[JN] CLC Filter Choke Choice

Guido Tent guido at tentlabs.com
Tue Jun 4 13:18:38 CDT 2019


hi Phil,

> On 4 Jun 2019, at 19:33, Philip Yates via Sound <sound at soundlist.org> wrote:
> 
> Well, 27 Hz and 22 uF implies 1.58 Hy, which could easily be the case when an iron core inductor has current going through it.

We agree here.

> The resistance is 1/(2*pi*27) = 268 ohms. For the inductor, subtract the resistance of the inductor from 268, and place the difference in series with the inductor. Put a 268 ohm resistor in series with the cap, and you have a phase-zero power supply. 268 ohms may be a little on the high side for a 300B amp, maybe not. With a 3K output tranny, the output impedance is increased by a factor of (3K + 268)/3K = 1.09. Of course, this impedance is constant over the amp's frequency range, whereas in a normal LC supply, the output impedance varies in both magnitude and phase over the frequency range.

that is an interesting though. A constant impedance power supply. I did look at those from another perspective once (actually I built one and took part of the line stage power supply shootout at ETF France).

> 
> In most cases, it LOOKS like the first stage should be either active -- basically an electronic inductor -- or a lower resistance (100 ohms or so) stage with large caps, maybe even electrolytic caps, that has a low resonant frequency (close to DC for the active inductor, maybe 2 to 5 Hz with electrolytics). This is followed by a second LC stage, with an air core inductor and a high quality cap that handles frequencies above 100 Hz or so, I don't know where an audible difference would occur.

I tend to keep all filter corner frequencies well outside the audio band.

Nevertheless, you proposed an interesting version of an LC filter; not as effective in reducing ripple (6dB only) but constant in terms of frequency (amplitude and phase wise)

best,
-
Guido

> 
> Phil
> 
> On Tue, Jun 4, 2019 at 9:07 AM Guido Tent via Sound <sound at soundlist.org <mailto:sound at soundlist.org>> wrote:
> hi,
> 
> I was working on an SE 300b amp yesterday. It has local filtering, 4H+22uF
> 
> Yes, that resonates around 27Hz where the HT showed max modulation :-)  The low frequency playback must suffer from that, I id not perform a listening test yet, will do.
> 
> best,
> -
> Guido
> 
> 
> 
> 
> 
> 
>> On May, 30, 2019, at 23:10, Joseph Levy via Sound <sound at soundlist.org <mailto:sound at soundlist.org>> wrote:
>> 
>> Greetings,
>> 
>> I have several transformers and chokes on hand that were stripped off a collection of somewhat crudely built (by someone else) DIY 300B amps. They include a pair of Tango Permalloy SET OPTs and a pair of Tamura Permalloy PP OPTs.
>> 
>> I also have several pairs of vintage filter chokes that meet or exceed the minimum specs for the designs I'm working from. The chokes have specs of:
>> 
>> 10Hy @ 200mA, DCR = 90 ohms
>> 8Hy @ 300mA, DCR = 80 ohms
>> 5Hy @ 400mA, DCR = 45 ohms
>> 
>> The rectifier is a 5AR4 / GZ34 followed by a 40uF cap, a choke, and then a higher value cap.
>> 
>> Some time ago, one of my local audio buddies told me that a lower DCR is preferred in an SET design rather than a PP where it's not as critical, but I don't recall him explaining why. If I were to use one of those chokes with either the SET or PP designs, is there a preferred combination?
>> 
>> Thanks!
>> JL
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