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31st Aug 2012, 11:41 pm | #1 |
Octode
Join Date: Mar 2006
Location: Watford, Hertfordshire, UK.
Posts: 1,270
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Home Brew RIAA Phono Amp
I have been searching a long while to find a good commercial circuit I could adopt, one that ideally had a buffer stage after the RIAA filter stages. I found one but it still needed a tweak to get the upper frequency characteristic correct plus I learnt a lot on the way with regards to the ideal supplies for the op-Amps. My initial stab at the project was with +/- 15V supplies but I found it does not leave much headroom at the bottom end when trying to deal with the required 40dB spread. Increasing the supply to +/-18V seemed to be the answer but the task of getting the whole working stereo circuit onto prototype strip-board lay ahead. My first effort was pretty outstanding and caused some arguments like "you are trying to fool us, you are really playing a CD". Yes it really does sound that good and Josh Ward now has the first build and seems to love it. So I built a second unit with some changes to the front end op-amps (x10 amplifiers) so that 18V rails could be used. With care you could probably build this for less than £25 and it sticks to the RIAA curve like glue. I am more than confident that the board layout is fully repeatable and provided you take care with the track cuts and soldering, should work first time. My home made 3 x ECC83 valve unit also sticks to the RIAA curve but the jury is still out as to which sounds best. The solid state unit seems very up-front and accurate but the valve unit has, well that warmer sound shall we say, but not quite the detail.
The power supply can be fairly simple, I used a twin 0-20 6VA transformer with a 7818 + 7918 combination, without heat sinks as the current draw is so small. The box for the Phono amp was a Hammond Stomp Box. I used non-polarised capacitors for C4,C9,C10,C17,C22 and C23. For C7,C8,C20 and C21 I chose polystyrene as can be seen in the photo. The non-polarised caps were huge! This led to some drifting away from symmetry but without any detriment, both channels are pretty well matched. I just added a final tweak to the board layout to bring the left channel output closer to the phono socket which avoids the long resistor reach you can see in the photo.
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1st Sep 2012, 2:04 pm | #2 |
Octode
Join Date: Mar 2006
Location: Watford, Hertfordshire, UK.
Posts: 1,270
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Re: Home Brew RIAA Phono Amp
Just a post to show the simple power supply I built. The inter-connecting cable was made of screened 4 core RS232 cable terminated with 3 pole connectors similar to microphone plugs. The RCA Jacks in the phono amp are insulated from ground as is the 0V rail, in fact the whole circuit including the power supply circuit is floating. The box is grounded by the screen connection of the inter-connect. I noticed a slight trace of mains hum when doing a quick test due to the phono amp sitting on top of the power supply (I know!). It all vanished when the units were just a few inches apart hence the reason for using separate boxes doh!.
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Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way! |
1st Sep 2012, 3:42 pm | #3 |
Triode
Join Date: Nov 2005
Location: South Manchester, UK.
Posts: 36
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Re: Home Brew RIAA Phono Amp
Very nice - looks like a really well implemented project...
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1st Sep 2012, 5:21 pm | #4 |
Heptode
Join Date: May 2005
Location: Oxfordshire, UK.
Posts: 852
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Re: Home Brew RIAA Phono Amp
That looks fantastic and so simple. I always think that the best sounding circuits are the ones that have the fewest components.
