Tore Seem Preamp
- Thread starter valco
- Start date
Help Support GroupDIY Audio Forum:
Just opening the gate and leaving it floating could cause odd behavior from stray leakage.valco said:
I might suggest lifting the diode(s) feeding the control voltage, so it will behave as if there is no signal and cut off the JFET.
JR
You have two different schematics, a simpler remedy is to lift the drain lead. That should take the JFET out of the audio path.valco said:
JR
Bo Deadly
Well-known member
As JR already said, unsoldering the "d" leg would deactivate it. Or just remove the transistor entirely if that's easier.valco said:
Just curious but why do you want to deactivate it? If it's causing undesirable effects, it's pretty easy to adjust the attack and / or threshold. If you just increased R10 from 51R to 100R or 220R maybe that would slow it down, put less stress on A2 (whih could be causing distortion in itself) and make the limiting much less audible. But of course then it wouldn't be a limiter and transients could clip. So it would be more like a compressor. If you're going to deactivate it, it doesn't hurt to experiment.
Or maybe it's broken and you think it's just a bad circuit but there's actually something wrong with it (like the zener was replaced with the wrong one).
HI!
Thanks for the reply, even though it's been a while.
I did a whole lot of work on the rack for the modules, but had a grounding issue and therefore it's been sitting for a while. Pulled it out now recently for a spring cleaning and begun working on these again.
It works fine to deactivate and remove the transistor, but the modules still have low headroom. I feel that they are cracking up to soon. Is that because the circuit is overloading the A2?
I also made a 20db pad on the inputs, but still they seem not to have enough headroom.
Thanks for the reply, even though it's been a while.
I did a whole lot of work on the rack for the modules, but had a grounding issue and therefore it's been sitting for a while. Pulled it out now recently for a spring cleaning and begun working on these again.
It works fine to deactivate and remove the transistor, but the modules still have low headroom. I feel that they are cracking up to soon. Is that because the circuit is overloading the A2?
I also made a 20db pad on the inputs, but still they seem not to have enough headroom.
Are you servicing NRK gear?
ruffrecords
Well-known member
Amplifier A1 in the limiter has a gain of of 62 times which is nearly 36dB. I expect under normal operation R1 and the FET form an attenuator which significantly reduces the overall gain. When you disconnect the FET the gain jumps up to 36dB so the headroom will certainly be compromised. If you are going to remove the limiter function then you probably need to replace the FET with a suitable resistor to bring the overall gain down to a sensible figues. What that figure should be is difficult to say because it depends on the gain of the mic pre stage that precedes it. If you have the schematic of that we could give you a better answer.
Cheers
Ian
ruffrecords
Well-known member
The answer depends on how much gain you want to have. There is a 1:5 transformer at the input which gives 14dB of gain. Let's assume we fix the output stage gain at 16dB giving us a total of 30dB without the input stage. If we want a maximum gain of 60dB then we need to arrange the frist stage to provide another 30dB of gain. You can work out the resistor values from that.
Cheers
Ian
Cheers
Ian
erik_DK
Well-known member
I just modified 3 of these:
Gain of input transformer is 14 dB
Gain of input amplifier is 15 - 27 dB, depending on position of gain-switch
Gain of second (limiter-) stage is 36 dB.
To turn off the limiter, I removed the FET and R6, R7 and R11.
To lower the gain of the second stage from 36 dB to 24 dB, I changed R3 from 1k to 4k12 (3k9 + 220R)
So now, instead of a gain range going from 46 - 76 dB, I have a range from 34 - 64 dB. This is a compromise, as I want to use the amps with my B&o BM6 ribbon microphones, which need lots of gain. It is also a fine range for many (most) dynamic mics. But I guess I will have to use a pad on the input if using a hot condenser mike.
Erik
Gain of input transformer is 14 dB
Gain of input amplifier is 15 - 27 dB, depending on position of gain-switch
Gain of second (limiter-) stage is 36 dB.
To turn off the limiter, I removed the FET and R6, R7 and R11.
To lower the gain of the second stage from 36 dB to 24 dB, I changed R3 from 1k to 4k12 (3k9 + 220R)
So now, instead of a gain range going from 46 - 76 dB, I have a range from 34 - 64 dB. This is a compromise, as I want to use the amps with my B&o BM6 ribbon microphones, which need lots of gain. It is also a fine range for many (most) dynamic mics. But I guess I will have to use a pad on the input if using a hot condenser mike.
Erik
Last edited:
erik_DK
Well-known member
erik_DK
Well-known member
looks really nice -
how to go about adjusting Threshold
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