Phono preamp without RIAA correction

[thomasdf]

I have thought about mono cartridges but they work with lateral movement (inconvenient) and they are pretty darn expensive. I have tested some 40ish euros brand news audio technica cartridges (green one, can't remember the exact ref atm ) and while I prefered old Shure for my records, it sounded very precise so I'm sure it's already a luxury option for a plate reverb as opposed to a piezo patch.

Beautiful Studer design ! I'll study that ! Thanks

 

[gyraf]

..you can get moving magnet cartridges really cheap on ali's express:

https://www.aliexpress.com/w/wholesale-moving-magnet-cartridge.html

 

[Gold]

I don't understand that. A 20Hz wave of a given physical amplitude on the disc takes up no more room than a 20KHz one.

Cheers

Ian

If that’s the case what is the purpose of the pre emphasis de emphasis curve? Why not just record flat on the disc?

For a given frequency the wavelength will change from the outside to the inside diameter due to the changing linear velocity.

 

[Mr. Moscode]

I'm not sure what your point is? In a disc recording system Input = Output with all the caveats of an analog system. If you play with the the angle the stylus contacts the disc you will get odd and heavily distorted results. The vertical/lateral encode and decode is a function of the way the cutterhead/phono pickup is constructed. The output of the cutterhead drive amps is Left/Right. There is no sum/difference encode/decode in lathe electronics or a phono preamp.

Thread starters isn't playing a record according to this statement in post #1: "For a project where I am toying with the idea of using a phono cartridge as a contact microphone, I'm looking for a good quality phono preamp design without RIAA correction." it has to touch the instrument to work and how it touches the instrument will give varied results. RIAA curve may be of little value here.
Thanks for asking.

 

[FIX]

I like turnovers.

 

[JohnRoberts]

I like turnovers.

got to love the internets......

JR

 

[abbey road d enfer]

I'm still investigating this, and really would like to try a pair of Audio Technica (entry level) audio cartridges (bang for the buck !) attached and decoupled to / from the metal structure, the stylus on the plate.

You would get much better response with guitar pickups. You don't need to attach them, just install them close enough to the plate.

 

[Gold]

Thread starters isn't playing a record according to this statement in post #1: "For a project where I am toying with the idea of using a phono cartridge as a contact microphone,

Ahh yes. Many of the cheap MM carts roll off a bunch of high end, which might help. My favorite cheap Audio Technica cart is no longer made. The VM-95C looks close. I like it.

A Stanton 500 starts rolling off high end around 6K.

 

[ruffrecords]

If that’s the case what is the purpose of the pre emphasis de emphasis curve? Why not just record flat on the disc?

The purpose is to correct for the properties of the electromagnetic cutting head. As it is current through the head that determines amplitude and the head is inductive, the higher the frequency the higher the impedance and the lower the current. So to achieve constant amplitude current you need a continuously rising response for a regular voltage input. Ideally you want a constant current drive and in tape recorders this is achieved by driving the head via a large value resistance. This works because the power required is very small but in a disc cutting head you need real power so this method is impractical. Instead they use a power amp with equalisation. I suspect the rising response being divided into two separate regions is one of practical implementation using negative feedback.

For a given frequency the wavelength will change from the outside to the inside diameter due to the changing linear velocity.

Indeed it does but this is a separate and additional issue and more to do with the frequency response of the medium itself.

Cheers

IAn

 

[abbey road d enfer]

If that’s the case what is the purpose of the pre emphasis de emphasis curve?

In the beginnings of phono mastering, it became obvious that constant velocity was making better use of groove width. In order to achieve that, the voltage drive of the cutting head would be constantly rising at 6dB/octave over the 9-10 octaves of audible spectrum, which would result in about nearly 60dB equalization, and also possibly dangerous acceleration of the head.
Constant velocity played with a magnetic pickup results in a 6dB/octave rise, so a counter EQ would be needed. The 60dB boost at 20Hz would prove dangerous for equipment when the stylus was inadvertently dropped, not mentioning possible LF howlback.

