Nobelium - tube bass preamp project

[matmonster58]

I'm waiting to get the right XLR jacks but I'll do some testing in the mean time.

What's a good software for measuring frequency response and THD?

I have REW and Multitone analyser but I haven't used them before.

 

[vigilante398]

What's a good software for measuring frequency response and THD?

I have REW and Multitone analyser but I haven't used them before.

REW is what I used when I was developing the spec for the Unagi E4305 and testing prototypes, but that was a while ago and I haven't used it since.

 

[SamuelG]

I've been in this same situation for the past few months.
Short answer: You can't get them right now

I've talked to Jensen, radial, and every other distributor and no one has them. I even talked to Jack from Nobel and he's having a hard time getting a hold of them. There is a Rode branded one on reverb in Australia right now but I haven't been able to get in contact with the seller.

What I ended up doing was just waiting until I found a JDI that was cheap enough and then I pulled the transformer out of it.
With how hard they are to find, I just wouldn't offer a Jensen transformer as an option. Cinemag, Unagi, and LBA are all great alternatives.

I ended up getting that RODE branded one and took on the project.
I have some Jensen’s on back, back, back order with a hope and a prayer that they come sometime this year. I want to make the most of the time it’s taken to design my UV print and find the parts
I’m really keen to see how the trannies compare - in circuit - with proper analysis

 

[matmonster58]

From my initial listening:

Unagi - Amazing, sounds very clear and balanced throughout the whole frequency range. I would choose the Unagi if you want a natural well extended sound

LBA - Also amazing but more colored. I hear a touch more heft in the low end and the highs are tucked down just a bit. Together you get a fatter, smoother tone.

Both the unagi and LBA give what I'm looking for in a "tubey" tone in the high end. I don't have a better way to describe it than that.

Jensen - The Jensen has a more "polite" tone. The lows and highs don't extend as much and the "tubey" character gets smoothed over a bit. I guess I would describe the Jensen as being true neutral, whereas the Unagi and LBA feel like they are enhancing your tone.

 

[Tassieviking]

I'm waiting to get the right XLR jacks but I'll do some testing in the mean time.

If you remove the nuts and washers from the switch on the XLR PCB it will be easier to align the switch with the slot on the enclosure.

 

[TillMack]

From my initial listening:

Unagi - Amazing, sounds very clear and balanced throughout the whole frequency range. I would choose the Unagi if you want a natural well extended sound

LBA - Also amazing but more colored. I hear a touch more heft in the low end and the highs are tucked down just a bit. Together you get a fatter, smoother tone.

Both the unagi and LBA give what I'm looking for in a "tubey" tone in the high end. I don't have a better way to describe it than that.

Jensen - The Jensen has a more "polite" tone. The lows and highs don't extend as much and the "tubey" character gets smoothed over a bit. I guess I would describe the Jensen as being true neutral, whereas the Unagi and LBA feel like they are enhancing your tone.

Thanks for the comparison.
You made me decide to go with the Unagi transformer for my build.
Ordered everything.
I will use the Neutrik NC3 MAV as XLR Output, looks promising from the datasheet measurements and maybe it's an option for everyone like me, cause in Europe it's hard to get the Amphenol.

 

[Tassieviking]

From my initial listening:

Unagi - Amazing, sounds very clear and balanced throughout the whole frequency range. I would choose the Unagi if you want a natural well extended sound

LBA - Also amazing but more colored. I hear a touch more heft in the low end and the highs are tucked down just a bit. Together you get a fatter, smoother tone.

Both the unagi and LBA give what I'm looking for in a "tubey" tone in the high end. I don't have a better way to describe it than that.

Jensen - The Jensen has a more "polite" tone. The lows and highs don't extend as much and the "tubey" character gets smoothed over a bit. I guess I would describe the Jensen as being true neutral, whereas the Unagi and LBA feel like they are enhancing your tone.

