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Thread: Paraphase inverter question

  1. #36
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    Quote Originally Posted by Gingertube View Post
    The Paraphase is great in simple /minimalist circuits - easy to add a 1 knob tone control
    See the GA40 for example of this
    https://music-electronics-forum.com/...ad.php?t=49627

    Cheers,
    Ian
    Yes, mine project is fixed bias and imply some nfb amount over it. It works pretty well. Thanks for material.

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    Quote Originally Posted by Gingertube View Post
    The Paraphase is great in simple /minimalist circuits - easy to add a 1 knob tone control
    See the GA40 for example of this
    https://music-electronics-forum.com/...ad.php?t=49627
    That's a Paraphase which is a little different from the Floating Paraphase in the OP. One difference is the Paraphase doesn't have negative feedback and therefore has equal output impedances meaning it won't be unbalanced by being loaded.

    I wonder why the Cathodyne and LTP became more popular in guitar amps when the Paraphase has more gain and output voltage than either of them? Is it something to do with overdrive performance?

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  3. #38
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    Quote Originally Posted by Dave H View Post
    I wonder why the Cathodyne and LTP became more popular in guitar amps when the Paraphase has more gain and output voltage than either of them? Is it something to do with overdrive performance?
    My guess is that it has to do with elegance. To an engineer, the paraphase looks (on paper) like a bit of a kluge compared to the cathodyne and LTP, both of which look like they would work well independent of tube characteristics.

    In practice I like the paraphase, especially in hi-fi amps.

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  4. #39
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    Perhaps the paraphase’s greater output signal swing is more of a drawback than benefit for most applications? As it may worsen bias shift / blocking distortion when overdriven.

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    Hello,
    I have a hard time to do this stage completely quiet.It still have a tiny hum. I tried almost everything : different ways of grounding, doubling filter caps...and did not help at all,still have a tiny amount of noise. The only thing it helps to reduce to none is to raise the negative feedback amount. The noise is pretty consistent, meant no dependant by what is happen in front (first stage of paraphase input is shorted to ground) and is related between paraphase inverter and power stage. Any ideas, please ? Thanks

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    Last edited by catalin gramada; 09-06-2019 at 01:36 PM.
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    This is the output

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    and these are floating paraphase outputs, (yellow inverted, blue non inverted)


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    paraphase input is shunted to the ground.

    The noise sound more like 100hz ripple, but doubling filter caps value have no effect, and the ground is clean. Have no ideea how to fix it.

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  7. #42
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    ...and think I get it...

    This is the ripple of bias circuit

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    and with filter cap doubled (100uF added in parallel)

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    I have to check it one more time, as with the same bias circuit and LTP inverter the amp was absolutely quiet. I wonder what did the paraphase to be noisy? That meant the ripple was injected in to the loop and did not cancelled in common mode but amplified instead...have sense.. ?

    And I still hear it in the speaker even with 200uF...

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    Last edited by catalin gramada; 09-08-2019 at 07:17 AM.
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    This is the ripple of bias circuit
    O.K., this makes sense as the fundamental frequency is 50Hz.
    With perfect power amp symmetry bias ripple cancels. So there must be asymmetry. Can you post a drawing of the complete power amp?
    Can you see the ripple at the power tube grids? Any difference between both sides?

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  9. #44
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    Thanks Helm, the last picture shows the ripple at power tube grids. There are two traces cause I splited the bias rail in two sections to can do individual adjustments. I,ll draw a sketch in a moment. Thanks

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    Click image for larger version. 

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    Last edited by catalin gramada; 09-08-2019 at 04:51 PM.

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    Quote Originally Posted by Helmholtz View Post
    O.K., this makes sense as the fundamental frequency is 50Hz.
    The noise pitch is more like 100Hz one, no 50. This induced me in error. I checked first the OT output with a dmm and shows remember 100hz ripple .The power tubes are pretty well balanced.
    Is a very tiny background noise, normal for a guitar aplication can be ignored, ...but should not be there, and definitely was not there with a LTP inverter in same power stage.
    I reworked the chassis few times in different ways till I found it comes from bias supply

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    Last edited by catalin gramada; 09-08-2019 at 05:19 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    The noise pitch is more like 100Hz one, no 50.
    Fundamental frequency of the signal in the scope pictures clearly is 50Hz, but it contains a lot of second harmonic (100Hz). Because of ear sensitivity at low levels and speaker/OT characteristics 100Hz is typically reproduced and perceived louder than 50Hz.

    I checked first the OT output with a dmm and shows remember 100hz ripple.
    How did you determine frequency with the DMM?


    Bias filtering improves with a resistor in series with the diode (of course will require to adjust shunt resistance). Remember a filter cap without enough series resistance is no good LP filter.


