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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 10th Jul 2019, 11:25 am   #121
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Default Re: Mullard 5-10 amplifier

Thanks guys,

I've installed a 470k resistor and the hiss is a lot better.
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Old 10th Jul 2019, 12:31 pm   #122
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Default Re: Mullard 5-10 amplifier

Originally Posted by vampyretim View Post
Thanks guys,

I've installed a 470k resistor and the hiss is a lot better.
That's good. As you've reduced the resistor by a factot of 4, I'd expect the noise to be 12dB lower than with the 2.2meg. That's a very noticeable improvement.

Of course, once the preamp is connected, its low output impedance will dominate the thermal noise picture at the power amp input, so the change from 2.2meg to 470k will have been a bit academic. It'll then be the input stage of the preamp that will generate most of the audible noise.

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Old 10th Jul 2019, 12:46 pm   #123
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Default Re: Mullard 5-10 amplifier

You shouldn't have to do this modification. I don't remember any of this excessive hiss problem with these. If there really is excessive hiss, then you need to find the real cause. You say again about the hum problems with the pre-amp, well I've already told you the most likely cause of that in a previous post. Keep up the good work!
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Old 10th Jul 2019, 8:14 pm   #124
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Default Re: Mullard 5-10 amplifier

Well the hiss is clearly coming from the grid resistor so the only other "problem" could be the gain of the amplifier as set by the feedback.

Different components are mentioned for different speaker impedances but they should be such that they give the same degree of feedback for the different output voltages from the amp depending on the speaker impedance. The design article only shows one secondary on the output TX so one would assume that the TX ratio was chosen for one particular load and the appropriate feedback network fitted. Possibly someone may have thought that to change the speaker load, all that was needed was to change the feedback network which would increase the gain if the 15R network was connected to a 3R output TX.

You could try measuring the actual amplifier gain and comparing it with what it should be which is quoted as 40mV input for full output.

Further digging may be required to measure output TX ratio if it's not multi-tapped and there's no helpful markings!

Also bear in mind that the hum and noise spec limit for the 5-10 is "only" -75dB relative to full output. Typical modern amps will manage about 20dB or more better than this!
.=.| _---\__|__|_---_|.

Last edited by Herald1360; 10th Jul 2019 at 8:27 pm.
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Old 13th Jul 2019, 5:28 pm   #125
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Default Re: Mullard 5-10 amplifier

I would put it back to the original 2.2M and wait to see what happens when you connect the pre-amp.

My guess is pickup of RF interference but checking the feedback sounds worthwhile.
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