Phono preamp without RIAA correction

[thomasdf]

Hello

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.

From what I understand, without RIAA a phono preamp is just a simple line preamp with a high input impedance. Is this true ? If the Audio Technica cartridge I want to use is said to have a 3K output impedance, what the proper way of building an appropriate preamp ? I think I'd use only the "side" part of the cartridge to amplify the vertical movement rather than the side, but I'll have to experiment with this too.

I was thinking of using a 12AT7, straight to the grid... I've found schematics... but for heat and power supply reasons I think it's better to go with IC / semi conductors.

Cheers

Thomas

 

[scott2000]

Doesn't Wayne Kirkwood have some flat phono transfer boards?

https://groupdiy.com/threads/ka-ele...semiconductors-power-supplies-and-more.82391/

 

[zamproject]

He Tom

I suppose any descent phono pre with the tilt EQ removed should do the job, at least at a first test ?

 

[zamproject]

Now...did you already try with a standard (RIAA corrected) phono pre ?
Maybe the way you plan to use the cartridge request post-ephasis too ?

 

[JohnRoberts]

Typical RIAA phono preamps are terminated with 47k resistive and 100 pF capacitive but the capacitance can vary.

RIAA gain stage EQ varies the gain roughly 40 dB between 20Hz and 20kHz (higher gain at LF).

You can probably accomplish what you want with a modern op amp.

[edit- typical RIAA is 40dB of gain at 1kHz, +20dB more at LF, and -20dB less at HF. /edit]

JR

 

[dmp]

You can try a JFET input Opamp. Just configure as a non-inverting amplifier.

 

[thomasdf]

Thanks all ! I didn't expect that many answers Regarding post treatment, I'll use a simple Lo / hi cut to clean up the signal.
The KA Electronics is right what I need but seem a bit overkill in terms of design and cost. I'm looking for something clean but not necessarily audiophile. I'll investigate JFET input opamp designs and will probably come back with further questions

Cheers !

 

[Tubetec]

Have a look at Ivans(E1DA) LNA and Cosmos ADC , its like a straight wire with gain ,
Can also be configured with RIAA curve via DSP
Presents the perfect load for an MM cartridge

 

[ruffrecords]

If you use a magnetic phono cartridge I am not sure you can get away without the RIAA characteristic or something like it. My understanding is that, contrary to popular myth, the amplitude of the excursions on a record are almost constant across the frequency range. If it wasn't. piezo cartridges would never have worked. The reason for the RIAA replay characteristic on replay with a magnetic cartridge is that the cartridge is a velocity sensitive device so its output is proportional to frequency. RIAA corrects this.

Similarly, when cutting the disc, the cutting head is also velocity sensitive so high frequencies would be recorded at a higher level and there would be no bass if the reverse RIAA characteristic were not applied.

Cheers

Ian

 

[Gold]

The RIAA curve makes frequencies below the 1K turnover frequency constant amplitude. The frequencies above the turnover frequency become constant velocity on the disc.

 

[Gold]

There are two types of piezo cartridges. The regular crystal type and the type also known as Strain Gauge. Piezo’s have a natural constant amplitude, constant velocity response so no RIAA decode network is necessary. A regular piezo pickup has a variable current output that needs to be converted to a voltage output.

The strain gauge type biases the piezo with a constant current source so produces a voltage output.

 

[ruffrecords]

The RIAA curve makes frequencies below the 1K turnover frequency constant amplitude. The frequencies above the turnover frequency become constant velocity on the disc.

RIAA has two turnover frequencies, one a 400Hz and one at about 2.5KHz. Apart the bit between those two frequencies it is constant amplitude all the way.

Cheers

ian

 

[JohnRoberts]

RIAA playback equalization is not a simple low-pass filter. It defines transition points in three places: 75 μs, 318 μs and 3180 μs, which correspond to 2122 Hz, 500 Hz and 50 Hz (rounded values)

As I recall poles at 75 uSec and 3180 uSec, and a zero at 318 uSec.

JR

 

[Disco Volante]

There are ample examples of simple RIAA preamps on the 'net. Many older designs just feature two transistors and a correction network either passive or in a feedback loop (Baxandall-style). I suppose it'd be a matter of removing a few Cs and Rs to get a linear response?
/TMI/ Bang & Olufsen springs to mind; they used to make a small RIAA amp with 2 BC549 per channel to install in turntables in case the amp didn't have a phono stage...

 

[Newmarket]

As basically said in this thread your starting point can be a simple non-inverting op amp stage with 47K // "some tens" of pF capacitance.
I have a feeling you might need more than simple Hi/Lo Pass filtering to get a usable signal but you'll find if that's the case quite quickly I imagine. Good Luck.

 

[k brown]

Why wouldn't a mic preamp work?

For example the direct-in of the DMP3 is 100k in z.

 

[Gold]

Here is the spec sheet for the RIAA characteristic. You can see unity gain for the resultant curve is at 1K. That's why I called it the turnover frequency.

 

[JohnRoberts]

Why wouldn't a mic preamp work?

For example the direct-in of the DMP3 is 100k in z.

Modern mic preamps are typically engineered to be bridging terminations to low Z microphones or 10x the nominal 150-200 ohm mic impedance. 1.5-2k ohm termination is low for a MM (moving magnet) phono cartridge.

JR

 

[ruffrecords]

Here is the spec sheet for the RIAA characteristic. You can see unity gain for the resultant curve is at 1K. That's why I called it the turnover frequency.

That is because 1KHz is the 0dB reference frequency. This has nothing t do with turnover frequencies. If you look at the first page of the spec it says "thee disc recording characteristic is defined as the modulation of the groove spiral as a function of frequency (expressed as a velocity ratio)"

So the recording curve shows an increasing velocity with frequency which is constant amplitude.

Cheers

Ian

 

[Newmarket]

Modern mic preamps are typically engineered to be bridging terminations to low Z microphones or 10x the nominal 150-200 ohm mic impedance. 1.5-2k ohm termination is low for a MM (moving magnet) phono cartridge.

JR

K Brown is talking about the DI input. Although IIRC Rupert Never put out some mic pres with a high input Z. Danger of overcomplicating it here though - OP just seems to be looking for a flat response input stage. Nothing complicated needed.