4.7's tend to be a bit bright, but I can do) and choose a coupling cap like a .0047 or .01uF or

I can use a larger bypass cap like a 25uF and choose a small-value coupling cap like a .0022uf or .001uF.

This is essentially the same thing many of you face when trying to keep the flabbiness out of your overdrive channels. I was wondering which approach you favor.

I sincerely appreciate your input as I think this will be an interesting exercise.

Regards, BTF.

[Ronsonic]

My inclination is a bit of both. Mostly rolling off with the cathode cap then you can use a smaller coupling cap. Don't like the idea of using the coupler as a tone control. Not that I have a valid theory here, just an aesthetic prejudice against making a tube amplify something that you then reject with the cap. Or something equally vague and technically meaningless.

[BTF]

I understand exactly what you're saying, Ronsonic. It's somewhat like the classic Marshall overdrive preamp where the coupling capacitors are moderately large, but Marshall scaled back the bass response by using the 2.7k/.68uF combination and then following with unbypassed cathodes on the next stages.

Rolling off from 150 Hz takes a bit of the muscle out of the guitar, but I figure that will be taken care of when the whole system is amplified by a PA (I don't play out anymore, this is mostly an exercise).

Thank you for your response! Regards, Bill.

[loudthud]

Remember that a cathode bypass cap creates a shelving network but the coupling cap is a true single pole high pass filter. A shelving network doesn't completely roll off the bass, it just turns it down a few dB.

Another thing to keep in mind; if the first tube has a large cathode bypass, say 25uF, the bass frequencies will generate harmonics that won't be rolled off as much as the bass frequencies that created them in a down stream high pass filter. You may or may not like the sound of this or may want to make it switchable somehow. You won't really know until you actually play through the amp with whatever pickup you have.

The Fender 5F6A style tone stack always sounds a little dark to me without a treble boost somewhere in the signal chain, and it has sort of a black hole effect on the mid range. A James tone network might be a better choice.

[BTF]

Thank you , loudthud. And I agree on your points. I know what you're saying about the 5F6A, and the effect you mentioned is the reason why the tonestack is the only section I'm not changing. I did some experiments with tonestacks and kept coming back to the 5F6. It has a higher mid quality and a warm tone that seems to really help piezo pickups. I have a flat-response Presonus acoustic preamp, but for this project I wanted not just reproduction, but augmentation.

Your point on downstream harmonics is also well-put. I use a very neat rackmount PA processor as a test device, and you can quickly grasp where your trouble spots are. That's how I determined 150 Hz was where I needed the knee. I built a 150 Hz notch filter, but I'm hoping to avoid generating too much in this area and therefore not need the thing.

Thanks again, Bill.

[octal]

Another thing to keep in mind; if the first tube has a large cathode bypass, say 25uF, the bass frequencies will generate harmonics that won't be rolled off as much as the bass frequencies that created them in a down stream high pass filter.

That's a very interesting point. So the tube's distortion products...2*F, 3*F, etc may fall above the pass band of a subsequent coupling cap high pass filter even if the fundamental doesn't, whereas if the bypass cap is smaller, the gain at frequency F (a frequency below the shelving point of the bypass cap) will be lower, the distortion will be less and the gain stage will generate fewer higher harmonic products.

I'm not sure I phrased that coherently, but I totally get what you're saying.

[Chuck H]

WHY is 150Hz bad? This may help others determine if the 6dB rolloff of the cathode bypass cap or the single filter of the coupling cap is more suitable for what you want.

Chuck

[BTF]

Forgive me, CH, for taking so long to reply!

The reason I need to attenuate at that frequency is that the guitar uses soundboard sensors. When amplified to appreciable levels, there is a hot spot sensitivity (the guitar wants to feed back) at 150 HZ. When that frequency is notched out, you can achieve substatially more gain before feedback resumes. Many commercial acoustic preamps have notches in the 100-200Hz range to prevent this as well. I built a 150 HZ Twin-T notch filter which I haven't installed but may if I get the urge. My parts arrived yesterday for the actual tube section of the preamp. I have been doing some calculations, and I believe I can achieve what I need by bypassing the 1.5K cathode resistor with a 1uF cap and using a .01uF coupling cap. That will put a moderately hard knee in the 150Hz area, but it may thin the guitar out too much. I'll need to try it to be sure.

One reason I wanted to post this is that most of the posts on these forums usually concern solidbody electric instruments. This type of thing makes an interesting break from that in that you not only have to deal with enhancing the tone of the guitar, but controlling feedback from the instrument as well.

I don't know if any of you have actually tried the 5F6A front end as a stand alone preamp, but it actually makes a quite good acoustic instrument preamp. Certainly it's a little hot in the low frequencies, but its tone-enhancing qualities really warm up piezo instrument pickups. A real testament to the original designers!

I look forward to hearing more ideas, and sincerely thank you all for your input. If I can get a chance today, I'll mod the preamp and post back with the results (I'm betting this arrangement will thin the sound too much, but I could be wrong).

Regards, Bill.

[Groover]

IMHO rolling off at 150 Hz seems kind of high, since the pitch of the low E string on a standard guitar is in the neighborhood of 88Hz, dropped D tunings are common, and a lot of body resonance lives in that low end. That being said, I have noticed on some guitar tracks I have recorded with pickup-equipped acoustic into a DI into the recording device have a lot of very low frequency "handling noise", so I have long thought that a good acoustic guitar pre-amp should incorporate a LF roll-off somewhere below 80Hz.

You might experiment with a roll-off in that neighborhood, and a notch filter at your feedback-prone frequency, instead of rolling all the LF off.

Just my $0.018 Euros. 8~)

[Chuck H]

I have to guess that if the OP has narrowed down the freq. to 150Hz then he is using a parametric or graphic EQ at present to determine this. So this would be the narrow band problem frequency. The rub is that you will lose tone and bass if you were to just arbitrarily roll off everything from 150Hz down. So a notch filter seems like the answer. This can be a tricky circuit to tune and it's probably easier in the long run to simply use an EQ for finite control like most players do.

JM2C

Chuck

[BTF]

Very true. I finished up the preamp a few days ago to good effect. I settled on a 100K plate resistor, 1.5K Rk, 4.7uF Ck and a .0022uF coupling cap and a .001uF plate-to-cathode attenuation cap for the first stage. This feeds my blend circuit and into a CF driven tone stack. This gives a tight bass while keeping the guitar relatively full-sounding. In my experiments, I also found that a .1uF Ck and .022uF coupling cap gave very good results, but the larger coupling cap made the guitar sensitive to low frequency feedback at loud volumes.

I actually designed and built the notch filter, but decided- much as you opined, CH- to use an EQ instead to attenuate any trouble frequencies further. This way the preamp seems warmer. However, I may decide to integrate it in the future. Many of the acoustic preamps out there use either a notch filter or a passive network to attenuate frequencies in the 100Hz to 250Hz range to give SBT's a little more headroom. I built the passive filter, but may try to come up with an active one. I think it would be an interesting project.

Regards and Thanks, BTF.