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B&O RIAA project

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Søren Hammer
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Søren Hammer Posted: Mon, Mar 5 2012 4:27 PM

Hello everybody on the new forum!

This thread is dedicated to a little thought that came to me over the course of the weekend. 

I want a RIAA preamplifier because my newly acquired Beomaster 6000 4 Channel does not have it built in, as it was made to suit a Beogram 6000 with a built-in combinated RIAA and CD4 decoder. I want to use a Beogram 4000 with it for normal stereo playback, so I'll need a RIAA to do the job Smile

I had previously been recapping two Beomaster 4400 receivers which are beautifully constructed, learned that the preamplifier section was located on the input module.

My vision became clear: build a RIAA with an additional auxilary input from the input module from a beomaster 4400. You may ask why it had to be 4400 of all B&O, it's just because I have a spare one that I received as a part of a trade some time back. My example has been abused a lot; All output transistors has been smoked, as well as defects on a lot of soldering pads because of bad repair work. It would truly hurt to use the module from a perfect working 4400, but this one is a wreck. The preamplifier was, as a big bonus, actually praised a lot back in the day (the "design story" folder does a really good job at explaining how exellent it is and how well it performs with a B&O cartridge.)

1. The RIAA

Blue: Phono input

Yellow and Green: signal output from RIAA, line level

2. Auxillary input

Pink: Tape 2 input

Purple and brown: originally outputs from tape 2, but I won't need that as the Beomaster 6000 would have a similar circuit, would rather take the signal directly at the DIN plug.

3. Power

Red and black are positive and negative, negative doubling as ground from all the other inputs. I still don't know the voltage, but I guess that it is 9 V.

I will find a suitable enclosure to house the whole module, output DIN plug, two switches and maybe a relay to handle the signals.

I will of course keep the module as original as possible to make it all reversible.

Does my idea sound good or crazy? I will be an almost free RIAA which is a very large advantage for a poor student like me!

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tournedos
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I can't see why it wouldn't work - but the supply voltage isn't 9V - it's 33V!

It also needs to be well stabilized or you'll probably get a bad hum. May not be that easy to arrange.

--mika

Søren Hammer
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tournedos:

I can't see why it wouldn't work - but the supply voltage isn't 9V - it's 33V!

It also needs to be well stabilized or you'll probably get a bad hum. May not be that easy to arrange.

Aha,  a non-standard voltage.

I assume that there would be some resistors in the way before reaching the RIAA part of the circuit Stick out tongue

It would be much easier to get a stabilized 9-18-24 V power supply that isn't switchmode based.

Vinyl records, cassettes, open reel, valve amplifiers and film photography.

Peter
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Peter replied on Mon, Mar 5 2012 5:06 PM

Er ... You do realise that the 6000 does have an RIAA already built in, don't you? You use the Aux socket and there is a switch underneath which selects low impedance pick ups. Very useful! I used the 6000 to compare the RIAA in my Beogram 6000 against the one in the amplifier. Couldn't tell the difference!

Peter

Søren Hammer
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Peter:

Er ... You do realise that the 6000 does have an RIAA already built in, don't you? You use the Aux socket and there is a switch underneath which selects low impedance pick ups. Very useful! I used the 6000 to compare the RIAA in my Beogram 6000 against the one in the amplifier. Couldn't tell the difference!

Oh, I did not know that! A pretty useful feature Smile

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Peter
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Peter replied on Mon, Mar 5 2012 5:26 PM

The user manual is on site - so is the technical manual and service manual. Both are interesting and worth downloading. The 6000 really was a tour de force - it is incredible what they fitted into this space age receiver.

Peter

Step1
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Step1 replied on Mon, Mar 5 2012 5:49 PM

Peter:

Er ... You do realise that the 6000 does have an RIAA already built in, don't you? You use the Aux socket and there is a switch underneath which selects low impedance pick ups. Very useful! I used the 6000 to compare the RIAA in my Beogram 6000 against the one in the amplifier. Couldn't tell the difference!

The 6000 uses a very high quality OP AMP for its RIAA amplifier the only downside is the extra diode switching etc. that the signal has to go through that I am sure must have a negative impact on any possible gains in sound quality! It might be interesting to construct a standalone RIAA preamp with just this IC to see how it stands up on its own!?

Olly

Søren Hammer
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Step1:

Peter:

Er ... You do realise that the 6000 does have an RIAA already built in, don't you? You use the Aux socket and there is a switch underneath which selects low impedance pick ups. Very useful! I used the 6000 to compare the RIAA in my Beogram 6000 against the one in the amplifier. Couldn't tell the difference!

The 6000 uses a very high quality OP AMP for its RIAA amplifier the only downside is the extra diode switching etc. that the signal has to go through that I am sure must have a negative impact on any possible gains in sound quality! It might be interesting to construct a standalone RIAA preamp with just this IC to see how it stands up on its own!?

I would love to try to make a RIAA based on such an IC, certainly possible if it isn't unoptanium!

I think that I will proceed with my project, as an extra RIAA in my Beosystem 8000 would be rather nice Smile

Vinyl records, cassettes, open reel, valve amplifiers and film photography.

chartz
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chartz replied on Mon, Mar 5 2012 6:00 PM

I'm sure the module will work fine with a stabilized power supply of some sort. You could use a simple zener circuit, just using three 11V zeners in series for instance (simple circuit like this one), or else using a 7824 and adapting the circuit as you say, I'm sure it'd work too. Or an LM317 from Texas, which works great up to 37V.

 

Jacques

tournedos
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I looked at the schematic and I also believe the circuit would work with something as low as 24V, although the noise etc. specifications will probably degrade a bit. It won't need much power either, I'd be surprised if the entire preamp module (with the now superfluous tape inputs included) used more than 100-200 mA.

The supply will need to have all remains of the 50/100 Hz mains ripple gone though, and although many switch mode power supplies are stable in that sense, the output can still be noisy in some other way. They would probably need some RC networks after them as well.

But this is all academic until you actually try it Smile

--mika

Søren Hammer
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It is maybe possible for me to find a 30V power supply and add a big(ish) filter cap to clean power? I agree that mains ripple would be the big killer in a sensitive circuit like this Smile

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chartz
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chartz replied on Mon, Mar 5 2012 9:39 PM

Yes you could do that. I once built an RIAA preamp fed by a 33,000 uF/40V cap and it was very quiet! The LED took 10 minutes to fully dim!

Jacques

Søren Hammer
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chartz:

Yes you could do that. I once built an RIAA preamp fed by a 33,000 uF/40V cap and it was very quiet! The LED took 10 minutes to fully dim!

I have salvaged two 6800 uF/40V caps from a dead Beomaster 3300 that would be perfect Wink

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Søren Hammer
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I have thought about integrating or making a seperate SQ encoder so it can be possible to make quadraphonic tapes. I'm curious about the old formats, it would be possible to try scince my Beomaster 3400 and 6000 does support it but what about the obscure circuit? Better go searching for a schematic.

Vinyl records, cassettes, open reel, valve amplifiers and film photography.

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