High Frequencies -- Signals Inside Mono/Poly

Continuing down the road of this post, I'm trying to put a little more sizzle into Mono/Poly.  I perceive that its highest frequencies aren't as present as they should be.  In previous posts, I showed how my Mono/Poly output started to gently roll-off starting around 4 kHz compared to an ideal sawtooth wave.  In this post, I open up the synth and start measuring the signal at various points in the synth's circuits.  I'm trying to isolate where the roll-off of the highest frequencies occur.

My Tools for the Job

To measure the signals at internal points in the circuit, I've chosen to use my trusty M-Audio handheld recorder.  It should be noted that the signals inside the synth can be very strong or very weak compared to the regular main output of the synth.  Therefore, one has to be careful to adjust the gain on the recorder so that it can properly handle the signal level at the given location in the synth.

The next issue is how to get the synth's internal signals out to the recorder.  Well, as you can see in the picture, I had a black coaxial cable with a BNC connector on one end and two clip leads on the other.  I then bought a BNC-to-Phono plug (1/4") so that I could plug this cable into my M-Audio recorder.  To record signals from within the synth, I just touch the point of interest with the clip leads (one to ground, the other to the point of interest).  Done.  If you don't have this kind of cable, you could take a guitar cable (1/4" on one end), cut it in the middle to expose the two internal conductors, and strip and tin the tips of the conductors.  Bingo!  Instant test cable.

Moving forward, I needed to decide where I was going to start measuring.  I already showed that the overall output was missing the highest frequencies.  So, I looked at the schematic (below) and chose to record the signal prior to the VCF at R1 (left side of the schematic) and I chose to record the signal after the VCA at R41 (ride side of the schematic).

I chose to record the signal prior to the VCF (left) and after the VCA (right).

I setup the synth to play one voice, sawtooth, with the filter wide open and no resonsnce.  I recorded a C1 (low) note at the R1 location.  Then I recorded a C1 (low) note at R141.  What did I see?  Well, I saw that I saturated my recorded because the signal was too strong, even with the M-Audio gain turned down to its lowest.  So, I turned down level of one oscillator using the knob on the front of the synth.  Then, I repeated my measurements.

What did I see this time?  In the time-domain (ie, like an oscilloscope would show), I got the tracings below. Notice that the blue trace (pre-VCF) shows those nice over-shoots at the vertical transition in the sawtooth. That means it'll have lots of sizzle.  The green race (post-VCA) lacks those overshoots.

Here is the Sawtooth Signal Inside the Mono/Poly from Before the VCF (blue) and from after the VCA (green)

When I measured the amplitude of all the harmonics, I got the frequency-domain plot below.  It clearly shows the roll-off of the highest frequencies in the post-VCA trace (green).  Notice that the signal before the VCF (blue) is totally flat.  It is as good a sawtooth wave as one could hope for.  Wow.  So, it appears that my sawtooth is loosing its edge somewhere in the VCF or in the VCA.

Comparison of the frequency content of the recorded signals to an ideal sawtooth.

What's the next step?  Well, there's a lot of circuitry between the two points that I measured.  Unfortunately, the signal levels are really low (20 mVpp) right after the filter (ie, the midpoint between the two points that I measured in this post), so it's really hard for my tools to measure the signal at this point.  My previous post discussed how I think that I've got the filter pushed open as much as it can be opened.  This suggests that the loss might be occurring in the VCA.  I guess that it's time to start probing the VCA.

Update: Here's my "Sizzle" Mod, where I solve my high-frequency problem!