PROPHET64 & MSSIAH Userforum

I am clueless about diodes but my first reaction would be that in order to have a diode regulate voltage the signal would either need to be, or become DC which is something you really dont want to feed into your amplifier. Then again I could be completely wrong in which case I hope this thread will enlighten me

Also, how would a diode circuit, or anything else that regulates the voltage like you said be different from a compressor? A limiter is just a compressor with a brickwall setting on it :-) At least the compressor would allow you to do some nice soft/hard knee tweaking and threshold/compression ratio tweaking.

An interesting solution might be to use a stereo pot meter. One channel can then be used to regulate feedback to the audio in pin of the SID while the other can be used in reverse to gradually decrease output volume coming from the audio out pin going into your external equipment. All by turning a single knob The rate at which this "volume decrease" occurs can easily be adjusted by adding a resistor in parallel to the "volume adjusting" channel of the pot in case the resistance is too large for subtle tweaking. I guess all you'd really need to do is measure the output voltage and see what resistance is required on the second channel of the pot to keep the signal it within safe limits. Voila, no compression, just volume adjustment.

Last edited by lynxxx (2009-03-24 21:11:38)

Check out the Poor Man's Brick Wall Limiter using LEDs:


Also interesting: DUAL-DIODE LIMITER


I used to install diodes to protect the SID out/in, it's pretty much the same idea:

but doesn't do clipping/limiting.

I think 5V is pretty much above the audio gain range for limiting purpose?!? 3Vpp should be more likely for audio but using ZF3 diodes might turn the clipping into audible distortion (which may be a nice side effect too).

I also thought about a stereo pot but two SIDs in my breadbox require two stereo pots (or does anyone hold a couple of quattro potentiometers in stock?). The IMHO perfect solution would be an active circuit, using OP-amps (or CA3080s;)) to control the feedback amount and reduce the overall gain at higher feedback levels.

djjonnyjams wrote:

Flemming....
The reply about a stereo potentiometer that reduces volume as the feedback mod is applied, sounds like it would help my problem. I read that big mech used a stereo pot, but I thought that was because he's running dual sids. I never thought to hook it up the way you suggest. The other circuits that were volunteered by the other replies were much appreciated, but I think for my purposes, the stereo pot would be the best way to keep the volume consistent throughout the range of the feedback pot, without effecting the natural qualities and characterisitic of the sound.
......as for a quad-pot, I don't know where you would find one, I don't believe I've ever seen one, but they might exist in some faraway land.

If you use a single SID only, then a stereo pot would suffice. for dual SIDS take a look at this

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi … 0372078546 (expensive)
There;s even a motorized version heheheh that would rock.. not useful whatsoever but still cool :-)
Hmmm... maybe I should do a midi controlled motorized pot mod on the C64 heeeeheeee


Then again, a lot of cheap pots use a plastic axis that you might be able to remove or extend yourself (in the old days stereo amplifiers used to have two single pots connected together mechanically as well I think.
Or maybe this http://www.omeg.co.uk/p4g2bub2.htm

I dunno, just a thought

Last edited by lynxxx (2009-03-25 17:54:49)

Ah I see.

Diodes have typically a step voltage when they start leading current. With in these limits the diode is a cut in circuit. In other words when the voltage arises or decreases " over " that step voltage the current is lead to ground. + and - are both arising voltages however the current has a different direction. This is why you have 2 diodes. The current can be + or - with in the limit of these 2 diodes step voltage. If the Up diode is +2.5V and Down diode is also (+2.5V) -2.5 then you got volume limited to +5V range.

Yes of course it works. It actually does not boost the sound. It just is called "Overdrive" or Distortion as we know it. Those who play electric quitar uses this very idea all the time. Overdrive may sound lauder but actually it is not same as amplification. For real overdrive you also need operational amplifier so you do not loose the " amplification " of the sound and really get the sound to boost over the diode limits.

The potentiometres simply works as an elevator for the step current so you can finetune when the " overdrive " takes step in and cuts off the " over voltage " or " below voltage " + or -..

I hope this helps.. To amplify your music you should consider getting transistor or tube amplifier.  =P

To amplify the sounds I would use 2 potentiometres and few resistors, maybe electrolytic capacitor and OP37 operational amplifier. =P


If you see TB-303 shematics then this very same thing has been done just after the analog switch .. and before the VCO. It is to keep the control voltage in range of +5V so it does not burn the VCO.

So indeed... This is a limiter in away. " Real limiter would drop the volume if it hits the tops . "

Last edited by ACrIkeD (2009-06-08 00:54:16)