Death by Audioo - Reverberation Machine

Glass_Hero · Post by **Glass_Hero** » 18 Oct 2015, 17:00

With a voltage buffer you have positive unity gain, the worst-case condition for stability.. especially if you put cap on Vref..

knutolai · Post by **knutolai** » 19 Oct 2015, 21:52

Is this a reply to my post? Maybe you misunderstood my drawing? A buffered reference voltage is pretty normal stuff, and quite stable as the reference voltage buffer has a very low impedance.

Glass_Hero · Post by **Glass_Hero** » 19 Oct 2015, 23:03

knutolai wrote:Is this a reply to my post? Maybe you misunderstood my drawing? A buffered reference voltage is pretty normal stuff, and quite stable as the reference voltage buffer has a very low impedance.

I've built/used voltage buffers before, mostly at work (industrial environment) and not so much on pedals.. if you are driving a "large" capacitive load they can oscillate, you have to check the datasheet for capacitive load stability.. if they oscillate, you might need a snubber circuit..

knutolai · Post by **knutolai** » 21 Oct 2015, 13:27

Well I've never had any issues with this and this is pretty standard practice used in numerous effect pedals. Could you show a example circuit where a buffered reference voltage would be problematic? I've never heard about this issue before, and I'm interested understanding it. I,m no electrical engineer...

Glass_Hero · Post by **Glass_Hero** » 21 Oct 2015, 23:16

knutolai wrote:Well I've never had any issues with this and this is pretty standard practice used in numerous effect pedals. Could you show a example circuit where a buffered reference voltage would be problematic? I've never heard about this issue before, and I'm interested understanding it. I,m no electrical engineer...

So the circuit was from work, i made a pic of the voltage reference buffer.. The issue was swapping to a less expensive opamp.. The guy running the project said they were equivalent opamps, both datasheets had similar information and numbers.. Well one opamp (the new "drop-in" replacement) had an oscillation when driving a 100nF cap, the original opamp had no problem driving the cap..

The circuit provided a reference voltage to a pressure transducer (Wheatstone bridge) and the return signal was fed into the remaining 3 opamps from the quad package into a roll-your-own instrument amplifier.. when i looked into the datasheets (the quad, dual, and single package) i found a section called capacitive load stability, where it stated about possible oscillations when driving large caps.. I also found enough information to prove they were not equivalent opamps.. Both opamps were precision opamps (like the tlc27m4) so when picking an amp look out for this..

I guess my point is all about opamp selection and misleading datasheets.. try to build this with a voltage buffer and see if you eliminate the circuit noise, or build it like this and measure the noise content then compare it to the noise content in the circuit with a voltage buffer..

I hope i wasn't too electrical engineer in my response..

Glass_Hero · Post by **Glass_Hero** » 22 Oct 2015, 22:02

Glass_Hero wrote:
knutolai wrote:Well I've never had any issues with this and this is pretty standard practice used in numerous effect pedals. Could you show a example circuit where a buffered reference voltage would be problematic? I've never heard about this issue before, and I'm interested understanding it. I,m no electrical engineer...
So the circuit was from work, i made a pic of the voltage reference buffer.. The issue was swapping to a less expensive opamp.. The guy running the project said they were equivalent opamps, both datasheets had similar information and numbers.. Well one opamp (the new "drop-in" replacement) had an oscillation when driving a 100nF cap, the original opamp had no problem driving the cap..

The circuit provided a reference voltage to a pressure transducer (Wheatstone bridge) and the return signal was fed into the remaining 3 opamps from the quad package into a roll-your-own instrument amplifier.. when i looked into the datasheets (the quad, dual, and single package) i found a section called capacitive load stability, where it stated about possible oscillations when driving large caps.. I also found enough information to prove they were not equivalent opamps.. Both opamps were precision opamps (like the tlc27m4) so when picking an amp look out for this..

I guess my point is all about opamp selection and misleading datasheets.. try to build this with a voltage buffer and see if you eliminate the circuit noise, or build it like this and measure the noise content then compare it to the noise content in the circuit with a voltage buffer..

I hope i wasn't too electrical engineer in my response..

In the TLC27m4 datasheet there is a section called Output Characteristics and Figure 41, this is situation i am describing.. It appears it cannot drive a capactive load bigger than few hundred pF without ringing or oscillation, this might not be good opamp to use as a voltage buffer..

jrfox92 · Post by **jrfox92** » 23 Oct 2015, 02:53

So, after actually giving a look over of the circuit board I noticed that you didn't include a 100k and 10k resistor.
One of them is probably for the LED, but I'm not sure what the other's for.
Also, is C12 what's hidden by the Blend pot?

