[ecir38]

What componets does the RIAA include?

[Brinkman]

Here's Jim's answer to a similar question:

Actually, almost all of the components in the circuit affect the RIAA equalization.  Everything except the 220 ohm grid stoppers and the RC filters in the power supply.  Only the cathode follower is exempt, as long as it presents a high impedance loading to the previous stage.

jh

And after seeing somebody go the extra mile on coupling caps:

If you're going to spend that much on coupling caps, you should upgrade the EQ caps too (47nF, 1nF).

jh

Meaning, upgrading C203 and C208 might be sufficient, but to get results beyond that, you'd have to also consider upgrading other caps in the main circuit (C200, C202, C204, C206).

Some people insist on using only carbon comp resistors as grid stoppers (R201, R208, R214). Seems like tube choice (12AX7s) would be the biggest sonic determinant.

Additionally, there are those who like to to target the quality of the power supply, and if you want to try this, you could probably opt for polypropylene caps in the 47uF positions rather than the stock electrolytics and mills wirewound for all B+ resistors. However, 47uF poly caps are BIG.

-Brinkman

[ecir38]

Thanks for the reply Brinkman, but the reason I asked was not to upgrade the capacitors.

What I am looking at possibly doing is to change the RIAA to NAB and or IEC to make the Cornet a Tape Head amp if it can be done. Bottlehead has done the same to his Seduction phono pre

http://www.bottlehead.com/et/adobespc/Seduction/bottleheadtapeheadpreamp.htm

[hagtech]

The EQ is done with passive RC filters.  The topology looks like

,

where each stage performs the transfer function

,

that is mathematically described as

,

giving time constants of

.

So, you just solve these functions for the desired turnover points and viola!  You're done.  The hard part is figuring out what comprises Ra and Rb.  For the CORNET2, Rb is simply R205.  Ra is more complicated...

For determining Ra, you calculate the ac small signal plate impedance of V200, which depends on R203.  Put that in parallel with R202, add R204 to the result, then put that in parallel with R207.  Everything is carefully balanced as compromises between grid current, parasitic capacitance, bandwidth, plate current, available supply voltage, noise, linearity of tube operation, and deviation from nominal response caused by rolling tubes.  This is my result, my compromise, my tuning.  You can do it differently.  In fact, most other designers do, basically by making R204 really high in value, thus dominating the response.  I don't like that approach.  I did exactly the opposite.

jh

[Brinkman]

Thanks for the reply Brinkman, but the reason I asked was not to upgrade the capacitors.

What I am looking at possibly doing is to change the RIAA to NAB and or IEC to make the Cornet a Tape Head amp if it can be done. Bottlehead has done the same to his Seduction phono pre

http://www.bottlehead.com/et/adobespc/Seduction/bottleheadtapeheadpreamp.htm

Sorry for being so presumptuous. The project looks like fun...
BTW, what reel-to-reel machine do you have?

On a similar note, my good friend Martyn has a Otari 2" 24-track and I told him that I'd like to one day build him some tube mic preamps, as well as help him calibrate his recording heads. He records his band and others from time to time. Maybe I can convince him to invest in some better microphones...

-Brinkman

[ecir38]

Thanks Jim, I will have to try and sought it out.

No problem Brinkman, I have a Otari MX5050 BII2.