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Old 22nd May 2011, 02:34 PM   #1
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Default motorboating noise due to transformer?

Hello its my first post here. And well, i need help...

I constructed this stereo amplifier with two EL84 in OTL SE triode mode fed by a SRPP 12ax7 gain stage. When the amplifier is turned on, a high frequency motorboating noise and an underlying 60hz hum is heard from the headphones. I used a rather large main filter capacitor 330uf.

I traced the root of the problem from the preamp to the power amp then the filter caps then rectifiers then transformer. When the amp is turned off and let the amp operate for about 5 seconds before emptying the filter caps, this turn off phase makes the amp run very quiet [no hum/motorboating].

Now about the power transformer, I rewound the transformer myself from a rather newly bought 60VA low voltage transformer. Secondaries are 220vac/100mA and 6.3v/3A. I have installed bell caps which were grounded via chassis and now I don’t know what to do. Below is the uhhh flowchart/details of the powersupply section.

Transformer HT – 1n4007 bridge rectifier each bypassed with a 4n7ceramic cap – 150ohm WW resistor – 330uf/400v + 100n/630v capacitor (star ground point) – 100ohm WW resistor – 250V zener shunt MOSFET regulator – 100uf/400v capacitor – power amps

The filament supply is filtered DC and i tried turning off the heater supply while operating the amp but the tzzzzzzzzzzzzzzz noise is still there. if you play electric guitar with a high gain distortion pedal, it sounds like the inherent noise of high gain distortion pedals.

i also tried tying to ground the audio input pins bu the sound just muted, the noise is still present.

How do I remedy a noisy transformer?

thanks in advance
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Old 22nd May 2011, 08:07 PM   #2
DF96 is offline DF96  England
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Circuit diagram? Motorboating is low frequency oscillation, so "high frequency motorboating" is a contradiction. What would you estimate the frequency to be?

Ordinary hum/buzz will disappear as soon as the mains is switched off. What makes you think the transformer is to blame? SET needs much better supply filtering than PP, so you may need a choke instead of the 100 ohm resistor. 100 ohm plus 100uF only reduces 100Hz by a factor of about 6. On the other hand, 330uF wll generate short sharp charging pulses so grounding and routeing has to be correct.
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Old 23rd May 2011, 02:18 AM   #3
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Thanks for the reply Mr. DF96

i used a mosfet shunt regulator (250v) right after the big supply cap. the noise is 120hz sawtooth-ish.

here are some pictures of my newbie build and the schematic.

the wires are to be Click the image to open in full size.

outside. the chassis is from an old-school modem
Click the image to open in full size.

schematic
Click the image to open in full size.
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Old 23rd May 2011, 10:14 AM   #4
DF96 is offline DF96  England
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I'm not clear how the FET regulator works. What establishes the current into the 250V zener on the gate? It seems to be fed from a 12V zener - shouldn't it be fed from a constant current source, not a voltage source? If this strange arrangement simply ensures that the FET is always turned on hard, then it does no smoothing.

What is the 18K feeding the SRPP upper anode doing? Should there be a decoupling cap to ground from here?
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Old 23rd May 2011, 11:04 AM   #5
SY is offline SY  United States
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I think he has the FET drawn backwards and is trying to use it as a source follower "regulator" rather than a shunt. The Zener then makes sense as G-S overvoltage protection. However, the Zener string still needs to be fed by a resistor from the higher voltage (raw supply) side.

A 470u coupling cap on the output? Why?
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Old 23rd May 2011, 11:26 AM   #6
DF96 is offline DF96  England
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Yes, he said "shunt" but drew "series" - I didn't spot that discrepancy. Is this an attempt at a hybrid of regulator and "active choke"? That might explain the extra cap.

Where does the circuit come from?
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Old 24th May 2011, 01:39 AM   #7
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@mr. DF95

oh sorry, a zener shunt feeding a series mosfet might be a more correct term, i think?
im a real ditz for terminology... this hobby gets very lonely you know. hahaha

i traced the circuit from a borrowed headphone amplifier from Mapletree audio. i only used the preamp section though. from what i know SRPP have a better power supply ripple rejection than common cathode designs.

the output stage is based here lampowy wzmacniacz s?uchawkowy OTL

i tried tapping the preamp signal out to a solid state amplifier, the noise is still there.
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Old 24th May 2011, 09:59 AM   #8
DF96 is offline DF96  England
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You need a resistor from the MOSFET gate to the +ve supply, to feed the 250V zener. The zener would then establish the gate voltage, which I assume is what the original designer intended.

SRPP and normal common cathode have similar PSRR - fairly poor. That is why you probably need a decoupling cap from the top anode to ground. The cathode follower output will have quite high input impedance so doesn't need an SRPP to drive it, but for some strange reason SRPP is currently fashionable.
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Old 24th May 2011, 02:38 PM   #9
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oh sorry i left that part out theres a 33k resistor there.

okay so common cathode and srpp have the same PSRR. what are my other options? i mean should i consider another preamp topology? i will add a cap to the preamp anode. another reason why i used srpp is the low output impedance [or so ive heard] so that i could use a lower grid resistor for the cathode follower el84.
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Old 24th May 2011, 03:19 PM   #10
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You don't need that much gain in the driver stage. A simple plate follower will do just nicely to drive a single EL84. Solve the PSRR problem by either applying additional filtering for the plate supply of the driver stage (a simple RC filter will improve things from the current situation) or feed the driver stage from a CSS, e.g. using a depletion mode mosfet such as the DN2540 or the even more popular Ixys 10M45.

But start by breadboarding a simple single triode plate follower driver stage with an RC filter for the plate supply and see how that works; essentially as per the Polish schematic you linked to. Then optimize from there. Half a 'AX7 will provide more than enough gain to drive your EL84 far into distortion, so you can even consider adding some global NFB to improve the circuit you linked to.

And also fix the MOSFET regulator, but that's obvious.

FYI: Wavebourn once made a quick&dirty concept for an OTL PCL84 headphone amp. The PCL84 consists of a triode with a mu of 65 and a pentode which bears some resemblance to a low-powered EL84. The Wavebourn Kaidan design is extremely simple and achieves roughly the same as your amp, but with fewer components. I like its elegance; I've breadboarded it once, and it works. Output power is low (I got maybe 15mW out of it into 32R), but sufficient for most high-efficiency headphones.
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