(And the ones you build yourself of course!) James. |
1st Sep 2012, 7:05 pm | #5 |
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
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Re: Home Brew RIAA Phono Amp
Looks very neat - quite a complex circuit, but nicely laid out on the Veroboard
Out of interest, have you tried Douglas Self's design from 1996? It's somewhat simpler than this - 99% of the RIAA is done in the first op-amp, which neatly avoids any headroom problems. In the accompanying article (detailed in his highly recommended books "Self on Audio" and "Small Signal Audio Design"), he examines some of the alternative topologies and concludes that the "all in one" approach is the best all round, apart from the difficulty of calculating the component values (he uses a spreadsheet for this) because the various time constants interact. I notice that most DIYers choose topologies that distribute the time constants to avoid this difficulty, but other parameters such as noise or headroom are usually compromised as a result. Using standard 1% E24 resistors and 1% E6 capacitors (polystyrene), he gets to within +/- 0.05dB. A sub-sonic filter is included as per the IEC amendment. My version is here (note that there are one or two component value errors thanks to poor printing in the article - these were checked with Douglas himself and corrected after the diagram was produced): http://www.mhennessy.f9.co.uk/preamp/Phono.pdf I'm interested to know how you produced the Vero layout diagrams, as I'm in the process of investigating the various design programmes that are out there. The move to 18V supplies does limit the op-amp choice, as within the canon of decent (and sensibly-priced) audio op-amps, only the NE5532/4 are happy with this - though I think these op-amps are very good, despite what people on the audio forums might say. The audio on many recordings and broadcasts travel through 100s of these before they reach us! Anyway, if you have tried the Self circuit, I'd be interested in your comments. It's been about 10 years since I last evaluated different circuits before settling on that one, but play a lot more records these days so perhaps it's time to experiment again... Cheers, Mark |
1st Sep 2012, 8:01 pm | #6 |
Octode
Join Date: Dec 2005
Location: Near Stowmarket, Suffolk, UK.
Posts: 1,962
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Re: Home Brew RIAA Phono Amp
As Les says he has kindly donated the first 'Les Box' (as I have christened it) to me.
It really does sound brilliant, very detailed (and therefore a superb match for my Ortofon 2M) although perhaps a little light on bass hence the latest experimentations. Hopefully if successful I will be able to modify mine. It sounds so much nicer than the VSPS I was using before (which I never did get to the bottom of, I suspect a poor PSU design above all else- very little smoothing!). The VSPS is now sitting awaiting me to post it to Les to see if he can tweak it a bit. Mark, thanks for sharing your link to the Douglas Self circuit, I think I may have most of those components here so I may have to knock one up sometime (I'll add it to the projects list ). Is your diagram updated with the correct values? Josh. |
1st Sep 2012, 8:35 pm | #7 |
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
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Re: Home Brew RIAA Phono Amp
Hi Josh,
Look to the feedback loop around the first op-amp - there is 51K in series with 2K7. These resistors should be 62K and 1K5. Cheers, Mark |
2nd Sep 2012, 6:25 pm | #8 |
Heptode
Join Date: Sep 2010
Location: Hythe, Hampshire, UK.
Posts: 626
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Re: Home Brew RIAA Phono Amp
Nice project, particularly like the box, I built something very similar a few years ago. If you want to improve the performance substitute the 5532's and 5543's with LME49860's. You can raise the supply voltage to +/- 22V, the signal to noise ratio will improve and drive capabilities of these OP-AMP's is far superior to the 5534's - they will quite happily drive a 600 ohm load to within a volt or two of the supply - also the slew rate is just over double. Of course you would have to contruct it from sctratch again but you will have one less physicla OP-AMP package on the board!
Peter |
7th Sep 2012, 4:40 pm | #9 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,636
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Re: Home Brew RIAA Phono Amp
A very neat job as always. Could you post your ECC83 preamp details/circuit please?
Andy.
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8th Sep 2012, 1:51 pm | #10 |
Octode
Join Date: Mar 2006
Location: Watford, Hertfordshire, UK.
Posts: 1,270
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Re: Home Brew RIAA Phono Amp
The valve amp is based on the WAD design, they went out of business before I could buy one. Fortunately they made public some of their circuits and the Phono Amp was one of them. Searching Forums gave some modifications and other tips so I decided to start collecting pieces to build one. I have attached the circuits I used for the amp and power supply, trying various valves for V3 like 6N1 and ECC82, finally adopting the ECC82. I was a bit optimistic with the enclosure, anyone with half a mind as me would have built it in a larger box. But we achieved the impossible and the performance of it made the project so very worthwhile. I used DC Heaters, elevated too and the remote PSU box keeps hum totally non-existent.
It was a good job I decided to build with skirted valve holders from the start, screening cans are Mandatory!
__________________
Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way! |