 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
68.190.171.116
In Reply to: RE: You're making a bigger mistake posted by Cougar on January 22, 2015 at 21:58:59
Okay, great to know that you're willing to learn. Some posters come here feeling they're smarter than the people who made their equipment.
I don't know if there is anything such as "too low," other than DC. What you want to do is NOT stop any part of the AC that is musical. Anything below 20hz is inaudible and possibly harmful. In order to pass 20hz without excessive phase distortion, you need a corner at least ten times lower, so 2hz is reasonable, and even lower than that is not crazy. A lot lower than that may be excessive from a cost standpoint: cap cost increases pretty fast above 1uf.
One reason the manufacturer may have selected such a large value is that electrolytic cap ratings are notoriously optimistic and woefully inaccurate, and they change for the worse as the cap ages. A film cap will be much more accurately rated and its value will never change with age, so you can select a value closer to what you establish to be the optimum.
Almost any film cap will yield a sonic improvement, but the better cap you can afford, the more it will be improved. Seek a brand name metalized polypropylene at a minimum, but you don't need to go crazy to get improvement over an electrolytic. If you're interested in value and good sound, check out Sonicap from Sonic Craft or Obbligato from various suppliers. There are plenty of other commercial grade caps that might be perfectly adequate for your needs. If you seek the ultimate, then an expensive true film and foil cap is necessary. Cap value/sonic performance is a complex topic.
Peace,
Tom E
Follow Ups:
I'm wondering why we are assuming the load is 33k ohms?Is it possible that the 33k ohm resistor is in parallel with another resistance to ground?
With a tube, the grid is almost an open circuit. Should we assume the same about the base of a transistor or the + input of an opamp?
" Input impedance varies considerably with the circuit configuration shown in Figure below. It also varies with biasing. Not considered here, the input impedance is complex and varies with frequency. For the common-emitter and common-collector it is base resistance times β. The base resistance can be both internal and external to the transistor. For the common-collector: Rin = βRE
It is a bit more complicated for the common-emitter circuit. We need to know the internal emitter resistance rEE. This is given by:
rEE = KT/IEm
where:
K=1.38×10-23 watt-sec/oC, Boltzman's constant
T= temperature in Kelvins ≅300.
IE = emitter current
m = varies from 1 to 2 for Silicon
RE ≅ 0.026V/IE = 26mV/IEThus, for the common-emitter circuit Rin is Rin = βrEE"
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/23/15
that is what I measured at the lead after the input coupling cap. :)
Yes. I'm thinking that is the value of the resistor to ground that follows the coupling capacitor.
I don't think the effective impedance to ground of the first transistor can be measured with a ohm meter and I'm thinking that impedance would only show up once the amplifier is turned on and current is flowing through the first transistor.
If I'm right, that would answer why they used such a large input coupling capacitor.
You could go about this empirically. If you replace the 330uf cap with a 1uf cap and then play the amp. Use a signal generator and measure the -3db point.
You can then do the math to calculate the impedance.
If, as an example, with a 1uf cap you find the -3db point to be 159Hz then the input impedance is 1000 ohms.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
when I took the measurement I measured from the output of the input coupling cap to ground. So that where I got the 33kohms. I'm glad I came here for help! :)
connected to and after the input coupling cap of 330uF, followed by a cap and another resistor both the cap and resistor are connected to ground. I can't see the values due to them being very small in size. I will have to get a magnifying glass to see them.
I will try my best to draw out a schematic of what I see of the input with the Long tail. This may help me do the formula for this to get a better idea. I have seen a site that had that equation for working out the Re and what size cap to use.
here's the site below.
with the other cap and resistor that is connected to ground. Sorry for not being more clear on my description.
"Sorry for not being more clear on my description."
That's OK. The bottom line, the input impedance (as explained in your link) is complicated.
That's why just simply testing it with a 1uf cap might be the easy way to go.
Once the input impedance is calculated, you can then use whatever value cap gives you the -3db point you want.
A -3db point of 2Hz will give you 20Hz with no amplitude loss or phase shift.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
the value is 22uF. So with the value of 22uf this is what I got and hopefully this will be ok .
1/(2*Pi*33k*22uf) = 0.219hz so that X10 comes out to 2.19hz
Thank You for the help, I really appreciate it.
Post a Followup:
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
