Hammond Chokes w/ Quickie - Cheap and Worthwhile!

[Dr. Toobz]

I've installed Hammond 150H 3.7k ohm chokes in place of the stock 4k ohm plate resistors, and the mod definitely seems worth the $30 or so it cost. Gain has increased noticeably, and the amp is even clearer and more punchy than ever! High-frequency response is very noticeably crisper over the plate resistors. This is a much different sound than the C4S S.E.X. - the Quickie is intimate, putting you right in the middle of the instruments, whereas the S.E.X. puts you a few rows back. In other words, soundstaging is still different, even with the chokes. But, that's ok. I actually sort of like the Quickie better for rock already.....maybe it has to do with the DHTs? It should make a great partner for my iMac, as it even sounds good straight from the headphone jack. If I want delicacy, I can always listen to the S.E.X. in the other room!

In any case, this was a worthwhile mod! Does anybody know where to get good transformer standoffs for the chokes, though? I haven't yet decided if I will mount these inside the wooden base I built, or hanging from standoffs.

I've been working on the wooden base lately, and even chiseled out some room on the inside of the front for a headphone jack to fit (I got down to about 1/4" and then drilled a hole for the neck of the jack). The old non-shorting jack from my S.E.X. amp worked perfectly, and looks good on the front of the wooden base (right under a brass Bottlehead badge). I'll post some photos in a few days, once I've put the clear finish over the mahogany stain.

[Grainger49]

I think I bought some "nylon" standoffs at Mouser.

[Len]

... Does anybody know where to get good transformer standoffs for the chokes, though? I haven't yet decided if I will mount these inside the wooden base I built, or hanging from standoffs.

http://www.audioasylum.com/forums/bottlehead/messages/13/138809.html

I decided not to use a headphone jack. I have a headphone jack to RCA adapter that I use. I also picked up a pair of Cardas clips for the Senns, and am planning of making a direct cable to RCA's (one day!)

I think Doc mentioned that the DHT's would be the killer attraction of the Quickies.

Thanks for the post on the chokes. Sounds like it's worth it. Maybe we could put in a mass buy through Doc and let him make the mark up on them?

[Paul Birkeland]

Does anybody know where to get good transformer standoffs for the chokes

I would go to McMaster Carr.  Those chokes will probably mount well with 6-32 hardware.  Do be sure to use a large flat washer between the bottom of the chassis plate and the standoff, as you want to spread the pressure that the standoff places on the chassis plate. 

[Len]

Does anybody know where to get good transformer standoffs for the chokes

I would go to McMaster Carr.  Those chokes will probably mount well with 6-32 hardware.  Do be sure to use a large flat washer between the bottom of the chassis plate and the standoff, as you want to spread the pressure that the standoff places on the chassis plate. 

You're right. I was thinking of those itty bitty chokes I'm using for my low pass filters. These chokes must be bigger.

I still plan to use the 4-40's if I mount the Hammonds, but with those really wide (pan?) washers (also available at McMaster). The Quickie is loaded with 1/8" holes and I don't want to change that in case I mod them again. I think the top plate would distort before the 4-40's give out, and I can probably use fiber shoulder washers when mounting the chokes to the standoffs to take up the slack in the mounting holes.

What do you think of using the Hammonds in a Quickie headphone amp to drive Senns, which IIRC are 300 ohms rated. Wouldn't the chokes raise the output impedance of the Quickies to 500 ohms, more than what should be used to drive 300 ohms input?

[Paul Birkeland]

What do you think of using the Hammonds in a Quickie headphone amp to drive Senns, which IIRC are 300 ohms rated. Wouldn't the chokes raise the output impedance of the Quickies to 500 ohms, more than what should be used to drive 300 ohms input?

The choke won't really cause any problems like that.  The current source that PJ drew up might raise the output impedance a little, but into the autoformer it suddenly isn't as important.

[Koda]

I am REALLY happy with the improvements the chokes made, such as better imaging and clarity.
However, now there is a fairly strong hum that was not audible (or just barely) before the chokes were put in.

The leads to the chokes have been trimmed as short as possible, and the frames are grounded.
I tried reversing the in and output wires to the chokes, with no success in quieting them.

