Phono Preamp Voltage Output vs Input voltage from Cart

sponger

I changed the stock cart on the Denon TT to an audio technica AT95E, which seems to be an upgrade despite costing only $40.

The phono input on the AVR seems to work OK. I'm not sure how much it matters, but I'm guessing it has a gain of 150mV. I'm guessing that because it doesn't sound much different from the TT integrated preamp, which Denon says has an output voltage of 150mV. But the stock Denon cart has 2.5mV of output. I don't know if that has an impact on the preamp output voltage.

So, whether or not output voltage is the reason, I'm throwing around the idea of a budget preamp like the $79 Pro Ject Phono Box MM.

Here's the thing: I'm not really sure how to interpret the output voltage rating. It seems like the $79 Phono Box MM has 500mV of output with 5mV from the cart? That's how I interpret the specs on the Pro Ject website anyway.

For example, the $99 Phono Box MM/MC has specs that say 300 mV/1 kHz at 3 mV/1 kHz (MM input).

I'm not sure what that means. I'm guessing it means that the lower the input from the cart, the lower the output from the preamp.

It seems like a lot of phono preamps <$500 don't list the output voltage, only operating voltage. They list decibel level like 40db for MM for example. Should that be all I should be focusing on?

Thanks.

mhardy6647

output voltage isn't gain - gain is a measure of the ratio of the output voltage to the input voltage, usually (and conveniently) reported in decibels.

Yes, the lower the cartridge output, the lower the phono premp/EQ output will be; given constant gain. The important figure for you is the input sensitivity and the gain of the phono input for your AVR.

Your "tt integrated preamp" serves the same purpose as the phono input on a full-function preamp ('linestage') integrated amp, or receiver -- it amplifies and equalizes the very low level output signal of the cartridge to a "flat" line-level signal to feed into a normal "line level" input (such as an AUX input) on a 'linestage', amplifier, or receiver. You bypass that preamp if you feed the signal from the cartridge directly to the phono input on your AVR.

Gain can be adjusted on some outboard (and, chances are, in some "onboard") phono preamps.

EDIT: I'm not sure exactly what question you are asking!

mhardy6647

Gain:

as an example: if you have a phono cartridge with a nominal output of 2.5 mV, and you use a phono preamp/EQ that amplifies the signal to an output voltage of 1.0 V (1000 mV), this is an amplification factor of 400 fold. 400 fold amplification (gain) is equal to about 26 dB (base 10 log of 400 = about 2.6 or 2.6 "Bel" and 2.6 x 10 = 26 dB).

Couple of things:

1) the output level specified for a cartridge is a reference point; the actual output varies, of course, because the signal "encoded" in the record groove varies!

2) the signal encoded in a record groove is "equalized" as a practical matter to make it easier to fit the audio signal onto a modulated groove molded into a piece of plastic! The equalization curve used on "modern" LP records is the RIAA curve and it looks like this:



The black curve is what is on the record.

A phono preamplifier doesn't just amplifiy, it also equalizes that very "distorted" (equalized) signal as it is "transduced" by the phono cartridge from the record groove, using an equal but opposite curve (the red curve in the image
above) to result in an output signal which is both amplified and "flat".

Notice that the RIAA curve is "flat" at exactly 1 kHz (i.e., there is no equalization applied at 1 kHz on recording, and thus also none on playback at 1 kHz).

The accuracy and quality of the preamp's EQ circuit is every bit as important as the quality and gain of its amplifier circuit.

sponger

Thank you very the very informative response.

I guess what I'm asking is how does a pre-amp meet its advertised gain of 40 db for MM carts when carts have varying levels of voltage output.

In your example, you use 1.0V of output as a reference point.

Using the stock Denon cart and TT integrated pre-amp for that same calculation, I divide 150 by 2.5 to arrive at 60, which comes out to 18 db? So, the stock cart & eq produces 18 db of gain? If that's the case, then that to me would explain why it sounds so cruddy out of the box.

So, that said, I was hoping a pre-amp rated at 500mV would result in more gain. The Pro Ject is rated at "40 db for MM." Should I interpret that to mean 40 db regardless of cart voltage?

Based on that formula, it looks like a pre-amp will need to produce 250V to result in 40db of gain from a 2.5mV cart (25,0000mV/2.5=10,000; Base 10 log of 10,000 is 4).

mhardy6647

sponger wrote: »

Thank you very the very informative response.

I guess what I'm asking is how does a pre-amp meet its advertised gain of 40 db for MM carts when carts have varying levels of voltage output.

In your example, you use 1.0V of output as a reference point.

Using the stock Denon cart and TT integrated pre-amp for that same calculation, I divide 150 by 2.5 to arrive at 60, which comes out to 18 db? So, the stock cart & eq produces 18 db of gain? If that's the case, then that to me would explain why it sounds so cruddy out of the box.

