need some advice on updating this one

[paba]

Hi all,

I found an old 1958 record player console fully functional. With a Monarch UA8 record player and the attached amplifier that is expecting a ceramic cartridge. It plays via two 6x9 oval alnico drivers.

I've already managed to install a MM cart on the tone arm (wasn't easy but it is done and found one that could handle a heavier tracking force.)

Now I wanted to see what is the easiest and cheapest way to update the amp to handle the lower voltage of the MM cart, since RIAA is already there.

- Replace R1 & R43 to 47k
- add some voltage gain (ideas welcome here), replace the V1 for 12AX7 and move the 12AT7 to V2...was my first idea.

Or maybe just build a small opamp solution with 20-30db gain and to match the input and output impedance and add that to the front of this amp. The RIAA curve is implemented in the circuit already and the amp has tone knobs to allow the user to dial in RIAA or deviate frm RIAA. The RIAA points are marked on the front panel where the knobs turn.

Any friend help or direction, suggestions...

Thanks
Paba

 

[Tom Bavis]

The RIAA equalization is approximated by the ceramic cartridge - the amp is more or less flat. You'll need a preamp with RIAA equalization.

 

[paba]

The RIAA equalization is approximated by the ceramic cartridge - the amp is more or less flat. You'll need a preamp with RIAA equalization.

Hi Tom,
the circuit already has RIAA implemented..I think by 1958, the RIAA had been set and deployed... I didn't check this point. As you can see in the circuit there is a network between V1 and the volume knob. In practice these are bass and treble knobs and Sparton has indicated on the front panel where to place the bass and treble knobs to give the RIAA curve response.

I agree that in Ceramic Cart days, there was no real need for RIAA network as the nature of the cart and the implementation usually provided a flat response in the preamp/amp.
I'm thinking that if I keep the bass and treble knobs at the positions indicated, I will have RIAA implemented. Ceramic carts were constant-amplitude and MM carts are constant velocity ...

My thinking is all I need to do is match the 47k load and provide some voltage gain.

 

[sesebe]

You need to have a amplify difference from 20Hz to 20KHz of 40dB.
Only from bass and treble knobs you will not obtain enough, and more than that will not be complementary to the correction used at recording.

My favorite schematic for this task is the schematic from figure no.1 from here: http://www.ti.com/lit/ds/symlink/lme49720.pdf

Or you can get inspired from here: http://www.tubecad.com/2007/09/blog0118.htm - if you like tubes

 

[Eli Duttman]

IMO, the tone controls will not provide even a 1st order approximation of the RIAA curve.

A very simple solution to the problem is Jim Hagerman's Bugle2 opamp based phono preamp. Please notice that net gain is selectable and you should have little difficulty in mating a Bugle2 to your console's gain structure.

 

[Eli Duttman]

I took a look at the schematic. A possible major rework would switch to SS rectified B+, dropping the tone controls, installing the "classic" RCA phono setup, installing 2X 250 Kohm log. pots. (for volume/balance control), and 12AX7 section drivers of the SE "finals".

In addition to freeing a socket up, eliminating the vacuum rectifier allows the phono section tube heaters to be fed with DC too.