J. Rockett Audio Designs - Lemon Aid  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
User avatar
TWSpedals
Breadboard Brother
Information
Posts: 53
Joined: 30 Dec 2020, 23:48
Has thanked: 18 times
Been thanked: 51 times

Post by TWSpedals »

Had one of these in for repair today.

Traced it out and found it had a very interesting current mirror topology. I'm not going to pretend to know what exactly is going on here but it's probably the most original gain stage in a pedal I've seen in a while. It's worth noting that there is no filter cap or polarity protection at all.

The pedal is loud, very clean, very stiff, and a little sterile which makes sense with the hifi/SS amp theory being utilised here. I don't find the cut switch useful at all. The pedal feels at home to me pushing an amp or drive pedal.

Have a nice day, Tom.
Attachments
Schematic I drew.
Schematic I drew.
Unit I traced
Unit I traced
Have a nice day!

User avatar
gigelmargel
Resistor Ronker
Information
Posts: 330
Joined: 16 Jul 2007, 20:57
Has thanked: 111 times
Been thanked: 23 times

Post by gigelmargel »

Hello, Tom!

Looking at the schematic, I am not sure that I understood the connections of the boost switch (I am not very good at reading schematics).
Please, can you write the numbers of the boost switch pins on the schematic? Usually, I saw that the pins of this kind of switch are numbered in this way:
1 - 4
2 - 5
3 - 6

I assume that in one position only the 1.5uF is connected, in the middle position no cap is connected and in other position only the 10uF is connected, but I am not sure (it seems to me that the 10uF is connected in parallel with the 10uF).

Thank you very much!

User avatar
TWSpedals
Breadboard Brother
Information
Posts: 53
Joined: 30 Dec 2020, 23:48
Has thanked: 18 times
Been thanked: 51 times

Post by TWSpedals »

gigelmargel wrote: 10 May 2021, 09:07 Hello, Tom!

Looking at the schematic, I am not sure that I understood the connections of the boost switch (I am not very good at reading schematics).
Please, can you write the numbers of the boost switch pins on the schematic? Usually, I saw that the pins of this kind of switch are numbered in this way:
1 - 4
2 - 5
3 - 6

I assume that in one position only the 1.5uF is connected, in the middle position no cap is connected and in other position only the 10uF is connected, but I am not sure (it seems to me that the 10uF is connected in parallel with the 10uF).

Thank you very much!
My mistake. This is the actual layout. I don't know why they put the 10u and 1.5u in parallel, it's not much of a jump up in value from 10u
Attachments
Correct boost switch wiring.
Correct boost switch wiring.
Have a nice day!

User avatar
gigelmargel
Resistor Ronker
Information
Posts: 330
Joined: 16 Jul 2007, 20:57
Has thanked: 111 times
Been thanked: 23 times

Post by gigelmargel »

Thank you very much, Tom! It's very clear now and as I supposed :)
I think that I will use a simple 1P3T instead of 2P3T because, as you said, there is a small difference between 10uF and 11.5 uF.

User avatar
TWSpedals
Breadboard Brother
Information
Posts: 53
Joined: 30 Dec 2020, 23:48
Has thanked: 18 times
Been thanked: 51 times

Post by TWSpedals »

That's what I'd do too. Personally I'd remove the cut switch and bias the first stage with just 1Meg but that's me.
Have a nice day!

User avatar
bmxguitarsbmx
Cap Cooler
Information
Posts: 510
Joined: 25 Mar 2010, 21:15
Has thanked: 704 times
Been thanked: 216 times

Post by bmxguitarsbmx »

Pretty cool circuit! My hat off to a company trying something.

User avatar
gigelmargel
Resistor Ronker
Information
Posts: 330
Joined: 16 Jul 2007, 20:57
Has thanked: 111 times
Been thanked: 23 times

Post by gigelmargel »

I finished my clone, but it's not working. Only a strong synth-like squelch. All pots and switches seems to work. I used 2N5484 instead of 2N5485 (no chance to find ones here). This could be a problem?

User avatar
TWSpedals
Breadboard Brother
Information
Posts: 53
Joined: 30 Dec 2020, 23:48
Has thanked: 18 times
Been thanked: 51 times

Post by TWSpedals »

gigelmargel wrote: 11 May 2021, 12:26 I finished my clone, but it's not working. Only a strong synth-like squelch. All pots and switches seems to work. I used 2N5484 instead of 2N5485 (no chance to find ones here). This could be a problem?
They do have different VGSoff value ranges so that may be an issue. If there's any external DC on the input that may also be a factor (maybe use a decoupling cap to be sure for now. I'll double check the schematic as I still have the pedal with me, though I don't think there's any errors beyond the boost switch one I corrected.
Have a nice day!

User avatar
gigelmargel
Resistor Ronker
Information
Posts: 330
Joined: 16 Jul 2007, 20:57
Has thanked: 111 times
Been thanked: 23 times

Post by gigelmargel »

I have some doubts regarding the Cut switch: in one position the resistance from gate of 2N5485 to ground is 10K and in the other one is around 100K. Both are much lower than the value of 1M that I usually saw in other schematics (but it's possible that I am wrong, because I don't have many knowledges about electronics).

If you can and/or if you have time, maybe you can post the voltages of the transistors. Thank you very much!

User avatar
TWSpedals
Breadboard Brother
Information
Posts: 53
Joined: 30 Dec 2020, 23:48
Has thanked: 18 times
Been thanked: 51 times

Post by TWSpedals »

gigelmargel wrote: 11 May 2021, 20:05 I have some doubts regarding the Cut switch: in one position the resistance from gate of 2N5485 to ground is 10K and in the other one is around 100K. Both are much lower than the value of 1M that I usually saw in other schematics (but it's possible that I am wrong, because I don't have many knowledges about electronics).