I think that Mr Sushi "Nathan" Box should borrow this post and the audio files for the C2CE website on the Unagi E-4305 page, and also on the Steddi Go and Nobelium pages as well to help people work out what transformer they want to use.
Well done young fellah. ( Unless the 58 is your birth year then make it old fart instead cause you would be 2 years older then me )

Cheers
Mick

EDIT: I might as well be the one that starts the "magic tone is in..." rumor, the reason that the LBA transformer sounds more colorful is because the purple PCB enhances and colors the sound more.
From now on the purple PCB's are going to be a collectors item because of those magic purple colors bleach through to the sound quality.

( I spelled colour wrong to help the overseas people understand better, it's colours not colors, you guys have to stop loosing things over there)

 

[matmonster58]

Those clips were done looping the bass with an Hx Stomp. The Nobelium was set with no hpf, bass maxed, treble at noon, and xlr volume maxed. I was hitting the transformers as hard as I could so that the differences would be more pronounced.

I haven't gotten a chance to compare with more neutral settings. The differences may be very subtle once you back the bass and volume off.

 

[Nerdridesbikes]

I am putting together my orders to go ahead with the nobelium, and have a couple questions about mods since I am 99.9% likely to never use the 1/4” output. New to understanding the details of the circuits so I appreciate any help. Sorry for being a noob and asking potentially dumb questions, I’m a mechanical engineer but never thought I’d want to know anything about electronics. I would have built the steddi go but I wanted the eq.

1) is it possible to just switch which leads go to the xlr out vs the 1/4”? What would the implications be? It would be the easiest way to put an accessible the volume on the xlr, and it would make use of both tubes.
2) if I add in a full pot instead of the trimpot, I assume I am looking for a 500k linear? Not in love with any of the locations to put it but I’ll make it work.
3) if I leave the circuit as is, is there any downside (besides not being able to use it) to just removing the tube for the 1/4” output?

 

[Audandash]

So I am not going to pretend like I know if it will function properly or not without the second tube (even though I do know using the xlr out bypasses it in theory). On other hand I can offer some solutions for the volume with the xlr. You could mount it on the same side as the xlr above the ground lift switch. There is plenty of room there just to hand wire it. Another option would be to put an xlr volume pot in place of the regular volume in the front (hand wired of course). Then on the backside of the pcb put a trim pot in place of the standard pot so you do have control over the volume if you ever do use the 1/4 inch. It would be a pain to adjust but at least it’s there if you needed it. You could also just move the standard volume to the side as stated above. You could use either a linear or logarithmic pot for it.
Mick or Nathan will have to address your technical questions.

 

[Tassieviking]

If you are not going to use the 1/4" output at all then you don't need C9 or any of the components to the right of C9 on the schematic, if that's the way you are going then replace the volume pot with an A500k pot and then pick the signal from the viper of the pot up on the right hole of C9 and run that to the XLR PCB, you still need to run the ground wire as well.
I think that the square pad on the volume pot is ground so that puts it right above the C9 hole you want to run to the XLR transformer.

I have no idea what the output to the 1/4" out socket would be so I can't say that it is suitable to feed to the XLR PCB.

The other option is to install an A500k log pot instead of the trimpot for the XLR output and install it wherever you can fit it in, just run some short wires to the pot from the trimpot holes on the PCB.

If you get a 9mm pot and cut the mounting legs off and straighten the terminals out it should fit between the PCB and the box, just place some tape between the pot and the board to make sure.

 

[SamuelG]

I've had success squeezing one of these between the enclosure and the PCB. I do cut the legs off though. https://www.taydaelectronics.com/ta...mic-taper-potentiometer-d-shaft-pcb-11mm.html

I like the above idea of replacing the xlr trim pot with a pot wherever you want it. However, you "might?" want that 1/4" output one day

OR...

It's my understanding that the second tube is a buffer with unity gain but I don't see how you'd fit another DI transformer into the case. However, you could use the buffer output by incorporating a DRV134PA to drive another xlr output? https://www.ti.com/product/DRV134/part-details/DRV134PA ... squezing this in might be feasable if you're up for some pcb building?

