Phono preamps....
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bcarso
Well-known member
jackies said:
I think Tim de Paravicini (of EAR) did a preamp with inductors a while back, although I never saw a schematic.
jackies said:
If you want to roll your own, from memory the RIAA playback EQ is defined as a pole at 3,180 uSec, zero at 318 uSec, and pole at 75 uSec.
Using inductors at that much gain suggests to me, you should probably shield them pretty well. Nominal gain at 1 khz is something like 40 dB for MM carts.
JR
Hi,therecordingart said:
I use this:
http://sweb.cz/hifivlk/preamp.jpg
for one year, I power it with two 4.5 V batteries in series, 20 mA
for powering two channel is not so much, bateries, switched on while playing, works for half year.
It is MK2 of more complicated construction, but thank bcarso
I have simplified it much.
This preamp makes synthetic resistance for MM phono cartridge damping
using RIAA correction and paralel reactance feedback.
Shielding two channel separately is rule.
mikep
Well-known member
Any suggestions for reference cartdriges? Looking for something to listen to and to use when auditioning/ developing phono preamps. Something nice, but also common, or should I say representative of what people listen on these days in the mid / high end. hopefully not TOO expensive. I have a technics SP-15 and a decent if utilitarian arm -- Audio Technica ATP-12T.
mike p
mike p
bcarso
Well-known member
(Moving iron) Grados are supposed to be pretty decent---I don't have a specific model number though.
mikep
Well-known member
I actually own a Grado prestige silver, not sure where it is though. I packed it up when my Lenco kicked the bucket and have moved a couple of times since. I just read about their company, looked at the "tour" on their website. I had no idea they were such a small, neat outfit. everything is done in-house. they even do all their injection molding on a small, antique press. very cool and inspiring!
I would think that a full understanding of the entire phono reproduction system would be a prerequisite for designing an ultimately effective preamp. But then again most recording equipment these days seems to be designed by engineers that have never set foot in a professional studio, nontheless worked as a recordist. personally, Ive already got quite a bit of experience with the lathe side of things (maintaining and cuttting sides on a VMS-70) but have never woried much about playback.
I was just reading the B&K AES paper about audible resonances. the fact that all those <10 Hz resonances show up as IMD sidebands throughout the audio band is enlightening. this is a must-read if anyone hasn't seen it:
http://www.merrillscillia.com/MechanicalResonances.pdf
Now Im thinking my ATP-12T arm is too massive to be a good middle-of-the-road reference. And I will need several cartridges. sort of like if one was designing a mic preamp and wanted to do subjective testing, one microphone won't tell you everything. for example, my choices would be something like SM57, COLES 4038 and KM84. one of each representative technology, each very commonplace in studios for years and years.
so a mid-grade grado would cover the moving iron, and guess you'd want a standard, well-liked moving coil and moving magnet as well. shure V15-III for moving magnet? maybe a denon DL-103 moving coil?
mike p
I would think that a full understanding of the entire phono reproduction system would be a prerequisite for designing an ultimately effective preamp. But then again most recording equipment these days seems to be designed by engineers that have never set foot in a professional studio, nontheless worked as a recordist. personally, Ive already got quite a bit of experience with the lathe side of things (maintaining and cuttting sides on a VMS-70) but have never woried much about playback.
I was just reading the B&K AES paper about audible resonances. the fact that all those <10 Hz resonances show up as IMD sidebands throughout the audio band is enlightening. this is a must-read if anyone hasn't seen it:
http://www.merrillscillia.com/MechanicalResonances.pdf
Now Im thinking my ATP-12T arm is too massive to be a good middle-of-the-road reference. And I will need several cartridges. sort of like if one was designing a mic preamp and wanted to do subjective testing, one microphone won't tell you everything. for example, my choices would be something like SM57, COLES 4038 and KM84. one of each representative technology, each very commonplace in studios for years and years.
so a mid-grade grado would cover the moving iron, and guess you'd want a standard, well-liked moving coil and moving magnet as well. shure V15-III for moving magnet? maybe a denon DL-103 moving coil?
mike p
IMO the preamp is far easier to design than the cartridge. Reasonably standard termination ( 47k and 150pF +/-) , well defined EQ curve (RIAA), etc.
