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 P9700S VCF/VCA distortion? 
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Joined: Thu Dec 26, 2013 4:35 pm
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Post P9700S VCF/VCA distortion?
Hello,

Ever since I have owned my P9700S synth, I've noticed that there is always a small amount of overdrive/distortion when using certain filter/Q settings on my VCF and it's also audible on the VCA even when the VCF is by passed...I believe this is due to the high output of the VCO's. I was reading the through some of the 9700 literature this weekend, and noticed a section that talked about the very high-level output of the VCO. There was mention in the documentation about making an "attenuator" type cable using 10K and 1K resistors in series to bring the VCO level down a bit prior to entering into the VCA or VCF. Would this help my issue with too hot of a signal coming from the VCO's?

Thanks for your time,

Nathan


Tue Jul 07, 2015 11:21 am
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Post Re: P9700S VCF/VCA distortion?
The attenuating cable can be constructed to bring down the signal level from a P9700S to be closer to what is expected for an amp/speaker, etc. having a nominal 0dBV input level (and without an input attenuator).

But also, it can be useful having the ability to reduce the signal from the oscillators, or the oscillators into the filter and then the VCA, to prevent the filter from breaking into oscillation or an overdriven feedback condition.

Just to patch the VCO waves through the VCA L and R inputs and take the L+R output (attenuated 3dB mix), before going into the VCF and then the selected VCF output to VCA A input helps in keeping the VCF in a stable range.

This modification to the VCF makes levels into, and control settings, less critical in terms of keeping things under control.

viewtopic.php?f=22&t=687

A pot with input and output connectors could be added into an FR-7 end-piece area to work as a general purpose attenuator. Wire the input, potentiometer, and output, referencing the VCO or VCF Modulator Level adjustment schematic section (signal into top of pot (cw terminal), signal out from wiper (center terminal) and the bottom of the pot (ccw terminal) wired to ground/circuit-common).

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Scott Lee
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Tue Jul 07, 2015 5:49 pm
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Joined: Thu Dec 26, 2013 4:35 pm
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Post Re: P9700S VCF/VCA distortion?
Scott,

Thanks for your reply. So are you suggesting to not use the attenuator resistor cables to come out of the VCO? I understand that it's recommended in the literature to come out of the VCA A output to go into an amp, but would it work okay in place of an FR-7 custom panel mount pot? Also, is the 9744 the same size as an FR-7 end piece? Instead of making a setup with a pot/jacks/etc., couldn't I just buy one of those to use as an attenuator coming out of the VCO before going to the VCF or VCA? I did try what you explained by going from the VCO to the VCA and then to the VCF last, (-3dB) and that did help! But I feel that it would still be better attenuated even more. Anyway, the 9744 looks cool and if it fits then perhaps I'll just get one! Also, I do have the 2 resistors modification added to my synth, which I know helps as well. I appreciate your time and help.

Thanks,

Nathan


Tue Jul 07, 2015 11:53 pm
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Post Re: P9700S VCF/VCA distortion?
It's best to keep the signal levels as high as is possible within the system, but if a patch or settings work best when the attenuation is in place, it is not a problem using it.

The PT topics below detail attenuators that can be built into, and in-line, patch cords.

http://paia.com/talk/viewtopic.php?f=22&t=689

http://paia.com/talk/viewtopic.php?f=22&t=688

If the VCA L and R sections of the 9710 aren't being used in a patch on your P9700S, either the L or R VCA can be used as an attenuator for the feed to the amp/speaker, etc. For example, if you want to hear the Pulse waveform from VCO A, patch it to the L VCA input, connect VCA L+R output to the amp/speaker, etc. and use the VCA A Pan control as the L+R output level control -- turned to R attenuates.

It would take some cutting of the end-piece, and sacrificing the rack-mounting holes, installing a 9744 CV Source + Attenuator/Mixer Module into the FR-7 End Piece Area.

https://www.paia.com/proddetail.asp?prod=9744K

The same lip with holes that can be seen in the rear of the FR-7 image linked below, exists on the front too. The FR-PWP (for using 9770R-15 power supply in end-piece) has holes for the side-mounted power supply LEDs and switch mounting.

https://www.paia.com/proddetail.asp?prod=FR-7

Compare it, with the single, upper and lower, mounting holes vs. the twin, edge-aligned holes on this FR-PWP used for mounting a 9770 series power supply in the FR-7 end-piece.

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Scott Lee
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Thu Jul 09, 2015 1:39 pm
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Joined: Thu Dec 26, 2013 4:35 pm
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Post Re: P9700S VCF/VCA distortion?
Scott,

Thank you for the helpful and detailed replies...I appreciate it. In looking at the link to the attenuated patch cords, it seems that attenuating 10v down to 1v is quite drastic, is it not? What 1/8w resistor combination would drop it down only by half, to around 5v? My issue isn't so much a too hot of a signal going out to the amp/speaker...it's the overdrive of the VCF and VCA themselves, even with the VCA mixer attenuation. I'm just guessing that attenuating the signal down to 10% of what it was might be too much?

Thanks,

Nathan


Sun Jul 12, 2015 11:11 pm
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Post Re: P9700S VCF/VCA distortion?
Hello Nathan.

Yes, the ten to one ratio for the resistors is a lot for patches within the system.

The 10K/1K parts are best when patching the VCO outputs direct to an amp/speaker (so the waves don't all sound the same -- loud square waves due to overdrive of amp input).

It can be useful having the extreme attenuation for realizing the most extreme dynamics which can occur with wide band modulation and higher Q settings when going from the VCO into the filter, especially with the two sections cascaded (VCF A into VCF B) and tuned to the same frequency .

To get less attenuation, just use closer to the same value, or the same value, resistors, ie both 10K.

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Scott Lee
scott@paia.com


Mon Jul 13, 2015 2:54 pm
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