Poundland 45

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Cressy Snr
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#46 Re: Poundland 45

Post by Cressy Snr »

OK,
So fitting the perforated steel baseplate has introduced a hardness into the sound. A characteristic I’d gotten rid of has returned. WTF how can this be? It doesn’t make any sense.
Anyway the baseplate came off toot fecking sweet, and normal service was resumed. I still don’t get it.
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Dave the bass
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#47 Re: Poundland 45

Post by Dave the bass »

I bet its rubbish at playing dub now.

There's deffo less base...

Arf!
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Cressy Snr
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#48 Re: Poundland 45

Post by Cressy Snr »

Crazy 🤔 :D
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IslandPink
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#49 Re: Poundland 45

Post by IslandPink »

Does this add weight to your arguments about the role of microphonics in the SE sound, Nick ? :)
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Nick
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#50 Re: Poundland 45

Post by Nick »

IslandPink wrote: Thu Jul 29, 2021 6:55 pm Does this add weight to your arguments about the role of microphonics in the SE sound, Nick ? :)
It could if I tried hard enough :-)
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pre65
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#51 Re: Poundland 45

Post by pre65 »

Steve, is it the base panel that's causing the problem, or the fact that it's grounded ?
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Nick
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#52 Re: Poundland 45

Post by Nick »

I guess you could experiment to see if it was the earthing and capacitive coupling by using a sheet a aluminium foil on the base so it would electrically look like the previous sheet, but not have anywhere like the mass to alter the chassis mechanical dynamics.
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ed
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#53 Re: Poundland 45

Post by ed »

perforated plastic sheet...but it's not cheap.
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Nick
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#54 Re: Poundland 45

Post by Nick »

I have seen a couple of Japanese built amps that use this sort of stuff on the bottom.

https://www.inoxia.co.uk/products/mesh/ ... 4AQAvD_BwE
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#55 Re: Poundland 45

Post by simon »

Piece of thin ply with lots of holes drilled in it? Offcut of nasty mdf radiator cover?
Cressy Snr
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#56 Re: Poundland 45

Post by Cressy Snr »

Well, having been out for my daily walk earlier today, I thought of flux leakage from somewhere, getting into the steel, then making its way from there into the audio circuit via capacitive coupling. The only possible culprit seemed to be the rectifier heater transformer, The 240V primary coil was very close to the steel base when it was fitted: only 1mm at most away from it.

The simple expedient of making a cutout around the offending transformer appears to have solved the issue. A/B’ing with and without the baseplate was tedious in the extreme (I hate doing that sort of thing) but on this occasion it was, I felt very necessary. I couldn’t tell any difference with or without the modified baseplate, whereas with and without the un-modded plate, the nastiness became pretty obvious. So it appears that the rectifier heater transformer was the source of the problem.
B445AC61-4010-40FF-ADC0-B0B5D7A8EB06.jpeg
B445AC61-4010-40FF-ADC0-B0B5D7A8EB06.jpeg (169.91 KiB) Viewed 2896 times
Thanks for the advice everyone. It was capacitive coupling as Nick suggested it might be. Just need to find a bit of insulating material to cover the hole. Either that or get some of that copper mesh Nick linked to.
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Nick
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#57 Re: Poundland 45

Post by Nick »

It may also haev been the transformer injecting 50Hz mechanical energy into the base which then is spread into the amp. I know from the turntable power supply how injecting low frequency and its harmonics can mess up the mid range.
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#58 Re: Poundland 45

Post by IslandPink »

Fascinating !
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#59 Re: Poundland 45

Post by RhythMick »

Fascinating. Glad you found it and well done for proving it with the cutout, and for posting it. We've found similar results but not followed through to prove it so thanks.

We've used material like Tufnol for non-conductive safety enclosures.
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#60 Re: Poundland 45

Post by Cressy Snr »

Had a little sort out of the 45 amp over the course of last week.
I played around for a while with a 12AT7 as a driver valve trying to accommodate the Cole phono and it sounded great, apart from the fact that the gain was too much once I went back to the MoFi phono. I went back to the ECC88, which was OK but still slightly too much gain.
In the end I steepened the load line from 27K to 12K, lowered the HT to the drive section by means of a series connected 0B2 glow tube and doubled the bias voltage by adding another series LED in the cathode circuit.
I removed the 2x220u Black Gate power supply cap, replacing it with a 2x100u JJ component and upped the first cap to 47uF, changing the rectifier to a 5U4G.
I now plan to use that BG cap in the forthcoming phono stage PSU, where it might be better employed.
Gain has dropped nicely and there are no penalties incurred in terms of the sound quality, even though quite a few changes have been made.

This is the current situation:
45cctV4.png
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45PSUV4.png
45PSUV4.png (71 KiB) Viewed 2598 times
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