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Old 27th October 2013, 11:43 AM   #1
el156 is offline el156  Portugal
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Default PUSH PULL SE ???

Maybe this a total idiot question, but would this circuit work ?
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Old 27th October 2013, 11:56 AM   #2
scholl is offline scholl  United States
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Yes, I don't know the advantage over single transformer P-P.
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Old 27th October 2013, 12:00 PM   #3
piano3 is offline piano3  United Kingdom
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Are there 2 separate transformers? What are you hoping to gain?

(sorry for overlap with scholl)

Last edited by piano3; 27th October 2013 at 12:03 PM.
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Old 27th October 2013, 12:12 PM   #4
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I have built just such an amp and it works like a treat. I did it so that i could run unmatched valves at constant currents with the B+ been a CCS with a capacitor after it to allow AC signal to develop. This was done so that i could have direct coupling - no cathode resistor and zero DC imbalance in the output transformer. It delivered on all counts.

However - in your proposed example i can see absolutely no advantage and since +B is at AC ground anyway it is functioning exactly like a conventional PP amplifier with a PP transformer. What are you trying to achieve here ?

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Old 27th October 2013, 12:19 PM   #5
el156 is offline el156  Portugal
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This was just a thought i had, i never tested that, and would like to know from you if i could get some advantages of combining PP with SE circuit like have better sound like in a normal SE amp. So, better not spending my time testing this idea...
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Old 27th October 2013, 12:24 PM   #6
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Hi!

The only minor advantage I see is a bit better PSRR against ripple on B+, assuming both OPTs are connected to the same B+. Disadvantage: twice the cost for the Opts. And the way the secondaries are connected you get twice the output impedance (poor damping factor).

I don't see why this would sound better.
You will certainly get better results if you spend the available budget for the iron on just one OPT per channel but a better one.

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Thomas
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Old 27th October 2013, 12:35 PM   #7
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Sorry didn't realize it was two separate SE transformers so not the same as my implementation which used PP transformers with separate primary windings.

You proposal shows all the signs of having all the disadvantages of SE without delivering any of the benefits (there will still be harmonic cancellation in the combined transformer secondaries) - so overall a bad idea really.

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Old 27th October 2013, 01:34 PM   #8
TheGimp is online now TheGimp  United States
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Esentially, you have bridged two SE amplifiers in the same chassis.

You might get more 2nd harmonic content as it won't be cancelled as well as it is in a PP amp.

You could use 4 ohm output taps with an 8 ohm load to minimize the reduction in output impedance and dampening.
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Old 27th October 2013, 06:48 PM   #9
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I wanted to try a differential 10 and had no 20-30K PP output transformers but there was a single ended stereo 10 breadboard under my bench so I tore it down and built one channel as roughly shown below. The input/driver plate loads are Wavebourn's SVCS circuit which to my ears makes the best SS plate load I've heard.

The circuit has been running for about a year without problems and still sounds great to me.

In addition, I still have the first amp I ever built on my own years ago using 6BX7>6336. Converting the amp to differential would be fairly simple. Doing it with single ended outputs takes care of one of the concerns that goes with the 6336 - badly matched sections. This way there is no issue around DC on the primary of a PP output transformer. On the other hand, getting output transformers that are well enough matched might be. At any rate, It's working with this one . The circuit has given no trouble and I like the sound of it a lot. And mono aint so bad either !
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Old 27th October 2013, 07:05 PM   #10
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Quote:
Originally Posted by el156 View Post
Maybe this a total idiot question, but would this circuit work ?
I've seen this done for RF amps. I don't see any bennies for audio. Two SE OPTs are likely to be pricier than a single, PP OPT, as the latter are easier to design than the former. You also lose the benefit of mutual coupling across the primary center tap, and the separate SE OPTs are likely not so well balanced.

At RF, these considerations don't apply since whatever harmonic distortion it produces is filtered out by bandpass filters and antenna tuners/match networks.
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