- Joined
- Feb 3, 2012
- Messages
- 73
aaquilato said:Has anyone built this kit?
Matador said:I find it almost impossible to believe that C2 is 47uF. C2 forms a LPF with R2, which is probably 100M or higher. In order to get a 3dB frequency of 1Hz, only about 1500pF is needed. The ELA M251 standard value here is about 5000pF/5nF. 47nF would give -3dB point of 0.03Hz which seems like overkill, but probably what was intended. 4.7nF is likely what would be specified, so perhaps you misplaced a decimal point?
ValveTone said:I have built this kit. I had the same 47uf electrolytic in my bag of parts and I suspect it was there to be used if you decided to cathode bias the circuit and be used as a cathode bypass capacitor. Or it could have been a mistake
Matador said:I find it almost impossible to believe that C2 is 47uF.
trans4funks1 said:The schematic you posted in the OP says C2 = 47nf. The original C12 Schematic shows 5000pf, which is a.k.a. 5nf, in the same position. I think Matador's suggestion that 4.7nf is what is intended seems like a most likely answer.
My guess is that the schematic linked to in the OP has a typo and is missing a "." where it says "47nf", but it seems like contacting AMI would be the easiest way to find out exactly what they intended.
aaquilato said:trans4funks1 said:The schematic you posted in the OP says C2 = 47nf. The original C12 Schematic shows 5000pf, which is a.k.a. 5nf, in the same position. I think Matador's suggestion that 4.7nf is what is intended seems like a most likely answer.
My guess is that the schematic linked to in the OP has a typo and is missing a "." where it says "47nf", but it seems like contacting AMI would be the easiest way to find out exactly what they intended.
Well, I have reached out to Oliver and various AMI associates but no answer.
I've started the build and used a 47nf (.047uf) Vishay Film cap per the circuit layout. I wouldn't use a electrolytic even if the right value had been supplied. Because this cap is "sandwiched between the two boards" once I finish this build changing it will be kind of difficult.
If I've got it correct, C2 is the decoupling capacitor which helps remove noise in the HT supply that may reach the capsule. So is 47nf going to be too high?
Yes, I am a noob with no EE background!
Thanks for your replies. They all help.
moamps said:Matador said:I find it almost impossible to believe that C2 is 47uF.
There are two main reasons why the C2 isn't 47uF electrolytic for sure:
- electrolytics have low insulation resistance (high leakage current)
- If we assume that the time for fully capacitor charging is 5RC, it will take more than 10 hours to change polar characteristic )
My guess is that the schematic linked to in the OP has a typo and is missing a "." where it says "47nf", but it seems like contacting AMI would be the easiest way to find out exactly what they intended.
aaquilato said:OK. Matador this is beyond me. I thought this cap dealt with noise. What has that got to do with changing the pattern?
Matador said:aaquilato said:OK. Matador this is beyond me. I thought this cap dealt with noise. What has that got to do with changing the pattern?
It is a LPF for the polarization voltage. The problem here is that even tiny disturbances on the capsule will happily couple to the grid and be amplified. So the capsule needs very clean DC to be quiet. This backplate voltage is what decides the pattern.
But having a very low pole in the LPF (formed from the 100M resistor and C2) also means the circuit is slower to respond to voltage changes. When you change from omni (0V on the backplate) to figure 8 (120V on the backplate), that cap has to charge from 0V to 120V, but it has to do so through that massive 100M resistor, which has only a tiny conductance.
So yes, it takes about 30 seconds (or about five RC time constants) to change with 47nF. But the noise filtering is better: the stock value of 4.7nF tries to strike a balance between speed of pattern changes and noise immunity.
aaquilato said:Matador said:I find it almost impossible to believe that C2 is 47uF. C2 forms a LPF with R2, which is probably 100M or higher. In order to get a 3dB frequency of 1Hz, only about 1500pF is needed. The ELA M251 standard value here is about 5000pF/5nF. 47nF would give -3dB point of 0.03Hz which seems like overkill, but probably what was intended. 4.7nF is likely what would be specified, so perhaps you misplaced a decimal point?
Thanks Matador. A 47uf electrolytic was supplied in the kit I got. As you said, it probably was a mistake. I do have a 47nf PIO I will try to fit in.
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