    I would have expected more difference between opposite grid signals. If they are identical, I am somewhat at a loss. Leaves power tube gm or OT primary asymmetry - not very likely.


    Do you also hear the hum without NFB?

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    Last edited by Helmholtz; 09-08-2019 at 05:45 PM.
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    Quote Originally Posted by Helmholtz View Post
    Fundamental frequency of the signal in the scope pictures clearly is 50Hz, but it contains a lot of second harmonic (100Hz). Because of ear sensitivity at low levels and speaker/OT characteristics 100Hz is typically reproduced and perceived louder than 50Hz.



    How did you determine frequency with the DMM?


    Bias filtering improves with a resistor in series with the diode (of course will require to adjust shunt resistance). Remember a filter cap without enough series resistance is no good LP filter.


    I would have expected more difference between opposite grid signals. If they are identical, I am somewhat at a loss. Leaves power tube gm or OT primary asymmetry - not very likely.


    Do you also hear the hum without NFB?
    Thanks

    It have a freq meter function.I tested again now.Just like that:

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    It shows the same 50hz fundamental.

    Ok, so have understand improving the filtering of bias supply will be the solution in this circumstances to get rid of noise, please ? I have to add the only other thing I tried to reduce the noise was to raise the amount of nfb. which of course is not something I wish.

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    Last edited by catalin gramada; 09-08-2019 at 06:02 PM.
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  14. #49
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    And Yes, the hum is much louder without nfb

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    Quote Originally Posted by catalin gramada View Post
    And Yes, the hum is much louder without nfb
    O.K., now I have an idea. Do you now see a significant difference in hum level between opposite grids without NFB? Not sure but maybe the explanation is that the wiring of the second PI tube injects some bias hum to its grid. It will then be amplified by a factor of 50 or so.
    This would require much better bias filtering.

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    Last edited by Helmholtz; 09-08-2019 at 07:21 PM.
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    This is what I get from power tubes grids. Yellow is first section, Blue the paraphase.

    Click image for larger version. 

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    At least the measured values are close there is a difference 136-120mV= 16 mV
    Global nfb was not applied

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    How much ripple do you measure at the speaker output without NFB?

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    Quote Originally Posted by Helmholtz View Post
    How much ripple do you measure at the speaker output without NFB?
    It shows 32 mV on scope

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  19. #54
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    Quote Originally Posted by Helmholtz View Post
    How much ripple do you measure at the speaker output without NFB?
    The real question.
    And how ripple manifests in speaker to sound.

    To such demanding "musicians" I have a habit of telling them ... bring the same model of amplifier that sounds different from yours ...

    EDIT 190908
    32 mV on scope, is this measured on the speaker?
    The GA 40 can output max 20W at 8 Ohm.
    That's 12V~ voltage on the speaker 8 Ohm.
    Dear colleague, calculate the signal/noise ratio for 12V/0.032V

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    Last edited by vintagekiki; 09-08-2019 at 08:04 PM.

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    The level of ripple in post #42 doesn't seem unreasonable. I'd expect getting on for 1V p-p on the raw bias supply with a diode and 100u capacitor straight off the winding. Try adding a 100u from each bias pot wiper to ground, that should be a lot more effective than adding 100u to the first cap (unless the wipers are right at the top end). Of course it's common mode and should be rejected as H. said.

    I think a common mode signal applied to the outputs through 220k resistors should produce out of phase signals at the actual outputs. The virtual earth triode has low output impedance compared with 220k so it will overpower a signal applied that way but its input is the output of the other triode and that signal should be inverted and appear at the virtual earth triode's output. Why doesn't that happen? Is there a shed load of phase shift going on because of the capacitor values and low frequency?

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    Last edited by Dave H; 09-08-2019 at 08:02 PM.

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    Quote Originally Posted by vintagekiki View Post
    The real question.
    And how ripple manifests in speaker to sound.

    To such demanding "musicians" I have a habit of telling them ... bring the same model of amplifier that sounds different from yours ...
    The noise as it is is not a real issue for a geetar amp, is extremely low, I just can heard it if is completely silent in my room. But as my project is not an amp for sale but for my instructable purpose I wanna push the things as far can be knowing why and how....?

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    Quote Originally Posted by Dave H View Post
    The level of ripple in post #42 doesn't seem unreasonable. I'd expect getting on for 1V p-p on the raw bias supply with a diode and 100u capacitor straight off the winding. Try adding a 100u from each bias pot wiper to ground, that should be a lot more effective than adding 100u to the first cap (unless the wipers are right at the top end). Of course it's common mode and should be rejected as H. said.

