Phase Splitter Question - Page 1
Forum section: electronics / beginners

author: Jwillis
author_rank: Super Contributor
timestamp: July 02, 2020, 10:08:10 pm
content: I'm building a push pull tube amplifier and I'm stumped about the phase splitter . Now from what I understand when a signal is applied to the input there you get an inverting signal at one output and a non inverting at the other output. This is what creates the push pull on the power amplifier tubes.Now I do indeed get a inverting and non inverting signal but they are not symmetrical . The inverting signal has a much higher amplitude than the non inverting signal and the difference increases with volume . I'm using a long tail phase inverter design and was under the impression that the 2 output signals should be symmetrical. I've tried various methods to get symmetry like changing voltages , changing tubes , varying cathode resistors , splitting cathode resistors , using different caps . Most ideas don't do anything at all. The amp sounds great even at highest volume with only a small amount of clipping. Well it sounds great considering I'm a little tone deaf.I'm missing something and can't put my finger on it.Should the Inverter outputs be symmetrical (balanced) ?PDf is the best I could due that is clear to see. Left Channel is not built . Some values on the power supply may be inaccurate. I tried to get multiple grounds to work in KiCad but can't .

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 03, 2020, 01:48:33 am
content: Is there a reason why the value of the two plate load resistor are different? One is 20K and the other 27K. This would surely create some unbalance in the output signal levels.

author: TimFox
author_rank: Super Contributor
timestamp: July 03, 2020, 02:01:39 am
content: The difference in plate resistors would only cause a 27/20 = 1.35:1 imbalance. You said “much higher”—how much do you mean?

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2020, 02:38:58 am
content: It can never be symmetrical. The sum of two symmetrical signals should be zero but R7 and R22 junction is used as the input signal to the lower driver tube. The gain of the lower tube needs to be greater than that of the upper in order to unbalance just enough to supply the required signal.I have agonized over similar circuits in the past and even as a young engineer back then I couldn't see how they expected symmetry. Using a control to vary the feedback signal and adjust it for symmetry is how I'd do it.

author: Jwillis
author_rank: Super Contributor
timestamp: July 03, 2020, 02:43:04 am
content: Quote from: eblc1388 on July 03, 2020, 01:48:33 amThe two values is to equalize the both anode voltages. Both are at 178V Making them equal doesn't seem to make any changes.Quote from: TimFox on July 03, 2020, 02:01:39 amIt depends on the volume set at the pre amp. But I took some pictures with settings at about 1/4 , 1/2 and full volume . I'm not sure but I thought I read something about increasing the cathode resistor to balance the out puts?Scope settings are 0.1ms/Div 0.5V/Div at 5KHz so the camera can pick it up. But frequency doesn't make a difference to the imbalance.As you can see the non inverting side which is the top wave is much smaller in all volume levels.The Power tubes are biased fairly close but changing the bias levels also makes no difference. Tone levels on the pre amp are flat . And signal was put directly to the input of the inverter with no change.

author: Jwillis
author_rank: Super Contributor
timestamp: July 03, 2020, 03:04:30 am
content: Quote from: bob91343 on July 03, 2020, 02:38:58 amI put the feedback to the final stage of the preamp . Would putting it to the splitter work better? But if you say it can never be symmetrical then maybe I'm over thinking this.It doesn't sound distorted until full volume.

author: Jwillis
author_rank: Super Contributor
timestamp: July 03, 2020, 05:51:50 am
content: I figured it out. I increased the values of C8 and C10 to 220nF.That balanced the inverting and non inverting signals better. Not perfect but much closer to being symmetrical. Thanks for the response.