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Old 3rd November 2005, 10:13 PM   #1
abj1 is offline abj1  Australia
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Default Input transformer & volume pot

Hi All,

Been struggling with my very limited understanding of input/interstage transformers. In a setup I have with an input transformer (humble Hammond 124B-works surprisingly well and suits my 'budget') on the input to an amp should the volume pot be across the primary or secondary winding (between the grids of the diff pair)? The Tx is doing phase splitting to a pair of 1F5G's as diff amp driving trioded PP 46's sitting on a 6F6G CCS (local feedback from screen taps on output Tx to plate resistors of 1F5G's). Amp input is direct from CD player at present... Now would I just be better off to insert my 71A linestage (which has a volume pot across its input) and just do away with the pot on the amp? I'd really rather be able to use the amp without necessarily having to hook up the preamp just for volume control +/- impedance matching issues...

My understanding of the frequency concequences of primary/secondary loading and impedance matching etc is sketchy at best - I could really do with some help from the learned and experienced to set me off in a sensible direction. At present I have a 100K pot across the primary as one would for a standard SE voltage amp which works but the result is perhaps a bit on the 'bright' side (bass is fine) and in fairness I haven't explored the question of loading of the secondary or anything like that as yet.

Cheers, Andrew.
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Old 3rd November 2005, 11:41 PM   #2
SY is offline SY  United States
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The volume control should be across the secondary. Take a look through the white papers on Jensen's site for some examples

www.jensentransformers.com
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Old 3rd November 2005, 11:54 PM   #3
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Hi Andrew,

you and me both, mine is to do with output and PP plate load values operating point &etc another story,

But if you think about it volume pot should be ahead of transformer other wise I think it gets difficult trying to keep o/p stage ballanced you could come up with shunt arrangement or dual gang pot after transformer.

without knowing your exact arrangement my preference is pre the transformer, mind you the source z will change from the low volume to mid and high, but would have less effect than changing the output load z on the transformer secondary.

part of the fun i guess, with op-amps not a big issue just add another noise inducing NFB buffer stage.....

Robert
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Old 4th November 2005, 12:26 AM   #4
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Hi Andrew,

Here is an example of a differential preamplifier with a couple of ways to implement the volume control.

http://www.raleighaudio.com/chapter_4.htm

Dave
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Old 4th November 2005, 03:21 AM   #5
abj1 is offline abj1  Australia
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Mmmm, 2 schools of thought with 2 to 1 in favour of pot across the secondary. Conceptually that is probably slightly more pleasing. The Raleigh Audio approach was what I was what I had in the back of my mind (nice to see it implemented by someone so at least I can be confident it will work)

SY - ta muchly for the ref to the Jensen site. Loads of info but mucho bewildering - could I trouble you to point out the more poignant papers to this Tx neophyte?

Presumably I will need to work out what resistance across the secondary sounds best (least ringing with these cheapo trannies) and then fashion a setup as in Fig 17 of the Raleigh Audio or pannos29 on this forum (that thread started after my search and I'm embarrassed to admit I didn't see it before my post....). Should I use my preamp (likely output impedance of perhaps 2K) in the chain with the current vol pot removed and then experiment until I get the sound I like or can I leave the pot across the primary for the time being and skip the preamp? Are the shunts across the primary and secondary likely to significantly interact or are the relatively independant? No sig gen/o-scope available so I can feel a bit of trial and error coming up!!

Cheers,
Andrew.
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Old 4th November 2005, 04:09 AM   #6
SY is offline SY  United States
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The maker of the input transformer will generally recommend a load. If your potentiometer is larger than the recommended load, you can either pad it down by paralleling a suitable resistor or use a series RC to tame any ringing. For the case of the Jensen JT11-P1, the recommended load is 10K; in my preamp (A Heretical Unity gain line stage), I used a 100K volume control, then just paralleled the control with a 15K resistor to bring the load closer to spec. IIRC, Jensen showed the series RC trick on the JT11-P1 datasheet.

Putting the pot on the primary of an input transformer will compromise the common-mode rejection.

I'll poke around and see if Hammond specs the best-case load for your transformer.
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Old 4th November 2005, 04:13 AM   #7
SY is offline SY  United States
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It appears that 100K should work fine as a secondary load.
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Old 4th November 2005, 05:54 AM   #8
abj1 is offline abj1  Australia
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Thanx SY - a lot of the info I need. Ta for the link to your preamp schematic. May I ask is the 1M resistor from pot wiper to ground just insurance in case the wiper ceases wiping?

I will jump straight into loading the secondary of the Tx with the pot tonight after the kids are fed, washed and in bed...

Would it be wise to find a good value of total load resistance and then split it in 3 series parts with the central bit being the pot (I don't mind changing from 100K that's just a value I use from habit for volume pots) and equal to half the total needed load R? This is basically the schema used by the Raleigh Audio design for push-pull grids in a differential setup like mine (the SE case is way easier to get my brain around - as in the Heretical Unity Gain Line Stage).

Cheers, Andrew.
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Old 4th November 2005, 01:22 PM   #9
Joel is offline Joel  United States
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Default re: the raleigh audio page

In the second (single pot) volume control configuration, I think it's wired wrong. The pot should be wired across the two grids with the wiper to one of them. In other words, all three lugs are used, and then the secondary load doesn't change with the volume control setting.

I believe that's the way Morgan Jones does it in his book as well, or am I remembering this wrong?

Joel
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Old 4th November 2005, 01:29 PM   #10
dhaen is offline dhaen  Europe
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Joel,

I think the method you mention in MJ's book requires a fully floating secondary, or source.
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