@ozorfis Hehehehe! The whole time I was thinking they were called Hellipot. It wasn't until I uploaded the photo I was like "wait , wuuut?"
@Mooly, Good to know those potential issues. I've come across discussions about some of the problems with pots so I wanted to go with the best stuff I could get my hands on, which ironically were more affordable than precision cermet pots at Mouser. At least this way if I do run into noise and whatever other problem I won't have to second guess whether I need a better pot.
Edit: I got real close to the monitor and squinted. We're ok everyone, it's helipot 😆
@Mooly, Good to know those potential issues. I've come across discussions about some of the problems with pots so I wanted to go with the best stuff I could get my hands on, which ironically were more affordable than precision cermet pots at Mouser. At least this way if I do run into noise and whatever other problem I won't have to second guess whether I need a better pot.
Edit: I got real close to the monitor and squinted. We're ok everyone, it's helipot 😆
Ideally the feedback network should be kept tightly connected. Remember the feedback node of the amp (the junction of the 18 and 100 ohm resistors is actually another 'input', in fact it is the 'inverting' input of what is similar to an opamp configuration.
Try it and see 🙂 Notice the phase inversion now. In theory the gain is R3/R4 which is 5.5 In practice it is a bit lower because the output is pulled down a little by the load and because the amp has a low open loop gain to correct such things... but all those are some of the reasons why they sound as they do:
Try it and see 🙂 Notice the phase inversion now. In theory the gain is R3/R4 which is 5.5 In practice it is a bit lower because the output is pulled down a little by the load and because the amp has a low open loop gain to correct such things... but all those are some of the reasons why they sound as they do:
And the input impedance is hmmm.. LOW at about 18 ohms (R4 value). So you need a power amp to drive the power amp...
I kind of think I understand. Tightly coupled, meaning that because of the inverting connection of the transformer you can't do much without throwing the phase (or balance for lack of a better word) relationship out of whack? Along those lines, you would also have to consider what happens at the input. So maybe having that adjustment feature is somewhat marginal?
Normally you want to keep wiring and PCB print short and direct around the inputs (both inputs remember). The low impedance at this particular node in this design works in your favour because it will be pretty immune because of the low impedance.
It just feels wrong and bad practice to add wiring and pots here... however... years ago many amps used to wrap the bass and treble controls around the feedback loop of the power amp.
I would say experiment by all means but do it with a view to fixing what you prefer.
Also this amp is different to most with having those transformers and I suspect the very low impedance at that node is a requirement for these to operate correctly and give a uniform response and minimal phase shift.
It just feels wrong and bad practice to add wiring and pots here... however... years ago many amps used to wrap the bass and treble controls around the feedback loop of the power amp.
I would say experiment by all means but do it with a view to fixing what you prefer.
Also this amp is different to most with having those transformers and I suspect the very low impedance at that node is a requirement for these to operate correctly and give a uniform response and minimal phase shift.
I did actually anticipate the trace length. Funny you mentioned the old gear because every time I start going down the rabbit hole of specifications I'm reminded of "the olden ways of wiry nests " at just about the point I'm going too far LOL. The tentative plan is to arrange the pots in a triangle with the tabs all pointing inward. The space in the center of the triangle is where the frontend circuits go. I'm looking at having a flexible PCB made that will allow the board to loop around the inner perimeter. I don't want to be too circuit specific so I'm looking for universal combinations that let me patch the pots in and out of various elements. Considering your point on possible noise issues I'm looking at encasing each pot in Mu metal as a mitigation strategy.
Like you say, this particular amp is unique so I'm not expecting anything out of making alterations. Currently I'm hashing out dimensions and geometry. Clustering the pots in a triangle tends to look reminiscent of a reel-to-reel tape machine if both channels are side by side. Should be fun to investigate some of those designs for inspiration. Going the water cooled custom PC style route by the way. Here's the current crude blob in cad:
Like you say, this particular amp is unique so I'm not expecting anything out of making alterations. Currently I'm hashing out dimensions and geometry. Clustering the pots in a triangle tends to look reminiscent of a reel-to-reel tape machine if both channels are side by side. Should be fun to investigate some of those designs for inspiration. Going the water cooled custom PC style route by the way. Here's the current crude blob in cad:
LOL! I think you're on to something; those Jensens need a little brass and driftwood to go with the massive 2" diameter pots-- plus a bunch of wires for "flair". 70s Akai seems to be much more my directional flavor, albeit entirely by accident.