Audio Asylum Thread Printer
Get a view of an entire thread on one page
|For Sale Ads|
In Reply to: RE: Pi filter resistors posted by Cousin Billy on January 02, 2016 at 04:46:47
Ralph uses very high quality resistors. The best you could choose would be Mills non-inductive wire wound or the Dales that I believe are used in your amplifier. Changing to Schottky diodes will raise the DCV by a few volts, which is trivial, but removing the resistors would diminish the filtering that comes after rectification. The two things work together to give you good DC. Furthermore, I challenge anyone to tell me he can "hear" the resistor in a pi filter, so long as its up to the job of carrying the requisite current, which is why Ralph recommends 1W rating.
Cousin, If you really want to get adventurous, you could try using an inductor in place of the 1K resistor. The problem you may encounter if you want to do that will be lack of space, but perhaps you can identify a physically small inductor that would work. Note also that since the inductor will probably have a DC resistance less than 1000 ohms, this will cause a very slight rise in voltages delivered to the circuit, probably not enough to throw anything out of whack, especially since physically small inductors tend to have relatively high-ish DC resistance, albeit probably not more than a few hundred ohms. I realized that I kept saying nay to all your ideas for tweaks, and this goes against my own sense of adventure when it comes to tweaking.
Thank you Lew
Your recommendation of the inductor is fabulous. This is the type of information I'm looking for.
I have lots of room, they will fit somewhere.
Now the hard part. What value mH? I obviously want high Rdc. This means really small guage wire inductors. Is this a 'choke'? application?
Thanking you in advance.
I'm so sorry, but we're cousins and should not get married.
You don't want "mH" (millihenries); you want Henries of value. Between 5H and 10H should be fine. I actually have some small-size 7H inductors with a DCR of several hundred ohms. Those would work. You need to know approximately how much current you are drawing. The inductor must be rated so as to tolerate the current across it. Here is where I am leary of asking you to do something potentially dangerous, but if you can safely measure the voltage across the 1K resistor, the calculation is done using Ohm's Law, V = current X resistance. In this case, R is known to be 1K and V is equal to the V on one lead of the resistor minus V on the other lead. Having said all that, based on my own familiarity with the circuit, probably a 100mA rated inductor is fine, overkill in fact. Check the Hammond catalog.
is just making sure that the power supply looks like a straight line on the 'scope.
If you decide to install a choke instead, before drilling any holes fire the amp up and put a high level signal through the voltage amplifier (amp on Standby IOW) and see what sort of noise is present on the filter caps feeding the voltage amp, using your choke instead of the resistor.
I think though that what you will find is that the CCS in the voltage amplifier is more important than the power supply. If you look at its design, it is built to reject power supply noise.
Was just trying to give Cousin Billy something to do. I know what it's like to get the itch.
This is the choke already in the amp.
Who knows it's value's?
The choke we see is paralleled with the 200uF cap.
I will assume any new chokes will attach as the resistors are attached?
I doubt that it is in parallel with a 200uF capacitor; more likely the capacitor is in parallel with the output from the full wave bridge rectifier, and the choke is in series with the FWBR. But as to value, I am sure Ralph can help.
Post a Followup:
Post a Message!
This post is made possible by the generous support of people like you and our sponsors: