Univibe Phase Differences between two identical builds

[aruberus]

Hi,
I have built 2 identical Harbinger 1.5 UniVibes, both using exactly the same components. I have actually built 4 but only two have the same identical components so for arguments sake ill refer to the two identical builds.

I am hearing a pleasing perceived difference between the two builds in that one has more depth / wobble / chorus / wahness to it. Im assuming the one I like is wrong because the other 3 builds sounds very similar to each other given the slight differences in components used.

My question is where should I search in the schematic to find what is creating this nicer sound (to my ears).

Testing on a dual channel oscilloscope i was able to visually see the differences when feeding a 220Hz test signal through and measuring the difference between source and the output of each phase shift network on the 4.7k resistor coming off each LDR.

LDR1  R19  (5.19v) A  180deg phase shift                                B  180deg phase shift
LDR2  R25  (5.16v) A  amplitude modulation and phase shift     B  amplitude modulation and phase shift
LDR3  R31 (5.19v)  A  -180deg phase shift                              B  -180deg phase shift and amplitude modulation
LDR4  R37  (15.8v) A  -180deg phase shift and slight DC Offset   B  -180deg phase shift and slight amp mod

The DC Offset of 'A' is what I suspect what is causing or adding to this difference but I cant work out why or where it is coming from.

Also of note is that all builds are working as I expect them to but they all have incorrect voltage readings on Q10 (Final Stage).
According to the madbean docs they should be (based on a 18V power supply)
C 17.5v
B 6.94v
E 6.5v

My Readings are (using the optional onboard Charge Pump )
Q10  2N5088
C  16.42
B  15.88   
E  15.47

Any assistance into identifying where this aural discrepancy may lie would be greatly appreciated.

A

[PRR]

> incorrect voltage readings on Q10 (Final Stage).  C  16.42  B  15.88    E  15.47

R39 wrong, missing, or not really connected.

As to your other question: LDRs are VERY variable one to the next.

[aruberus]

Thanks PRR, R40 68k was 680 ohms. Changing now.

Regarding the sound difference the other builds have different LDR's in them but the difference is not that great and I dont believe is contributing to the sound difference im hearing. (I could be wrong and maybe its just the difference between the same brand being less or more matched)

One has the Banzai Univibe set, the others have the Banzai PHR-1526D which I found a little better. I trialled the Banzai PHR-2406 but it produced a weak output so put that one aside.

Im still keen to know if there is something else that could be effecting the sound difference and where I should look. Does the modulating DC offset measured on this one build offer any clues?

Here is a pic of the Build.



Thanks,
A

[PRR]

> Thanks PRR, R40 68k was 680 ohms.

Does not make sense. (680 would make it near zero.) 680k?

> being less or more matched)

LDRs are extremely variable. Even in the same part number and the same production lot.

[PRR]

For some reason I was thinking pre-made optos. Here's a test-jig to compare four optos at four different currents. I suspect +/-30% variation.


I now realize you use one lamp and four naked LDRs in a box. IMHO four LDRs are even less likely to match than four good Vactrols; and the lamp and box and LDR relations add more variability. You can use the jig above, but with power off, and use external DC supply and limiting resistor to compare the four LDRs.