So, that said, I was hoping a pre-amp rated at 500mV would result in more gain. The Pro Ject is rated at "40 db for MM." Should I interpret that to mean 40 db regardless of cart voltage?

Based on that formula, it looks like a pre-amp will need to produce 250V to result in 40db of gain from a 2.5mV cart (25,0000mV/2.5=10,000; Base 10 log of 10,000 is 4).

Gain is gain; whatever voltage comes in would be amplified by 40 dB, within the limitations of the amplifier.
If you put in 1 mV, a preamp with 40 dB of (voltage) gain "should" result in an output of 10 Volts (40 dB = 40 Bel = 4 orders of magnitude; 1 mV * 10^4 = 10000 mV = 10 V).

EDIT: OOPS!!!! I stand corrected, there is a factor of 20, not 10 to relate log (Vout/Vin) to gain in dB that I neglected!

http://www.sengpielaudio.com/calculator-FactorRatioLevelDecibel.htm

40 dB gain = 20 * log (Vout/Vin)

for Vin = 1 mV, then, for 40 dB of amplifier gain:

40/20 = 2.0 = log (Vout/Vin) so, for Vin = 1 mV; Vout = (10^2) = 100 mV

Sorry!

mhardy6647

For 2.5 mV = Vin and a gain of 40; the output V (Vout) would thus be

Vout/Vin = 100 , so Vin = 2.5 mV at 40 dB gain Vout = 250 mV

I am so, so sorry I was off by two orders of magnitude in my original reply!

Viking64

mhardy6647

So does mine, but it's a head cold in my case.

Aha, here's a fairly reasonable explanation of why the factor is 20 rather than 10
for voltage (it is 10 for ratios of power) and how to deal with impedance differences between in and out.

https://www.radio-electronics.com/info/formulae/decibels/decibel-formulae-equation.php

I am really sorry I led all y'all (or, at least, the OP) astray!

jdjohn

To the OP, I would actually recommend an ART DJPre-II in that price range for a few reasons.

1) It lists 45db of gain, but there is an adjustment knob where you can tweak it a bit up or down. The PhonoBox is fixed at 40db.
2) The ART has two capacitance settings (100 & 200pF), whereas the PhonoBox is set at 150pF.
3) The ART is a bit cheaper. Trust me, you will not miss anything sonically with the ART. It has a reputation of being a great budget phono-pre. I still keep one on-hand as a backup, and whenever I put it in service, it always amazes me how good it sounds for its price...or at a price much higher.

sponger

Thank you, mhardy, for the updated calcs. I haven't run those numbers yet. But basically it looks like you've demonstrated that it's possible to prove that the stock cart & preamp were grossly inadequate using specs alone.

jdjohn: the ART is only $50! What an incredible value. I almost feel like what's the catch. But I looked it up and haven't been able to find anything negative being said about it. Will definitely give it due consideration.

But, while researching the ART, I noticed that a lot of people were adding the Schiit Mani to their list. It's $129 and made in the USA, the latter being another selling point. It's way beyond the $80 I had set aside for the Pro Ject, and almost triple the ART's price. Also, Amazon no longer carries Schiit products. So I'm concerned about that.

Thanks again.

mhardy6647

The Mani seems like a nice package with some reasonable adjustability, but I haven't heard one and -- not everybody -- is crazy about the sound of Schiit's products. And then there's their marketing strategy



I will say that dollar for dollar, the ART phono preamp does indeed look pretty good, and it gets good press from folks who own or have heard them -- but it is entry level and certainly won't be the be-all and end-all. It may (or may not) better what you already have, though. Haven't heard one of these, either but keep getting tempted to invest in one just to see how they do.

tonyp063

@sponger
If you are interested seriously in the Schiit Mani, keep an eye on this site.
http://www.schiit.com/b-stocks

They sometimes show up for less than $100

mhardy6647

tonyp063 wrote: »

@sponger
If you are interested seriously in the Schiit Mani, keep an eye on this site.
http://www.schiit.com/b-stocks

They sometimes show up for less than $100

jdjohn

I tried a POSchiit Mani, and had terrible RFI with it. I bought it used, but still reached-out to Schiit about this KNOWN issue, but I got crickets.

Other owners with no RFI issues love the Mani, so I can't discount that.

Before purchasing, I would recommend checking your FM signal strength at this site: http://www.fmfool.com/?option=com_wrapper&Itemid=29
You only need to enter your zip code at a minimum to get a report, but you can enter your entire address for more precision...supposedly. I have a station coming in at -6.5 Rx(dBm), which I have come to realize is crazy strong, so no surprise about my RFI with the Mani. I would say if you have any stations coming in at -10 or less (more?), be wary.