tardishead
Well-known member
Hi guys
I am finishing a Fairchild style comp for myself after making a few units for friends over the years which have all worked extremely well.
Typically when setting up my own unit I came across a problem I have not experienced before.
Basically AC audio signal is appearing on the control voltage line after the bridge rectifier.
It is particularly present from low bass to 1khz and then attenuates up to nothing at 10khz.
It completely messes with the limiting effect and there is audible distortion on the faster settings.
On the time constant switch when I switch in larger capacitors only then the AC bleed is flattened.
Its been doing my head in for a few days. I have rechecked my build over and over paying particular attention to the CV line after the control amp transformer and to the grids of the vari mu tubes.I was convinced I had made a mistake.
Now I did mention that I have built this design and works perfectly. 3 times before.
And I suddenly realised that the only difference with the current one is that all previous units have had the in/out transformers mounted on the back of the unit with holes connecting wiring to the inside electronics. On this one I mounted the control amp transformers on the INSIDE for aesthetic purposes. They are open frame Edcor types.
I suddenly thought could the CV bleed be coming from the control amp transformers themselves.
My CV wiring to the Time Constant switch on the front panel is not screened as I have never found a need to do this before. But like I said the CA transformers I have always previously mounted the on the outside.
Could the CV wiring be acting as a large aerial and picking up the field of those transformers?
Could this be the source of the problem?
Or is the positioning of the control amp output transformers on the inside of the unit (pretty close to the signal amp input circuitry and transformer) more of a obvious problem?
I was going to try wiring the cv with screened cable but wanted to just get some insights before I did so.
Is normal unbalanced ok?
Or should I use twisted pair?
I am finishing a Fairchild style comp for myself after making a few units for friends over the years which have all worked extremely well.
Typically when setting up my own unit I came across a problem I have not experienced before.
Basically AC audio signal is appearing on the control voltage line after the bridge rectifier.
It is particularly present from low bass to 1khz and then attenuates up to nothing at 10khz.
It completely messes with the limiting effect and there is audible distortion on the faster settings.
On the time constant switch when I switch in larger capacitors only then the AC bleed is flattened.
Its been doing my head in for a few days. I have rechecked my build over and over paying particular attention to the CV line after the control amp transformer and to the grids of the vari mu tubes.I was convinced I had made a mistake.
Now I did mention that I have built this design and works perfectly. 3 times before.
And I suddenly realised that the only difference with the current one is that all previous units have had the in/out transformers mounted on the back of the unit with holes connecting wiring to the inside electronics. On this one I mounted the control amp transformers on the INSIDE for aesthetic purposes. They are open frame Edcor types.
I suddenly thought could the CV bleed be coming from the control amp transformers themselves.
My CV wiring to the Time Constant switch on the front panel is not screened as I have never found a need to do this before. But like I said the CA transformers I have always previously mounted the on the outside.
Could the CV wiring be acting as a large aerial and picking up the field of those transformers?
Could this be the source of the problem?
Or is the positioning of the control amp output transformers on the inside of the unit (pretty close to the signal amp input circuitry and transformer) more of a obvious problem?
I was going to try wiring the cv with screened cable but wanted to just get some insights before I did so.
Is normal unbalanced ok?
Or should I use twisted pair?