Input signal clipping, need to clean it up...

[Vesperado]

I have a chorus very similar to this one, the FX68 Super Chorus, and want to clean up the input as I suspect the transistor is responsible for clipping the signal. I have already replaced all the ICs, and a NE5532 for a LM1458, as well as replacing all electrolytics. This helped a little. My only concern is disturbing the 1st half of the TL062 and perhaps upsetting the balance on its output, dry and wet. Does anyone know if it would be possible to bypass the transistor into the positive input without causing harm? I am wondering if this will require a different RC combo at the input. Any work around would be greatly appreciated!

[dschwartz]

I use the same buffer for my designs..
If biased with 4.5V, the headroom is decreased because the output is not at 4.5v..it will be somewhere around 3v DC, which may clip with hot pickups...
I use a dedicated Vref for it, with 6k8/10k divider and 22uF filter, so the output is near 4.5V...it has more headroom that way.
You cannot bypass this buffer on this design because the next stage input impedance is too low..
You can also replace the bjt with a jfet.

[Vesperado]

Thanks for the info.

I will check the voltages and see what is going on with that. I was afraid the Z would be low...thanks for confirming.

[antonis]

Clipping depends on input signal amplitude..

As Daniel said above, you better alter bias configuration to leave more headroom for undistorted signal, 'cause Emitter followers don't exhibit linear behavior..(symmetrical Emitter quiescent point swinging ..) 

IMHO, you must bias Emitter at a point higher than 4.5V 'cause R9 forms a voltage divider with R10//(R11+C8) for signal negative waveform, resulting into lower than 4.5V output clipping..
(a rough estimation of Emitter biased at about 5.5V should be OK..)

That said, you'll need a bias configuration to set Base at 6.2V or so..

[ElectricDruid]

The simplest way to get more headroom would be to run the pedal from 12V. The BBD/Clock are ok up to 15V, so that's your upper limit. Typical op-amps of the era were designed for +/-15V rails, so can easily cope with 30V across them.

A 12V supply would be 30% more headroom. 15V supply would be 60% more.

Actually, no, I'm wrong: The simplest way to reduce clipping would be TO TURN THE DAMN GUITAR DOWN!!!

[KarenColumbo]

The simplest way to reduce clipping would be TO TURN THE DAMN GUITAR DOWN!!!

I'll take the T-Shirt

[iainpunk]

i recommend you grab an audio probe and check where the distortion starts, i honestly doubt its that first transistor.


cheers

[Vesperado]

Great suggestions, gents.

I didn't mention that I already adjusted the clamp down on the power so when feeding it 15.1V it reads 14.35V at the BBD. There is a trimmer as well, and when I can get some time I will see if that affects the base voltage any.

The 2N5088 datasheet says 3V emitter, but I assume as long as it operates and doesn't fail it should be fine. I will order a new PNP just in case the old one doesn't show improvement.

I do not have an oscillascope so I am limited to what I can do, otherwise I would look to see if the 47k feedback at the opamps is too high. I might try lowering this value to see if I can hear a difference or no.

[antonis]

I didn't mention that I already adjusted the clamp down on the power so when feeding it 15.1V it reads 14.35V at the BBD. There is a trimmer as well, and when I can get some time I will see if that affects the collector voltage any.

Trimmer sets Vbias and has nothing to do with Collector voltage..
Just trim Vbias at about 7.5V and you'll be OK..
(both from buffer point of view and op-amps symmetrical headroom..)

The 2N5088 datasheet says 3V emitter...

Could you plz elaborate..??

I will order a new PNP just in case the old one doesn't show improvement.

Don't do it 'cause 2N5088 is NPN type..

[Vesperado]

too early for me...thanks!

[Vesperado]

Ok, so I made some strides. I attempted to adjust the Vbias tap for Q3, a 100k with 1M: nothing changed. So I resorted to reduce Vbias voltage by increasing R5 2.2k further. It appears that Q3, a J113, is not clamping down much on Vbias. In fact, the range is offset to that of Q5, a 2N5088, where at higher Vbias the offset augments further in proportion to a difference of Vbias: they do not correlate at any point set over P1. Therefore, if the Vbias is say 7V, Q5 gets 6.1V and Q3 shows 6.9V, but at 6.5V Q5 gets 5.4V and Q3 shows 6.4V. So in effort to reduce voltage at Q3, I will stick another 5k on R5 and reduce the value at R8 to see if I cannot bring both Q5/Q3 closer, for it is evident that the clipping is not occurring at the dry signal, but at the wet. I think if I can get them both 6V or a little under that, the clipping might be at a minimal.

