choked

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ed
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#1 choked

Post by ed »

I've started to put the sv572/811 back in it's(not so smart anymore) box.

I swapped the 10h 150ma shrouded choke(193G) for a 10h 125ma frame choke(159p) so I now have 2 matching 159P chokes...taking the big choke off the top and putting it inside. It isn't inside yet cos I thought I'd better check it on the breadboard first.......

well...........It now hums. Just a smidgeon, but enough to hear with the ear hard against the driver. It wasn't thus before.

Anybody idea what's occurring.

I gather from the lack of feedback at Owston that it wasn't that impressive. Please guys, if there's anything you didn't like I'd really appreciate the feedback....I know from experience there are things unheard until they're pointed out by other people.
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#2

Post by IslandPink »

If that was the amp you played on Saturday before the Pass F2, then it sounded really nice. I didn't notice any probs, and the tone in mids and treble was excellent . I tend to concentrate on mids and HF when confronted with a new amp, so I may have missed something in bass or timing - I expect Nick or James can spot anything there if there was .

I remember commenting it had that 'thoriated' sound - that was a compliment :D
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#3

Post by Ali Tait »

Yup, thought it sounded very nice Ed, nowt wrong there.
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#4

Post by Mike H »

+1! :D
 
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Nick
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#5

Post by Nick »

Maybe the 125ma choke is saturating a bit more than the previous one, and creating a bit more of a hum field thats getting into the rest of the amp.
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#6

Post by ed »

Maybe the 125ma choke is saturating a bit more than the previous one, and creating a bit more of a hum field thats getting into the rest of the amp.
sounds like a good maybe. This choke is obviously closer to reality than the shrouded one....the amp draws 115ma.

I'm not sure what the ma spec on a choke really signifies...I have a vague memory that I read somewhere it is the figure at which the Henries rating is correct, rather than a max current....but I'm hazy...do ou know the truth?

It's a bit disappointing to have eliminated the hum completely then for the pesky thing to creep back in because I wanted both chokes inside the chassis.....the god of small things is obviously playing with me.
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#7 Re: choked

Post by Mike H »

ed wrote:I swapped the 10h 150ma shrouded choke(193G) for a 10h 125ma frame choke(159p) so I now have 2 matching 159P chokes...
That may be the reason, the shrouded choke would mean it also has fully screened windings. :?:

The framed one won't so you'll have a field coming out of each side.
 
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#8

Post by steve s »

The inductance varies with the current going through the choke, some of the parmeko chokes have a working range of inductance as their spec, in reality the lower current transformer would have less inductance at the same current has the higher rated one .if that equates to less hum ...has the ripple changed?..

Dont see the frame making a difference but you never know. I think nicks on the right track
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#9

Post by Nick »

The rating should be the flux level where the combination of core and air gap starts to saturate, so inductance falls off.

Just remember though, that just because the amp is drawing 115ma, the current through the choke can be much higher. The total current through the choke includes the 115ma, but it also includes the AC current that's shunted to ground by the cap following it.
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#10

Post by ed »

I've done some measurements, at speaker terminals. All I can find is cycling between 1.5 and 6mv at 27 khz on the scope. No other frequencies detected.

Fluke says 3mv.

These are exactly the same measurements that I got with the shrouded choke when there was no hum whatsoever.

with the HT off and the heaters still on I get 5mv at 5mhz.

anybody any ideas of the significance of 27khz. I have a 10k grid stopper on the sv811s.

I've moved the 2nd choke around a bit but it makes no difference...it's a good 3inches from the first one.

*****edit: re the 27khz stuff, I've just re-read this(bottom paragraph) so I might have answered my own question:

http://www.audio-talk.co.uk/phpBB2/view ... c&start=13
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#11

Post by Nick »

The meter will not show you the shape of the hum. If its a nice sine it will be less noticeable than if it has lots of edges (harmonics)
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#12

Post by ed »

the shape of my hum?

I'm inspired to write poetry.....

but seriously, as the hum version measures exactly the same as the non hum version I deduce that my equipment is not capable of measuring this particular hum......

I'm probably over reacting because it really is hard to hear it, i.e one has to get right up close to the driver. My neurosis forces me to write about it though.

to put it to bed though I will say that the shape of the 27k ac component is halway to a sawtooth, nasty looking thing, but I don't think it's what I'm hearing.
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#13

Post by IslandPink »

Is it a 'dynamo hum' ?
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