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Effects Book

2/2/2000 7:11 PM
JPD	Effects Book Picked up a copy of "Music Projects" by R.A. Penfold a few weeks ago. There are some interesting circuits in here (no less than four tremolo designs, a pretty simple fuzz, a double-tracking unit, two treble boosters, a headphone enhancer, and some MIDI stuff) but its all British...he uses alot of what I've been told are internal trimpots. Any experience with these designs? Can I easily sub a regular pot for a trimpot?

2/2/2000 7:11 PM

JPD

Effects Book

Picked up a copy of "Music Projects" by R.A. Penfold a few weeks ago. There are some interesting circuits in here (no less than four tremolo designs, a pretty simple fuzz, a double-tracking unit, two treble boosters, a headphone enhancer, and some MIDI stuff) but its all British...he uses alot of what I've been told are internal trimpots. Any experience with these designs? Can I easily sub a regular pot for a trimpot?

2/2/2000 8:23 PM
Mark Hammer	"...he uses alot of what I've been told are internal trimpots. Any experience with these designs? Can I easily sub a regular pot for a trimpot?" Robert Penfold is a pretty prolific designer, and his name has graced the pages of many an interesting project for 2 decades or more. Personally, I find his projects to be too simple sometimes, and too complex at others. Not in terms of difficulty, but in terms of getting to the heart of the effect in an efficient and maleable manner. Sometimes too complicated for what it has to do, and other times not enough control to get the job done. To answer your question, use of trimpots is often, though not always, for set-and-forget purposes. For example, many delay devices require trimpots for setting the bias voltage for the delay chip. Once set, you want the trimpot setting to NEVER budge from there, so chassis mounting it is contraindicated. Other uses of this type may be setting the gain or input attenuation of the first stage to match the level of the signal source. The flexibility is nice to have, but you may find yourself setting it once and never having a need to adjust it after that. The inclusion of a trimpot is simply avoid the nuisance of having to identify the right fixed resistor value. Other trimpot uses are for controls that are deemed one-size-fits-all by the manufacturer, and are set for the "average user". For instance, many phase shifters have hidden resonance controls in trimpot form on the PC board. Several old Electro Harmonix devices have additional controls in trimpot form on the PC board, and only a single pot and switch on the chassis, simply because a pot, a knob, and assembly costs raise the selling price more than soldering a trimpot to a board. When it comes to kits, the cost factor plays less of a role, but it still surprises me how many magazine projects I've seen that place perfectly usable controls on the board as trimpots. At other times, trimpots are used in a combination of these two ways. I recently put together an EH Doctor Q from the posted schematic. It has a trimpot mounted on the board. About a third of the range of the trimpot provides real usable control over the effect, whereas the remainder either has no effect, or screws things up. Obviously, EH would prefer NOT to have to explain proper use of such a control to your average 16 year old string basher in 1978, and rather than have them turn the control too far and proclaim "This thing sucks", they elected to board mount it as a trimpot, adjust it for the average user, and forsake flexibility. Many phasers have control voltage bias trimpots that function the same way. You CAN chassis mount them as "real" pots, but have to be cognizant of the fact that less than 100% of their rotation will result in usable changes to the sound. This is the long way of saying that when it comes to DIY projects in magazines and books: 1) trimpots are more likely to be of the set and forget type, since no one is keeping tabs on assembly costs, 2) where a trimpot is not of the set and forget type, chances are pretty good that not all settings of the trimpot are useful, so chassis-mounting brings with it the obligation of prudent use of the control.

2/2/2000 8:23 PM

Mark Hammer

"...he uses alot of what I've been told are internal trimpots. Any experience with these designs? Can I easily sub a regular pot for a trimpot?"

Robert Penfold is a pretty prolific designer, and his name has graced the pages of many an interesting project for 2 decades or more. Personally, I find his projects to be too simple sometimes, and too complex at others. Not in terms of difficulty, but in terms of getting to the heart of the effect in an efficient and maleable manner. Sometimes too complicated for what it has to do, and other times not enough control to get the job done.

To answer your question, use of trimpots is often, though not always, for set-and-forget purposes. For example, many delay devices require trimpots for setting the bias voltage for the delay chip. Once set, you want the trimpot setting to NEVER budge from there, so chassis mounting it is contraindicated. Other uses of this type may be setting the gain or input attenuation of the first stage to match the level of the signal source. The flexibility is nice to have, but you may find yourself setting it once and never having a need to adjust it after that. The inclusion of a trimpot is simply avoid the nuisance of having to identify the right fixed resistor value.