Glass_Hero · Post by **Glass_Hero** » 23 Oct 2015, 23:21

jrfox92 wrote:So, after actually giving a look over of the circuit board I noticed that you didn't include a 100k and 10k resistor.
One of them is probably for the LED, but I'm not sure what the other's for.
Also, is C12 what's hidden by the Blend pot?

there is a 100k pull-down resistor off pin 14.. it is on my hand drawn schems but not on the computer drawn schem..

10k current limiting resistor for the LED.. the schem is a functional circuit only and does not include switching or LED status..

jrfox92 · Post by **jrfox92** » 24 Oct 2015, 01:21

Glass_Hero wrote:
jrfox92 wrote:So, after actually giving a look over of the circuit board I noticed that you didn't include a 100k and 10k resistor.
One of them is probably for the LED, but I'm not sure what the other's for.
Also, is C12 what's hidden by the Blend pot?
there is a 100k pull-down resistor off pin 14.. it is on my hand drawn schems but not on the computer drawn schem..

10k current limiting resistor for the LED.. the schem is a functional circuit only and does not include switching or LED status..

Yeah, okay, the 100k resistor is the one I was thinking of. I just wasn't in the mood to go and plot out the entire circuit board.

knutolai · Post by **knutolai** » 28 Oct 2015, 22:16

That's certainly possible. Would you like to toggle all the pots or just one of them? Looking at it now it should be possible to toggle all three at the same time with a 3pdt. I could draw up a dirty sketch for you if you like.

knutolai · Post by **knutolai** » 29 Oct 2015, 23:02

Here you go:
https://i.imgur.com/vkeHSkr.png

Couldn't figure out how to make the image smaller in the post so there's the link to it instead.

I've added POT lug numbers. Hopefully that makes it easier if you are used to wiring diagrams (I prefer schematics). To have a LED indicator and all three pots switchable you need a 4pdt switch. Don't think they are too uncommon. You might consider swapping the Blend pots for A20k and the Volume pots to A200k to preserve the current impedances. The Gain pot size does not need to be changed as only the one in use is "seen" by the rest of the circuit.

knutolai · Post by **knutolai** » 01 Nov 2015, 17:44

You would wire it up like this (looking at the lugs of the 4PDT foot-switch):

This is how I've numbered the Potentiometer Lugs:

tabbycat · Post by **tabbycat** » 05 Nov 2015, 03:59

got mine running good last night. it's damn tasty as a reverb when you get it into a sweet spot. am pretty happy with it. definitely lives up to the best of the demos i've heard. very big plate feel too it. when you unplug a jack with it switched on it sounds like someone dropping a safe down a set of stairs in a massive empty warehouse, a tone i'm particularly fond of. so am definitely boxing.

as to reservations, the gain control may as well be called 'mud'. or 'supercharged mud'. since it seems to instantly throw the thing into a muddy distorted racket unless you spend a lot of time tweaking both the other controls to try to recover the same volume and tonal territory. it can be overcome, but it is a fiddle. i can see why the above mod would be a really useful feature (am going to integrate it into my build). this has a handful of great tones, but unless you want to be on your hands and knees twiddling the controls for half your set, you may as well consider it a 'one tone per set' box. the above mod will give you at least two immediately accesible alternatives at the flick of the switch.

am also considering adding some kind of tonestack or filter to counter the muddy excesses of the gain control, but not entirely sure which type would most suit and where it would best work. any ideas?
am guessing the where is between the gain and volume pots, replacing the cap and resistor with a proper stack.
as to which type, maybe the swtc?

many thanks to everyone who contributed to this thread, especially glasshero for the schematic and tech info and knutolai for the ideas for mods.

i used alex's layout (thank you alex) at gfx and a long belton brick as recommended.

jrfox92 · Post by **jrfox92** » 06 Nov 2015, 03:36

tabbycat wrote:got mine running good last night. it's damn tasty as a reverb when you get it into a sweet spot. am pretty happy with it. definitely lives up to the best of the demos i've heard. very big plate feel too it. when you unplug a jack with it switched on it sounds like someone dropping a safe down a set of stairs in a massive empty warehouse, a tone i'm particularly fond of. so am definitely boxing.

as to reservations, the gain control may as well be called 'mud'. or 'supercharged mud'. since it seems to instantly throw the thing into a muddy distorted racket unless you spend a lot of time tweaking both the other controls to try to recover the same volume and tonal territory. it can be overcome, but it is a fiddle. i can see why the above mod would be a really useful feature (am going to integrate it into my build). this has a handful of great tones, but unless you want to be on your hands and knees twiddling the controls for half your set, you may as well consider it a 'one tone per set' box. the above mod will give you at least two immediately accesible alternatives at the flick of the switch.

am also considering adding some kind of tonestack or filter to counter the muddy excesses of the gain control, but not entirely sure which type would most suit and where it would best work. any ideas?
am guessing the where is between the gain and volume pots, replacing the cap and resistor with a proper stack.
as to which type, maybe the swtc?

many thanks to everyone who contributed to this thread, especially glasshero for the schematic and tech info and knutolai for the ideas for mods.

i used alex's layout (thank you alex) at gfx and a long belton brick as recommended.