The RIAA suggested a compromise, with some boost (relative) at 50Hz, and a plateau in the midrange, so in the end, the ambitus is limited to 40dB. This was based on the observation that energy below 50Hz and above 10k was somewhat restrained.
The actual combination of current drive and RIAA preemphasis gives flat-ish response, with some 50Hz boost and a negative HF shelf.
Just the same, the combination of the pick-up's rising response and the RIAA de-emphasis is not too far fom a flat line.
It's that property that makes piezo or ceramic cartridges usable, even though they don't have the RIAA EQ.

Why not just record flat on the disc?

It would work somewhat (not mentioning all the HF limiting and resonance compensation), but would probably put more strain on the cutter head.

 

[zamproject]

@abbey road d enfer , I think Paul (as ME that cut vinyl) was gently cynical by asking those questions
But I'm grateful you offer crystal clear answer, as usual, that enlight everyone.

Cheers
Zam

 

[Gold]

From Handbook For Sound Engineers Vol 2

 

[ruffrecords]

From Handbook For Sound Engineers Vol 2

Thanks for that. This explains why there are two parts to the RIAA characteristic. The lower part was the one first introduced to alter the frequency response of the cutter head to be constant amplitude rather than constant velocity below 1KHz. At that point they left the upper response constant velocity and you can see the result is the amplitude decreases as the frequency increases. So the second (right hand) part was introduced to change the upper frequencies also to constant amplitude.

Cheers

ian

 

[Gold]

Right, the second graph shows the response is constant amplitude in the low end and constant velocity in the high end, with the turnover frequency(their terminology) at 500hz.

 

[Mr. Moscode]

Thank you, gents. (Am I assuming gender incorrectly?)
I'm learning a lot. My stuff used the RCA passive RIAA EQ net, same as the Hafler but high impedance for tubes. I thank the universe I didn't have to think about all this jazz to build a preamp. Except the jazz part, which I think about all the time.

 

[Gold]

The author of the Handbook For Sound Engineers article is George Alexandrovitch. He designed the Fairchild cutterhead. He also worked for Stanton designing phono pickups. Last I checked he was still alive but he's in his 90's now.

 

[moamps]

I had no intention of responding to this thread, but I think that some misconceptions should be corrected.

For the OP: If you want to use a magnetic pickup, you should first try it with the RIAA preamp as it is, with equalization.

Constant velocity played with a magnetic pickup results in a 6dB/octave rise, so a counter EQ would be needed. ....

Isn't that against Faraday's Law of Electromagnetic Induction?

From the book Handbook for Sound Engineers Fourth Edition page 1016:



From Galo's article:



For further reading who is interested:

 

[ruffrecords]

I had no intention of responding to this thread, but I think that some misconceptions should be corrected.

For the OP: If you want to use a magnetic pickup, you should first try it with the RIAA preamp as it is, with equalization.


Isn't that against Faraday's Law of Electromagnetic Induction?

The output voltage is directly proportional to rate of change of flux which is itself directly proportional to frequency. For a constant amplitude of flux the higher the frequency the higher the output. It is the same reason magnetic tape head output voltage is proportional to frequency.

Cheers

Ian

 

[abbey road d enfer]

Isn't that against Faraday's Law of Electromagnetic Induction?

Agreed. I mixed up excursion and velocity.
Thank you for correcting me.
Just replace velocity with excursion (or amplitude) and I believe it's correct.

 

[abbey road d enfer]

From Handbook For Sound Engineers Vol 2

Carefully analyzing this text shows double transition, first between constant amplitude and constant velocity, then back to constant amplitude, so the overall shape is basically a differentiator for cutting and an integrator for playback.

Right, the second graph shows the response is constant amplitude in the low end and constant velocity in the high end, with the turnover frequency(their terminology) at 500hz.

Not in the high end, only in the midrange between 500Hz and 2.1kHz.