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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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Old 2nd Jul 2020, 9:44 am   #41
mole42uk
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Default 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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Old 2nd Jul 2020, 1:10 pm   #42
Ted Kendall
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Default Re: Help! JLH RIAA pre-amp

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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.
Which pickup cartridge are you using?
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Old 2nd Jul 2020, 3:32 pm   #43
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Default 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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Old 2nd Jul 2020, 4:45 pm   #44
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Default 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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Old 2nd Jul 2020, 4:57 pm   #45
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Default Re: Help! JLH RIAA pre-amp

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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.
Which pickup cartridge are you using?
Shure M75ED2. Reckons to be good into 47kΩ @ 400-500pF but the cable alone is 300pF and I don't know how to calculate the amplifier input capacitance. I have read that the Shure cartridge sounds better into a lower capacitance so maybe there's room to play around there.

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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Old 2nd Jul 2020, 6:20 pm   #46
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Default 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.

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Old 2nd Jul 2020, 6:33 pm   #47
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Default 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

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Old 2nd Jul 2020, 8:32 pm   #48
Ted Kendall
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Default Re: Help! JLH RIAA pre-amp

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Originally Posted by mole42uk View Post
Shure M75ED2. Reckons to be good into 47kΩ @ 400-500pF but the cable alone is 300pF and I don't know how to calculate the amplifier input capacitance. I have read that the Shure cartridge sounds better into a lower capacitance so maybe there's room to play around there.
There's always somebody on a forum (not usually this one, to be fair) who thinks they know better than the manufacturer. Shures, most pickups come to that, work best and give the flattest response into the recommended load. This series did have the vestiges of the mild upper mid range dip which was evident in the V15/ii and ameliorated in the V15/ii Improved and 75t2 series, but reducing the load capacitance doesn't improve matters. That said, there's no harm in having some margin to play with - SME supplied the III series with low capacitance cables and a little 300pF cap in the phono plugs which you could snip out if you were using a cartridge which wanted to see a lower capacitance, like the contemporary Ortofons.
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Old 2nd Jul 2020, 11:28 pm   #49
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Default 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.

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Old 3rd Jul 2020, 9:45 am   #50
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Default Re: Help! JLH RIAA pre-amp

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That's a hefty cable capacitance, or very long cables. Low capacitance coax is typically 40pF-60pF per metre
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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Old Yesterday, 7:18 am   #51
mole42uk
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Default 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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