The other thing I would like to figure out is, if the Depth range would extend if I changed the B100k for a B150k?

[antonis]

Q3 works as a series switch for engaging-bypassing dry signal and it has nothing to do with input signal clipping headroom..

[Vesperado]

Granted it is, but the wet signal passes through it. The only other place I can think of Vbias affecting the clipping is on the output opamp, however, there is no clipping whatsoever when I set the Mix to full-dry. So my reasoning stands that Q3 is likely the culprit.

From what I am hearing is I can set the headroom high on Q5 without any problem (up to 8V), but Q3 seems to be causing the clipping (above 5.9V). I have been able to further rectify the clipping by taking R5 up to 17k and lowering R8 down to 43k, where now there is a half-volt difference between Q5 and Q3. I have been able to tame the clipping to a satisfactory level now:  Q3 is set at 5.9V and Q5 correlates to 5.4, Q3 being the limiting factor. I feed the pedal 15V and have the clamp set for 14.5V. So after much tinkering I have the best headroom achievable now.

Last thing I need to do is extend the range on the Depth control some...

[Phend]

?

[Phend]

.

[ElectricDruid]

If you short Drain and Source on Q3, does the clipping go away? If not, it's not Q3.

Your reasoning that when you turn the mix to fully-dry you get no clipping only shows you that the problem lies in the wet path, not that the problem is Q3. It could be the delay line itself, or potentially the post-delay filter.
I second Iain's suggestion of tracing the signal with an audio probe. You need to know exactly where the problem is coming from before you have much chance of fixing it.
I also second Antonis suggestion of 7.5V for the Vbias. Honestly, anywhere in the centre of the range is going to be fine, and if you're still getting clipping with a 15V supply, either something is wrong somewhere or you're feeding in an *extremely* hot signal. At 15V supply, you could run 10Vpp and not get any clipping. That's a *massive* signal, modular synth level not guitar level!
The BBD is more likely to be the bottleneck, and is also more likely to be sensitive about the bias level, and in my view, that's why there is a bias trim there at all. The op-amps and buffers aren't so fussy.

[Vesperado]

I suppose one could bypass Q3 to see where else any clipping might be present in the circuit. So far the clipping is pretty much gone, but at the trade for some headroom. Exactly where in the filtering would you suspect it? I run strait in with my ASAT Classic, then amp; no hot signal here.

I also am working on cleaning up a DOD 460 Mini Chorus. At the + input I replaced the 470k with an 820k (or a 91k in the fb should work, too), and the output feedback with a 58k, up from the 47k, along with some other things on the backside of that opamp. This hints that the 2N4124 off BBD pin 4 passes a signal less-than unity-gain, so perhaps lowering the 33k's might help clean up some. As you mentioned, the filtering may need some improvement as well. What is rather peculiar is when powering up this pedal it is clippy, but after a while the clipping reduces. Here's a drawing I've found:



So while on the topic of filtering, what would be your recommendations?

[ElectricDruid]

I suppose one could bypass Q3 to see where else any clipping might be present in the circuit. So far the clipping is pretty much gone, but at the trade for some headroom.

I'm not surprised if the bias finishes up being set somewhere off-centre. After all, the schematic suggests the designer knew the optimal bias point was a bit above the midway point. So a little bit of headroom gets eaten. But still, on 15V supply you should have *bags* of headroom. This is not a high gain pedal.

Exactly where in the filtering would you suspect it? I run strait in with my ASAT Classic, then amp; no hot signal here.

UIB, the post-delay filter (half a TL062). But I don't really suspect it. It's a possibility is all. Get your audio probe on it and find out. My bet is the distortion comes out of the BBD itself, not the filter.

What is rather peculiar is when powering up this pedal it is clippy, but after a while the clipping reduces

That sounds like a cap charging up and bringing a bias point to the right level. It may well have always been like that.

HTH

[antonis]

Could you plz inform us about input signal amplitude..??

[ElectricDruid]

Could you plz inform us about input signal amplitude..??

+1 agree. Any kind of clipping problem on a 15V supply implies some crazy input level or something seriously wrong. So far, you've given no idea that either of these is the case. I have no idea what an ASAT Classic is or what level it puts out, so that doesn't help me.