Help With New Layout Glass Blower Booster Please

[midwayfair]

According to what you say Q3 is then necessary to maintain the pedal original nature right?

I still can't figure out why it seems to be on all the time and Jones suggestion seemed an easy way out....

The FET has no effect on the sound. It's being used as a switch. It's either 'a lot' of resistance when the pedal is on -- and the feedback loop resistance is controlled by the resistor in parallel -- or it's 'almost no' resistance as if you'd just shorted the op amp pins.

The other transistors performance RM a different function and have nothing to do with the bypass.

If removing the FET made your LED stop working, then you have a mistake somewhere. The LED's cathode is connected to ground in one position -- so it turns on -- or 9v iin the other -- so it turns off. The LED has nothing to do with the audio.

Q3 has three connections: the source and drain connect to the op amp pins, and the gate connects to a resistor and diode for the voltage to control the switching. None of that affects the LED.

Get the LED working a and then return to the bypass switching.

In the meantime, you only use two of the switch's lugs for mechanically switching. They're just shorting the lugs, instead of using the FETvto do the same function.

 

[sonolink]

Jon
Thanks for the concept explanation. It makes it easier to grasp things

The LED is now working without Q3 on the board. I apologise but I'm afraid it was the language barrier again.
The Switch is now connected to the vero board as it was before.
It's pinout is:

1 - free
2 - free
3 - free
4 - 9v
5 - connected to LED (-) and R12 (pad called Stomp SW2 on layout)
6 - ground

Could you please tell me what connections I should make?

Thanks a lot for your patience
Cheers
Sono

 

[midwayfair]

Jon
Thanks for the concept explanation. It makes it easier to grasp things

The LED is now working without Q3 on the board. I apologise but I'm afraid it was the language barrier again.
The Switch is now connected to the vero board as it was before.
It's pinout is:

1 - free
2 - free
3 - free
4 - 9v
5 - connected to LED (-) and R12 (pad called Stomp SW2 on layout)
6 - ground

Could you please tell me what connections I should make?

Thanks a lot for your patience
Cheers
Sono

Like this:

1 - free
2 - op amp pin 2
3 - op amp pin EDIT: 1
4 - 9v
5 - connected to LED (-) and R12 (pad called Stomp SW2 on layout)
6 - ground

If this works, then you'll know there was a problem with the JFET (it's why I didn't have you remove anything else that's part of the FJET switching).

 

[sonolink]

Hi Jon

I connected as you indicated pole 2 to TL072 pin2 and pole 3 to TL072 Pin3.

When the LED is off, the pot still acts as a booster controlling volume, when the LED is on there is no sound at all...

I've had another look at Merlin's  schem and the Jfet connects to pins 2 and 1 of the opamp. I didn't try to connect pole 3 of the switch to TL072 pin 1 just in case I fry something.
What do you reckon? If that's not a good idea any other suggestion?

Thanks again for your time and help

Cheers
Sono

 

[midwayfair]

Hi Jon

I connected as you indicated pole 2 to TL072 pin2 and pole 3 to TL072 Pin3.

When the LED is off, the pot still acts as a booster controlling volume, when the LED is on there is no sound at all...

I've had another look at Merlin's  schem and the Jfet connects to pins 2 and 1 of the opamp. I didn't try to connect pole 3 of the switch to TL072 pin 1 just in case I fry something.
What do you reckon? If that's not a good idea any other suggestion?

Thanks again for your time and help

Cheers
Sono

Sorry about that, the switch connects to 1 and 2.

 

[sonolink]

No problem Jon

It DOES work indeed now! And it sounds EXACTLY the same as the other 2 with Merlin's layout!

I've put it in the enclosure and the pot is behaving weird but I think it's because it's really tight in there so I'll have to recheck joints and spray some isolation.

Thanks A LOT for your help mate!!! I owe you a beer! 

One last question out of curiosity: if it works the same without Q3 why did Merlin put it in the circuit? Isn't it always better to keep it simple and the less components the purest?

Cheers
Sono

 

[midwayfair]

One last question out of curiosity: if it works the same without Q3 why did Merlin put it in the circuit? Isn't it always better to keep it simple and the less components the purest?

(a) It allows the use of a single-pole switch. Theoretically you could get a much better/heavier duty switch for the switching scheme. (b) Again, the FET has no effect on the sound, it's just an electronic switch. (c) I'm not sure why "pure" would even be a concern when you're blasting the front of the amp with an extra 8 volts and now distorting the first preamp tube as well as the second ... the whole point is to help the amp mangle the guitar sound.

I know you've already boxed it, but I think it might be worth it to track down what was causing the FET switching to fail, if nothing else than to learn where the mistake happened. Re-check the datasheet for your FET to make sure you had it oriented correctly and re-check the rows where it was sitting. Make sure you had the right FET with the right pinout. etc. Or do that on your next build.

