Sunn Beta Bass preamp schematic

[daustinhall]

Pardon me if it's here in the thread somewhere and I just can't find it, or understand it if I did see it, but if you make the two channel version, how do you go about tying the inputs for the "both" channel?
I see how to do A or B, with the summing and master dual gang pot, but I don't understand how to do the A + B.
Can anyone help?

[DiscoVlad]

Because the A and B channels are identical (and my schematic is multi-sheet) I've attached the input section for ONE channel of the 2-channel version. The second channel is the same, and you'll have two copies of the pre-amp circuit.

In this case you need 3 input jacks: A, AB (both channels), and B.

sunn_beta_ch_inp.png

Each Ch_in1 is connected to one of either the A or B jacks depending on which channel it is. The Ch_in2 input of both channels is connected to the AB/Both jack.

Overall it should end up something like this hopefully helpful block diagram

Beta-Pre.pdf

Each channel block is an identical copy of everything up to the output of the EQ, the Master block contains the each channel's Volume/level control and the Master volume, this saves a couple of opamps (or needing to use singles) and some board space.

[maciek336]

Hi everyone,
here are my Eagle version of the schematic, should be correct to last version... hope so

http://www.mediafire.com/download/s666l ... _Eagle.rar

cheers and good luck!

Thanks for the redraw! I made a PCB based on it and it turned out that there are two errors in your version. One near the second half of the gain pot and the other in the mids section of the tone stack. I'll post updated version below (yeah, I've used paint and I'm proud of it)

https://image.ibb.co/ixskTp/Sunn_Beta_Bass_Single_Channel.png

[fontaine0304]

I know someone already asked, but is there a way to use polarized electrolytics for the 2.2uF caps? Someone told me that it isn't possible because they are coupling caps, but it was hinted earlier in this thread that it was actually possible, although there was no actual confirmation. So I guess my question is, can I use polarized electrolytics for the 2.2uF caps, if so, how?

Thanks a lot !

[DiscoVlad]

1) Use a single 2.2uF and connect the +ve lead to whichever side has the highest voltage.
2) Use 2x 4.7uF in series back to back (+ve leads together) to make a 2.35uF (close enough) non-polarised electrolytic cap.
3) Just use non-polarised 2.2uF film caps because Mouser/Element14/RS Components/Digikey/etc. all have a wide selection of them and they are neither difficult to obtain nor expensive?

[fontaine0304]

1) Use a single 2.2uF and connect the +ve lead to whichever side has the highest voltage.

The anode toward +15v and the cathode toward -15v, is that it ?

3) Just use non-polarised 2.2uF film caps because Mouser/Element14/RS Components/Digikey/etc. all have a wide selection of them and they are neither difficult to obtain nor expensive?

Their shipping fees to where I live would be 5x what my order would cost. I'll look there as I last resort if I really have to.

Thanks for the help, it's appreciated.

[induction]

2) Use 2x 4.7uF in series back to back (+ve leads together) to make a 2.35uF (close enough) non-polarised electrolytic cap.

You'll want to use 2u2 caps in series.

The math for caps in series breaks down for polarized electrolytics. Polarized caps only block DC in one direction and behave like jumpers in the other direction, so putting them back to back blocks DC in both directions. When a signal attempts to reverse-bias one of the caps, the other cap blocks the current flow, which prevents damage to the reversed cap. At either polarity, a given signal will forward bias one cap so it acts like a cap, and reverse bias the other one, so it acts like a jumper. In effect, there is only one cap being used at any given time, so the capacitance of the series pair is the same is that of the individual polarized caps.

[DiscoVlad]

You'll want to use 2u2 caps in series.

The math for caps in series breaks down for polarized electrolytics. Polarized caps only block DC in one direction and behave like jumpers in the other direction, so putting them back to back blocks DC in both directions. When a signal attempts to reverse-bias one of the caps, the other cap blocks the current flow, which prevents damage to the reversed cap. At either polarity, a given signal will forward bias one cap so it acts like a cap, and reverse bias the other one, so it acts like a jumper. In effect, there is only one cap being used at any given time, so the capacitance of the series pair is the same is that of the individual polarized caps.

