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Post by Deleted on Dec 18, 2017 10:55:57 GMT -6
I have a Radial J33 phono pre for my turntable, pleasure listening and client transfers to digital. I love it, it sounds great. However, its "rumble" filter is -3dB at 100Hz, which is very, very audible, too audible for me to be able to use on most records, especially if I am doing a transfer to digital. I have no idea how easy or difficult it would be to mod this, so that the cutoff went down to something like 20Hz instead. It would be far less audible, but still be getting rid of all that sub bass DC crap down there (I can see it on an analyser). I am hoping it might be something like a simple capacitor or resistor swap, to lower the cutoff frequency. I have no idea if it's an active or passive HPF. There are some deets on the Radial page: www.radialeng.com/j33-specs.phpBut last time I contacted Radial about modding it (I wanted to change the impedance to better match my cartridge), they poo pooed the idea, and so I am pretty sure they'd be unhelpful again in this case. Any modder/solder monkeys here have any advice? |
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Post by svart on Dec 18, 2017 11:30:59 GMT -6
Those are the questions!
The problem is that it's not as simple as swapping parts, even if you have the schematic.
Phono filters are notorious for being finicky and ultimately hard to design.
The high impedance makes swapping parts harder since you essentially have to "tune in" the parts because the loading is so touchy.
There is also the case of the filter might be shown separately on the block diagram, but be designed into the riaa filter in practice.
Looking at the graphs, I see the distortion rise greatly below 40hz, which makes me wonder if the filter knee isn't actually at 40hz instead of 100hz..
There's no way to know if it's active or passive without a schematic or pictures of the guts.
I wish my friend Brad was still around, he designed many different phono front-ends for some high-class designs and he'd know exactly what to do. Sadly he was one of the last great phono design gurus.
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Post by Deleted on Dec 18, 2017 14:38:52 GMT -6
Thanks, seems there's no simple tweak then! I'll either have to use the Bax EQ in the chain before the ADC, or just use software HPF after capture, for client transfers. I kinda thought it might not be so easy.  All I know is I can't bear to have the rumble filter in, because with many genres/albums most of the deep bass just disappears, not good. |
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ericn
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Post by ericn on Dec 27, 2017 11:23:32 GMT -6
Yeah your best bet is to use an analog filter before the AD so the rumble and other LF noise isn't eating up all your headroom!
Great application for a pair of Chopshops!
There was a very cool pre for 78 transfers built by the guy who did all the test bench stuff for Audio Magazine, it had a variable RIAA curve and filters!
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Post by Deleted on Dec 27, 2017 13:43:18 GMT -6
What are Chopshops, Eric?
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Post by ericn on Dec 27, 2017 14:15:56 GMT -6
What are Chopshops, Eric? 500 series variable HPF & LPF with single knob EQ from LOUDER THAN LIFTOFF! One of those little things we should all have a bunch of! |
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Post by jimwilliams on Dec 27, 2017 18:18:15 GMT -6
Double the value of the filter caps and the roll off is 50 hz. Double them again and you are at 25 hz. Do transfers quietly to avoid speaker interaction.
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Post by Deleted on Dec 28, 2017 9:09:12 GMT -6
Thanks Jim, quadrupled value filter caps sound like the way to go then. Now just to try and work out which ones they are, haha. Might try hitting Radial up with a support request again.
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Post by Deleted on Jan 9, 2018 10:48:52 GMT -6
From Radial:
"Unfortunately, we do not have a modification that I can recommend to the J33 to change the cutoff point of the HPF. I would instead suggest disengaging the HPF entirely and passing the outputs of the J33 through a separate filter that rolls off below 20Hz."
So, the same kind of unhelpful response I got from them when I asked about better impedance matching with my phono pre. Oh well.
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Post by svart on Jan 9, 2018 12:38:10 GMT -6
From Radial: "Unfortunately, we do not have a modification that I can recommend to the J33 to change the cutoff point of the HPF. I would instead suggest disengaging the HPF entirely and passing the outputs of the J33 through a separate filter that rolls off below 20Hz." So, the same kind of unhelpful response I got from them when I asked about better impedance matching with my phono pre. Oh well. A lot of manufacturers won't help on things like this since it's hard for them to keep track of modding. If they started helping people void warranties, it becomes a quagmire of people screwing things up and then sending in broken equipment with the excuses of "but you told me to do it" and then it becomes legal he said/she said.. It's an unfortunate but not unexpected result of the litigious world we live in now. |
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Post by Deleted on Jan 9, 2018 13:34:31 GMT -6
Yep, and for obvs. reasons they won't supply you with the circuit diagram either, so you can't even get a competent third party tech to look at it...
From Jim's info, I'm gonna have a hunt for all the caps on the PCB and see if my brain can work out which the filter caps are and their values... Wish me luck!
