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Double Bass Stagg EUB stock preamp mod

#41
M1234 said:
Thanks for testing this and sharing your findings. I've never experienced clipping with mine and to be honest I find it strange that it happened with yours. Since my mod doesn't affect the input buffers I think if you have clipping after doing the mod that probably means you've had it before as well. Or is there a chance you've also made other changes (maybe accidentally?)
Click to expand...

Yes I agree. I don't think I changed anything that I shouldn't have... With my limited knowledge of buffers, the clipping seemed to me to be the result of the input exceeding the FET threshold voltage. I tried to simulate the circuit, showing similar behaviour.

Anyways, I like the sound of the E side way more than the G side, but still wanted to have some control over the G side, so I decided to give my own twist to the mod. I've removed the internal pots and replaced them effectively by the Volume/Subbass pots, so now Volume controls the G Piezo, and Subbass the E piezo. Pictures:

20181023_131254.jpg
20181023_131307.jpg
 
#47
DoubleMIDI said:
As far as I can remember it is a 100 KOhms logarithmic pot. But I haven‘t found a replacement part that fits and can use the old knob yet.
Click to expand...

I see, looks like a straightforward part, but the devil might be in the details.

I just had a look here, the first one that came to mind:
Alpha 9mm

Perhaps it's an option to replace the knob-type, if the exact shaft & right number of 'rims' (of the knurled shaft) can no be found?
Or replace even both pots while at it, so that also two same knob types can be used.
 
#48
clintrubber said:
I see, looks like a straightforward part, but the devil might be in the details.

I just had a look here, the first one that came to mind:
Alpha 9mm

Perhaps it's an option to replace the knob-type, if the exact shaft & right number of 'rims' (of the knurled shaft) can no be found?
Or replace even both pots while at it, so that also two same knob types can be used.
Click to expand...
These are the right ones, but the standard position) axis in parallel to the PCB) and a split shaft.

Changing both and replacing the knobs would be an option too. Since I got a Clevinger 5-string EUB I only seldom use the Stagg and only at home with no amp. So I can wait until I find a source for it (in Germany or Europe).
 
#51
I just stumbled across this thread. I have had a Stagg EUB for a while, I got it for silent practice but I so disliked the tone I hardly ever use it - it always had far too much low bass for my liking, even with 'Sub Bass' on zero. I wish that sub-bass was a HPF control...

After reading this thread so far, I unplugged each pickup one by one and listened on headphones. I don't even know which one is now disconnected, but it has improved the tone a lot, and I can even tolerate turning the sub bass up a little for a fatter,warmer sound. I left the 2.5mm plug inserted into a small block of foam to prevent any rattles or shorts before I closed things up.

One downside is that the max volume is now reduced, so I thought I could open the bass up and tweak the trimpot for a little more signal. Then I had the idea - what if I just zeroed the trimpot for the unplugged pickup and re-plugged it in? Has anyone tried that, I prefer to know that there is not something loose inside...

If not - when I get time, I will give it a try and report back. @DoubleMIDI can you foresee any benefits or potential problems from leaving both pickups connected and trimming one right down?

Thank you for all the trial results contributed so far in this thread. I am trying to suppress my GAS for a Yamaha Silent Bass!
Steve
 
#52
Happy Steve said:
@DoubleMIDI can you foresee any benefits or potential problems from leaving both pickups connected and trimming one right down?
Click to expand...
It might be possible that the pot cannot reduce the level to zero, that depends on the control, but it is worth a try.
If you can reduce the level to zero you connect the output of the impedance buffer to ground but I think there was a resistor in between, just not sure about it. If there is a resistor in between, dialing the pot to zero output is safe.
 
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#53
Happy Steve said:
I have had a Stagg EUB for a while, I got it for silent practice but I so disliked the tone I hardly ever use it - it always had far too much low bass for my liking, even with 'Sub Bass' on zero. I wish that sub-bass was a HPF control...
Click to expand...

Had so far assumed/hoping the 'Sub Bass' control was exactly that, controlling the corner-frequency of a HighPassFilter, so it's doing something else?

(Note 'my' EUB is not here yet, so no hands-on experience here yet.)
 