No it doesn't, electrolytics in series behave exactly the same as normal capacitors.

IMG_0066.jpg

This is 2x 10uF in series, with the + leads connected between them, it measures 5.38uF whichever way the meter is connected.
The individual capacitors measure 11.1 and 10.44uF

Two caps in series Ctotal = (C1 x C2) ÷ (C1 + C2) -> 11.1 x 10.44 ÷ (11.1 + 10.44) = 5.38

[induction]

Fair enough. I won't argue with your meter.

I will, however, direct you to this thread about the subject, including a post by R.G. that was the source of my assertion. R.G. is rarely wrong about this sort of thing (can't say the same about myself), so I suspect there's something to it, but I haven't done an empirical study on real caps.

I did notice this quote from R.G. in his post: "There is a small region around 0V where both are acting like caps but it's best avoided." Which suggests the possibility that your meter is using a very low voltage to check the capacitance. Or maybe there are other details we're neglecting.

In any case, now I don't know what to believe.

[DiscoVlad]

Just to add to the confusion... RG is right, Electrolytics do have wide tolerances, and won't behave predictably outside of their normal operating conditions, but as he always says, the devil is in the details.

The capacitance range of my meter outputs a 5ms long 3V pulse every 400ms, which means things I guess

I haven't physically built this to test, but according to the spice simulations I've done (.op to get some voltages to see which way electros should be oriented, because I couldn't find NP caps of that value which weren't physically enormous at the time), and with the usual caveats about spice component model accuracy or straight up mistakes in drawing the circuit both leads of the 2uF and 2.2uF caps were both 1-2 microvolts away from [opamp virtual or real] ground anyway, so in terms of DC voltages it might not even matter which way you orient them.

Providing the AC levels are low enough (below 1V should be ok?), then if the capacitors are reverse biased in this application, it doesn't seem to be enough to cause the oxide layer dielectric to break down, and make the capacitor explode

edit, because this just occurred to me: The "avoid using near 0V" thing which RG was saying I take to mean that your DC Bias voltage needs to be higher than the AC ripple voltage to avoid reverse biasing the capacitor, and running the risk of it failing (explosively usually )

[induction]

edit, because this just occurred to me: The "avoid using near 0V" thing which RG was saying I take to mean that your DC Bias voltage needs to be higher than the AC ripple voltage to avoid reverse biasing the capacitor, and running the risk of it failing (explosively usually )

I took it to mean that there are two capacitance regions for series electros: small voltages give C_effective = C/2, while larger voltages give C_effective = C, and that the larger voltage range is more appropriate, capacitance-wise, if you require a specific value. I don't think he was saying avoid using this arrangement near 0V, because the whole point of non-polar caps is that you can safely reverse-bias them, or use them unbiased. With a bias voltage, there's no need to construct a non-polar cap out of two polar ones, as you could just go ahead and use the polar caps directly.

For clarity, I'll post the relevant RG quotes from the link. The tolerance issue wasn't what I was referring to.

In the series-NP connection, the capacitance value is funny. Normally caps in series are a smaller capacitance than either cap by itself. If you had two 3.3uF polyester caps, then the expected value for two of them in series is 1.65uF. However, electrolytic caps actually conduct in the reverse direction, so two 3.3uF polarized aluminum electrolytic caps act like they each have a diode in parallel with them that conducts when the voltage is backwards for that one cap. So two 3.3uF caps hooked up as series non polar (i.e. negative to negative) look like a single 3.3uF NP cap.

