Afterglow PS Choke

[Ken]

In anticipation of rebuilding/rebirthing my Afterglow kits I have been doing some reading and study.  I noticed that the Triad CX7 power supply choke seems to have an unusual R rating.  There are quiet a few PS chokes with 10H inductance and ~100mA current capability but the  CX7 rating of 270R resistance seems to be unique and substantially higher than others.   Why is that the case?

[Paul Birkeland]

Look at the dimensions of those 10H/100mA chokes with lower DCR, as they are probably too big to even fit into a channel frame. 

[Ken]

Some of them, like this Hammond have much lower DCR and are channel mount:

https://www.hammfg.com/electronics/transformers/choke/153-159

Was size the main consideration?  Or is the 270 DCR a necessary value for the Afterglow circuit?

[Paul Birkeland]

The difference between 270 ohms and and 195 ohms in a circuit drawing 60mA is 4.5V.  That's about 1% of the B+, so certainly you could use either one.  There are other Hammond chokes with similar inductance but with DCR that's around 100 ohms, and those will end up being pretty large for the job.

The consideration for the C-7X is that it was well made (originally wound on a paper bobbin), affordable, usually in stock, and up to the job when we started using it.

You don't actually need to use a choke at all in the Afterglow.  If you use a CRC filter in each amp with a pair of these and a 270 ohm 5W resistor, that will also work nicely.

[Paul Joppa]

As I recall the choke was chosen for low cost, to go in the first Bottlehead product, the S.E.X. monoblocks. That would be in 1996. After that, it was in inventory and used whenever a choke was needed - I even used it for a plate choke in the original Paramour.

[Doc B.]

I don't recall the exact nubers but I recall that were were pushing the limits of the filter cap peak voltage ratings in the Afterglow. If you reduce the DCR of the choke down to 100 ohms I'd suggest watching the startup voltage surge to see how how it swings and where it levels off at warmup. Actually I would uprate the caps in any Afterglow that hasn't had that done already. PB and I have usually replaced the 100uF 450V rated caps each with two in series 220uF 250V or 270uF 350V caps, with 100Kohm bleeders across each to equalize the cap voltages.

[Ken]

Thanks to all for this info.  The Triad Magnetics CX-7 is still available and appears to be the least expensive choke with values in this general range.

That begs two questions:  (1)  are the more expensive options "better" quality?; and (2) will they make a discernible tonal difference in the Afterglow circuit?

[Paul Birkeland]

In the Afterglow, the last power supply capacitor is very much in the signal path, the filter choke will see nearly no signal current.  Spending more money on the filter choke feels a little wasteful, but I often deal with people deciding whether to spend $300 or $3000 on a mains fuse, so do what makes you (and not us) happy.

[Ken]

In the Afterglow, the last power supply capacitor is very much in the signal path, the filter choke will see nearly no signal current.  Spending more money on the filter choke feels a little wasteful, but I often deal with people deciding whether to spend $300 or $3000 on a mains fuse, so do what makes you (and not us) happy.

I hear you Paul.  I'm not into spending money on esoterica but I will spend a little extra if there is a direct benefit to be had, hence my question.  Of course, what my ears think they hear starts with the source and works its way through the entire system and out the speakers, so whether the PS choke would make any discernible difference in my system would seem to be highly debatable.

I don't know enough about the subject to go much beyond that.  I do note, however, that among the mainstream manufacturers like Triad, Hammond, Edcor, etc. there is quite a wide difference in pricing for the same or very similar unit.  Hammond being about twice the price of Triad.  I assume that someone knowledgeable specs one over the other for substantive reasons, but even so those reasons may not amount to an audible difference in this application.

I'm just asking Colombo-like questions and trying to learn as I go 

[Paul Birkeland]

Hammond (I am pretty certain) is making the Triad chokes now.

[Ken]

One last question please Paul.  How critical is the inductance rating?  The original Triads have a nominal rating of 10H.

[Paul Birkeland]

As inductance increases, the impedance of the choke at 120Hz (the AC ripple frequency in the HV DC supply) also increases, so that source of noise is reduced.  Since the afterglow is an AC heated 2A3 amp, at some point heater hum will be much greater than power supply noise, so there very much is a point of diminishing returns.  The 220uF->270R->220uF filter arrangement will push the high voltage noise low enough that you should only hear some heater hum.  A 100uF->10H->100uF is also acceptable.

Beyond that it's redesigning an amp that hasn't been made for 25 years, so you would need to learn PSUD and decide what your design goals are vs. what you're willing to pay for in terms of parts.

[Ken]

Thanks Paul.  I have nowhere the knowledge or competence necessary to design anything from scratch.  I'm just curious about how this one works and trying to learn a bit so that I'm not doing a completely "paint by numbers" exercise 

[2wo]

If you are rebuilding the amps, don't you have the original chokes?  I'd use them...John

[Ken]

If you are rebuilding the amps, don't you have the original chokes?  I'd use them...John

I do have the originals but the paint I put on them many years ago is pretty grungy and chipped, so I thought I would look around and see what new/clean ones would cost and that's what got me started inquiring.  However, if swapping them is going to be too expensive for no discernible tonal benefit then I will sand down the existing paint on the frames and re-spray them.