Question about coupling cap size

[Robert Twotone]

I am building a clone of the EAR834 phono amp and was wondering if it would affect the circuit negatively if I resized C3 from 0.15uf to 0.22uf?

0.15uf film caps are not as readily available is why I am asking this. If it will negatively affect the circuit then I will make up the value with two caps (0.1uf and a 0.047uf)

 

[merlin el mago]

It seems the coupling cap between stages, if you increase you will have more bass.

 

[GoatGuy]

I am building a clone of the EAR834 phono amp and was wondering if it would affect the circuit negatively if I resized C₃ from 0.15 μf to 0.22 μf?

You could use larger. But I don't see that they're particularly rare, 0.15 μF: ECW-FD2W154KQ Panasonic | Mouser

45¢ ea, quantity 1

GoatGuy

 

[JMFahey]

In any case , I very very very much doubt you can hear an y difference .
make that even *measure* any difference , in the audible band.
.15 + 2M means 0.53 Hz crossover frequency.
.22 + 2M means 0.36 Hz.

Maybe a whale can sense some difference, and even so I doubt it.

That said, you have been kindly suggested a supplier in the post above

 

[TonyTecson]

if you have the 220nf cap on hand use it, you may also want to try 100nf as well,
heck even a 47nf will work just fine in that position...

look at the resistors used, i wonder why the use of highish coupling caps to begin with...

rp//R1//R8 will not be so low to use such a big cap...

 

[6A3sUMMER]

The saving grace is that the negative feedback network has a 330pF in series driving the rest of the RIAA parts. 330pf does not load a 0.15uf which is driving the 2 Meg Ohm and grid.

If C6 had been much much larger, then the 750k Ohm would have raised the low frequency pole versus the 2 Meg Ohm.

 

[Hearinspace]

Just curious about the circuit and what the voltage on the input 12AX7 will be. How much current does it draw? With 500KΩ in series with the plate circuit it sure can't be much.

 

[TonyTecson]

universal gain formula, Av =gm*Rl, or Rl/rk

if assuming 100 volts plate, then plate current is 280/500k, or 0.56mA
what is gm at cathode current of 0.56mA?
add the 1/gm to the 2.2k cathode resistor to get the total rk,
the Rl = rp//330k//2M

 

[Hearinspace]

Well if Vp=100V then there won't be 280V to work in the formula but I get it. Somewhere between 50 and 100 volts at less than 1mA. I was mostly interested in how low a voltage rating the OP could use for his coupling cap. Low voltage rated film caps can be a lot cheaper.
Looks like the OP has more than a few possibilities to choose from.

 

[TonyTecson]

ah, when it comes to voltage ratings of coupling caps, there is 400 or 600 volts available to use....i based mine on the B+ so if it is 280v, then not lower than 250 volts dc caps which are available..

 

[spaceistheplace]

Well if Vp=100V then there won't be 280V to work in the formula but I get it. Somewhere between 50 and 100 volts at less than 1mA. I was mostly interested in how low a voltage rating the OP could use for his coupling cap. Low voltage rated film caps can be a lot cheaper.

Why such a low operating point in the first place would my question. Seems like different tube choices could yield quite different results, being in a less linear range if I understand correctly.

Surely tdp was thinking when he designed this. He chose these values intentionally. He is not an amateur designer and has a noted background. It is also quite a well-liked design.

What is the logic to unpack from this?

How would you approach bringing this design into more conservatively operating territory?

 

[anatech]

Just use the 0.22uF if you have it and smile. It's close to the design value and you may even see a production change to the 0.22uF just to keep the number of skews down on parts.

Anyway, not enough of a difference to be worried about.

-Chris

 

[6A3sUMMER]

C3 Voltage rating?
Think Reliability.
Tubes are cold, then B+ is applied; or U1 dies; or U1 is pulled out.
The B+ at the plate of U1 may be 250V or more, and C3 will see all of that.
Be conservative and potentially save a capacitor.

 

[TonyTecson]

Why such a low operating point in the first place would my question. Seems like different tube choices could yield quite different results, being in a less linear range if I understand correctly.

Surely tdp was thinking when he designed this. He chose these values intentionally. He is not an amateur designer and has a noted background. It is also quite a well-liked design.

What is the logic to unpack from this?

How would you approach bringing this design into more conservatively operating territory?

even at a 100 volts plate, you have potential of 50 volts peak to peak, a 25 volt peak or 18 volt rms, the second stage may not even eat 1 or 2 volts of input swing....