Bonson Praemp  [documentation]

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dylan159
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Post by dylan159 »

bonson.PNG
Hello, this is a circuit based on the Benson Preamp, called the Bonson praemp, because I'm refusing to call it a preamplifier. This is part of a streak of me going through circuits I see and even like, to conform them more to my standards while trying to keep the same timbre and character. A more thorough explanation is give at the beginning of the Clown thread.

Up for some circuit analysis? The Bonchild (name changes every time, it's a rule) consists of the same building block repeated over and over, the jfet common source amplifier, using the odd duck of jfets, the J201, very appreciated in the pedal world for the low Vgs and the ease of overdriving it, leading to a streak of jfet adaptations of tube preamplifier of which this and the Benson, together with the very very similar Plexi Drive and some circuits on ROG.

The tube legacy of this circuit reflected in the high impedance design followed which was the main thing i addressed: common cathode triodes have an output impedance which is many times higher than common source usually, and that creates the need to use higher value resistors in tone networks and such, which can lead to higher Johnson noise (not sure how much it matters here but it makes me feel better) and worse output impedance.

The first stage is an unbypassed common source amplifier, with input impedance of 1M and a flat frequency response. My first change was removing a big resistor in series with the input, which reminds of the grid stopper usually welcome in tube amplifiers, but which I think is of no consequence here except adding noise right where it matters most. 1K or so can do well for ESD protection instead if wanted. In my case this stage biased with the drain at about 6V, which is off from giving maximum clean output and instead closer to the biasing suggested by ROG. I say in my case because already there's some variation between mine and other values I found online. I decided to stick with the decision of not having a way to adjust bias on this stage because it's close enough and this stage will clip only when pushed hard anyway.

After this comes the volume and bass network. In short the Gain is a volume control (voltage divider) to attenuate the signal going into the next stage, and between lugs 2 and 3 there's a conventional "bright cap", which lets high frequencies bypass the attenuation depending on the pot setting. The series resistor tames this effect. C2 in series with C3 form an high pass RC filter with the volume pot's total resistance. What the bass control does is progressively short out the 3.3n cap so instead of a filter with 3,19nF and 100Kohm, we have a filter between 100nF and 100Kohm which lets all audio frequencies pass. Actually the finite pot resistance means when the bass is cut it forms a shelving filter because lower frequencies pass, altough greatly attenuated. That's why you can't use a pot that's too small.

Here the change was getting rid of that awful antiquate 1M potentiometer and redesigning it for lower impedance. I started by scaling down resistance by 10 and capacitances up by 10, then working in LTSPICE to try and get the same response across the whole range. I also removed another big series resistor, which other than noise meant there was always a minimum of -3dB attenuation, which is a small amount on the log volume scale (and hey, if you crank the pot, it can give you a bit more "gain"), but also meant reworking the rest of the passive network because now the bass cut could be more effective. Also hey, I like to save on parts that aren't needed. And hey, this all meant I could get rid of that ugly 2M bass pot and instead use a much more common 100K or 250K, which is log and wired as bass cut and gives a much smoother response. You can also use a rev log wired in reverse. I guess C2M pots are hard to find…

The bright cap and resistor, the bass capacitors were tweaked to get a similar response, always within 2dB of the original, with common values. 250K bass cut gives you more range than the original, 100K a bit less but a nicer sweep.
Comparison of stepped bass pot in the volume network (100K pot). With volume at full:
bonsonfull.png
With volume 6dB lower:
bonsonhalf.png
With volume at half:
bonsonlow.png
With volume very low:
bonsonmin.png
Then there's another common source stage, this time fully bypassed, which means more gain. We also see a trim pot on the drain resistor. This is to account for the large variability between each single jfet, by letting you set the drain voltage to a desired value, in this case 4V which is again slightly off from a symmetrical swing and maximum output headroom. This is usually the preferred method, but keep in mind you're varying the load resistance and hence voltage gain of the stage! But then if the bias was off transconductance was too and you have the "gain" attenuator so things kinda take care of themselves. On a bypassed stage like this, a source trimmer would work well too.

The output of this stage goes into the final common source, again unbypassed (it was bypassed but they decided it was too much gain I guess). The input impedance is 100k, which forms a RC highpass with the 22n which barely invades audio and especially guitar range. I decided to keep it but 100n is probably fine. If we consider that the output resistance of the previous stage is roughly 10k + trimmer resistance, there's also a small but negligible amount of attenuation.