Any suggestions? Did anyone else pick up more noise when doing this modification, or is it just my luck?
I sure like (most) of what the chokes do, but want to get my Quickie as quiet as it was before if at all possible.
Thanks in advance for any ideas or helpful pointers.

[Len]

I am REALLY happy with the improvements the chokes made, such as better imaging and clarity.
However, now there is a fairly strong hum that was not audible (or just barely) before the chokes were put in.

The leads to the chokes have been trimmed as short as possible, and the frames are grounded.
I tried reversing the in and output wires to the chokes, with no success in quieting them.

Any suggestions? Did anyone else pick up more noise when doing this modification, or is it just my luck?
I sure like (most) of what the chokes do, but want to get my Quickie as quiet as it was before if at all possible.
Thanks in advance for any ideas or helpful pointers.

Did you try ungrounding the chokes? Obviously it's not AC filament hum or unfiltered HT, so I would guess a ground loop. Is it on both channels? Are the grounds short and to their respective channels? Just guessing until the heavy hitters get here.

[Grainger49]

I am REALLY happy with the improvements the chokes made, such as better imaging and clarity.  .  .  .   

I don't know if this is the right place to ask but would plate chokes be good in the FP 2 or Seduction (already has C4S)?  This because I am an admitted imaging freak!

I can start a thread about this in the Legacy Products area.

[Len]

I am REALLY happy with the improvements the chokes made, such as better imaging and clarity.  .  .  .   

I don't know if this is the right place to ask but would plate chokes be good in the FP 2 or Seduction (already has C4S)?  This because I am an admitted imaging freak!

I can start a thread about this in the Legacy Products area.

I think Gary Pimm went into this in one of his iterations. IIRC, his opinion was that for a small frequency range the choke helped get rid of a little graininess of his CCS, but otherwise did not do much. The question became where to put the choke, and I think his answer was after the CCS.

When I was looking into it, I couldn't get many replies about it. But the fact that the choke would allow storage of current whereas the CCS keeps it constant seems, to me, to show that the two effects would be fighting one another.

I'm interested (academically) in what other people have found, too.

[Grainger49]

Len, I started a thread here to keep this one from skidding sideways:

http://www.bottlehead.com/smf/index.php/topic,79.msg279.html#msg279

[booangler]

I've installed Hammond 150H 3.7k ohm chokes in ...

Dr. T,

Would mind giving out where you ordered the chokes from and what part number they were?

Thanks,

Alan

[Grainger49]

Alan, looks like a 156C from his numbers.  I'll bet Parts ConneXion stocks them.

[Paul Joppa]

...
However, now there is a fairly strong hum that was not audible (or just barely) before the chokes were put in.

This is almost certainly the choke picking up a magnetic field, probably from a nearby power transformer. Move the Quickie away from any such device and see if that does not make it quieter.

Shileding is the old-fashioned commercial answer. The best shields are mumetal and completely surround the choke; these are hard to come by but can give some 30dB attenuation of hum fields.

Remember, mumetal loses its performance if physically bent. The cans are tempered after forming. The Seduction power transformer uses a high-permeability tape that can be bent if you are careful; it's called "molyperm" and has a high molybdenum content. That's all I know about it - I don't know a source for it. My measurements were that this reduces hum by 6 to 20 dB depending on the relative position and orientation of the hum source. The variation is because the tape does not fully surround the iron.

Some people have used iron pipe (like 2" diameter drain pipe) for homemade magnetic shields. Because the permeabilty of iron is hundreds of times less than mumetal, it will I believe provide only 5-6dB attenuation, and for that the pipe needs a thick wall, like 3/16 inch or more. Very heavy.

Best solution is still to move the Quickie away from power transformers in other devices.

[John Swenson]

You can actually do magnetic shielding with aluminum! Thick aluminum (say 1/4" to 1/2" thick) has a high enough conductivity that external fields induce eddy currents which generate their own fields which counteract the original field. You can get 5-6db or so at 120Hz with 1/4" thick aluminum. It actually gets more effective as the frequency goes up.

One of the best places to get small amounts of magnetic shielding material is lessemf:

http://www.lessemf.com

They have several flexible foil type products. I've used the metglass ribbon to shield preamp tubes and it worked very well.

John S.