#54
DoubleMIDI said:
It might be possible that the pot cannot reduce the level to zero, that depends on the control, but it is worth a try.
If you can reduce the level to zero you connect the output of the impedance buffer to ground but I think there was a resistor in between, just not sure about it. If there is a resistor in between, dialing the pot to zero output is safe.
Click to expand...

If you want to be 'totally safe' for killing the sound of that disconnected piezo-side without any worry of killing the circuit, you could consider to add (insert) a shorted dummy plug in the open input-PCB-connector.

It'll result in [1] no remaining chance of any incoming disturbances or noises still making it past the set-at-zero pot (in case it's not fully-reducing signal to zero, as DoubleMIDI pointed out could be possible).
And [2] in case the set-at-zero pot is loading down the preceeding buffer (as also pointed out by DoubleMIDI could be possible, and which in fact would not be the best circuit design), no incoming signal, even when amplified, is still little to nothing at the output, so no shorting worries.
 
#55
clintrubber said:
Had so far assumed/hoping the 'Sub Bass' control was exactly that, controlling the corner-frequency of a HighPassFilter, so it's doing something else?
Click to expand...

PCB-pic from Huubrutjes at top of this page shows both the 'external' pots are 'mono', so my assumed/hoped-for HPF-function of the 'Sub Bass' control is either unlikely or at best not what it could/should be. As in: for a reasonably effective/noticable HPF function you'd want to have at least a 2nd order filter, so a dual-pot for filter-frequency would be required.

So curious to what the actual function/principle of the 'Sub Bass' is, anyone knows more?

Bye/thanks
 
#56
The sub bass seems more like a boost than a cut, I keep it on 0 usually. @clintrubber I get what youre saying about the dual pot - I built a very rough FDeck some years ago. I must try the Stagg through that into my powered speaker.

Experiment-wise,tweaking the trimpots with both pickups plugged in has achieved some sort of improvement from the original setup. I very close to zero'd the side which was previously unplugged. It was possible to make the remaining side distort with too high a setting, but I am able to get a bit more signal now without any audible distortion. It is hard to say how trimpot mods compare to unplugging, as it takes some time to pop a few screws back in to prevent rattles and potential scratches from a dangling preamp unit.

I am still not that pleased with the sound, but maybe I need some strings with a bit more 'zing'. It is currently fitted with an old set of Belcantos. Strangely, arco sounds very thin and buzzy in comparison to pizzicato.

I may mess around a bit more if I get an idle moment. At least I could have a quiet practice with headphones now without being distracted by the super deep bass tone.
 
#57
Hi,

Thanks for mentioning the 'FDeck' name, looked it up and happened to be returning to the very same website I stumbled upon long ago.

(for own easy reference let me paste a link:
hpftechllc - HPF-Pre Series 3 technical info )

Which is to say that with the amount of opamps on the HDB-200 PCB I was assuming they'd realized an alike circuit, but as turned out they're not, and doing something else.

OK, so 'Sub Bass' at CCW is actually 'flat' then? And CW it's adding lots of BABOOM ?

I've contacted Stagg support for the schematic, need to hear back from them.

Have a good weekend!
 
#58
Some info w.r.t. the electronics:

-The 'SubBass'-control is indeed not a highpass corner-freq control, but a conventional bass-control (cut & boost)

-No adjustable treble control, but the circuit acts as if a treble-control is set at max-cut.
(possibly to tune/tame the response of the piezo-pickups)

- Input impedance for each piezo input is a healthy 10 MOhm each

- The trimmers for each piezo can go fully down to zero, without any adverse loading effect of the preceeding JFET 'unity gain' buffers. So they're no gain-controls, but conventional volume-controls

- In addition to being sent to the headphone in stereo, the stereo CD-input is also summed to mono, and sent to the output-jack as well

- To protect for battery reversal at minimum voltage drop, a Germanium diode in series is employed


- Yep, I have the schematic, but with the condition of no forwarding, uploading etc, you'll understand. Hence the form of the info above, which I figure summarizes most of the relevant bits.

Bye
 
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#59
clintrubber said:
…

- Yep, I have the schematic, but with the condition of no forwarding, uploading etc, you'll understand. Hence the form of the info above, which I figure summarizes most of the relevant bits.
Click to expand...
Thanks for the information you habe us.

Could you please tell us the value and characteristic of the volume and sub-bass pots, since they are the ones that might be damaged first?

I tried to measure it in-circuit, but that was not very successful.
 
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