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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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19th Nov 2018, 11:10 pm | #1 |
Octode
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
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What if I take my DC feed to an output transformer CT off the reservoir capacitor?
Mulling over the circuit for the rework of the EL34 amp.
Now as things stand, the volts to the output transformer CT are taken after the smoothing choke on a standard CLC filter. All well and good. But ideally I would like a bit more voltage and straight after the reservoir capacitor off the rectifier would indicate its about right. As I understand how these things work, a push pull output stage has a kind of natural noise and hum cancellation property, at least of common mode noise, so theoretically taking the power straight off the reservoir cap should be ok. There are some older designs that show this, either theres no filter or its a simple RC filter to the preceding amplifying stages. Can anybody see a reason why I can't do this, bearing in mind if I find empirically that theres a bit too much hum I can simply move the wire to the downstream end of the choke. Might be a silly question but It was something that struck me when reading up on push pull amps. Andy. |
19th Nov 2018, 11:31 pm | #2 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
It's pretty much standard practice on guitar amps for the OT to connect to the reservoir cap so don't see why it should be a problem.
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19th Nov 2018, 11:40 pm | #3 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
Tetrodes/pentodes have a high anode impedance and are relatively uninfluenced by a hummy anode supply, at least compared to triodes. The screen grid, however, can make quite an effective modulating electrode, so is best connected post-choke in the typical CLC PSU set-up. This obviously implies that ultra-linear operation is ruled out, or at least "difficult", but I don't suppose many guitar amps used U-L.
Certainly suck-it-and-see- it'll only be shifting a wire or two! |
19th Nov 2018, 11:41 pm | #4 |
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
In theory if the output valves are closely balanced there should be little hum, even with a significant ripple on the center tap as the plate hum currents phase cancel in each half of the primary. This was one of the many advantages cited for push pull over class A operation. In addition there is no net DC magnetizing flux, so the output transformer cores can be smaller than for class A output transformers.
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19th Nov 2018, 11:44 pm | #5 |
Dekatron
Join Date: Sep 2007
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
Cancellation of the hum which arises from HT rail ripple is, as you say, a feature of push-pull amps. But it does depend on the quiescent currents in the two output valves being matched. In the real world there is always some mismatch. The worse this mismatch is, the worse the hum will be.
A more intractable issue is that the HT voltage variation will impose a corresponding variation in the gain of the output stage, and the push-pull configuration will not cancel this. The consequence of this gain variation is that the audio signal will become (inter)modulated by the ripple so, for example, a 1kHz signal will acquire sidebands at +/-100Hz, +/-200Hz etc. The resulting frequencies will not be harmonically related to the fundamental and this will make them sound unusually offensive (it's been known for a long time that intermodulation distortion is much more objectionable than harmonic distortion). Back in the day relatively simple distortion analysers didn't have the selectivity needed to measure these 100Hz sidebands and I suspect that at least some manufacturers believed that the distortion in their amps was actally lower than it really was. I've seen such sidebands present at levels comparable with the harmonic distortion i.e. tenths of a percent. Cheers, GJ
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19th Nov 2018, 11:55 pm | #6 |
Octode
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Location: Penrith, Cumbria, UK
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
right that does explain why you generally see the supply to CT after a CLC filter.
That decides it, I shall stay conventional. |
20th Nov 2018, 1:12 am | #7 |
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
True but you would need substantial ripple to amplitude modulate the output from a push pull output stage. So a few volts ripple on a 400V supply would be insignificant in gain alteration of the stage. If you were not happy with the ripple level say if it was over 10 or 20V simply increase the filter capacitor within allowable limits depending on the rectifier (if thermionic) and add a small series current limiting resistor to keep the valve rectifier's peak cathode current down to recommended limits, if silicon rectifiers, don't worry.
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20th Nov 2018, 1:24 am | #8 | |
Dekatron
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
Quote:
It's actually more common to see the OP transformer CT fed direct from the reservoir capacitor in modern hifi amps. The reason is that solid-state rectifiers can now be very hefty and can therefore drive a very large reservoir capacitor. And large capacitors are much cheaper than they once were. Cheers, GJ
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20th Nov 2018, 12:58 pm | #9 |
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
Hum intermodulation from the supply rail is more likely with a triode or UL PP ouput than a pentode PP with clean g2 supply.
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20th Nov 2018, 2:09 pm | #10 | |
Dekatron
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
Quote:
To check the impact of this 7V RMS ripple I disconnected the negative feedback and ran the amp (nominal capability 100W/ch) at 8W output at 1kHz. The fundamental signal was at 18dBV. The 900Hz and 1100Hz sidebands were measured at -16.1dBV, the 800Hz and 1200Hz ones were at -25.3dBV and the 700Hz and 1300Hz ones were at -37.4dBV. By comparison the harmonic distortion components were at -21.9dBV (2kHz), -23.5dBV (3kHz) and -50.4dBV (4kHz). So the first set of ripple-driven sidebands were substantially larger than the largest harmonic component. Even the second set of sidebands, given that there were two of them, were comparable with the largest harmonic. Bearing in mind that the anharmonic distortion is also more offensive than the harmonic I think it's clear that the ripple-driven features are the herd of elephants in the room. Assuming the intermodulation was linear I could have reduced the ripple distortion by, say, 10dB by tripling the reservoir capacitors. But 600uF 450V electrolytics would have been quite a big ask back in the 1960s. Instead I fitted a C-L filter (50uF and 2.5H) before the existing 100uF capacitor set. This dropped the 900Hz/1100Hz components by 33dB and the 800Hz/1200Hz ones by 40dB. That really did render them insignificant . Cheers, GJ
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20th Nov 2018, 3:32 pm | #11 |
Rest in Peace
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
Your IM figures are, within a factor of 2, what a back-of-envelope estimate would suggest. Always good when theory and practice tell the same story!
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20th Nov 2018, 3:39 pm | #12 |
Dekatron
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
Thanks, that's good to know. I confess I didn't attempt the theory on this. It's worth pointing out that the distortion levels are not, overall, high. Without feedback each sideband is 34dB below the fundamental. The amp has 26dB feedback so when it's applied each sideband would correspond to 0.1% distortion. The harmonic distortion is a good deal less than this at 8W out but it will increase with output power and so may, at some point, overtake the intermodulation. Then again as the amp moves out of class A it will draw more average current from the power supply and the ripple voltage will increase too.
Cheers, GJ
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20th Nov 2018, 5:11 pm | #13 |
Rest in Peace
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
10V pk AC on 600V DC is roughly 1.7%. This could cause gain of the output to vary by maybe 2.5% (roughly 1.7% from direct transconductance, about half as much again from anode impedance). 2.5% gain variation will cause 1.25% 2nd-order intermods. 1.25% is -38dB. You found -34dB.
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20th Nov 2018, 9:28 pm | #14 | |
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Re: What if I take my DC feed to an output transformer CT off the reservoir capacitor
Quote:
Last edited by Argus25; 20th Nov 2018 at 9:34 pm. |
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