Hello. I have a phono pre built many years ago. It was an expensive project and worked very well. Problem is I do not use it since I switched to strain gauge system. It is the shame to keep it idle on the shelf.
I was thinking to redo it as a tape head pre for NAB decks. I saw many threads about doing it. Could someone take a look and direct me how to change an equalization from RIAA to NAB? I can follow a scematic but not much more.🙂
Thanks for help. Robert.
I was thinking to redo it as a tape head pre for NAB decks. I saw many threads about doing it. Could someone take a look and direct me how to change an equalization from RIAA to NAB? I can follow a scematic but not much more.🙂
Thanks for help. Robert.
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You would use a 50uS zero for the treble, and a 3180uS pole for the bass.
T(s) = ( 1 + s x 50uS ) / ( 1+ s x 3180uS )
Use a series R followed by a shunt RC network ( Rshunt in series with C ) to ground.
Circuit analysis gives the following transfer function:
T(s) = ( 1 + sC x Rshunt ) / ( 1+ sC x ( Rshunt + Rseries ) )
Equating like terms in s for both the numerator and denominator, we have:
C x Rshunt = 50uS
C x ( Rshunt + Rseries ) = 3180uS
C x Rseries = 3130uS
C x Rshunt = 50uS
Rseries / Rshunt = 3130uS /50uS = 62.6
If Rseries = 31.6k then Rshunt = 31.6k / 62.6 = 505R
and C = 3130uS / Rseries = 3130uS / 31.6k = 99nF
If you keep the 31.6k 1% series resistor, then remove the rest of the RIAA parts,
and all you will need is a series RC to ground.
The R is 500 or 510 ohms 1% metal film ( ideally 505R ).
The C is 0.1uF 1% ( ideally 99nF ) polypropylene or polystyrene.
You might want to retain the old parts for a possible future restoration of the RIAA.
T(s) = ( 1 + s x 50uS ) / ( 1+ s x 3180uS )
Use a series R followed by a shunt RC network ( Rshunt in series with C ) to ground.
Circuit analysis gives the following transfer function:
T(s) = ( 1 + sC x Rshunt ) / ( 1+ sC x ( Rshunt + Rseries ) )
Equating like terms in s for both the numerator and denominator, we have:
C x Rshunt = 50uS
C x ( Rshunt + Rseries ) = 3180uS
C x Rseries = 3130uS
C x Rshunt = 50uS
Rseries / Rshunt = 3130uS /50uS = 62.6
If Rseries = 31.6k then Rshunt = 31.6k / 62.6 = 505R
and C = 3130uS / Rseries = 3130uS / 31.6k = 99nF
If you keep the 31.6k 1% series resistor, then remove the rest of the RIAA parts,
and all you will need is a series RC to ground.
The R is 500 or 510 ohms 1% metal film ( ideally 505R ).
The C is 0.1uF 1% ( ideally 99nF ) polypropylene or polystyrene.
You might want to retain the old parts for a possible future restoration of the RIAA.
An RIAA needs one zero and two poles, so you need one less capacitor for NAB.
Your choice of 31.6k for the RIAA was fortunate, and allows a standard value for the capacitor.
Connect the outer foil of the capacitor to ground, with the 505R between the grid and other lead of the 0.1uF.
In the 60s it was common to have both RIAA and NAB in a preamp, with the feedback selected by a switch.
Post back here about your results.
Your choice of 31.6k for the RIAA was fortunate, and allows a standard value for the capacitor.
Connect the outer foil of the capacitor to ground, with the 505R between the grid and other lead of the 0.1uF.
In the 60s it was common to have both RIAA and NAB in a preamp, with the feedback selected by a switch.
Post back here about your results.
Note also that if you need IEC EQ at 35us or 70us set the LF pole at around 7000us and do the calculation for 35us /70us (15ips/7.5ips) as Ray describes for NAB.
NAB is most useful if you are playing antique quarter track stereo tapes, modern 2 track reissues are mostly IEC. (I have multiple 2 track machines and a small library of Analog Productions Ultra Tape, Tape Project, STS, home made dubs.)
NAB is most useful if you are playing antique quarter track stereo tapes, modern 2 track reissues are mostly IEC. (I have multiple 2 track machines and a small library of Analog Productions Ultra Tape, Tape Project, STS, home made dubs.)
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Contact me privately by PM. I am not sure whether I have boards left or not at this point, if not I can provide the gerbers and you can get some made fairly inexpensively at JLCPCB - there are no more power transformers unfortunately.
The last EQ iteration was a large improvement over my prior attempts. What cartridge are you using?
The last EQ iteration was a large improvement over my prior attempts. What cartridge are you using?
Hello. Today I changed equalization in the pre according to directions. It is not 100% precise but very close. It works very well but it was clear that this pre does not have enough amplification. Luckily when I was building the original phono one input was going through a 10db Shure sut for lower output cartridges. As you can guess that solved the issue. I am listening to it now and I hear no reason to complain. It is silent and no hum when all parts are properly grounded.
Thank you for help.Robert.
Thank you for help.Robert.
