Hi Folks !
So I got a bassdude kit which I built for my bassist son. He loves the sound of it ! The only small change we did after trying a few different ones is the V1 input tube, which we swapped for a 12AT7. It still crushes at high gain, but seems a bit easier to tame and the kid likes the sound that way.
Now, one of my long term plan is to use it (either as-is or get inspired from the design) to build him an amp head using a tube preamp and a class D module (I have an IcePower ASP 250 and a Hyper UcD 400 around). This however led to a couple of issues for which I would like opinions, mostly to enlighten myself
I am at best a hobbyist when it comes to analog electronics and have a lot to learn, so feel free to call out anything stupid I might be saying here, I'm happy to be educated ! This is also a learning journey for me.
So ... there are two main problems as I see things with the Bassdude as a preamp for the aformentioned power amp modules:
* The output voltage of the bass dude is too high. Nathan measured something like 65V PP. I haven't yet measured the one I built with the 12AT7, I plan to do that and also do the math as an educational exercise in the next few days (I managed to buy a copy of Merlin B's tube preamp book 2nd edition, so that helps). More below on this, but TL;DR is at least the hypex will die if fed more than +/-12V (I checked with them, it will be damaged).
* The output impedance. Now this is getting close to the limits of my understanding (still learning...). What I *think* I understand so far is that the output impedance is fundamentally the ability of the circuit to deliver current without impact on the signal (voltage) and matters mostly if what is connected to the output has itself a low'ish input impedance (that I funndamentally think of as a current draw, treating the reactance component as a frequency-related element). The hypex has a "poor" input impedance of 100kOhm afaik and the IcePower is way worse at 8kOhms (that is really bad from what I understand).
So let's start with the first one. To be completely safe, I'll probably add a pair of 11V Zeners to ensure we clip rather than burn the amp (maybe in serie with a resistor as to not burn the zeners either) to the input of the amp itself. But beyond that, once I have properly characterised the circuit (math and/or measurements), I'm thinking of replacing the existing 1M volume pot with a resistor in series with a small(er) volume pot, to act as a voltage divider and ensure that the max output voltage remains within range. However unless I misunderstand things, this won't particularly help with the impedance issue, on the contrary.
As for the latter, well ... the bassdude has a cathode follower feeding the tone stack if I understand properly. This should be ok in most case, but between the impedance added by the tone stack and the max impedance added by the tone pot (afaik 1/4 of the pot, so 250kOhm), we are way above the input impedance of either power amp.
One thing with the Bassdude is that there's an unused triode in V1. (12AX7 in the original circuit and 12AT7 in mine). Would it make sense to wire it up as an additional cathode follower *after* the tone pot to provide a lower output impedance and thus avoid any possible problem with whatever's connected downstream ? I'm not sure anything can be really done for the bloody IcePower without involving a transistor or an op-amp but I'm not sure I care, I might end up sticking with the HyperX.
Does that make any sense ? Anything fundamentally wrong with my reasoning ? Alternatively I could I suppose put a 5532 or TL072 fed off 9V with a 4.5V BIAS but it gets messy, I'd probably have to try to fit another PCB in the box, and I would have to reduce the max volume further. Plus I like the idea of using the unused triode...
Thanks in advance for any constructive comment
So I got a bassdude kit which I built for my bassist son. He loves the sound of it ! The only small change we did after trying a few different ones is the V1 input tube, which we swapped for a 12AT7. It still crushes at high gain, but seems a bit easier to tame and the kid likes the sound that way.
Now, one of my long term plan is to use it (either as-is or get inspired from the design) to build him an amp head using a tube preamp and a class D module (I have an IcePower ASP 250 and a Hyper UcD 400 around). This however led to a couple of issues for which I would like opinions, mostly to enlighten myself
I am at best a hobbyist when it comes to analog electronics and have a lot to learn, so feel free to call out anything stupid I might be saying here, I'm happy to be educated ! This is also a learning journey for me.
So ... there are two main problems as I see things with the Bassdude as a preamp for the aformentioned power amp modules:
* The output voltage of the bass dude is too high. Nathan measured something like 65V PP. I haven't yet measured the one I built with the 12AT7, I plan to do that and also do the math as an educational exercise in the next few days (I managed to buy a copy of Merlin B's tube preamp book 2nd edition, so that helps). More below on this, but TL;DR is at least the hypex will die if fed more than +/-12V (I checked with them, it will be damaged).
* The output impedance. Now this is getting close to the limits of my understanding (still learning...). What I *think* I understand so far is that the output impedance is fundamentally the ability of the circuit to deliver current without impact on the signal (voltage) and matters mostly if what is connected to the output has itself a low'ish input impedance (that I funndamentally think of as a current draw, treating the reactance component as a frequency-related element). The hypex has a "poor" input impedance of 100kOhm afaik and the IcePower is way worse at 8kOhms (that is really bad from what I understand).
So let's start with the first one. To be completely safe, I'll probably add a pair of 11V Zeners to ensure we clip rather than burn the amp (maybe in serie with a resistor as to not burn the zeners either) to the input of the amp itself. But beyond that, once I have properly characterised the circuit (math and/or measurements), I'm thinking of replacing the existing 1M volume pot with a resistor in series with a small(er) volume pot, to act as a voltage divider and ensure that the max output voltage remains within range. However unless I misunderstand things, this won't particularly help with the impedance issue, on the contrary.
As for the latter, well ... the bassdude has a cathode follower feeding the tone stack if I understand properly. This should be ok in most case, but between the impedance added by the tone stack and the max impedance added by the tone pot (afaik 1/4 of the pot, so 250kOhm), we are way above the input impedance of either power amp.
One thing with the Bassdude is that there's an unused triode in V1. (12AX7 in the original circuit and 12AT7 in mine). Would it make sense to wire it up as an additional cathode follower *after* the tone pot to provide a lower output impedance and thus avoid any possible problem with whatever's connected downstream ? I'm not sure anything can be really done for the bloody IcePower without involving a transistor or an op-amp but I'm not sure I care, I might end up sticking with the HyperX.
Does that make any sense ? Anything fundamentally wrong with my reasoning ? Alternatively I could I suppose put a 5532 or TL072 fed off 9V with a 4.5V BIAS but it gets messy, I'd probably have to try to fit another PCB in the box, and I would have to reduce the max volume further. Plus I like the idea of using the unused triode...
Thanks in advance for any constructive comment