If you can and/or if you have time, maybe you can post the voltages of the transistors. Thank you very much!
Cut switch is correct, I'm aware it's not the standard values for a JFET gate, as I said I don't see much point in it.

Voltages for the FETs are as follows
FET 1: Gate = 0V, Source = 2.5V, Drain = 8.6V
FET 2: Gate = 0V, Source = 1.9V Drain = 8.5V
Have a nice day!

User avatar
bmxguitarsbmx
Cap Cooler
Information
Posts: 510
Joined: 25 Mar 2010, 21:15
Has thanked: 704 times
Been thanked: 216 times

Post by bmxguitarsbmx »

Series feedback amplifiers are tough to bias up sometimes. I'm almost thinking these would need to be matched FET's. Though, getting the right part number may get you close enough.

What is the voltage at the output transistors (2n3906)?

The FET drains will be locked at essentially the forward voltage of the 2n3906. (Supply voltage - (diode drop + the emitter resistor voltage drop))

User avatar
gigelmargel
Resistor Ronker
Information
Posts: 330
Joined: 16 Jul 2007, 20:57
Has thanked: 111 times
Been thanked: 23 times

Post by gigelmargel »

TWSpedals wrote: 12 May 2021, 15:24
gigelmargel wrote: 11 May 2021, 20:05 I have some doubts regarding the Cut switch: in one position the resistance from gate of 2N5485 to ground is 10K and in the other one is around 100K. Both are much lower than the value of 1M that I usually saw in other schematics (but it's possible that I am wrong, because I don't have many knowledges about electronics).

If you can and/or if you have time, maybe you can post the voltages of the transistors. Thank you very much!
Cut switch is correct, I'm aware it's not the standard values for a JFET gate, as I said I don't see much point in it.

Voltages for the FETs are as follows
FET 1: Gate = 0V, Source = 2.5V, Drain = 8.6V
FET 2: Gate = 0V, Source = 1.9V Drain = 8.5V
Thank you, Tom!

My voltages are the following (bolded):
Battery: 8.85 V
FET 1: Gate = 0V / 0V, Source = 1.45V / 2.5V, Drain = 7.83V / 8.6V
FET 2: Gate = 0V / 0V, Source = 1.87V / 1.9V, Drain = 7.87V / 8.5V

My voltages seems to be closed to yours, excepting the source of FET 1. I tested and the squelch is present at the source of FET 1, so maybe I will find the error :)

User avatar
gigelmargel
Resistor Ronker
Information
Posts: 330
Joined: 16 Jul 2007, 20:57
Has thanked: 111 times
Been thanked: 23 times

Post by gigelmargel »

I wrote on the schematic all my voltages:
Lemon Aid voltages.jpg

User avatar
gigelmargel
Resistor Ronker
Information
Posts: 330
Joined: 16 Jul 2007, 20:57
Has thanked: 111 times
Been thanked: 23 times

Post by gigelmargel »

bmxguitarsbmx wrote: 12 May 2021, 17:01 I'm almost thinking these would need to be matched FET's. Though, getting the right part number may get you close enough.
I have no chance to find 2N5485 here :(
I will search in datasheets for a closer equivalent.

User avatar
TWSpedals
Breadboard Brother
Information
Posts: 53
Joined: 30 Dec 2020, 23:48
Has thanked: 18 times
Been thanked: 51 times

Post by TWSpedals »

The voltages at the collectors of the 2nd and 4th 2n3906 transistors in the unit I traced are as follows:

Q3: 5.75V
Q6: 4.3V

Hope this helps you. Though I do fear that it's a case of starting with the right FETs and going from there.
Have a nice day!

User avatar
gigelmargel
Resistor Ronker
Information
Posts: 330
Joined: 16 Jul 2007, 20:57
Has thanked: 111 times
Been thanked: 23 times

Post by gigelmargel »

TWSpedals wrote: 13 May 2021, 13:35 The voltages at the collectors of the 2nd and 4th 2n3906 transistors in the unit I traced are as follows:

Q3: 5.75V
Q6: 4.3V

Hope this helps you. Though I do fear that it's a case of starting with the right FETs and going from there.
It seems that you are right, Tom :(

Thank you very much for your help!

User avatar
gigelmargel
Resistor Ronker
Information
Posts: 330
Joined: 16 Jul 2007, 20:57
Has thanked: 111 times
Been thanked: 23 times

Post by gigelmargel »

I replaced the 2N5484 with other FETs (BF256 and 2SK30A), but with no luck. I got the same squelch. I recheck all for 2 times and it seems to be OK.

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1672 times
Been thanked: 1344 times

Post by Manfred »

I ran a simulation with some JFET types, only the 2N4221 and the 2N3822 came with the DC voltage values close to the read values with the 2N5485,
the diffrences were about 15% to 20%. Unfortunately, the two types are difficult to get and if then at very high prices.
There seems to be no suitable replacement type for the 2N5485.

User avatar
TWSpedals
Breadboard Brother
Information
Posts: 53
Joined: 30 Dec 2020, 23:48
Has thanked: 18 times
Been thanked: 51 times

Post by TWSpedals »

Have you considered depletion mode MOSFETs like the LND150?
Have a nice day!

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1672 times
Been thanked: 1344 times

Post by Manfred »

I just ran another simulation with the BF256 you have ruled out and found that the BF256B gives the same DC voltage values as the 2N5458.
BF256A and BF256C did not work.
Due to the large variations of the JFETs even in the same selection group, one must select the appropriate types.

Post Reply