    I think a common mode signal applied to the outputs through 220k resistors should produce out of phase signals at the actual outputs. The virtual earth triode has low output impedance compared with 220k so it will overpower a signal applied that way but its input is the output of the other triode and that signal should be inverted and appear at the virtual earth triode's output. Why doesn't that happen? Is there a shed load of phase shift going on because of the capacitor values and low frequency?
    Thanks Dave. There are a lot of things which complicate my life regards the options in bias circuit. First the voltage is almost in limit to provide a decent ecart. Second ,it is a low current source (see how I did the supply just for the reason to not draw to much current) .Third raising the caps value can be a serious problem for effective charging time, leaving by accident (disrupted supply) the power stage unbiased while caps still charging. Now with all that infos You was kind to provide it is clear what should be done, the next step is how...? Thank You All !

    Full rectification can be an option...will see.

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    Quote Originally Posted by catalin gramada View Post
    This is what I get from power tubes grids. Yellow is first section, Blue the paraphase.

    Click image for larger version. 

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    At least the measured values are close there is a difference 136-120mV= 16 mV
    Global nfb was not applied
    There's some phase shift there. Perhaps that's why the ripple isn't rejected more?

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    Quote Originally Posted by Dave H View Post
    There's some phase shift there. Perhaps that's why the ripple isn't rejected more?
    It was a question at the beginning of the thread. The reason I opened this thread regards how I can configure the floating paraphase adding some caps into circuit. The main reason was to know how I can keep the outputs into right phase over audio frequency range. Maybe shifting poles can help a little bit, but for the moment I.m pretty happy with values I choosed from a sound perspective.

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    It shows 32 mV on scope
    Now this seems to indicate that there should be another reason as power amp gain is typically below 1. Can you please measure grids' and output ripple with both PI coupling caps disconnected.

    And yes, the phase shift by the 47nF cap is an issue. Does your scope provide differential mode (Channel A - Channel B) operation? If so, what is the differential ripple voltage between the grids without NFB?

    Regarding bias filtering, please consider that inserting a series resistor before the filter cap in the k Ohm range might be much more effective than doubling capacitance.

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    Last edited by Helmholtz; 09-08-2019 at 09:06 PM.
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    Quote Originally Posted by Helmholtz View Post
    Regarding bias filtering, please consider that inserting a series resistor before the filter cap in the k Ohm range might be much more effective than doubling capacitance.
    Roger that, Thank You

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    Since you are expecting a hi-fi amplifier performance from a guitar amplifier, I'm afraid you will have to do a lot of work on grounding and shielding and revision the parts, as oldtimer built-in parts are a reflection and spirit of his time.

    EDIT 190908
    When the amplifier is brought with the measuring instruments "under the thread", the question arises ... what's left of the sound ...

    Tip
    Try for comparison purposes to perform identical measurements on another similar guitar amplifier.

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    Last edited by vintagekiki; 09-08-2019 at 10:25 PM.

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    OT output with grids disconected from PI (4.7mV ripple)

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    Grids not coupled to PI (there are identical superimposed )


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    Sorry I have to learn what differential mode scope meant. Please to explain for a truck driver way of understanding. Thanks

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    Last edited by catalin gramada; 09-08-2019 at 09:53 PM.
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    Do You think the ripple is injected into the PI through feedback loop, than returned to power grids with different phase shifts the difference will be amplified instead to be cancelled, please ?

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    Wow, so disconnecting the PI significantly increases ripple voltage at the grids without NFB??

    What about differential measurements? Point is that the phase shift causes the differential voltage between the grids to be larger than the difference between peak values.

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    Quote Originally Posted by catalin gramada View Post
    Do You think the ripple is injected into the PI through feedback loop, than returned to power grids with different phase shifts the difference will be amplified instead to be cancelled, please ?
    No, there is no such indication as ripple is stronger without NFB.

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  32. #67
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    A seemingly stupid question.
    Is there ripple when all tubes from the amplifier are removed and the amplifier is on?

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    I couldn't understand why it didn't work as I described (badly) in post #55 (outputs out of phase) so I decided to simulate it by injecting a 50Hz sine wave where the bias supply connects. The first plot below is as the schematic Catalin posted. It shows a small phase shift. I then changed the capacitors to 4u7 to push the effects of their phase shift well below 50Hz and produced the second plot which now has the outputs 180* out of phase.

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    Quote Originally Posted by vintagekiki View Post
    A seemingly stupid question.
    Is there ripple when all tubes from the amplifier are removed and the amplifier is on?
    Are You asking about OT output with tubes removed ? Yes it is . This is what I get:

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    Well, that is direct PT to OT leakage field coupling.

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