I hear you like (pre)amps

[ChannelingHemingway]

Hello! I previously posted under Adamadamadam but no longer have access to the associated email address (.edu problems). Fortunately I found the trash username for my usual email. Anyhow.

I've been working on this for a while and it's finally working well enough that I don't feel too embarrassed to share. I'm still not 100% on the frequencies for the tone section and I shouldn't have removed the second gain stage (whoops), and there's some background hum I'm confident I can reduce. Also, the taper for the volume pot is backwards? That's probably my fault but I'm not sure how.

So it's obviously SS, but V+ is 250vdc (or 208/2099 because the transformer is out of spec). It's just a preamp, but I've a single-ended and push-pull power sections in different stages of 'the works' at 12ish and 90ish watts respectively. For now I'm using a JC as my power section for testing and it's hilariously clean.

The board has 'normal' and -6db inputs but I'm only using the normal here. The tone sections are independent and can boost or cut and can be stacked almost indefinitely. I have five on the board but will draw up a sub-board with another five in case a 10-band eq ever seems like a good idea. The frequency is determined by a pair of capacitors so my testing is for standard/A standard guitar, but it could absolutely be tailored for any frequency range. I guess the drawback is that it works more like a bandpass than a shelf.

I have two pcbs left so the next step is a dual/stereo unit to correct a few problems and get the circuit 'production ready.' I realized yesterday that the enclosure I ordered is too small so we're off to a good start.

Ask away? I unfortunately don't have any sound clips, my usb interface kicked the bucket and replacing it hasn't been a priority, but it's on the list.

Also also, if you regularly order pcbs who do you use? I'm pretty new to this side of things and I've been very happy with Oshpark's quality, but they seem a bit pricey. Do they scale better with quantity? I'm using kicad for what it matters.

[crochambeau]

Oh, that's pretty.

250 volt rail you say? Out of curiosity, what magnitude of clean signal output is it capable of? (I feel like I butchered grammar on that last sentence, let me grab some tea before proceeding)

[a moment no one else will notice passes]

I spy what appears to be a partially populated parallel of actives, output section?

I like OSHpark, and the price per board acre does go down when you select the larger run option. OSHpark is fantastic with Kicad, because - as I expect you are aware- you can just upload the .kicad_pcb file and be done with it. The wait on the larger runs is not too terrible, but does not sit well with instant gratification fans.

That said, I have also been using JLCpcb, which is considerably cheaper if you are comfortable navigating the conceptual spiderwebs of offshore production. JLC wants gerbers, the industry standard, and that's another step of processing, resulting in an array of files that work together. I have found the wait to be on par with the larger run of OSH boards, though I have not ordered anything at this present stage of shipping mayhem.

Anyway, that looks cool, thanks for sharing!

[ChannelingHemingway]

So there's no breakup, which is what I was going for. I generally don't like preamp distortion and I wanted to be able to run this off a tap of a tube power supply. I'm just not getting the volume I want/was expecting: I was using a different test rig when breadboarding because I didn't want to blow up the roland if something went wrong.

You do, but the parallel sections are the frequency controls and have nothing to do with the output. I'm just using three but I know some bassists like having more control over their low and high end, and it doesn't take up much room and there are no drawbacks if they're not populated. They're basically bandpass filters in a negative feedback loop. The frequency is determined by the two capacitors and messing around with the resistors changes the Q.

Kicad's great. I prefer Eagle because you can update the schematic from the pcb, but the free version restricts the PCB size and the 'pro' version is a subscription instead of a one-time purchase. I'll definitely check out JLCpcb, thanks for the recommendation. I'm still in the 'messing around in my basement' stage of things so cost-effective larger runs aren't a pressing concern, but maybe one of these days.

[ibarakishi]

this is a cool thread, subscribed

[Chankgeez]

Yeah, that looks pretty neat.

[crochambeau]

Is this derivative of another circuit, or one of original devising? I have curiosity based questions, such as wanting to see the schematic and so forth - but I get one wanting to keep some sort of shroud of mystery over it, so I'll try to not get pushy.

What actives are you using?

Can we see the schematic?

[ChannelingHemingway]

It's derivative of some old hi-fi stuff I've found, but as far as I'm aware it's not being used pretty much anywhere. If you've ever poked around inside an Ampeg you'd probably recognize the input section, etc. This does have a few changes from what's on the board and I need to stick another gain section at the end, but it's close enough.

The transistors are all Supertex LND150s, which is a high-voltage n-channel depletion mosfet, so they behave pretty similarly to triodes.

[crochambeau]

Awesome, thanks! That looks cool, I can't wait to see it with power sections as well.

[ChannelingHemingway]

Neither can I. The push-pull has technically been on the bench since 2016ish but single-ended power sections are way less complicated. Unfortunately transformers aren't exactly cheap, and the more budget-friendly options I've found don't have ultralinear taps which is something I'm keen on.

[ChannelingHemingway]

I've ordered the single-ended transformers but the lead time is practically forever so I've got some time to get everything else together. Still waiting on some backordered parts for preamp v2. I'm reasonably sure at this point that the hum is because I had to leave an RC filter off the power board due to space reasons, which is pretty obvious in retrospect.

Not really sure when I'm going to be able to get back to tinkering though? I was accepted to law school which is neat, but they've already started hitting me up for seat deposits and whatnot which I had definitely not factored into the budget.