Author Topic: Things to do with that extra hole  (Read 364 times)

Mark Hammer

Things to do with that extra hole
« on: June 07, 2021, 10:44:28 AM »
I ordered some CA3260 dual op-amps from a Chinese supplier.  Much to my dismay, only one of the 6 seem to work.  For all I know, they could be relabelled CA3160 single op-amps.  I'll stick them into something with a 741 socket today or tomorrow and see.

But, that said, I bought them to make a clone or two of the Ibanez MT10 Mostortion.  I made one for my modular setup, and it sounds decent, though I have no idea how essential THAT particular op-amp is to the sound.  There are a bunch of other things that improve it beyond a Tube Screamer, not the least of which is a great 3-band EQ.

In any event, I etched an MT10 board from one of the posted layouts and populated it.  By swapping the 3260 for a simple TL072 or JRC4558, I learned the nonfunctionality was a result of the chip not the board.  Great.  So I have a working MT10.  I have a few empty enclosures I had gotten from Tayda pre-pandemic, and thought I'd install it in one of them.  But they were predrilled for 6 holes, plus LED.  The MT10 has 5 controls.  What can go in the 6th hole?

Couldn't think of anything that really needed adding; it's that good and complete of a pedal.  Then it occurred to me.  The difference between the sought-after TS-808 and the TS-9 is essentially the output buffer.  Ibanez changed the resistance values from what the 808 had to what one sees in the 9, which is a higher value series resistor (470R vs 100R) and terminating resistor (100k vs 10k).  I imagine the lower-value terminating resistor loads down the signal, if one is feeding it straight to an amp, that it removes some of the fizziness of a distorted signal (in somewhat the same way that a curly cable might), and is probably what makes it desirable. Of course, if one is NOT feeding a cable to an amp from the pedal, whether directly  or through another pedal, that loading is probably inaudible.  But the pedal's interaction with the capacitance of the cable and input impedance of the amp likely accounts for why many prefer the 808 over the 9.

Why the change?  Keep in mind that both pedals are electronically/FET-switched.  The output buffer is always on, no matter what the state of the pedal.  What that means is that whatever loading might improve the tonal quality of the drive sound, is also going to suck tone from the clean/bypass sound.  So, my thinking was that the 808-to-9 change in buffer values was to improve the bypass tone.

Looking at the MT10 schematic, I realized it had the exact same output buffer and component values as the TS-9.  A-HA!  And since the 9 only differs from the 808 by those two resistance values, I could use a DPDT toggle in the 6th hole to add in parallel resistances to drop the 470R and 100k down to 100R and 10K (or close enough).  Since I'm using a stompswitch, rather than FET switching, changes to the buffer ONLY affect the drive sound and not the bypass sound.

I'll be curious to learn if there is any audible difference.  I guess this makes the pedal an MTS-1008.

stallik

Re: Things to do with that extra hole
« Reply #1 on: June 07, 2021, 01:06:14 PM »
I have, in the past, filled such a hole with a pot (of any value) wired to nothing. I’ve then labelled it ‘mojo’ or ‘magic smoke’, stood back and listened to other guitarists wax lyrical about how effective it is. I’ve found it’s particularly effective if there are real wires going somewhere and even more so if I tag a shiny diode across a couple of lugs ;)
Insanity: doing the same thing over and over again and expecting different results. Albert Einstein

Vivek

Re: Things to do with that extra hole
« Reply #2 on: June 07, 2021, 01:24:33 PM »
« Last Edit: June 07, 2021, 01:50:36 PM by Vivek »

Elektrojänis

Re: Things to do with that extra hole
« Reply #3 on: June 07, 2021, 01:24:48 PM »
About that loading of the terminating resistor... Are you sure it would actually cut treble if the loading is "heavier"? It would be in parallel with the cable capacitance, so I think it would make the total output tiny bit quieter and move the cutoff tiny bit higher compared to maximum output.

The loading is usually thought to cut treble responce because signal from guitar pickups react like that. Passive pickups are inductive and loading down inductive source will cut treble. But is that an universal rule for all sources?

Vivek

Re: Things to do with that extra hole
« Reply #4 on: June 07, 2021, 02:10:12 PM »
Mark's comments on MT10 in 2006

A few things jump out about this particular pedal:

1) Uses a 2+2 diode complement in the gain/clipping stage.  As you might expect, it aims for more gain than a TS-9 does (x186 vs x118) to get closer to a higher clipping threshold produced by use of a 2+2 diode arrangement.

2) Uses a somewhat different passive 3-band EQ than one often sees on Ibanez and other pedals.  Bass control appears to be cut only, though the treble seems to be boost/cut type.

3) Second op-amp is uses as gain-recovery after tone controls rather than as part of active tone control as in other Ibanez pedals.

4) The clipping stage differs from the TS-9 in terms of rolloffs.  C5 and R8 give low-end rolloff around 268hz, compared to 720hz for TS-9.  Retention of more bottom is also why gain doesn't have to be substantially greater than TS-9 even though clipping threshold is effectively doubled by diodes.

A quick glance at the schem, and consideration of what we know from other pedals, suggests a bit more dynamics than a TS-9 and SD-1, not as much clipping as an SD-1, and a fuller less strident sound than a TS-9 or SD-1.  My guess is that the reason Parnell spoke so highly of it is because of these features.  Whether the CA3260 is a big player in all of this....who knows.  ut the general design approach seems interesting for those who like to fiddle with TS-9/SD-1 variations.


Mark Hammer

Re: Things to do with that extra hole
« Reply #5 on: June 07, 2021, 03:34:19 PM »
Thanks.  Forgot I had written that; especially 15 years ago!  Took me a while to get around to making one.

Phend

Re: Things to do with that extra hole
« Reply #6 on: June 07, 2021, 05:10:05 PM »
Take one of the pots value, divided by 2, and wire the two of them in series. Hence 2 pots equal to 1.
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iainpunk

Re: Things to do with that extra hole
« Reply #7 on: June 07, 2021, 06:21:11 PM »
you might be somewhat lucky if its an CA3160 since the strobe/compensation pin can be used for the output for a feedback loop so the gain can be controlled there. take note that the feedback loop you build has to terminate to the non-inverting input since there would normally be one phase inversion more due to the output stage you now bypass.
you can then use the cmos inverter open loop for natural soft clipping with the advantages a) of not having 5 leftover cmos inverters and b) you have an opamp in front which is way more versatile than another cmos inverter.

i have been experimenting with a simple but super great sounding overdrive using this trick. its tight, touch sensitive and raw sounding. you can play everything from Bo Diddley to Discharge with the gain cranked, while getting quite clean with edge of distortion sounds on the lower side of the gain control's settings.

cheers
« Last Edit: June 07, 2021, 06:24:49 PM by iainpunk »
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287m

Re: Things to do with that extra hole
« Reply #8 on: June 07, 2021, 06:31:22 PM »
Out of topic, im sorry

We know Son of screamer, but where Son of Mostortion?