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Post by svart on Jan 9, 2018 14:01:04 GMT -6
Yep, and for obvs. reasons they won't supply you with the circuit diagram either, so you can't even get a competent third party tech to look at it... From Jim's info, I'm gonna have a hunt for all the caps on the PCB and see if my brain can work out which the filter caps are and their values... Wish me luck! I'd look for the largest value caps in series with the audio signal. If the filters are active, I'd wager that they'd be butterworth with the typical Sallen-Key 2 or 3 pole filter arrangement: sim.okawa-denshi.jp/en/Sallenkey3Hikeisan.htmOr they could be more simple than that. |
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Post by Bob Olhsson on Jan 9, 2018 15:39:58 GMT -6
The HPF was added to the RIAA spec in the '70s. A lot of us prefer the old spec.
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Post by Deleted on Jan 10, 2018 7:32:46 GMT -6
svart Thanks, you are a star! I'll have a look at it today. @bob Olhsson It has a switchable HPF filter (apparently -3dB at 100Hz). As far as I know, with the filter switched out it's regular old RIAA, but the information in the manual or on the website is not very detailed. All I know is it's HUGELY audible and no good for client transfers to digital, whereas if the cutoff was much lower (around 20Hz) it would be much more usable for me. [Edit] Current manual states the HPF is: "200Hz cut off, -3dB at 100Hz" which suggests a single pole filter at 6dB per octave at 200Hz?
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ericn
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Post by ericn on Jan 26, 2018 14:23:03 GMT -6
Easiest solution I can think of is a bunch of Harrison Labs HPF's on a patch bay!
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Post by johneppstein on Jan 27, 2018 1:42:33 GMT -6
Yep, and for obvs. reasons they won't supply you with the circuit diagram either, so you can't even get a competent third party tech to look at it... From Jim's info, I'm gonna have a hunt for all the caps on the PCB and see if my brain can work out which the filter caps are and their values... Wish me luck! I'd look for the largest value caps in series with the audio signal. If the filters are active, I'd wager that they'd be butterworth with the typical Sallen-Key 2 or 3 pole filter arrangement:sim.okawa-denshi.jp/en/Sallenkey3Hikeisan.htmOr they could be more simple than that. I could definitely be in error on this, as I really have not done any work in the area of phono compensation in probably half a century, but IIRC the RIAA filter curve is a complex S shaped curve that doesn't really adhere to any of the standard filter topologies. But I was very young back them and may have misinterpreted the schematics I was working with. I do believe that it does take the inductance and/or impedance of the magnetic cartridge into account in the design of the circuit. |
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Post by svart on Jan 27, 2018 8:44:39 GMT -6
I'd look for the largest value caps in series with the audio signal. If the filters are active, I'd wager that they'd be butterworth with the typical Sallen-Key 2 or 3 pole filter arrangement:sim.okawa-denshi.jp/en/Sallenkey3Hikeisan.htmOr they could be more simple than that. I could definitely be in error on this, as I really have not done any work in the area of phono compensation in probably half a century, but IIRC the RIAA filter curve is a complex S shaped curve that doesn't really adhere to any of the standard filter topologies. But I was very young back them and may have misinterpreted the schematics I was working with. I do believe that it does take the inductance and/or impedance of the magnetic cartridge into account in the design of the circuit. Well, you're not wrong. In fact, you'll scroll up and see that I advised against it in my first post, for exactly those reasons. However, the RIAA curve doesn't necessarily include a HPF, so I would hope that the designers of the circuit would have just tacked it on separately instead of designing it into the filter as a system.. But OP seems determined to mod it, so I thought I'd at least offer some kind of help so that they don't outright destroy their circuit.. |
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Post by johneppstein on Jan 27, 2018 13:17:17 GMT -6
I could definitely be in error on this, as I really have not done any work in the area of phono compensation in probably half a century, but IIRC the RIAA filter curve is a complex S shaped curve that doesn't really adhere to any of the standard filter topologies. But I was very young back them and may have misinterpreted the schematics I was working with. I do believe that it does take the inductance and/or impedance of the magnetic cartridge into account in the design of the circuit. Well, you're not wrong. In fact, you'll scroll up and see that I advised against it in my first post, for exactly those reasons. However, the RIAA curve doesn't necessarily include a HPF, so I would hope that the designers of the circuit would have just tacked it on separately instead of designing it into the filter as a system.. But OP seems determined to mod it, so I thought I'd at least offer some kind of help so that they don't outright destroy their circuit.. Yeah, I'd do what you suggested too - put a simple HPF in series with the existing RIAA filter. Much simpler and more straightforward, less chance of seriously mucking things up. After thinking about it I'd probably have the HPF follow the RIAA section. |
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Post by Deleted on Jan 27, 2018 15:19:09 GMT -6
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