Dealing with very LF is an unresolved issue with the standards groups. RIAA defines the playback EQ as a series of time constants and bandwidth of interest. They later extended the LF BW from 30 Hz to 20Hz but offered no insight for what to do below. IEC proposed adding a pole at 30Hz (7950 uSec) but the RIAA never adopted that so the recording encode process never accommodated the extra dB or so loss in band. About the only practical way to meet the letter of the RIAA curve and provide significant attenuation of warped and off center records, is to add an under-damped 2 pole HPF in the low end.
Not to swerve off topic but I recall an interesting problem back when I was designing companding NR for tape recording. This infrasonic information off records, would get compressed during the tape record encode process, but wouldn't print to tape, causing a phantom modulation on playback when the envelope of the audio was missing the same infrasonic content. It caused an unusual AM of the full range signal at the very low (missing) frequency.
I can't say that I miss analog tape or vinyl all that much.
JR
Dealing with very LF is an unresolved issue with the standards groups. RIAA defines the playback EQ as a series of time constants and bandwidth of interest. They later extended the LF BW from 30 Hz to 20Hz but offered no insight for what to do below. IEC proposed adding a pole at 30Hz (7950 uSec) but the RIAA never adopted that so the recording encode process never accommodated the extra dB or so loss in band. About the only practical way to meet the letter of the RIAA curve and provide significant attenuation of warped and off center records, is to add an under-damped 2 pole HPF in the low end.
Not to swerve off topic but I recall an interesting problem back when I was designing companding NR for tape recording. This infrasonic information off records, would get compressed during the tape record encode process, but wouldn't print to tape, causing a phantom modulation on playback when the envelope of the audio was missing the same infrasonic content. It caused an unusual AM of the full range signal at the very low (missing) frequency.
I can't say that I miss analog tape or vinyl all that much.
JR
mikep
Well-known member
the B&K paper implies that all cartridges are underdamped and concludes that choosing the compliance/effective mass combination for a higher than normal ~18Hz resonance is a good idea, to get the resonant peak up above the unavoidable 4-10Hz noise originating from various imperfections in the record surface.
adding mechanical damping to the arm won't help, that will increase the apparent mass of the arm. an electronic high-pass of any type is not getting to the root of the problem*, intermodulation with the signal will have already occured. A high-Q band-reject filter would seem more appropriate anyway, but at what frequency and Q? it will change for every cart/arm combination. Perhaps a synthetically generated complex impedance (using a 3-opamp constant amplitude phase shift network) could be used to present a low-z load only at the resonant frequency, 47k resistive at all other frequencies. coming up with a way to somhow automatically servo the f and Q would seem daunting. requiring the user to measure the resonance and dial it in would be probably be impractical but might work.
* unless you consider an R-L highpass, the input Z gets lower, approaching the series R at low frequencies. that would require quite an inductor.
Mike P
adding mechanical damping to the arm won't help, that will increase the apparent mass of the arm. an electronic high-pass of any type is not getting to the root of the problem*, intermodulation with the signal will have already occured. A high-Q band-reject filter would seem more appropriate anyway, but at what frequency and Q? it will change for every cart/arm combination. Perhaps a synthetically generated complex impedance (using a 3-opamp constant amplitude phase shift network) could be used to present a low-z load only at the resonant frequency, 47k resistive at all other frequencies. coming up with a way to somhow automatically servo the f and Q would seem daunting. requiring the user to measure the resonance and dial it in would be probably be impractical but might work.
* unless you consider an R-L highpass, the input Z gets lower, approaching the series R at low frequencies. that would require quite an inductor.
Mike P
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