Some what OT, but use REW to test TT isolation.

[srbaker]

I think this will work. I would like to use REW to test various isolation ideas for my TT. I currently have my Rega P3 on a stock Target wall rack. When I am using the Klipshorns the wall and the Target rack and to a much lesser extent the TT all vibrate with deep bass passages. So this is my plan: put a record on the TT with the needle on the record, but not spinning. Run the Record out (I am using an Adcom GFP-710 as my Phone preamp) from the preamp into my sound card (SB Live! 24-bit external) line in and my line out to my integrated amp; basically substituting the TT and preamp for the mic. Then run sweeps and graph make changes run sweeps graph and compare. I guess the main questions I have are, how to calibrate the whole thing and what would be the best sweep tones and other settings to use?

Rich

[Otto]

Hey there srbaker,

So I think I got this right -- you basically want to run sweeps and measure the response of a non-spinning TT to see how the bass is affecting your TT that's mounted to the wall. I think that's a great idea, and one in which I'm interested.

I recently had my Thorens TT sitting atop my rack (kinda high at about eye height), but it is simply unusable in that position. I had a ~100 lb person unknowingly walk into the room, and the stylus jumped all over the place. I'm working on getting a two-shelf wall-mount Target TT rack from a friend, and may well be experiencing the same issues you are.

I'm wondering -- rather than a non-spinning TT, might there be a calibration record out there that would simply output a constant tone? Then, you could measure it without the sweeps, then with the sweeps, and compare. I would imagine that there would be some frequency modulation with respect to the original signal.

Of course, measuring a non-spinning TT output may also show disturbances in the signal. I'd give it a try first (especially since I don't have any calibration records).

It may be easier to initially run straight tones through REW, rather than sweeps. It might be easier to identify a pure tone disturbance in your measured output.

Good luck, and let us know how it goes. I'm mounting my TT on the opposite side of the room as my sub, and somewhat away from the corner.

Interesting...

[srbaker]

I have thought about using my test LP spinning in the blank track, and I will try that. The idea is to play sweep tones, or music from a CD and see (record) the effect on the TT independently.

[srbaker]

I guess another option would be to use a test CD to play tones and just use a sound recorder on the laptop and compare those graphs. However, since I already have REW and it can play and record various tones I though it might be easier to use.

[brucek]

Wouldn't it be easier to simply estabish what frequencies cause vibrations in the turntable by feeding tones from REW into your system? You could use the "frequency tracks cursor" feature of REW's signal generator and move the cursor around until you found the offending frequencies.

The feedback could simply be your finger held lightly on top of the tonearm.

Once the peak offending frequency has been found I guess you could then try to isolate the turntable while that frequency plays.

brucek

[srbaker]

Brucek: In a way that is what I am doing now; just with music playing and my finger. I did not know about the "frequency tracks cursor" feature of REW's signal generator. Thank you, I will try that. However, since I will be generation tones with REW why not record and graph them also, if it is possible.

[brucek]

However, since I will be generation tones with REW why not record and graph them also, if it is possible.

Yeah, I just don't know how reliable the cartridge will be as a feedback device compared to your finger. As I'm sure you know, the voltage output of the cartridge is proportional to the velocity of the stylus, so to reproduce records without frequency discrimination, the phono preamp must exhibit the inverse of the frequency velocity characteristics in which the record was cut.

In general, the higher frequencies produce a larger output voltage than do the lower frequencies on the record. Records are cut this way because of the very large swing it would require to produce low frequency signals at the same output amplitude of higher frequencies. I think this R.I.A.A playback equalization curve that is applied to the cartridge output will skew the results from the phono preamp.

My guess is that your finger will provide a more reliable transducer than the cartridge itself.....

brucek

[srbaker]

Hmmm, forgot about RIAA playback equalization. If I found the values for the RIAA curve and put those in say a mic. cal. file, do you think that would work?

[brucek]

If I found the values for the RIAA curve and put those in say a mic. cal. file, do you think that would work?

Yes.

It would go something like:

10.00 -20.00
20.00 -xx.xx
30.00 -xx.xx
etc. up to
1000.00 0.00

and then up to wherever it ends....... you get the idea

brucek

[srbaker]

What are you thoughts on this. Thinking I need a the equivalent of a Mic. Cal. curve. This is my Idea: with TT playing track 6 sidr 2 of the HiFi News Analogue test LP "Residual system noise (unmodulated groove) run the Mic. Cal. test to get a base line. I tried it with a sweep from 15-1000hz, and I got an error. Edit: use the sound card calibration but use that file as the mic. cal. file.

[srbaker]

Ok that didn't work, I got an error saying the trace was not reliable and the graph looked like a feedback graph.

[brucek]

What are you thoughts on this

I think you are trying to establish the frequencies that cause unwanted vibration in your turntable.

The simplest thing to do is to use the proper soundcard.cal file and use the signal generators 'frequency tracks cursor' feature to test for the worst vibrations.

If you want to make up a microphone.cal file that matches the RIAA curve, you could do that and test an REW sweep up to 200Hz and hope the cartridge is microphony enough to provide an appropriate level to satisfy the input line-in of the soundcard from your preamp. I just don't see how you could use a playing record to get any meaningful data to stop vibrations...

brucek