If you're getting muddy distorted sounds from the Altitude control, I'd assume it's related to whatever chip you ended up getting. That or you're hitting that thing with some super low frequencies and maxing everything out.
I say that even though I've never seen one in real life, but based on the various videos out there, something doesn't sound right with that gain control.

BUT, if you want to get some interesting tones out of it you should throw in the Evil Filter's circuit minus the fuzz to see what you get.

Also, can you post whatever layout you used (be it vero or whatever).

albru80 · Post by **albru80** » 06 Nov 2015, 12:17

Here it is:

tabbycat · Post by **tabbycat** » 08 Nov 2015, 09:43

jrfox92 wrote:If you're getting muddy distorted sounds from the Altitude control, I'd assume it's related to whatever chip you ended up getting. That or you're hitting that thing with some super low frequencies and maxing everything out.
I say that even though I've never seen one in real life, but based on the various videos out there, something doesn't sound right with that gain control.

BUT, if you want to get some interesting tones out of it you should throw in the Evil Filter's circuit minus the fuzz to see what you get.

Also, can you post whatever layout you used (be it vero or whatever).

hey jrfox, good to hear from you.

it might be that i was playing from my mp3 player into it rather than a direct guitar input, though i kept my eq fairly flattish (bass boost off). just leaves me more hands free for twiddling and switching. also my guitar is in wiring surgery atm.
i used a b1m instead of a c1m gain pot (numbered as alex's layout) thinking that it would just work backwards, which is fine for testing. maybe that's relevant? will trace what i've got compared to the schematic. it's 100% correct (gain pot as mentioned) with the vero layout, double checked, and reverb end seems to work great. just boomy, like the dark-heavy setting in the demos but thick-thick. anyway, will check schemo to my build and report back.

re the evil filter, that's something i was seriously considering anyway. the dirt not convincing as far as my tastes go but the filter sweeps menacingly wide. just checked the schemtic and it looks a completely separate section, no shared ics or anything messy. re a vero, maybe just a case of stripping the fuzz components out of alex's layout at gfx and shuffling things around to save space and tidy up loose ends.

jrfox92 · Post by **jrfox92** » 08 Nov 2015, 18:56

tabbycat wrote: hey jrfox, good to hear from you.

it might be that i was playing from my mp3 player into it rather than a direct guitar input, though i kept my eq fairly flattish (bass boost off). just leaves me more hands free for twiddling and switching. also my guitar is in wiring surgery atm.
i used a b1m instead of a c1m gain pot (numbered as alex's layout) thinking that it would just work backwards, which is fine for testing. maybe that's relevant? will trace what i've got compared to the schematic. it's 100% correct (gain pot as mentioned) with the vero layout, double checked, and reverb end seems to work great. just boomy, like the dark-heavy setting in the demos but thick-thick. anyway, will check schemo to my build and report back.

re the evil filter, that's something i was seriously considering anyway. the dirt not convincing as far as my tastes go but the filter sweeps menacingly wide. just checked the schemtic and it looks a completely separate section, no shared ics or anything messy. re a vero, maybe just a case of stripping the fuzz components out of alex's layout at gfx and shuffling things around to save space and tidy up loose ends.

Using a linear pot instead of reversed logarithmic will change the dynamic in a HUGE way. I learned that when I switched out the gain control on my Colorsound Overdriver. It's important to figure out comparable values when you switch between sweep types.
It's possible that using an A1M pot would be closer to what you're looking for rather than the B1M, but I find that sticking what was originally used is important when it comes to Death by Audio anything.

With the Evil Filter, it's two completely separate circuits which are connected only by the voltage. So, you just just remove the fuzz components from the vero layout and it would work as just the straight up filter.
I don't really care for the fuzz on it because it's really bright and has a bit of that octave sound like the Soundwave Breakdown, but it works out alright with the filter, too.

storyboardist · Post by **storyboardist** » 07 Dec 2015, 19:44

Here's a PCB/perf layout for this. The Belton brick can be directly mounted to the board from the solder-side. Should fit in a 125B nicely.

Post by **audioguy** » 05 Jan 2016, 11:43

2 quick questions..
Has anyone got a source for the TLC27M4ALN? I've googled for a while and came up with just datasheets
Has anyone validated Storyboardists layout?

Dirk_Hendrik · Post by **Dirk_Hendrik** » 05 Jan 2016, 18:05

Use a TL074.

Death by Audioo - Reverberation Machine [traced]