The output of the final stage goes into the final lowpass network and the volume control. We can consider the tone control a second order RC network in which one half is variable, composed by the treble pot, R11, C8, C10. The variable lowpass is done with what I call "guitar style", adding series resistance to the capacitor leg to reduce the attenuation to almost zero. I like this kind of filter here because it doesn't add series resistance to our output and instead exploits the output resistance of the last stage, meaning at the frequencies of interest its gain is basically reduced, resulting in lowpassing. Again the output resistance of the stage depends on the trimmer setting, and that's in series with both sections of the filter, but again our main filter is variable so we can account for most of that. An output attenuator works as volume control and pulldown for C9, the DC blocking capacitor.

My changes here were concerned with output impedance. We don't start well from the 10K+ of the common source, but we can do our best. I reduced R11 to 10k, which is still more than i wanted but a lower value would have led to less attenuation of treble. I also found that a more common 100K for the treble pot still gives all the treble and so also a better sweep. C9 has been changed to a 10u so that impedance remains as low as possible at low (mains) frequencies, with no audible change. The overall response is very close to the original again. The 100K volume still hampers our output impedance but we can't really go lower without losing output or loading down the last stage.
bonsontone.png
Demo:
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Lani
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Post by Lani »

Nice work here Dylan!! Question... Do you think this circuit will work with a BF862 jfet without changing much? I suppose a breadboard is in order. Again...sounds great!! Cheers!!

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Post by dylan159 »

Lani wrote: 23 Mar 2021, 11:27 Nice work here Dylan!! Question... Do you think this circuit will work with a BF862 jfet without changing much? I suppose a breadboard is in order. Again...sounds great!! Cheers!!
Thanks! I checked their specs, and while Vgsoff is as close as we can ask (important for the input headroom), the transconductance is about 10 times higher! That's a lot of transconductance :) Thankfully we have drain trimmers for that, accounting for both bias and gain in the unbypassed stage. You probably want to lower the 10K stopper to 1K or omit it entirely. The bypassed stage is another deal: if you get the bias right, the gain will be too high and vice versa. If you don't have a j201 to spare, I suggest also adding another trim in series with the bypass cap, so that you can get the gain right with a scope after the bias, or get the gain right first and the bias good enough and then finetune the bias by making the source resistor a trim (gain shouldn't change). Anyway you want to make the first stage trimmable too because it won't be close enough for you to avoid it. Try anyway, maybe on breadboard, I think you can do it :applause:
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dylan159
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Post by dylan159 »

The vero layout I used... No clue why I haven't posted it before, it's a decent 1590B friendly layout
bonson.png
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Post by gryffsmyth »

If I wanted to reduce the overall bass response of the circuit, could I just reduce C2's value? Or C3? Understand this'd likely change the character some.

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Post by dylan159 »

gryffsmyth wrote: 24 Jun 2021, 13:00 If I wanted to reduce the overall bass response of the circuit, could I just reduce C2's value? Or C3? Understand this'd likely change the character some.
I'd say the circuit already allows you to cut as much as needed, as you can see from the plots. The bass cut pot progressively shorts out C3, but when it's at max resistance it's (almost) as if c2 and c3 are in series, giving an effective 3.2nF cap highpassing with the 100K pot (in practice it's not so nice because it's shelving but close enough). That's enough cut even for me, and the goal was to keep it close to the original. we can do more though, by decreasing C3 (no need to decrease C2, that's the maximum amount of bass but doesn't affect what's happening with C3 in series much), decrease C7 (fixed amount and not before most clipping though), or mess with C4 and R5 (mostly R5) to let more treble through at low gains.
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Post by Vurguuz »

Hi @dylan159, what an interesting project and admirable approach!

Have you a/b-ed your praemp against the original at all or is this more of an academic exercise?

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Post by dylan159 »

Vurguuz wrote: 26 Mar 2023, 14:23 Hi @dylan159, what an interesting project and admirable approach!

Have you a/b-ed your praemp against the original at all or is this more of an academic exercise?
I haven't. In some other cases I have, but for this one I've explained why I think it would sound the same and what would be the small, able to be tuned out differences in a naive "same settings" comparison. Then anybody is free to check if they want.
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