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Old 12th April 2003, 03:31 AM   #1
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Default "Parafeed trafo inverted phase"

Will this work??

I cobbled it together, plugged it in and sound (badly distorted with heavy hum) came out.

After you quit laughing, any comment appreciated....just trying to learn by doing...

Rick
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Old 12th April 2003, 07:16 AM   #2
stigla is offline stigla  Norway
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Man over board!!



Well, let's start with the beginning;

a 117V : 250V CT, that calculates to about a 1 : 1+1 (or 1 : 2) ratio. I dont know what CD player you got, but with a normal 2V output it's probably from here you get that hefty distortion since the player have troubles driving the iron. If your CDplayer is other than the averedge it is possible that it either saturates the core of the IT or overdrives the Ouput tubes themselves.

Distorted or not, if you've connected the IT correctly, you'll get phase inversion, and the different phases go to a seperate SE amplifier and speakers. So, what happends at the speakers is the same as if you connetcted the + and - reversed at one speaker in a normal stereo, the phases cancels to a certian degree, having the one speaker cone going outwards while the other going inwards.

Despite the results I want to congratulate you for doing this, it's what DIY is all about!
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Old 12th April 2003, 02:56 PM   #3
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Default Thanks for the thought!

Appreciate the encouraging words--I am a newbie afterall.

When you say the cdp may have trouble driving the iron, does that refer to input impedance to the iron and output impedance from the cdp? If so, it seems to me the reflected impedance to the cdp will not be so large because the IT is about 1:1 (I'm just guessing here, as you know). I thought the center tap divides the trafo so that it is about 117:125 ratio for each side?

The cdp has no trouble driving the output tubes directly i.e. without the IT. So, I'm a bit confused here.

As far as phase, wouldn't each output tube "see" only 1/2 of the waveform much like a push-pull system? The only difference is this is single ended but easily could be converted to push-pull with the appropriate trafo?

IT saturated? With what? Should be no DC from the cdp--right? Could stick in a coupling cap if that is the case?

Oh well, thanks for reading and commenting--anybody else? Seems like a very simple setup and should work??

Aspiring diyer,
Rick

ps thanks again Stigla!
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Old 12th April 2003, 03:14 PM   #4
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Default SOME QQS.

Hi,

If the two speakers in your drawing represent a single stereo channel than one of the two will be driven with reversed polarity with respect to the other.

No big deal, just reverse the polarity of your speaker lead and that's it.

If however circuit shows a stereo setup and you input both left and right channels into the same transformer than again you would have the same polarity reversion at the out put of one achannel and the same solution applies as noted above.

The most practical way to solve this is to use two identical input xformers, one for each channel or none at all.

Ciao,
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Old 12th April 2003, 03:54 PM   #5
stigla is offline stigla  Norway
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I thought the center tap divides the trafo so that it is about 117:125 ratio for each side?

Yep, that's right. That gives you a 1:1+1 or 1:2 CT if you may.

IT saturated? With what? Should be no DC from the cdp--right? Could stick in a coupling cap if that is the case?

Ahem, sorry 'bout that. Your CDP has no DC of course, I was merely considering the AC voltage, but thats a pretty numb thing too I realice since you obviously are using a Power transformer (able to handle 117V AC or more!).

When you say the cdp may have trouble driving the iron, does that refer to input impedance to the iron and output impedance from the cdp? If so, it seems to me the reflected impedance to the cdp will not be so large because the IT is about 1:1.

I was thinking of the IT providing an pretty low impendace to the CDP, too low obviously, since:

The cdp has no trouble driving the output tubes directly i.e. without the IT.

I'm not really sure if I understand what your goal is, so I'm just guessing also. Is it to have a stereo amplifier? Or a single channel with two outputs for different drivers perhaps? Kind of Bi-amping? That's pretty interesting...
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Old 12th April 2003, 05:17 PM   #6
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Default The Goal

for me is to learn by doing and reading with your help. But more to the point, this is just a "tune-up" for a push pull scheme.

You see, I thought this center tapped trafo would split the signal with a + signal going to one output and a - signal going to other output. But, I am starting to think that the entire waveform goes to both outputs but out of phase by 180 degrees? That would be a phase inverter wouldn't it? And not a phase splitter?

But anyway, if it is indeed a phase inverter then I could get the output back in phase as Fdegrove suggested by switching the polarity of the speaker terminals--thus--twice the output power but in mono.

Here's my question: Isn't this configuration essentially the same as if I substituted the single ended trafos for a push pull trafo that receives both output tube signals. (See Joel's "tiny, DHT amp" thread--thank you Joel).

I guess it boils down to this: What is going on with this setup?

Thanks for trying to straighten me out,
Rick
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Old 12th April 2003, 05:41 PM   #7
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Rick

Before inventing new topologies it may be a good start to get a grasp of the basics. First, your input phase-splitting transformer should have sufficient primary inductance as that's what will constitute the cdp load in the absence of secondary load. Start by measuring the primary inductance or just using the first transformer i got my hands on as an example. 100VA and 20H. At 20Hz this is a slightly marginal load and one which will not be appreciated by the majority of CDP. You may well try to calculate it. Whether this is responsible for the distortion you hear is difficult to say as you haven't shown your PS. You may have such a high level of hum that everything gets modulated by the hum.
Is your valve getting the recommended bias? As i don't know anything about that particular valve it will be interesting to know whether it's got sufficient mu to get by with a single stage.
Yes, it is very possible to create a PP amp using a phase splitter at the input. You can even go on using your SE transformers and connect the load between the two. You will get something like a bridged SE amp, very similar to PP.


cheers

peter
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Old 12th April 2003, 10:13 PM   #8
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Default Thanks Peter

So the reflected impedance from the tubes do not constitute the load? Rather, it is the inductance of the primary?

X=1/(6.28*L*20 hz)

Unfortunately, I cannot measure the inductance and have no basis for estimating.

The PS consists of full wave bridge rectifier->10 mf cap->7H choke, 150ma->100 mf cap->150 ohm R->150 mf cap->load

CLCRC (that is)

This PS worked just fine (with very faint hum, ripple=0.00025v, PSUD modeling) in the SE stereo mode using the same tubes, Rk, and OPTs. I'm assuming that it is ok because the PS "sees" the same tubes, bias, OPT's, etc--but I could be wrong (probably).

I tried switching the polarity of the speakers but that did not do it.

This hum sounds like a lot of unfiltered power (120 hz) getting through?? And, only one speaker is working?? Very strange?? Because this is breadboarded I have re-configured the amp back to SE several times and it works beautifully that way (1.4 watts in pentode).

Thanks for your comments, Peter.
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Old 12th April 2003, 10:33 PM   #9
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Default Oops

Wrong formula; should have been :

L=2pi*f*XL

then about 3-4 H inductance on the primary, no?

Rick
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Old 12th April 2003, 11:40 PM   #10
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Hmmm...25EH5 is a pentode...what's the screen connecting to?

If it's floating, the worst that could happen is practically no output, so that's not the problem anyway...
Is it possible you connected something else to the heaters, somehow? That would get a lot of hum inside.
Otherwise, yeah it would have to be the input tranny, can't think of anything else in the circuit that would be bad. As long as it's wired correctly, SE is pretty fool-proof.

Tim
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