It would be a lot of work, but I'd be curious to hear the difference between a Transformer DI xlr and a tube buffered SS xlr.

 

[Tassieviking]

I was thinking more of how the "Steddi Go" was wired so both outputs shared the volume pot, that way you would not need the trimpot.
Just tap of the volume pot to go to the XLR PCB, C9 on the right terminal goes to the viper of the pot and the square pad of the pot is a ground.

 

[Nerdridesbikes]

Audandash, Tassieviking, SamuelG,
Thanks for your replies. I’m not intending to reengineer this whole thing, and while I am good at building, I am still learning the circuit theory.

I think the most reasonable path is use the additional pot you recommended and keep the circuit as is. If I can mount above the board that will allow a decent layout.

Thanks again!

 

[Audandash]

I don’t recall where I saw it (it may even be toward the beginning of this thread) but I am pretty sure Nathan has his XLR output volume mounted on the side near the ground lift switch. I may have also made it up, lol.

 

[vigilante398]

I don’t recall where I saw it (it may even be toward the beginning of this thread) but I am pretty sure Nathan has his XLR output volume mounted on the side near the ground lift switch. I may have also made it up, lol.

Yup, all of my builds with a D.I. use a different XLR daughter board that has an XLR output volume control down there by the ground lift switch.

 

[retty]

I just finished building my nobelium but it has a weird popcorn type noise on the output. I've done a few things to try and troubleshoot and have ruled out cables and the bass, itself. I've read that this could be caused by cold solder joints so I've also reflowed a bunch of solder joints onthe tube daughterboards. both the angled connector and the tube sockets.

Other internet advice says it could be a grounding issue. I also wonder if it's power-supply related. I am using a "cheap" 9v 1A switch mode thing from Amazon so I suppose that could be the issue. I'd need to know a better psu to order if I'm to rule that out.

Symptoms

1. The noise happens more often if tapping on or around the tubes. I don't think this is microphonics. tonally it sounds like an 8-bit snare drum (link to audio below)
2. it happens more when deflecting the narrow pcb "finger " between the two tubes (I've reflowed the resistors here in case it was a cold joint)
3. If the volume is <= 85%, you only hear the noise when playing a note on the bass
4. between ~85-90% volume, the noise manifests randomly, like popcorn popping, and comes through like it's just passing some threshold to make it through to the output. Kinda sputtery.
5. Above 90% volume, there is a persistent noise floor (unsurprising) and the "popping" noise comes through the output whether or not anything is connected.

Testing

1. The noise gets louder when you turn the volume knob, and its timbre IS affected by the tone controls.
2. I ran a sinewave into the pedal and took a look on an oscilloscope. As soon as the signal touches the input jack it gets small ~47kHz spikes on the carrier. I'll add some pics once I figure out how.
3. I've probed around the 555 pins to see if there was anything out of the ordinary but wasn't sure what to look for. The waveforms looked normal, save for some ringing(?) on the off cycle of the square wave on pin 6 or 7.

I don't know if the high frequency spikes on the sinewave is indicative of anything. Looking at the PSU on the same oscilloscope, however, I don't see any of that high frequency noise on its output. But I'm not an EE.

Has anyone else experienced this? Is it possible the noise could be coming from another part of the circuit?

Here's a link to what this noise sounds like

 

[SamuelG]

Hi Retty,
Thanks for reaching out. There are much smarter people who will ask more questions
Two things to begin with.
1. Please post some good photos of the pcb board.
2. Have you tried swapping out the tube. Any 12AX7 compatible tube will work.

 

[SamuelG]

I meant “Smarter than me”

 

[retty]

Both are true

Pardon the flux.

I Hope it's not the tubes.... I don't have others to test atm and they are both brand new. I also paid the upcharge to have them both tested before shipping to specifically avoid situations like this.