 

[sonolink]

Thanks for the explanation

An electronical switch is the most understandable to me. My use of this pedal is not to blast the front end of the amp. It's a means to have a "louder channel" (like for solos) and having that dynamic range makes it possible to work on the lower part to avoid dostortion compression and coloration (not pushing it basically). The greatest thing I find about Merlin's pedal is that a) it doesn't change the sound at all b) it serves as a buffer and being transparent is great since you don't have to worry about true bypass.

I'll try to track down the problem but I'll do it on the next build. I'll build the original first and test it and  from there I'll introduce the mods.

I'd like to thank you again for your time and help and your knowledge. This is a great hobby for me (and also building tube amps) and I wish I had the time to learn from scratch properly the way electronics work
I find it SO amazing beautiful and magic and need to work more on the "point of view" (the way you look at things and understand them), the concept...

Thanks and I hope I can buy you that beer some time soon
Cheers
Sono

 

[Michael Tibes]

> make sense out of what the transistors are doing here

sonolink really should include a link to the Project Page.

http://www.valvewizard.co.uk/glassblower.html

We have 9V supply on the pedal-board. With losses we get maybe 2Vrms out of an effect box. Some tube amp inputs make one overload sound with 2V in but a deeper overload sound with 4V(!) in.

Merlin ties the power rails to a buffered (transistors Q1 Q2) copy of the signal to *double* the maximum signal voltage "possible with one 9V supply".

(Transistor Q3 selects unity-gain or up to 20:1 gain; a verse/solo switch.)

You can ask why not a 18V or 47V supply? Or why not just turn-up the amp? Well, guitar-pedal is a funny world, and there is a niche for this.

Thank you for this

I haven't heard the pedal, but stripping away the fet switch and the extension of the max output level it remains a booster with 2 TL072 opamp stages - which doesn't sound like a revolutionary idea to me. My gut says that soundwise this could maybe even  be done better with 2 x 9V, a single opamp stage  and a true bypass, but maybe I'm wrong? This would definetely make things a lot easier. Does the transistor part add anything desirable to the sound? I personally prefer true bypass, but that might just be my personal preference. If you don't want true bypass you could still keep the fet arrangement and have your bypass.
Thinking about it, what is the fet on resistance and how much gain does this really create while in 'bypass'? To have 0 dB gain it would need to be 0 ohms which certainly won't be reached. Maybe the 'magic' of the pedal is that it still boosts a bit while 'off' and since even a little bit louder is easily perceived as 'better', is is better....

Michael

 

[sonolink]

Hello Michael

My personal opinion is that the magic of this pedal is that it has no magic at all, meaning that it does NOT change the sound at all. When the pot is at the minimum position you can change the switch position and the sound does not change.

When you crank the pot up the volume increases BUT without changing the sound, without compression nor distortion until reaching all the way up (where it does change BUT it's SO loud that I think the idea is to use the first 3/4  range of the pot. I never go past 9-10 o'clock).
In addition since it does not change the sound it allows for a large chain of pedals.

That's MY idea of a Booster anyway. It respects the nature of my sound only increasing volume.

Cheers
Sono

 

[midwayfair]

My gut says that soundwise this could maybe even  be done better with 2 x 9V, a single opamp stage  and a true bypass, but maybe I'm wrong? This would definetely make things a lot easier. Does the transistor part add anything desirable to the sound?


Thinking about it, what is the fet on resistance and how much gain does this really create while in 'bypass'? To have 0 dB gain it would need to be 0 ohms which certainly won't be reached. Maybe the 'magic' of the pedal is that it still boosts a bit while 'off' and since even a little bit louder is easily perceived as 'better', is is better....

A few things: Two 9V batteries is in no way more convenient than a pair of transistors boostrapping the power rail. Most people use a 9V adapter with their pedals, not a battery. Especially DIYers, who rarely run their pedals off batteries at all.

The other option for 18V is a charge pump. Compared to a charge pump, this method has  higher current capability, costs a lot less, and doesn't produce switching noise (which isn't a problem in many situations, but it's still a bug that you avoid). It also requires less PCB space overall (though not by much).

As far as the FET resistance goes, no, it's not 0. There is a small amount of loss at each stage, though, so a couple hundred ohms of resistance (likely no higher than 200R with that FET) is probably putting you back at unity. But the "louder = better" comparison doesn't really matter here. The pedal is buffered in bypass, so the comparison in use is between the stage in buffer mode and the stage in booster mode. The only change is the severity of the low pass (from the feedback stabilization cap between 1 and 2) and the high pass boost (from the negative feedback cap to ground on the boost pot).

 

[Michael Tibes]

A few things: Two 9V batteries is in no way more convenient than a pair of transistors boostrapping the power rail. Most people use a 9V adapter with their pedals, not a battery. Especially DIYers, who rarely run their pedals off batteries at all.

The idea would be to simplify and to help someone finish a booster who is obviously struggling with the more complex version he's attempting to build. I'm running all my pedals on batteries and know many players who also do so (especially in studio sessions), but I sure see the point you make. To me there's no ultimate truth here, whatever get's you going 

Michael

 

[sonolink]

Out of curiosity since I've eliminated Q3, I don't need DE (d3 on schem) right?

Cheers
Sono