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bajaman wrote:

Baja AC30 Normal amplifier overdrive 9v low gain rev 2 210218.GIF

Thanks!

What do you think about to use the TL074 instead of the TL064?
The '74 is less noisy...

Thanks again

I have just stumbled upon this project, and I have only one question: is it possible to have both the Top Boost and the Normal channels going into the same "power amp" section? Basically, could you have these circuits combined into a single, two-channel circuit? I have an idea of how to combine them but I'm not sure if it's correct.

Could the BC547B's be replaced by 2N2222's? The hFE is min 200 for the BC547B and min100 for the 2N2222. Everything else looked to be in the ballpark. Please forgive my ignorance, but how would that affect the circuit?

rpc66 wrote:Could the BC547B's be replaced by 2N2222's? The hFE is min 200 for the BC547B and min100 for the 2N2222. Everything else looked to be in the ballpark. Please forgive my ignorance, but how would that affect the circuit?

As far as I know, the BC547Bs behave as diodes when their collectors are disconnected. Hence you could sub these transistors easily with a priori any BJT, or even with regular silicon diodes. As for their role in the circuit, I'd say they introduce some hard clipping, but I'm no expert.

You're the Baja MAN!

Hi Bajaman, for your updated vero layout is the tone control stack wired off the board like your other layouts? Am a bit confused with comparing the vero and the schematic.

Thanks.

I'd like to listen some demo of these circuits. Are there some links, please?

Edit: i missed rev2! It seems you fixed my main issue, but ill leave my innitial response as i still think there are improvements that can be done in terms of accuracy.

Original post:
This is an idea i've been messing around with, and there are some good resources with ac30 analysis. I don't want to come of as rude or complaintive, i just want to give some feedback. I can't se how this hits the mark though, except for the tone stacks..
I'd say my primary issue is with the preampclipping. Those diodes are fed directly by an op amp (very low impedance despite the feedback resistance). Maybe the voltage never passes Vf for those LEDs? In that case they will approximate to having no effect as the opamp will compensate for any small current leakage. (Or am i missing something??) However if those leds open up I think the op amps gain goes straight to the rails. Which in the best case results in a super hard knee, but it will probably also induce very questionable behaviour from the op amp as it tries to compensate for an almost infinite voltage loss. (This would cause those very square waves, not how an ac30 behaves). I would definitively feed the diodes with a much higher impedance and, if the voltage is to low for the LEDs, swap to lower Vf diodes. For a tube-like clipping knee i'd go as high as 500k-1M resistive impedance and 2-3 low-Vf diodes in each direction.. the signal would then probably need to be buffered again before the tonestack however.

As a fan of the AC amps i would love to se a version that nails the clipping behaviour as well! The amps clip innitially in the final preamp, or possibly the phase inverter, as gain is multiplied with each stage. But as input is increased the first stages will saturate and the clipping will have several different cutoff curves so to say. I think this can be approximated fairly well with a series of parallel diodes and resistors, or a "softclipper" in the opamp followed by shunt diodes.

Next thing to tackle is the voltage sag from the rectifier tube will share if i ever finish up my own sagging amp pedal project!

bajaman wrote: ↑18 Feb 2018, 03:45 Here you go - built it and verified - works very well - not a cabsim but an a speaker response load sim designed to duplicate the frequency response caused by the speaker load on a tube output stage.
Baja AC30 Top Boost amplifier overdrive 9v rev 1 with speaker response loading 170218.GIF
I had completely forgotten this design, so many thanks to HENK for pointing this one out
cheers
bajaman

Don't suppose there is a vero layout for the speaker load part knocking about to save me some time with the graph paper lol