Other trimpot uses are for controls that are deemed one-size-fits-all by the manufacturer, and are set for the "average user". For instance, many phase shifters have hidden resonance controls in trimpot form on the PC board. Several old Electro Harmonix devices have additional controls in trimpot form on the PC board, and only a single pot and switch on the chassis, simply because a pot, a knob, and assembly costs raise the selling price more than soldering a trimpot to a board.

When it comes to kits, the cost factor plays less of a role, but it still surprises me how many magazine projects I've seen that place perfectly usable controls on the board as trimpots.

At other times, trimpots are used in a combination of these two ways. I recently put together an EH Doctor Q from the posted schematic. It has a trimpot mounted on the board. About a third of the range of the trimpot provides real usable control over the effect, whereas the remainder either has no effect, or screws things up. Obviously, EH would prefer NOT to have to explain proper use of such a control to your average 16 year old string basher in 1978, and rather than have them turn the control too far and proclaim "This thing sucks", they elected to board mount it as a trimpot, adjust it for the average user, and forsake flexibility. Many phasers have control voltage bias trimpots that function the same way. You CAN chassis mount them as "real" pots, but have to be cognizant of the fact that less than 100% of their rotation will result in usable changes to the sound.

This is the long way of saying that when it comes to DIY projects in magazines and books:
1) trimpots are more likely to be of the set and forget type, since no one is keeping tabs on assembly costs,
2) where a trimpot is not of the set and forget type, chances are pretty good that not all settings of the trimpot are useful, so chassis-mounting brings with it the obligation of prudent use of the control.

2/3/2000 9:30 AM

Joe Fuzz

quote:
"Obviously, EH would prefer NOT to have to explain proper
use of such a control to your average 16 year old string basher in 1978, and rather than have them turn the control too far and proclaim "This thing sucks", they elected to board mount it as a trimpot, adjust it for the average user, and forsake flexibility. Many phasers have control voltage bias trimpots that function the same way. You CAN chassis mount them as "real" pots, but have to be cogniscent of the fact that less than 100% of their rotation will result in usable changes to the sound."

Just to get slightly off topic here, but I think you've summed up what it means to be a DIYer right there. I'm able to push my effects to the hairy edge, get that extra 10%, because I use a pot value that could never be put in a commercial effect because it's unusable for the last quarter turn. And I know I'm not alone on this! Probably everyone reading this can claim something similar.

Getting back to the subject of trimpots: I find I use them quite a lot while experimenting with a circuit on the breadboard but when I build the circuit, about half the time I'll replace the trimpots with a fixed value resistor.

2/3/2000 10:32 AM
paul perry	Firstly, on Penfold: A lot of his books are riddled with errors (especially his earlier electronic music stuff, one of mine even has an errata slip!) which is bad anywhere, but CRIMINAL for a book that beginners might use. Secondly, Mark & Joe are 100% right about 'difficult' controls. I manufacture an envelope follower that goes ...further... that you would want & while it's very nice indeed on the edge, we get lots of negative feedback (no pun intended) from people who cant get it to go in 3 seconds.. "no safety wheels" is the motto here at Frostwave BTW it's called the Funk-a-Duck!

2/3/2000 10:32 AM

paul perry

Firstly, on Penfold: A *lot* of his books are riddled with errors (especially his earlier electronic music stuff, one of mine even has an errata slip!) which is bad anywhere, but CRIMINAL for a book that beginners might use.
Secondly, Mark & Joe are 100% right about 'difficult' controls. I manufacture an envelope follower that goes ...further... that you would want & while it's very nice indeed on the edge, we get lots of negative feedback (no pun intended) from people who cant get it to go in 3 seconds.. "no safety wheels" is the motto here at Frostwave ;-)

BTW it's called the Funk-a-Duck!

2/4/2000 3:37 PM
JPD	I also appreciate a slightly "bad" sound now and again. As a beginner, I have little knowledge or skill to excecute the sounds in my head, but thats what reading and experimenting is for right? For example, I love the sound of a distortion box just as the battery begins to decay: in my trusty DOD 250, it gives a bit more compression, and a crackly breakup that I would one day love to be able to harness somehow. In addition, there is the sound of an overdriven mic input that I've never been able to find (on the Beatles' "Revolution" most notably). In the long run, like all of us who do this (to the detriment to our wallets and perhaps even relationships with friends, loved ones) is to sound unique, better than the next schlub in the band that goes on after yours. If I can't play like Duane Allman, I'd at least like to have good tone....a never-ending quest. So the bottom line on Penfold is to be cautious, I geuss.

