EA trem below unity gain!

[Mugshot]

i recently built the EA Trem using ROG's schem and layout, http://www.home-wrecker.com/eatremolo.html. pretty neat! lacking some of the components needed to complete the circuit, i substituted parts i had on hand (later on that ).

ok, on to the debugging sop:

1.Name of the project: EA tremolo
2.http://www.home-wrecker.com/eatremolo.html
3.i followed ROG's PCB for it!
4. now here's where it gets nasty:

the schem:



heavy part substitutions here.

1) i used a 2n7000 mosfet. didnt have the bs170 specified in ROG's schem. pinout is correct (i think).
2) i lowered the 4.7K drain resistor off the 2n7000 to 3.3K.
3) i didnt have the required 180ohm Source resistor for the mosfet stage, i used a 100ohm instead. next value i have is 300ohm. hmmm.
4) the 68K and 120K resistors limiting the Depth control have been removed (jumpered), and used a 1Meg Depth pot.
5) i dont have a 15K resistor so i used a 10K off the 1uf caps.
6) i used 1uF electros, oriented correctly.

5. negative ground, positive supply

6. ok. it tremolos alright depth and rate pots respond as they should. can get choppy likewise. volume control does what it's supposed to. however, the output is below unity, there is volume drop. the LED blinks in response to the rate pot (nive feature!). but there is considerable volume loss, or more specifically there is high-end attenuation, the high frequencies are significantly cut.

i took measurements of the pins.

Q1 (2N7000)
D - 3.2V
G - 3.2V (hmmm)
S - 1.0V

Q2 (MPF102) - i forgot the readings!  silly me, will measure tonight.

Q3  (2N5088)
C - voltage swings! 3.0-4.0V, in the ballpark
B - .08V
E - 0.0V

battery and PSU used to power the circuit. it is already boxed btw. oh, it's the same PSU i use everytime, so im sure it's not busted.

thoughts:

a) im quite certain the oscillator section works alright, i dont know what lowering the 15K resistor (Q3) to 10K does to the effect. also, the use of electros for the 1uF caps, how does it affect the effect?

b) what about the 180ohm resistor? what effect does lowering/raising its value have on the circuit?

c) ive tinkered with the 100K bias control. no luck. what i noticed though is that the Gate voltage is always similar to the Drain voltage no matter how i adjust the bias. hmmm.

any help i'd really appreciate thanks!

[Mugshot]

to add: i used a 10K volume pot!  i didnt have higher value pots as of the moment. could this be why there is high end attenuation?

[MarcoMike]

to me this is the usual mosfet failure... it happened to me several times already... try substituting it and let us know!

[Mugshot]

to me this is the usual mosfet failure... it happened to me several times already... try substituting it and let us know!

hmmm *scratch chin* no matter how i bias Q1 using the 100K trimpot, always it's Gate voltage = Drain voltage. will substitute a 2n7000 tonight, after OT work. 

[CynicalMan]

The 10k pot is probably your problem. This stage has a output impedance of around 4.7k, so having the 10k to ground will reduce your signal by around 3.3 dB.

Edit: I see that you are using a 3k3 drain resistor. That means you are losing around 2.5 dB.

[cobra94]

I agree the 10K is the problem, try putting a 50K resistor between the lug and ground to see if that helps raise the volume.

Also, for the 180ohm source you can use two of your 300ohms in parallel to get 150ohms.

Cheers,
Peter

[Mugshot]

I agree the 10K is the problem, try putting a 50K resistor between the lug and ground to see if that helps raise the volume.

Also, for the 180ohm source you can use two of your 300ohms in parallel to get 150ohms.

Cheers,
Peter

as advised, i desoldered the mosfet tranny and installed a fresh one. guess what? charraaannn, it's fixed! marcoMike was right, it was a bad mosfet 

it can get pretty loud too. since there is no minimum depth resistor, the trem can also be used as boost with the depth pot at full CCW. idk, but with a bass guitar the boost isnt that pronounced. but with a six-string guitar, it can get loud.

as for the 180 ohm, does the lower value of 100ohm raise the gain of the mosfet stage? mine doesnt distort either, so  maybe it can stay?

i was too tired last night i had no time to wire in a new volume pot. it works as it should with the 10K last night, but i'll be replacing it with a 1Meg later this week. im thinking this will give me more highs and lower output impedance, no? im confusing myself 

[CynicalMan]

I agree the 10K is the problem, try putting a 50K resistor between the lug and ground to see if that helps raise the volume.

Also, for the 180ohm source you can use two of your 300ohms in parallel to get 150ohms.

Cheers,
Peter

as advised, i desoldered the mosfet tranny and installed a fresh one. guess what? charraaannn, it's fixed! marcoMike was right, it was a bad mosfet 

it can get pretty loud too. since there is no minimum depth resistor, the trem can also be used as boost with the depth pot at full CCW. idk, but with a bass guitar the boost isnt that pronounced. but with a six-string guitar, it can get loud.

as for the 180 ohm, does the lower value of 100ohm raise the gain of the mosfet stage? mine doesnt distort either, so  maybe it can stay?

i was too tired last night i had no time to wire in a new volume pot. it works as it should with the 10K last night, but i'll be replacing it with a 1Meg later this week. im thinking this will give me more highs and lower output impedance, no? im confusing myself 

If it sounds OK with 10k, I'd say leave it. I can't see any way increasing the pot's value would affect high end and a 1Meg pot would increase output impedance when it wasn't set at maximum.

[Mugshot]

I agree the 10K is the problem, try putting a 50K resistor between the lug and ground to see if that helps raise the volume.

Also, for the 180ohm source you can use two of your 300ohms in parallel to get 150ohms.

Cheers,
Peter

as advised, i desoldered the mosfet tranny and installed a fresh one. guess what? charraaannn, it's fixed! marcoMike was right, it was a bad mosfet 

it can get pretty loud too. since there is no minimum depth resistor, the trem can also be used as boost with the depth pot at full CCW. idk, but with a bass guitar the boost isnt that pronounced. but with a six-string guitar, it can get loud.

as for the 180 ohm, does the lower value of 100ohm raise the gain of the mosfet stage? mine doesnt distort either, so  maybe it can stay?

i was too tired last night i had no time to wire in a new volume pot. it works as it should with the 10K last night, but i'll be replacing it with a 1Meg later this week. im thinking this will give me more highs and lower output impedance, no? im confusing myself 

If it sounds OK with 10k, I'd say leave it. I can't see any way increasing the pot's value would affect high end and a 1Meg pot would increase output impedance when it wasn't set at maximum.

unity gain with mine (with the 10K pot) is around 12-1oclock. just to be sure, in case.

[Mugshot]

a follow-up to my original post:D 

a) im quite certain the oscillator section works alright, i dont know what lowering the 15K resistor (Q3) to 10K does to the effect. also, the use of electros for the 1uF caps, how does it affect the effect? Cheesy

b) what about the 180ohm resistor? what effect does lowering/raising its value have on the circuit?

*scratches chin*