... except for tiny region near zero volts where they withstand a tiny reverse voltage, so they look like 1.65uF there

When someone pointed out that he tried it with tants, and his cap meter gave C_eff = C/2, RG's response was:

Tantalum will withstand a reverse voltage of a few volts before the leakage comes up on the reverse biased side. So with tantalum the "funny area in the middle" is much bigger, maybe as much as 4-5V depending on the cap. Also, most DMM testers test with a small voltage across the cap so they give better answers when the caps are tested in-circuit. You're probably getting both effects.

In your conditions, I would expect just the results you're getting.

You measured electros, not tants, and 3V may not be sufficiently near zero for electros to stay in the funny area in the middle (or it may, I'm not sure), so I don't know if that explains your measurements or not.

[DiscoVlad]

In the spirit of DIY it was using what I had available

Tantalum electrolytics might behave differently, or they might not depending on the application. I think from an engineering standpoint it makes much more sense to use parts which are going to behave in a predictable manner because then you tend to have fewer weird edge cases to worry about - In this example if tants did behave differently, I would use something else that behaved as expected.

I can only suggest that you try a range of capacitors in series and see under what circumstances these effects happen, but that exercise is kind of off topic...

For the voltages and signal levels present in the Pre-amp it seems very unlikely that putting two equal value caps in series for the coupling caps won't result in ones that are half the value. ymmv

[fontaine0304]

Is there any reason a 470uF cap wouldn't work instead of that 680uF ? The one coming from +15v and going to ground.

[georg667]

Hey guys!
finally I managed to build my beta lead preamp. I used the vero layout from tagboardeffects. I just finished it and went through some troubleshooting. Guess I solved most of my problems as all of the pots react kind of correct and the sound is quite similar to the original amp. I got one original by myself and so it's easy to compare the sound.
The only issue I've got is that there are some sound artefacts the higher the gain knob is turned. Turning it half the way the sound is nice and crunchy as expected. If I go higher the distorted signal reacts kind of weird depending on the strength of my guitar strum. when I play all strings and hit them really soft it's a crackling distortion which sounds shit. when I hit the strings harder there is a volume jump and it sounds more like it should but not as nice as the real amp.
So I went on troubleshooting and noticed that some parts in the tagboardeffects version are different than shown in the original schematics. Looking at the version morain51 posted here some time ago I also noticed the same differences compared to the tagboard version.
I talk about the 2uF capacitors on the positions C102, C103, C110, and C145. I took some 2,2uF instead as my vero-layout told me. Could it be that these capacitors affect the sound as I mentioned above?

I'll attach the two layouts where I highlighted the capacitors so you can see the differences.
Any advice is appreciated. Maybe I manage to record some sound files tomorrow

cheers georg

[fontaine0304]

Hey Georg, I just finished building this 5 minutes ago with 2.2uF caps and it sound great. The folks over at tagboard often use values more common/easy to find. Anyway for this layout it doesn't matter, at least not to make it behave like you described.

[georg667]

Yeah thanks man.
That's what I found out as well, as those 2uF caps are hard to find ...
Anyway, I will record a tune later so maybe some of you guys could give me a hint in which direction I have to look. Could be a fault in the drive circuit itself...
So long
Georg

PS: @fontaine, Did you build the bass or lead version? Could you also record some samples?

[fontaine0304]

I build both, can't record samples right now but they sounded really close to what people have been posting on here and on tagboard. The EQ is really powerful so try to test your circuit with everything at noon.

[georg667]

holy shit!

I finally found the mistake... Although I triple checked everything I have overseen that I mixed up some cables to the drive pot, which I found using an audio probe. I switched connection 4 and 5 and that led to a very strange distortion.

Now everything seems to work correct and man! this thing sounds absolutely amazing!!
positive side effect... when I use the original amp the overdriven signal has a very loud background noise which is far more silent in this build!

Next: the second channel and of course the housing.
here a little glimpse of what is in the pipeline

PS: thanks to everyone who worked on the veroboard layout!!!

[georg667]

Hey guys here is a little test-run of my preamp. Have fun!

https://youtu.be/MXjmMMn5Yp4

PS: language is german. Preamp sound at about 1 min