2/4/2000 3:37 PM

JPD

I also appreciate a slightly "bad" sound now and again. As a beginner, I have little knowledge or skill to excecute the sounds in my head, but thats what reading and experimenting is for right?

For example, I love the sound of a distortion box just as the battery begins to decay: in my trusty DOD 250, it gives a bit more compression, and a crackly breakup that I would one day love to be able to harness somehow. In addition, there is the sound of an overdriven mic input that I've never been able to find (on the Beatles' "Revolution" most notably).

In the long run, like all of us who do this (to the detriment to our wallets and perhaps even relationships with friends, loved ones) is to sound unique, better than the next schlub in the band that goes on after yours. If I can't play like Duane Allman, I'd at least like to have good tone....a never-ending quest.

So the bottom line on Penfold is to be cautious, I geuss.

2/4/2000 4:15 PM
Mark Hammer	How to be cautious, and dying batteries Two points raised by and in your posting. 1) RG Keen has a schematic for an AC adaptor that can, to some extent, mimic "dying batteries", by being able to provide a regulated 7.5 volts. You can find it at GEOFEX. I say "to some extent", because dying batteries not only change with respect to voltage, but one assumes in terms of ability to deliver current as well. There was a (what else?) distortion project in Electronic Musician last year that deliberately used a current regulator to "starve" the distortion element. Perhaps that's what you want to mimic the battery conditions you need in a reliable manner. 2) A good procedure to follow when building magazine or book-based projects is to corroborate the schematic against the parts layout, the PC mask, and the parts list. Even in high quality publications, with respected authors, there are often little glitches that go unnoticed. These are often things like shifting decimals places in parts values (e.g., calling a cap 2u, instead of 2n) and orientation of polarized caps. The same kinds of things that show up in net postings. In the case of magazines, they can publish a correction next month. In the case of books, correcting information post hoc is considerably harder to do. I suppose one of these days, books purchased electronically will have the feature of automatically e-mailed errata/corrections, but until then WE need to make sure on our own. Case in point: Went to perfboard the ring modulator from Anderton's EPFM II the other night (Made one years ago, liked it, but sold it.) because I wanted to use a different op-amp package. When I looked at the PC-mask, I found traces connecting IC pins that were not in the drawn schematic.

2/4/2000 4:15 PM

Mark Hammer

How to be cautious, and dying batteries

Two points raised by and in your posting.

1) RG Keen has a schematic for an AC adaptor that can, to some extent, mimic "dying batteries", by being able to provide a regulated 7.5 volts. You can find it at GEOFEX.

I say "to some extent", because dying batteries not only change with respect to voltage, but one assumes in terms of ability to deliver current as well.

There was a (what else?) distortion project in Electronic Musician last year that deliberately used a current regulator to "starve" the distortion element. Perhaps that's what you want to mimic the battery conditions you need in a reliable manner.

2) A good procedure to follow when building magazine or book-based projects is to corroborate the schematic against the parts layout, the PC mask, and the parts list. Even in high quality publications, with respected authors, there are often little glitches that go unnoticed. These are often things like shifting decimals places in parts values (e.g., calling a cap 2u, instead of 2n) and orientation of polarized caps. The same kinds of things that show up in net postings. In the case of magazines, they can publish a correction next month. In the case of books, correcting information post hoc is considerably harder to do. I suppose one of these days, books purchased electronically will have the feature of automatically e-mailed errata/corrections, but until then WE need to make sure on our own.

Case in point: Went to perfboard the ring modulator from Anderton's EPFM II the other night (Made one years ago, liked it, but sold it.) because I wanted to use a different op-amp package. When I looked at the PC-mask, I found traces connecting IC pins that were not in the drawn schematic.

2/4/2000 4:48 PM

R.G.

quote:
"1) RG Keen has a schematic for an AC adaptor that can, to some extent, mimic "dying batteries", by being able to provide a regulated 7.5 volts. You can find it at GEOFEX.

I say "to some extent", because dying batteries not only change with respect to voltage, but one assumes in terms of ability to deliver current as well."

Actually, I did worry about the current delivery as well. That comes out as an increased internal resistance. The adapter shown at GEO also includes a series resistor to fake the increased internal resistance too.

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