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

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Old 12th Jul 2020, 2:10 pm   #21
paul1962
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Default Re: Dansette Monarch Amplifier Issues

Hi Edward,
Yes, the hum level is pretty much gone apart from a slight mains hum that is barely audible.
When I use the term microphonic, I come from a musician background and to me it means that an induced current is being supplied. It means that for example, you have a preamp input (such as a guitar lead) that picks up noise, which then goes through an amplifier (or speaker coils) and then gets amplified, which can make one hell of a noise. The controls of the Monarch are so close to one of the speakers, that a slight noise from the control panel is in effect amplified by the speaker coils and also must in theory go back from the speaker wires and affect the output stage of the main amplifier. I hope I have explained this as best as I can (but probably not) hahaha.
I guess my main theory is that output comes from the speaker (such as mains hum), then picked up by the controls and fed back to the amplifier (thus the amplified effect of the mains hum) like an endless loop.

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Old 12th Jul 2020, 3:59 pm   #22
Edward Huggins
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Default Re: Dansette Monarch Amplifier Issues

I don't think your explanation quite works in practice as you describe, but never mind.
I know what you mean by Microphonic as I'm a Muso too. I suggest you gently tap the ELC82 valve and if you hear a screechy/feedback type of sound, then that shows that the valve itself has become microphonic, as can sometimes occur, especially over time.
This might be what you are hearing. Let us know how you get on.
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Old 12th Jul 2020, 4:21 pm   #23
AC/HL
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Default Re: Dansette Monarch Amplifier Issues

Do you get a noise whilst rotating the control? If so, it may need cleaning. What you describe sounds more like feedback than microphony. Neither are especially common in these sorts of player.
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Old 13th Jul 2020, 12:33 am   #24
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Default Re: Dansette Monarch Amplifier Issues

Many thanks for your replies Edward and AC/HL

The problems with my particular Monarch is cured, but again, many thanks for further ideas.
Just to clarify, the pots are ok and run without any scratching noises and appear to be fine, with both volume and tones running smoothly and the valve that I have replaced was a new old stock that had been tested on a leakage Valve tester with no known issues.

In regards the mains hum issue, what I found was that if the control unit was removed from the cabinet and rotated at 90 degrees, the mains hum was drastically reduced, (To only about 1 1/2 times that of valve heater hum) but the minute I put the controls back above the speaker, the noise went back to it's original loud state. The control unit was properly earthed and all fine, so the only conclusion I could draw was that the speaker itself was a major factor in the noise. If I kept the 2 apart, the issue went away, but together was back to square 1. After making the shielding box (thus keeping an effectual barrier between the 2, the problem was resolved

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Old 13th Jul 2020, 10:40 am   #25
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Default Re: Dansette Monarch Amplifier Issues

If a component (be it a valve or wire or whatever) is described as "microphonic" it simply means that it converts mechanical disturbance by sound or physical movement into an (unexpected) electrical signal. This could result in unwanted feedback if the offending component is close to the loudspeaker or just odd noises if the component is disturbed.

In this case, I suspect the proximity to the loudspeaker is a red herring, it is more likely proximity to other wiring or components in the same general area.
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