1176 Build, Release knob affecting gain reduction without signal

Help Support GroupDIY:

ottosteer

Member
Joined
May 16, 2021
Messages
10
Location
San Jose
Just finished my second hairball 1176 build and I’m having an odd behavior. The unit passes signal and works great when the release knob is set to the fastest setting. Without a signal when i set the release slower, the VU meter starts to indicate reduction. It goes from zero and all the way to 20db at the slowest release setting and everything else in between. The gain reduction is not just the meter but actual GR as confirmed with input vs output signal. Any thoughts? I’ve checked the obvious, like values and voltages. Also calibrated and that went as planned. I am very perplexed.


View attachment IMG_8034.MOV
 

Philip_BlueFX

Active member
Joined
Feb 27, 2020
Messages
27
Location
Ελλάδα
Well hello mate, I came here looking an answer to the same question.

I was probing around the 1176 detection circuit and it seems to me that the Release pot can heavily affect the compression ratio.
It actually affects the control voltage to the FET's gate and it's starting point. From the circuit design it seems like this is normal behaviour since the release pot is directly tied to the Q' bias voltage which controls the starting point for the FET's control voltage.

Anyone can confirm this? I am curious on how much of a role it plays on the compression ratio. It looks to be affecting the threshold a lot.
 

ottosteer

Member
Joined
May 16, 2021
Messages
10
Location
San Jose
Well hello mate, I came here looking an answer to the same question.

I was probing around the 1176 detection circuit and it seems to me that the Release pot can heavily affect the compression ratio.
It actually affects the control voltage to the FET's gate and it's starting point. From the circuit design it seems like this is normal behaviour since the release pot is directly tied to the Q' bias voltage which controls the starting point for the FET's control voltage.

Anyone can confirm this? I am curious on how much of a role it plays on the compression ratio. It looks to be affecting the threshold a lot.
Is yours a hairball build as well?
 

dmp

Well-known member
Joined
Oct 28, 2009
Messages
3,095
Location
Madison, WI
Without a signal when i set the release slower, the VU meter starts to indicate reduction.
Doesn't sound right. With no signal, there shouldn't be any voltage changing the Qbias as the release is changed.
You might have oscillations? You could try grounding the upstream attack pot / diodes junction to see if this stops the behavior.

Release pot shouldn't change the ratio or threshold. The ratio switches do change the threshold when changing the ratio. The original URIE manual explains this well.

I would check everything on the ratio board, make sure the resistances are correct, and that the -10 v is arriving correctly to R64.
 

Gustav

Well-known member
Joined
Jun 4, 2004
Messages
2,222
Location
DK
Ive also had luck removing small leaks by replacing the 6.8uF tants in conjuction with the diodes to a different type, but Ive never seen more than 0.5dB movement or so on the full range of the release before replacement in those cases.

Gustav
 

Philip_BlueFX

Active member
Joined
Feb 27, 2020
Messages
27
Location
Ελλάδα
perhaps leaky fd333 diodes?

have you searched our giant help thread for similar problems and solutions?

/Jakob E.

I didn't manage to get my hands on some FD333s so I went with a couple of 4148s as someone stated in the help thread, temporarily.
Do you think this might be the issue? I was actually wondering what the effect would be of a non low-leakage diode as the 4148s to the circuit. Someone suggested using a FET like the BF245 (?) instead of the FDs, maybe Ill try that when I get the time and see what's cooking.
 

dmp

Well-known member
Joined
Oct 28, 2009
Messages
3,095
Location
Madison, WI
If you are using new 4148s it probably isn't the problem, assuming they aren't backwards or something.
Most likely there is a wrong component value somewhere.
Have you gone over every resistor again and checked all the wiring?
 

Philip_BlueFX

Active member
Joined
Feb 27, 2020
Messages
27
Location
Ελλάδα
Update:

I have just tried substituting the 4148s with BC546 BJTs and the result is amazing! I would love to know more about that, what's the issue with low-leakage diodes on this design?

Note: The BJTs make a good difference when the sidechain is triggered (signal goin to the peak detector) but if you disconnect, the release still messes with the FET's gate voltage. However, maybe I can do better with a different transistor.

OneMoreNote: By observation, my FET conducting point begins at about -3V gate voltage so that is where I have put my Q-bias point to be. Just above the limit. With no signal to the peak detector, the Release messes with this voltage quite a lot causing the FET to act ressistively (am I sayin that right? hope so).
 
Last edited:

gyraf

Well-known member
Staff member
Moderator
Joined
Jun 4, 2004
Messages
10,383
Location
Aarhus, Denmark
..you NEED the low-leakage type diodes in there, if there is ANY leakage, the release pot will fight the Q-bias setting iirc..

/Jakob E.
 
Top