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Old 7th January 2003, 03:39 PM   #1
lohk is offline lohk  Europe
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Default Simple RIAA amp from c't magazine

In the German computer magazine "c't" a simple high-class RIAA amplifier was published and is maybe of interest here.
It was mainly constructed for recording/transferring LPs to the digital world. But the design is quite capable and can be used as a good general MM-RIAA amp (very good indeed, and much better than the usual "primitive" one-opamp circuit. It is nevertheless maybe not in the class of the Pearl and other highend designs, but only very few people have the subsequent highend turntables anyway ...)

German speaking members can download more information (article, soundfiles, software) from http://www.heise.de/ct/ .

There is a good quality PCB available for 8 EURO only.

I did two amps with some modifications:

1. Input
I ommitted the 3k9 series input resistor (R1) and changed the 43k input resistor (R2) to the nominal value 47k. To get an input RF filter it seems better to me to use a 3winding ferrite bead instaed of R1. I used a 0R link.

2. Output:
I paralled the output capacitor C7 (in the first version a 10yF Philips K030, in the second version a 100yF Roderstein) with a 1yF Philips MKT-HQ capacitor. Then there is a 100k resistor to ground and a 100R series resistor, driving the cable.

3. Opamps:
Although this RIAA amp works very good with the LM833 (and the NE5532A), I personally think that the OPA2134 sounds far better, too good almost... It reveals the finest distorsions from the cartridge etc. (And nobody should tell me anymore that there is no sound difference between opamps...)

4. Gain:
I ommitted P1, because adjustments have to be permanent, I think.
I used a 27k resistor (40db gain at 100Hz ) first but found that more gain (less feedback) sounds much better. So I replaced P1 with a 47k resistor and a link.

5. Input lowpass:
I used a y33 MKP and a y47 MKT-HQ with equal success.

All resistors and capacitors are selected and measured of course; resistors are metal film from Roederstein and capacitors mostly are MKT-HQ from Philips.

The circuit is orginally constructed for the use with 2 x 9V batteries, I used (beta phase) a standard single LM317/337 psu.
The whole amp is very quiet, almost no noise and completely hum free.

good luck
Klaus
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Old 7th January 2003, 04:03 PM   #2
lohk is offline lohk  Europe
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Sorry typo.
5. should read: Input highpass (rumble filter)
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Old 7th January 2003, 04:08 PM   #3
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If you want to check the RIAA network accurancy I have a Excel spreadsheet.


It's a good designing pratice to not let any capacitor hanging loose the output capacitor in you case. They can pick up charge and/or won't get discharged when the amp is disconnected.

Do you have a photo over the wonder?
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Old 7th January 2003, 09:48 PM   #4
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peranders,

in my modfications list you will see that I changed the schematic at the output.
The output capacitor in this circuit can also be ommitted if the following amp has an input cap anyway.

I must state that I did not develop this schematic not did I calculate the component values. I trusted this c't develloper...

But this schematic - in my opinion - did get some design elements right for a simple riaa-amp:
1. active operation for the "bass" side and passive for the treble side of the riaa curve.
2. do not attempt to use just one opamp
3. do not apply the "upper part" equalisation at the output (like Rod Elliots RIAA amp)

The result is a remarkable noise and distortion free riaa-amp for little money.

I do not have a digital camera nor a scanner, so no photos here (- I am an audio man, btw.). I tested this amp in an open test setup only, but I will do a proper case in the next days ( a classic euro sized alu box I suppose).
But believe me, it is no "wonder", just a little amp with decent sound. I use it with my Systemdek I-920 / Rega250 / AT95 at the moment to test it with a basic setup. I will change the cartridge to something slightly better tomorrow (a LinnK9 from my box or a Goldring 1024). I cannot test it with my LinnLP12 because I do not have a MC step-up amp at the moment.

Klaus
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Old 7th January 2003, 09:54 PM   #5
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The whole c't article can be found here (with diagrams and photos etc.):
http://www.heise.de/ct/02/13/224/default.shtml
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Old 7th January 2003, 09:55 PM   #6
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But attention:
They mixed up the plus and minus symbols for the opamps - not like "my" schematic.
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Old 8th January 2003, 07:21 AM   #7
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Quote:
Originally posted by lohk
I must state that I did not develop this schematic not did I calculate the component values. I trusted this c't develloper...
I thought you maybe wanted to check how good the designer had done his job. It's rather difficult to get an optimum RIAA curve with a non-inverting amp. I made this Excel file in order tweak in the best values for my RIAA amp
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Old 8th January 2003, 07:28 AM   #8
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Quote:
Originally posted by lohk

1. active operation for the "bass" side and passive for the treble side of the riaa curve.
2. do not attempt to use just one opamp
3. do not apply the "upper part" equalisation at the output (like Rod Elliots RIAA amp)

The result is a remarkable noise and distortion free riaa-amp for little money.
There are approx. a dozen different ways to make a RIAA amp. Each one has it's pros and cons. As alsways certain topologies are very sensitive when it comes to choosing the right parts. It's quite possible to make a MC RIAA amp with only one opamp but the amp have to have high open-loop gain and also be rather fast.
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Old 8th January 2003, 07:48 AM   #9
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A good example of a single-opamp MC preamp is the LFD MM0/Mistral Phonostage (essentially the same thing).

It has 56dB of gain, uses one NE5534 per channel, alongside junk like 7815/7915s and even output coupling through an elcap, which incidentally is also needed for a small correction to the bass response, and guess what ... sounds quite wonderful.

It is also remarkable for its tantalum input load resistor which costs probably as much as all of the other components put together
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Old 8th January 2003, 07:58 AM   #10
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I'm amazed that so many commercial vinyl amps have NE5534??? Is it all about the money?
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