phono riaa measurements

[Henryyyy]

Hi, can anyone help me measure phono riaa curves with rew (or point me to the relevant posts). I have built a tube preamp with SUT and passive RIAA filter but I am not too happy with it. I am hoping that I can use rew to connect on either end of the preamp to measure how the filter is working . I have used rew with miniDSP mic for room measurements but want something more precise (especially as I am just using my macbook sound card). Is there a USB sound card that will do input/output with levels suitable for mc phono preamps? Thanks! Henry

 

[Matthew J Poes]

Hi, can anyone help me measure phono riaa curves with rew (or point me to the relevant posts). I have built a tube preamp with SUT and passive RIAA filter but I am not too happy with it. I am hoping that I can use rew to connect on either end of the preamp to measure how the filter is working . I have used rew with miniDSP mic for room measurements but want something more precise (especially as I am just using my macbook sound card). Is there a USB sound card that will do input/output with levels suitable for mc phono preamps? Thanks! Henry

I would start with a baseline measurement by driving it directly. The impedance and capacitance may be wrong, but what you do then is take a second measurement loading it. Just make a cable with the loading caps and resistors built into it, if need be. It may not be needed depending on how this was designed. In terms of adjusting for the voltage (which will be far too high), you simply lower the output volume of your source to match that of the cartridge. To do this, you just need to measure the AC voltage of a sinewave coming out of your source. I use an oscilloscope to check this, but a multimeter works too.

 

[John Mulcahy]

You will need to pad down the soundcard output according to the gain of the preamp, probably around 60 dB for MC. A resistive divider will do the trick.

 

[Henryyyy]

Hi, thanks for the advice. If I understand, right, for the input into the preamp I will feed that with the output from my laptop adding a resistive divider to get the voltage lower (Any suggestion on rough values?). I wasn't really clear on what Matthew meant by loading - the cartridge is moving coil so I just have it loaded with a 20K resistor (no caps). I have attached a pdf showing the input of the preamp. I will use an oscilloscope on the output of the preamp to make sure that I am not clipping sine waves and adjust the resistive divider accordingly. Then I want to connect the preamp output to the input of my computer - can you recommend a reasonably priced USB analogue to digital converter? I would then do a sweep and subtract the RIAA values. Is there an easy way to do this in REW? Thanks! Henry

 

[John Mulcahy]

If you load the theoretical RIAA response as a cal file in REW it will be subtracted automatically so any deviation from flat would be a deviation from the RIAA curve. 10k and 10 ohms should be OK for the divider. 20k is on the high side for an MC load, might get some HF peaking from that. More typically would expect 100 ohms or so. There are a couple of audio interface recommendations on the website.

 

[Henryyyy]

Hi John,

I'll try that divider. The loading resistor is on the other side of the step up transformer (10X), so the 20K resistor is equivalent to 200R load (http://www.sowter.co.uk/phono-cartridge-transformers.php). That is what is recommended for the Benz MG cartridge. I hope I've got it right!

I looked at the REW website but I was hoping to spend less on a USB input (I'm not a perfectionist ). I guess I'll try amazon.

Thanks!

 

[John Mulcahy]

Always worth trying the PC's own built-in audio interfaces. You can do a loopback calibration of it first.

 

[Henryyyy]

I will, thanks for the excellent advice!

 

[Matthew J Poes]

Always worth trying the PC's own built-in audio interfaces. You can do a loopback calibration of it first.

I did one on my laptop, it's surprisingly (maybe not so surprising) clean.
The biggest issues I noticed were less than stellar channel separation and sometimes a little bit of leaking between the input and output.
Ignore my loading comment. John's advice is right on.

 

[Henryyyy]

I think I still have to get a usb input device - macbook air doesn't have an audio input. Ta

 

[Matthew J Poes]

I think I still have to get a usb input device - macbook air doesn't have an audio input. Ta

Doesn't it have a TRRS headphone jack? If so then it should have a combo line in and line out on the TRRS. You need a splitter that separates the headphone out (your line out) from the mic in (your line in).
A separate USB interface is still probably a good idea. The Behringer models are very clean and cheap. They have some problems for acoustic measurements, like latency, but I'm not sure it's a huge problem for this. If you have a local shop that carries their interface, it might be worth trying. They start around $30-$40 each. If not, the $99 focusrite is the next cheapest good option.