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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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2nd Jul 2020, 9:44 am | #41 |
Nonode
Join Date: Aug 2010
Location: Resolfen, Wales; and Bristol, England
Posts: 2,588
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Re: Help! JLH RIAA pre-amp
Okay, to get back to reality and think about my JLH amplifier, I decided overnight to put a 20dB gain section in between the RIAA pre-amp and the input selector switch. That should give me about 600mV which matches quite well with the other sources. I have the Mullard Transistor Circuits book and there's a nice little 2xBC108 gain stage that should do the job nicely and will save sorting a seperate power supply for the Baxendall NE5534 idea.
I do need to do something about the screened cable from the pickup to the amplifier though. The cable on it's own measures 300pF.
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2nd Jul 2020, 1:10 pm | #42 |
Dekatron
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Location: Kington, Herefordshire, UK.
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Re: Help! JLH RIAA pre-amp
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2nd Jul 2020, 3:32 pm | #43 |
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Re: Help! JLH RIAA pre-amp
I am always amazed by the length of reports/papers etc. the same can be said in 20% of the words. I have to write the odd report (technical) and try my best to make them short, understandable and slightly humorous. It makes them more likely to be read and used.
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2nd Jul 2020, 4:45 pm | #44 |
Moderator
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Location: Fife, Scotland, UK.
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Re: Help! JLH RIAA pre-amp
I know that they're not paid by the word, but I wonder if the peer-reviewed published word is the base unit of academic kudos ?
David
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2nd Jul 2020, 4:57 pm | #45 | |
Nonode
Join Date: Aug 2010
Location: Resolfen, Wales; and Bristol, England
Posts: 2,588
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Re: Help! JLH RIAA pre-amp
Quote:
I'm not into audiophool territory but I'd like to use this amplifier build to understand a bit more about things like input capacitance and matching without transformers.
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2nd Jul 2020, 6:20 pm | #46 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
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Re: Help! JLH RIAA pre-amp
That's a hefty cable capacitance, or very long cables. Low capacitance coax is typically 40pF-60pF per metre, a typical length, which means that you can select an additional capacitor in the input to the phono stage to tune the cartridge HF response.
Craig |
2nd Jul 2020, 6:33 pm | #47 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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Re: Help! JLH RIAA pre-amp
This is actually a guitar cable by Van Damme. 55pF per metre.
I'd go for that or something similar, and make up your own leads to suit what it connects to at the deck and amp. https://www.worldwidemusic.co.uk/van...tre-1451-p.asp Craig |
2nd Jul 2020, 8:32 pm | #48 | |
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Re: Help! JLH RIAA pre-amp
Quote:
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2nd Jul 2020, 11:28 pm | #49 |
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Re: Help! JLH RIAA pre-amp
Plan B would be to shorten the cables greatly and locate the RIAA stage close to the arm pivot. You then have low enough capacitance for anything and can pad it up to where it needs to be.
David
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3rd Jul 2020, 9:45 am | #50 |
Nonode
Join Date: Aug 2010
Location: Resolfen, Wales; and Bristol, England
Posts: 2,588
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Re: Help! JLH RIAA pre-amp
Yes, for some reason I fitted cables of 3m length some years ago, I don't remember why but that makes them 100pF per metre which isn't too bad. I think I'd better shorten them a bit.
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6th Jul 2020, 7:18 am | #51 |
Nonode
Join Date: Aug 2010
Location: Resolfen, Wales; and Bristol, England
Posts: 2,588
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Re: Help! JLH RIAA pre-amp
I’ve put together a 10dB gain stage which fits after the RIAA pre-amp and lifts the output of that to the same level as the other inputs. I’m happy enough, now I just need to see to adequately cooling the output devices.
Thanks for all your help.
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